Publications

2022

A Augusto, A Srinivasa, SG Bowden, Influence of the bulk resistivity on silicon heterojunction solar cells and module reliability, Solar RRL 6 (5), 2100519 (2022)

J Karas, B Phua, A Mo, N Iqbal, K Davis, S Bowden, A Lennon, A Augusto,Copper Outdiffusion from Copper-Plated Solar Cell Contacts during Damp Heat Exposure, ACS Appl. Mater. Interfaces, 14 (10), 12149-12155 (2022)

2021

Maggie M. Potter, Megan E. Phelan, Pradeep Balaji, Phillip Jahelka, Haley C. Bauser, Rebecca D. Glaudell, Cora M. Went, Michael J. Enright, David R. Needell, André Augusto, Harry A. Atwater, and Ralph G. Nuzzo, Silicon Heterojunction Microcells, ACS Appl. Mater. Interfaces, 13, 38, 45600–45608 (2021)

A. Killam, J. Karas, A. Augusto, and S. G. Bowden, Monitoring of Photovoltaic System Performance Using Outdoor Suns-V_OC, Joule 5, 210-227 (2021)

E. E. Looney, Z. Liu, A. Classen, H. Liu, N. Riedel, M. Braga, P. Balaji, A. Augusto, T. Buonassisi, and Ian M. Peters, Representative Identification of Spectra and the Environment (RISE) using K-means, Prog. Phot. Res. Appl. 29 200–211 (2021)

2020

J. Karas, L. Michaelson, K. Munoz, M. J. Hossain, E. Schneller, K. Davis, S. Bowden, and A. Augusto, Degradation of copper-plated silicon solar cells with damp heat stress, Prog Photovolt Res Appl. Prog.1–12 (2020)

A. Augusto, J. Karas, P. Balaji, S. Bowden, and R.R. King, Exploring the practical efficiency limit of silicon solar cells using thin solar-grade substrates, Journal of Materials Chemistry A 8, 16599 (2020)

P. Balaji, W. J. Dauksher, S. G. Bowden, and A. Augusto, Improving surface passivation on very thin substrates for high efficiency silicon heterojunction solar cells, Solar Energy Materials and Solar Cells 216, 110715 (2020)

P. Balaji, S. G. Bowden, and A. Augusto, Studying edge losses in silicon heterojunction solar cells, IEEE 47th Photovoltaic Specialists Conference (2020)

A. Srinivasa, R. R. King, S. G. Bowden P. Balaji, and A. Augusto, Effect of substrate resistivity, defects and temperature on silicon heterojunction solar cells performance, IEEE 47th Photovoltaic Specialists Conference (2020)

A. Srinivasa, S. Herasimenka, A. Augusto and S. G. Bowden, Effect of ingot variability on performance of silicon heterojunction solar cells, IEEE 47th Photovoltaic Specialists Conference (2020)

S. Kim, A. Augusto, S. G. Bowden, and C. B. Honsberg, Ultrathin SiO2/Al2O3 passivation for silicon heterojunctions using rapid thermal annealing, IEEE 47th Photovoltaic Specialists Conference (2020)

2019

J. Karas, A. Sinha, V. S. P. Buddha, F. Li, G. TamizhMani, S. G. Bowden, and A. Augusto, Damp Heat Induced Degradation of Silicon Heterojunction Solar Cells with Cu-plated Contacts, IEEE Journal of Photovoltaics, October 1-6 (2019)

D. Alonso-Álvarez, A. Augusto, P. Pearce, L. F. Llin, A. Mellor; S. Bowden, D. Paul, and N. Ekins-Daukes, Thermal emissivity of silicon heterojunction solar cells, Solar Energy Materials and Solar Cells 201, 110051 (2019)

P. Balaji, W. J. Dauksher, S. G. Bowden, and A. Augusto, Flexible silicon heterojunction solar cells on 40 µm thin substrates, IEEE 46th Photovoltaic Specialists Conference, 1089-1092 (2019)

A. Augusto, A. Srinivasa, R. R. King, and S. G. Bowden, Performance of silicon heterojunction solar cells using high resistivity substrates, IEEE 46th Photovoltaic Specialists Conference, 0300-0303 (2019)

S. Kim, P. Balaji, A. Augusto, S. G. Bowden, and C. B. Honsberg, Ultra thin Al2O3 passivation for hetero-junction Si solar cell, IEEE 46th Photovoltaic Specialists Conference, 2684-2687 (2019)

2018

A. Augusto, P. Balaji, J. Karas, and S. G. Bowden, Impact of substrate thickness on the surface passivation in high performance n-type solar cells, 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC), 2792 - 2794 (2018)

P. Balaji, W. J. Dauksher, S. G. Bowden, and A. Augusto, Development of 40 µm thin flexible silicon heterojunction solar cells, IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC), 2100-2103 (2018)

A. Srinivasa, J. Karas, A. Augusto, and S. G. Bowden, Performance of Silicon Heterojunction Solar Cells Under Low Illumination Conditions, IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC), 2070 - 2073 (2018)

2017

A. Augusto, S. Y. Herasimenka, R. R. King, S. G. Bowden, and C. Honsberg, “Analysis of the recombination mechanisms of a silicon solar cell with low bandgap-voltage offset,” Journal of Applied Physics, vol. 121, no. 20, p. 205704, May 2017 [Online]. https://doi.org/10.1063/1.4984071

A. M. Jeffries, L. Ding, J. J. Williams, T. L. Williamson, M. A. Hoffbauer, C. B. Honsberg, and M. I. Bertoni, “Gallium nitride grown by molecular beam epitaxy at low temperatures,” Thin Solid Films, vol. 642, no. Supplement C, pp. 25–30, Nov. 2017 [Online]. https://doi.org/10.1016/j.tsf.2017.07.066

Thin Solid Films

Abstract

Growth of gallium nitride at low temperatures broadens the opportunity for its integration into optoelectronic devices that contain thermally sensitive substrates or active layers. As temperature is a very critical growth parameter‚ changes in crystallinity‚ defect density‚ optical‚ and structural properties are expected as temperatures fall below those typical of molecular beam epitaxy growth. In this contribution‚ energetic neutral atomic-beam lithography and epitaxy‚ a molecular beam epitaxy method that utilizes energetic neutral atomic nitrogen as the active nitrogen species‚ is used to grow gallium nitride directly on nitridized sapphire at temperatures between 800 and 200°C. Photospectroscopy‚ photoluminescence‚ Raman spectroscopy‚ scanning electron microscopy and X-ray diffractometry are applied to determine changes in optical‚ morphological and structural properties induced by the unconventional low-temperature growth process. As anticipated‚ we observe that defect density‚ disorder‚ and light absorptance increase as growth temperature decreases. Interestingly‚ X-ray diffraction and photoluminescence reveal the presence of the cubic phase of gallium nitride in films grown at low temperatures under a nitrogen-rich regime‚ which differs from growth conditions reported by plasma-assisted molecular beam epitaxy and metalorganic molecular beam epitaxy. These discrepancies are presented in a critical review of several studies reporting the stabilization of the cubic phase over the energetically-favored hexagonal phase‚ with emphasis on relation to growth temperature‚ Ga/N flux ratio and surface kinetics during growth.

C. B. Honsberg, S. Y. Herasimenka, Y. Zou, M. K. Cotton, and J. Lee, “Auger Recombination Impact for Limiting Efficiency of Silicon Solar Cells,” in 33rd European Photovoltaic Solar Energy Conference and Exhibition, 2017, pp. 849–852 [Online]. https://doi.org/10.4229/EUPVSEC20172017-2CV.2.34

Abstract

The detailed balance equations including material parameters are important to predict the theoretical efficiency for specific materials. The temperature dependence of recombination processes in silicon solar cell are discussed due to sensitivity of electrical parameters with single junction and multiple exciton generation solar cells (MEGSC). Among these characteristics‚ the impact of Auger recombination (AR) is significant in high doping or high injection level so that the effects of AR are applied into the detailed balance equations of silicon solar cells for single junction and multiple exciton generation concept. After we parameterize the material properties‚ the temperature sensitivity of the open circuit voltage shows as the most significant impact to the efficiency. Further‚ we investigate various Auger coefficient modeling to see the effect of temperature sensitivity. After detailed analyzation‚ we could also find the potential benefits of using MEGSC in comparison to SJ approaches.

E. Vadiee, E. Renteria, C. Zhang, J. J. Williams, A. Mansoori, S. Addamane, G. Balakrishnan, and C. B. Honsberg, “AlGaSb-Based Solar Cells Grown on GaAs: Structural Investigation and Device Performance,” IEEE Journal of Photovoltaics, vol. 7, no. 6, pp. 1795–1801, Nov. 2017. https://doi.org/10.1109/JPHOTOV.2017.2756056

IEEE Journal of Photovoltaics

Abstract

GaSb and alloys based on the 6.1 Å family can be grown metamorphically on substrates such as GaAs allowing for the realization of several multijunction solar cell designs. This paper investigates the molecular beam epitaxy growth‚ crystal quality‚ and device performance of AlxGa1-xSb-based single-junction solar cells grown on GaAs substrates. The focus is on the optimization of the growth of AlxGa1-xSb on GaAs (001) substrates in order to minimize the threading dislocation density resulting from the large lattice mismatch between GaSb and GaAs. Utilizing optimum growth conditions‚ solar cells with absorbing layers of different AlxGa1-xSb compositions are studied and compared to control cells grown on lattice-matched GaSb substrates. GaSb‚ Al0.15Ga0.85Sb‚ and Al0.5Ga0.5Sb solar cells grown on GaAs substrates show open-circuit voltages of 0.16‚ 0.17‚ and 0.35 V‚ respectively. Furthermore‚ the lattice-mismatched cells demonstrate promising carrier collection with comparable spectral response to lattice-matched control cells grown on GaSb.

T. C. R. Russell, R. Saive, A. Augusto, S. G. Bowden, and H. A. Atwater, “The Influence of Spectral Albedo on Bifacial Solar Cells: A Theoretical and Experimental Study,” IEEE Journal of Photovoltaics, vol. 7, no. 6, pp. 1611–1618, Nov. 2017. https://doi.org/10.1109/JPHOTOV.2017.2756068

IEEE Journal of Photovoltaics

Abstract

Summary form only given. Strong light-matter coupling has been recently successfully explored in the GHz and THz [1] range with on-chip platforms. New and intriguing quantum optical phenomena have been predicted in the ultrastrong coupling regime [2]‚ when the coupling strength Ω becomes comparable to the unperturbed frequency of the system ω. We recently proposed a new experimental platform where we couple the inter-Landau level transition of an high-mobility 2DEG to the highly subwavelength photonic mode of an LC meta-atom [3] showing very large Ω/ωc = 0.87. Our system benefits from the collective enhancement of the light-matter coupling which comes from the scaling of the coupling Ω ∝ √n‚ were n is the number of optically active electrons. In our previous experiments [3] and in literature [4] this number varies from 104-103 electrons per meta-atom. We now engineer a new cavity‚ resonant at 290 GHz‚ with an extremely reduced effective mode surface Seff = 4 × 10-14 m2 (FE simulations‚ CST)‚ yielding large field enhancements above 1500 and allowing to enter the few (<;100) electron regime. It consist of a complementary metasurface with two very sharp metallic tips separated by a 60 nm gap (Fig.1(a‚ b)) on top of a single triangular quantum well. THz-TDS transmission experiments as a function of the applied magnetic field reveal strong anticrossing of the cavity mode with linear cyclotron dispersion. Measurements for arrays of only 12 cavities are reported in Fig.1(c). On the top horizontal axis we report the number of electrons occupying the topmost Landau level as a function of the magnetic field. At the anticrossing field of B=0.73 T we measure approximately 60 electrons ultra strongly coupled (Ω/ω- ||

C. Zhang, Y. Kim, N. N. Faleev, and C. B. Honsberg, “Improvement of GaP crystal quality and silicon bulk lifetime in GaP/Si heteroepitaxy,” Journal of Crystal Growth, vol. 475, no. Supplement C, pp. 83–87, Oct. 2017 [Online]. https://doi.org/10.1016/j.jcrysgro.2017.05.030

Journal of Crystal Growth

Abstract

The GaP crystal quality and Si bulk lifetime of GaP/Si heterostructures‚ grown by molecular beam epitaxy‚ are investigated. The Si bulk lifetime is reduced by over one order of magnitude after thermal deoxidation at high temperatures (>700°C). This significant reduction of the bulk lifetime is not observed when 150nm-thick SiNx film is present on the backside of Si wafer‚ which can act as a diffusion barrier and/or getter. In addition‚ a 15nm-thick GaP layer grown on the front side of Si wafer with SiNx on the backside shows a high crystal quality of GaP with a low crystalline defect density of 1.1×105cm−2. Moreover‚ the Si bulk lifetime is determined to be 1.83ms with a-Si:H passivation at an injected minority-carrier density of 1×1015cm−3‚ indicative of no bulk lifetime degradation. The high crystallinity of GaP and improved Si bulk lifetime are beneficial to improve photovoltaic device performance of III–V compound solar cells integrated with Si solar cells.

A. Augusto, E. Looney, C. del Cañizo, S. G. Bowden, and T. Buonassisi, “Thin silicon solar cells: Pathway to cost-effective and defect-tolerant cell design,” Energy Procedia, vol. 124, pp. 706–711, Sep. 2017 [Online]. https://doi.org/10.1016/j.egypro.2017.09.346

Energy Procedia

Abstract

Thinner silicon wafers are a pathway to lower cost without compromising the efficiency of solar cells. In this work‚ we study the recombination mechanism for thin and thick silicon heterojunction solar cells‚ and we discuss the potential of using more defective material to manufacture high performance thin solar cells. Modelling the performance of silicon heterojunction solar cells indicates that at open-circuit voltage the recombination is dominated by Auger and surface‚ representing nearly 90% of the total recombination. At maximum power point‚ the surface is responsible for 50 to 80% of the overall recombination‚ and its contribution increases inversely with the wafer thickness. The experimental results show that for lower quality CZ material with 1 ms bulk lifetime‚ 60 µm-thick cells perform better than 170 µm-thick cells. The potential efficiency gain is 1% absolute. The gains in voltage of using thinner wafers are significantly higher for the lower quality CZ material‚ 25 mV‚ than for standard CZ material‚ 10 mV.

T. U. Nærland, S. Bernardini, N. Stoddard, E. Good, A. Augusto, and M. Bertoni, “Comparison of iron-related recombination centers in boron, gallium, and indium doped silicon analyzed by defect parameter contour mapping,” Energy Procedia, vol. 124, pp. 138–145, Sep. 2017 [Online]. https://doi.org/10.1016/j.egypro.2017.09.321

Energy Procedia

Abstract

In this work‚ we are showing that iron (Fe) related defects in mono-silicon have very different recombination characteristics depending on the doping element employed. While the defect characteristics of the Fe in its dissociated state is comparably the same in the materials of investigation‚ the defect characteristics of the associated state vary considerably. By using‚ defect parameter contour mapping (DPCM)‚ a newly developed method for analyzing temperature and injection dependent lifetime data‚ we have for the first time‚ been able to show that in the case of gallium doping it is the orthorhombic state of the Fe-acceptor complex that is dominating the lifetime.

S. Y. Herasimenka, M. Reginevich, L. Michaelson, K. Munoz, T. Tyson, R. Yessa, E. Khokhlov, S. Nastochkin, V. Shiripov, J. Karas, W. Dauksher, and S. G. Bowden, “Cu-SHJ cells with 22.7% efficiency and a roadmap to 24%,” presented at the 27th Workshop on Crystalline Silicon Solar Cells & Modules, Breckenridge, Colorado, 2017.

27th Workshop on Crystalline Silicon Solar Cells & Modules

Allison Wolf, Milton Johnson, Scott Currier, Elliot Hall, Stuart G Bowden, and A. Killam, “Creating an Inexpensive Instrument to Accurately Measure Solar Irradiance,” presented at the 27th Workshop on Crystalline Silicon Solar Cells & Modules, Breckenridge, Colorado, 2017.

27th Workshop on Crystalline Silicon Solar Cells & Modules

Abstract

Solar irradiance measurement presents a unique challenge: although there are a plethora of devices available‚ they are typically expensive or highly inaccurate. We present the solar irradiance device (SID)‚ an apparatus that can be constructed from readily available materials for under fifteen dollars and is especially suited to photovoltaic applications. We discuss the methods for the calibration of the device and the likely sources of error. To date‚ SID has shown a repeatability of 1% and further measurements are underway to determine the repeatability throughout the year and at alternate locations.

Scott Currier, Elliot Hall, Allison Wolf, Milton Johnson, Stuart G Bowden, and Nicole Bowers, “Educating Teachers to Create Solar Engineers,” presented at the 27th Workshop on Crystalline Silicon Solar Cells & Modules, Breckenridge, Colorado, 2017.

27th Workshop on Crystalline Silicon Solar Cells & Modules

Abstract

The purpose of this paper is to inform the reader about the Quantum Energy and Sustainable Solar Technologies (QESST) research experience for educating teachers in order to expose students to photovoltaic education. The Research Experience for Teachers (RET) participants are paired with REUs (Research Experience for Undergraduates)‚ and high school students to complete a two-week Solar 101 course‚ which takes them from a silicon wafer to a completed heterojunction solar cell. They then complete a three-week research project‚ and work on creating curriculum for their grade specific classrooms that will end up being published for the general public.

Y. Kim, I.-W. Cho, M.-Y. Ryu, J. O. Kim, S. J. Lee, K.-Y. Ban, and C. B. Honsberg, “Stranski–Krastanov InAs/GaAsSb quantum dots coupled with sub-monolayer quantum dot stacks as a promising absorber for intermediate band solar cells,” Applied Physics Letters, vol. 111, no. 7, p. 073103, Aug. 2017 [Online]. https://doi.org/10.1063/1.4999437

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Applied Physics Letters

Journal of Applied Physics

L. B. Bliss, “Optimization of Front Contact Design on Nickel-Plated Si Solar Cells,” Barrett Honors College Thesis, Arizona State University, 2017 [Online].

Abstract

As global population and demand for electrical power increase‚ humanity is faced with the growing challenge of harnessing and distributing enough energy to sustain the developing world. Currently‚ fossil fuels (coal/natural gas) are our main sources of electricity. However‚ their cost is increasing‚ they are nonrenewable‚ and they are very harmful to the environment. Thus‚ capacity expansion in the renewable energy sector must be realized to offset higher energy demand and reduce dependence on fossil fuels. Solar energy represents a practical solution‚ as installed global solar capacity has been increasing exponentially over the past 2 decades. However‚ even with government incentives‚ solar energy price (/kWh) continues to be highly dependent on political climate and raw material (silicon and silver) cost. To realistically and cost effectively meet the projected expansions within the solar industry‚ silver must be replaced with less costly and more abundant metals (such as copper) in the front-grid metallization process of photovoltaic cells. Copper‚ while offering both higher achievable efficiencies and a raw material cost nearly 100 times cheaper than silver‚ has inherent disadvantages. Specifically‚ copper diffuses rapidly into the silicon substrate‚ requires more complex and error-prone processing steps‚ and tends to have less adhesive strength‚ reducing panel robustness. In this study‚ nickel deposition via sputtering was analyzed‚ as well as overall potential of nickel as a seed layer for copper plating‚ which also provides a barrier layer to copper diffusion in silicon. Thermally-formed nickel silicide also reduces contact resistivity‚ increasing cell efficiency. It was found that at 400 ºC‚ ideal nickel silicide formation occurred. By computer modeling‚ contact resistivity was found to have a significant impact on cell efficiency (up to 1.8%). Finally‚ sputtering proved useful to analyze nickel silicide formation‚ but costs and time requirements prevent it from being a practical industrial-scale metallization method.

E. LeBeau, “All About Solar,” Barrett Honors College Thesis, Arizona State University, 2017 [Online].

Abstract

This is a lectures series on photovoltaics. As the need for electrical energy rises‚ mankind has struggled to meet its need in a reliable lasting way. Throughout this struggle‚ solar energy has come to the foreground as a complete solution. However‚ it has many drawbacks and needs a lot of development. In addition‚ the general public is unaware of how solar energy works‚ how it is made‚ and how it stands economically. This series of lectures answering those three questions. After two years doing photovoltaic research‚ and an undergraduate degree in Electrical Engineering‚ enough expertise has been acquired present on at a late high-school to early college level. Education is key to improving the popularity of using solar energy and the popularity of investing in photovoltaic research. Solar energy is a viable option to satisfy our energy crisis because the materials it requires can quickly be acquired‚ and there is enough of material to provide a global solution. In addition‚ the amount of solar energy that hits the surface of the earth in a day is orders of magnitude more than the amount of energy we require. The main goal of this project is to have an effective accessible tool to teach people about solar. Thus‚ the lectured will be posted on pveducation.com‚ YouTube‚ the Barrett repository‚ and the QUSST website. The content was acquired in four ways. The first way is reading up on the current papers and journals describing the new developments in photovoltaics. The second part is getting in contact with Stuart Bowden and Bill Daukser at Arizona State University’s Solar Power Lab as well as the other faculty associated with the Solar Power Lab. There is quite a bit of novel research going on at their lab‚ as well as a student run pilot line that is actively building solar cells. The third way is reading about solar device physics using device physics textbooks and the PVEducation website made by Stuart Bowden. The forth way is going into ASU’s solar power lab.

K. G. Nelson, A. F. McKenna, S. K. Brem, J. Hilpert, J. Husman, and E. Pettinato, “Students’ Misconceptions about Semiconductors and Use of Knowledge in Simulations,” Journal of Engineering Education, vol. 106, no. 2, pp. 218–244, Apr. 2017 [Online]. https://doi.org/10.1002/jee.20163

Journal of Engineering Education

J. Eng. Educ.

Abstract

Background Little research exists on students’ misconceptions about semiconductors‚ why they form‚ and what role educational resources like simulations play in misconception formation. Research on misconceptions can help enhance student learning about semiconductors. Purpose (Hypothesis) This project sought to identify students’ misconceptions about three semiconductor phenomena – diffusion‚ drift‚ and excitation – and to determine if prior knowledge‚ knowledge acquired from watching animated simulations‚ or both were related to students’ misconceptions. We hypothesized that students would hold misconceptions about those phenomena and that students’ prior knowledge and knowledge acquired from watching animated simulations would be associated with their misconceptions. Design/Method Forty-one engineering students completed an instrument that asked questions about three semiconductor phenomena after the students had observed the animated simulations. Responses were analyzed and coded using two frameworks: misconception and knowledge use. Results Misconceptions were prevalent for all three phenomena. Misconceptions were associated with use of incorrect prior knowledge‚ a combination of correct or incorrect prior knowledge‚ and the knowledge acquired from watching the animated simulations alone or in combination with correct and incorrect prior knowledge. Misconceptions indicated a lack of understanding of chemistry and physics concepts. Conclusions Findings indicate that students hold many misconceptions about semiconductor phenomena. These misconceptions were common among our participants. The knowledge acquired from the animated simulations alone or in combination with prior knowledge could reinforce or contribute to misconception formation. Our findings can guide instructors to use or create better simulations to aid student learning.

E. Khokhlov, S. Nastockin, S. Yasunas, V. Shiripov, K. Miasnikov, S. Y. Herasimenka, and M. Reginevich, “Lab-scale vacuum equipment for HJT solar cell production,” presented at the 33rd European Photovoltaic Solar Energy and Exhibition, Amsterdam, Netherlands, 2017.

33rd European Photovoltaic Solar Energy and Exhibition

A. M. Maros, “Modeling, Growth and Characterization of III-V and Dilute Nitride Antimonide Materials and Solar Cells,” Ph. D. Thesis, Arizona State University, 2017 [Online].

Abstract

III-V multijunction solar cells have demonstrated record efficiencies with the best device currently at 46 % under concentration. Dilute nitride materials such as GaInNAsSb have been identified as a prime choice for the development of high efficiency‚ monolithic and lattice-matched multijunction solar cells as they can be lattice-matched to both GaAs and Ge substrates. These types of cells have demonstrated efficiencies of 44% for terrestrial concentrators‚ and with their upright configuration‚ they are a direct drop-in product for today’s space and concentrator solar panels. The work presented in this dissertation has focused on the development of relatively novel dilute nitride antimonide (GaNAsSb) materials and solar cells using plasma-assisted molecular beam epitaxy‚ along with the modeling and characterization of single- and multijunction solar cells. Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films‚ leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states. The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2‚ a fill factor of 64 % and an open-circuit voltage of 0.39 V‚ corresponding to a bandgap-voltage offset of 0.57 V‚ comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented.

J. J. Williams, H. McFavilen, A. M. Fischer, D. Ding, S. Young, E. Vadiee, F. A. Ponce, C. Arena, C. B. Honsberg, and S. M. Goodnick, “Refractory In_xGa_1-xN Solar Cells for High-Temperature Applications,” IEEE Journal of Photovoltaics, vol. PP, no. 99, pp. 1–7, 2017. https://doi.org/10.1109/JPHOTOV.2017.2756057

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IEEE Journal of Photovoltaics

Abstract

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J. Oh, B. Dauksher, S. Bowden, G. Tamizhmani, P. Hacke, and J. D’Amico, “Further Studies on the Effect of SiN Refractive Index and Emitter Sheet Resistance on Potential-Induced Degradation,” IEEE Journal of Photovoltaics, vol. PP, no. 99, pp. 1–7, 2017. https://doi.org/10.1109/JPHOTOV.2016.2642952

IEEE Journal of Photovoltaics

Abstract

We present the impacts of silicon nitride (SiN ) antireflection coating refractive index and emitter sheet resistance on potential-induced degradation of the shunting type (PID-s). Previously‚ it has been shown that the cell becomes more PID-s-susceptible as the refractive index decreases or the emitter sheet resistance increases. To verify the effect of refractive index on PID-s‚ we fabricated cells with varying SiN refractive index (1.87‚ 1.94‚ 2.05) on typical p-type base solar cells with ∼60 Ω/sq emitters. However‚ none of these cells showed output power degradation‚ regardless of the refractive index. Further investigation of the emitter showed that the PID-s was suppressed at ∼60 Ω/sq due to the extremely high surface phosphorus concentration (6 × 1021 cm−3)‚ as measured by secondary ion mass spectrometry. Furthermore‚ PID-s was observed on cells possessing a high emitter sheet resistance (∼80 Ω/sq). The emitter surface phosphorus concentration plays an important role in determining PID-s susceptibility.

2016

S. Y. Herasimenka, W. J. Dauksher, M. Boccard, and S. Bowden, “ITO/SiOx:H stacks for silicon heterojunction solar cells,” Solar Energy Materials and Solar Cells, vol. 158, Part 1, pp. 98–101, Dec. 2016 [Online]. https://doi.org/10.1016/j.solmat.2016.05.024

Solar Energy Materials and Solar Cells

Abstract

A method of reducing optical losses in the transparent conductive oxides (TCO) used in silicon heterojunction solar cells without compromising with series resistance is described. In the method the thickness of a TCO is reduced two-three times and a hydrogenated dielectric is deposited on top to form a double layer antireflection coating. The conductivity of a thin TCO is increased due to the effect of hydrogen treatment supplied from the capping dielectric during the post deposition annealing. The optimized cells with ITO/SiOx:H stacks achieved more than 41 mA/cm2 generation current on 120-micron-thick wafers while having approximately 100 Ω/square sheet resistance. The paper also considers integration of ITO/SiOx:H stacks with Cu plating and using ITO/SiNx/SiOx triple layer antireflection coatings.

A. Maros, N. N. Faleev, M. I. Bertoni, C. B. Honsberg, and R. R. King, “Carrier localization effects in GaAs1−xSbx/GaAs heterostructures,” Journal of Applied Physics, vol. 120, no. 18, p. 183104, Nov. 2016 [Online]. https://doi.org/10.1063/1.4967755

Journal of Applied Physics

Abstract

We investigated the structural and optical properties of GaAs1−xSbx/GaAs heterostructures grown by molecular beam epitaxy on GaAs (001) substrates for Sb concentration up to 12% by means of high-resolution X-ray diffraction and photoluminescence. The correlation between our structural and optical analysis revealed that compositional fluctuations induced localized states which trap carriers at low temperature. Under low excitation power‚ the photoluminescence (PL) spectra are composed of two competing peaks in the temperature range of 30–80 K. The lower energy peak is associated with transitions from localized states in the band-tail of the density of states while the higher energy peak corresponds to transitions from free carriers. A model based on a redistribution process of localized excitons was used to reproduce the S-shape behavior of the temperature dependent PL. Reducing the growth temperature from 500 °C to 420 °C suppressed the S-shape behavior of the PL indicating a reduction in compositional variation.

K.-Y. Ban, Y. Kim, D. Kuciauskas, S. P. Bremner, and C. B. Honsberg, “Investigation of carrier dynamics in InAs/GaAsSb quantum dots with different silicon delta-doping levels,” Semiconductor Science and Technology, vol. 31, no. 12, p. 125010, Nov. 2016 [Online]. https://doi.org/10.1088/0268-1242/31/12/125010

Semiconductor Science and Technology

Semicond. Sci. Technol.

Abstract

The optical properties of InAs quantum dots (QDs) embedded in a GaAsSb matrix with different delta ( δ )-doping levels of 0‚ 2‚ 4‚ and 6 electrons per dot (e − /dot)‚ incorporated to control the occupation of QD electronic states‚ are studied by photoluminescence (PL) spectroscopy. The time-resolved PL data taken at 10 K reveal that the increase of δ -doping density from 2 to 6 e − /dot decreases the recombination lifetime of carriers at ground states of the QDs from 996 ± 36 to 792 ± 19 ps‚ respectively. Furthermore‚ the carrier lifetime of the sample with 4 e − /dot is found to increase at a slower rate than that of the undoped sample as temperature increases above 70 K. An Arrhenius plot of the temperature dependent PL intensity indicates that the thermal activation energy of electrons in the QDs‚ required for carrier escape from the dot ground state to continuum state‚ is increased when the δ -doping density is high enough (>4 e − /dot). These results are attributed to the enhanced Coulomb interaction of electrons provided by the δ -doping‚ leading to reduced thermal quenching of the PL.

S. H. Lee, J. S. Kim, S. Yoon, Y. Kim, S. J. Lee, and C. B. Honsberg, “Investigation of Localized Electric Field in the Type-II InAs/GaAsSb/GaAs Structure,” Acta Physica Polonica A, vol. 130, no. 5, pp. 1213–1216, Nov. 2016. https://doi.org/10.12693/APhysPolA.130.1213

WOS:000389065300016

Acta Physica Polonica A

Acta Phys. Pol. A

Abstract

The effect of localized electric field (F) was investigated in the type-II InAs/GaAsSb/GaAs structures. To compare type-I to type-II‚ two types of samples with different Sb contents was grown by molecular beam epitaxy‚ whose Sb contents are 3% (type-I) and 16% (type-II)‚ respectively. In the both samples‚ we performed excitation power dependent-photoreflectance at 10 K and the result showed that the period of the Franz-Keldysh oscillation‚ revealed above the band gap (E-g) of GaAs‚ was broadened in the only type-II system‚ which means that F was also increased because it is proportional to the period of the Franz-Keldysh oscillation while the period of the Franz-Keldysh oscillations stayed unchanged in type-I system. This phenomenon is explained by that the F was affected by the band bending effect caused by the spatially separated photo-excited carriers in the interface between GaAsSb and GaAs. The F changed linearly as a function of square root of excitation power as expected for the F. Moreover‚ F was calculated using fast Fourier transform method for a qualitative analysis‚ which is in a good agreement with the theory of triangular well approximation.

M. S. Bailly, J. Karas, H. Jain, W. J. Dauksher, and S. Bowden, “Damage-free laser patterning of silicon nitride on textured crystalline silicon using an amorphous silicon etch mask for Ni/Cu plated silicon solar cells,” Thin Solid Films, vol. 612, pp. 243–249, Aug. 2016 [Online]. https://doi.org/10.1016/j.tsf.2016.06.011

Thin Solid Films

Abstract

We investigate the optimization of laser ablation with a femtosecond laser for direct and indirect removal of SiNx on alkaline textured c-Si. Our proposed resist-free indirect removal process uses an a-Si:H etch mask and is demonstrated to have a drastically improved surface quality of the laser processed areas when compared to our direct removal process. Scanning electron microscope images of ablated sites show the existence of substantial surface defects for the standard direct removal process‚ and the reduction of those defects with our proposed process. Opening of SiNx and SiOx passivating layers with laser ablation is a promising alternative to the standard screen print and fire process for making contact to Si solar cells. The potential for small contacts from laser openings of dielectrics coupled with the selective deposition of metal from light induced plating allows for high-aspect-ratio metal contacts for front grid metallization. The minimization of defects generated in this process would serve to enhance the performance of the device and provides the motivation for our work.

A. Augusto, K. Tyler, S. Y. Herasimenka, and S. G. Bowden, “Flexible Modules Using <70 μm Thick Silicon Solar Cells,” Energy Procedia, vol. 92, pp. 493–499, Aug. 2016 [Online]. https://doi.org/10.1016/j.egypro.2016.07.132

Energy Procedia

Abstract

Highly flexible modules using thin 153 cm2 silicon crystalline cells and transparent fluoropolymer foil are demonstrated. The modules can be flexed 200 times around a bend radius of 4 cm without change in efficiency. The silicon crystalline heterojunction solar cells are 65±5 μm-thick with efficiencies up to 18.4%. Cracks in the solar cells and interconnections that are induced by mechanical stress during module bending are examined using electroluminescence. Two interconnection solutions are discussed: ribbons affixed to the busbars using a conductive adhesive‚ and indium coated wires directly bonded to the cell fingers. Modules using wire interconnection are found to be highly flexible with efficiencies greatly exceeding existing commercial flexible modules using thin films and have potential applications in light-weight modules for building integrated and portable photovoltaic power.

L. Ding, C. Zhang, T. U. Nærland, N. Faleev, C. Honsberg, and M. I. Bertoni, “Silicon Minority-carrier Lifetime Degradation During Molecular Beam Heteroepitaxial III-V Material Growth,” Energy Procedia, vol. 92, pp. 617–623, Aug. 2016 [Online]. https://doi.org/10.1016/j.egypro.2016.07.027

Energy Procedia

Abstract

A major hindrance to the development of devices integrating III-V materials on silicon‚ where it is an active component of the device‚ is the preservation of its electronic quality. In this contribution‚ we report on our effort to identify the mechanism behind the severe decrease in the bulk minority-carrier lifetime of silicon after heteroepitaxial growth of gallium phosphide‚ in our molecular beam epitaxy (MBE) system. We identify that the drop in lifetime occurs at a threshold temperature of 500 °C; we assign the increased recombination rate to extrinsic‚ fast-diffusing impurities coming from the MBE chamber environment. Impurities can be gettered by phosphorous diffusion‚ leading to a lifetime recovery. Moreover‚ we narrow the list of contaminants based on specific experimental observations and compare our hypothesis to modeling of injection-dependent lifetime spectra. Finally we show that coating the silicon wafer with a sacrificial silicon nitride film helps significantly to reduce contamination and provides a path to successful III-V growth on silicon.

Y. Kim, N. N. Faleev, K.-Y. Ban, J. O. Kim, S. J. Lee, and C. B. Honsberg, “Growth of highly dense InAs quantum dots with improved crystal quality embedded in an InGaAsSb quantum well,” Journal of Physics D: Applied Physics, vol. 49, no. 30, p. 305102, Jul. 2016 [Online]. https://doi.org/10.1088/0022-3727/49/30/305102

Journal of Physics D: Applied Physics

J. Phys. D: Appl. Phys.

Abstract

We investigate the structural and optical properties of InAs quantum dots (QDs) embedded in In x Ga 1− x As 0.96 Sb 0.04 layers with different In content. Indium incorporation from 0 to 9% increases QD areal density up to 9.3 × 10 10 cm −2 and decreases QD diameter and height by the reduction of the island–island interaction. The elastic strain of the InGaAsSb layers‚ surrounded by a GaAs matrix‚ increases with higher In content. Further‚ the increase of In content from 5 to 9% reduces the density of dislocation loops in the InGaAsSb and GaAs layers almost by half due to improvement of the InAs/InGaAsSb interface quality. The photoluminescence peak from the QDs is redshifted with increasing In content as a result of reduced strain inside the QDs.

E. Vadiee, C. Zhang, N. N. Faleev, S. Addamane, S. Wang, F. A. Ponce, G. Balakrishnan, and C. B. Honsberg, “AlGaSb based solar cells grown on GaAs by Molecular Beam Epitaxy,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 2313–2316. https://doi.org/10.1109/PVSC.2016.7750050

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

A goal for concentrating photovoltaics is to realize efficiencies over 50%. Recent 4J bonded solar cells show a path to such high efficiency devices by separately growing the top and bottom solar cells. Present experimental devices use InP-based materials for the bottom junctions. III-Sb solar cells can be good candidates for bottom solar cells. Sb-containing III-V alloys have shown high electron mobility‚ wide band gap range including small band gaps‚ flexible band alignment‚ and small effective electron mass [1]. In addition‚ GaSb alloys can be grown with low defect densities on GaAs. This paper investigates GaSb-based solar cells. We show AlGaSb based solar cells grown directly on semi-insulator GaAs (001) substrates by Molecular Beam Epitaxy (MBE). Device and structural investigations have been performed to assess the electrical properties and material quality. Devices in the GaSb material system show Woc of 0.30‚ a very high value for a low band gap solar cell. To control the device properties‚ GaSb based solar cells grown on GaAs (100) substrates were compared to the devices grown on GaSb substrates.

D. B. Needleman, A. Augusto, A. Peral, S. Bowden, C. del Cañizo, and T. Buonassisi, “Thin absorbers for defect-tolerant solar cell design,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 0606–0610. https://doi.org/10.1109/PVSC.2016.7749669

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Thin silicon wafers provide a pathway to lower cost and lower capital intensity PV module manufacturing. They can also produce higher-efficiency devices with less expensive feedstock and crystallization processes because they require shorter diffusion lengths and operate at higher carrier injection. Through simulation‚ we show that thin Si wafers can be incorporated into high-efficiency cells with greater defect tolerance than thick wafers. Experimentally‚ we demonstrate the importance of excellent surface passivation to realizing the efficiency potential of thin silicon solar cells and show that such passivation can be achieved in silicon/amorphous silicon heterojunction devices.

A. F. Routhier and C. Honsberg, “Thermal energy storage to increase solar photovoltaic penetration,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 1857–1861. https://doi.org/10.1109/PVSC.2016.7749943

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

This research focuses on a novel management strategy of a thermal energy storage (TES) system‚ along with the associated chillers‚ in order to increase solar photovoltaic (PV) penetration at Arizona State University’s (ASU) Tempe campus. By charging the TES while the PV panels are producing power‚ and monitoring the maximum charging rate of the TES‚ it enables ASU to increase the amount of solar PV they’re utilizing‚ and decrease their overall carbon footprint‚ while not sending any energy to the grid. Using this new management method‚ ASU will be able to increase their current PV generation by a factor of 3.46 and generate 48% of their annual energy needs.

J. J. Williams, H. McFavilen, A. M. Fischer, D. Ding, S. R. Young, E. Vadiee, F. A. Ponce, C. Arena, C. B. Honsberg, and S. M. Goodnick, “Development of a high-band gap high temperature III-nitride solar cell for integration with concentrated solar power technology,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 0193–0195. https://doi.org/10.1109/PVSC.2016.7749576

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The III-N material class of semiconductors exhibits desirable properties for construction of a cell for integration with the thermal receiver of a concentrated solar plant. We design a GaN-InGaN based solar cell for operation at 450 °C. An MQW structure for the InGaN absorber is selected to improve voltage through improved material quality. Cell performance shows a VOC of 2.4 V for room temperature and 1.7 V at operating temperature and 300x suns. EQE measurements show little cell performance decrease up to 500 °C. Repeated measurements indicate the device to be thermally robust.

S. Kim, S. Bowden, and C. B. Honsberg, “Fabrication of nanopillar structure by silica nanosphere lithography and passivation with wet chemical oxidation cleaning,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 2922–2924. https://doi.org/10.1109/PVSC.2016.7750192

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

This work focus the fabrication and passivation of Si nanopillar structure. Si nanopillar structures are fabricated by well controlled silica nanosphere lithography with metal assisted chemical etching (MACE). For high quality passivation‚ nanopillar structures are cleaned using general Si cleaning process with wet oxidation. The wet oxidation process helps to reduce surface state density of nanopillar structure. Nanopillar structure is passivated using several methods as thermal‚ wet oxide‚ a-Si:H‚ organic and aluminum oxide (Al2O3). Al2O3 passivation shows the highest lifetime and implied open circuit voltage (Voc) as 31.9 μs and 595 mV.

A. Killam, T. Reblitz, A. Augusto, and S. Bowden, “All silicon tandem solar cell,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 2448–2450. https://doi.org/10.1109/PVSC.2016.7750082

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Crystalline silicon is consistently the dominant material for commercial photovoltaic devices. Exploiting the direct and indirect bandgap of silicon results in a silicon-silicon tandem solar cells with possible efficiency benefits over standard single-junction silicon solar cells. Epitaxial growth offers a way to make such cells and the resulting devices have higher voltage and lower currents leading to much lower module losses. All silicon tandem devices were modeled in PC1D using precise solar spectrums generated with SMARTS. The optimal layer thicknesses found when the input spectrum is AM1.5G for a silicon-silicon device are: 3.3 μm for the top absorber and 172 μm for the bottom absorber. The modeled device produces an efficiency of 21.3%‚ a 1.1% relative increase over a model for a commercial silicon cell.

P. Luppina, S. Bowden, P. Lugli, and S. M. Goodnick, “Modeling of a gallium phosphide/silicon heterojunction solar cells,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 2467–2472. https://doi.org/10.1109/PVSC.2016.7750087

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Here we use a coupled optical and electrical model to study the performance of heterojunction Si (HJSi) solar cells based on gallium phosphide (GaP)/crystalline Silicon (c-Si) structures in comparison with Si (c-Si)/amorphous Si (a-Si) HIT solar cells. The simulations are based on a numerical driftdiffusion model performed with the Sentaurus TCAD tool. We investigate the impact of highly n-doped indium tin oxide (ITO n+) window layer for the case of flat and textured surface with different ITO thicknesses. Simulation results indicates that GaP used in the top layer of a HJSi solar cell is a good candidate to improve the performance and reach efficiencies in excess of of the 25.6% currently reached for a HIT cells with a-Si. We perform a detailed simulation study of a fabricated solar cells structure for various emitter designs‚ extracting key figures of merit like efficiency‚ short-circuit current and open circuit voltage; our values are in good agreement with recently reported solar cells. After having validated our simulation approach‚ different optimization techniques are investigated in order to maximize the performance of the solar cell.

A. Maros, N. Faleev, S. H. Lee, J. S. Kim, C. B. Honsberg, and R. R. King, “1-eV GaNAsSb for multijunction solar cells,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 2306–2309. https://doi.org/10.1109/PVSC.2016.7750048

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

We report on the growth of 1-eV GaNAsSb lattice- matched to GaAs as an alternative material to the most commonly used GaInNAs(Sb). GaNAsSb layers were grown by plasma assisted molecular beam epitaxy with different N and Sb compositions. The electronic and optical properties of the layers were probed using photoluminescence and photoreflectance spectroscopy and compared to the band anticrossing model. The incorporation mechanism of the group-V elements were investigated using secondary ion mass spectrometry. It was found that Sb does not affect the N incorporation. Moreover increasing the N flux increased the N composition at the expense of the Sb composition. Post-growth annealing was investigated and found to greatly improve the photoluminescence intensity.

S. N. Dahal, J. A. LeBeau, S. Bowden, and C. Honsberg, “Sub-surface laser damage in sapphire and silicon: A path towards laser wafering,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 0628–0630. https://doi.org/10.1109/PVSC.2016.7749674

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

A variable wavelength nanosecond pulsed laser is used to create and characterize the subsurface damage in Sapphire and Silicon. The high intensity laser light of wavelengths that are transparent to crystalline Sapphire and silicon is used. The depth and size of the damage spots are compared with a ray-optics model and electron plasma breakdown model. The effect of focusing optics‚ number of shots‚ numerical aperture (NA) of the focusing optic on the experimentally measured subsurface damage spot size is presented. A range of spot sizes were formed from 100-300 μm in the bulk of sapphire without damaging the surface. Preferential chemical etchants and or cleaving methods will be implemented to peel thin wafers from the thicker ones. These results and the understanding in the fields of bulk material modification and internal laser micromachining of semiconductors will be implemented towards the laser wafering.

P. Muralidharan, S. Bowden, S. M. Goodnick, and D. Vasileska, “Multiscale modeling of silicon heterojunction solar cells,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 3547–3551. https://doi.org/10.1109/PVSC.2016.7750331

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

In recent years‚ silicon photovoltaic technologies utilizing amorphous silicon (a-Si) to form heterojunction solar cells with thin intrinsic (HIT) passivating layers have consistently demonstrated high efficiencies (>20%) including a world record efficiency of 25.6%‚ high fill factor’s and high open circuit voltages (VOC > 700 mV). Further improvements in efficiency require a rigorous approach to better understand and improve device behavior. In this work we analyze the transport and device performance of heterojunction cells by applying a multiscale simulation methodology. Our multiscale solver consists of three primary domains‚ namely; the drift-diffusion (DD) domain‚ the ensemble Monte Carlo (EMC) and the kinetic Monte Carlo (KMC) domain. Using our multiscale methodology we investigate the role of midgap defects in the a-Si and interface defects at the crystalline silicon (c-Si) and a-Si heterointerface. Simulations indicate that recombination at the interface is a key limiting factor in device performance and contributes to the ‘S’ shaped current voltage characteristic. We have also used commercial device simulator SILVACO to investigate the role of surface potential at the heterointerface.

C. Zhang, N. N. Faleev, L. Ding, M. Boccard, M. Bertoni, Z. Holman, R. R. King, and C. B. Honsberg, “Hetero-emitter GaP/Si solar cells with high Si bulk lifetime,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 1950–1953. https://doi.org/10.1109/PVSC.2016.7749966

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

III-V/silicon solar cells which have an active silicon bottom solar cell are promising for multi-junction solar cell applications. In such solar cell structures‚ a high minority carrier lifetime in the bulk silicon substrate is necessary. Annealing silicon wafers at high temperature (> 500oC) in the molecular beam epitaxy (MBE) high-vacuum chamber revealed significant lifetime degradation. In this work‚ we developed a practical method to maintain high Si bulk lifetime. SiNx layer deposited on Si back side helps maintain millisecond level minority carrier lifetime. By this procedure high minority carrier lifetime in the Si substrate is preserved while high quality thin GaP layer is achieved. We demonstrate GaP as a hetero-emitter layer with high Si bulk lifetime in GaP/Si structure solar cell with 524mV open circuit voltage.

A. Aguilar, S. Y. Herasimenka, J. Karas, H. Jain, J. Lee, K. Munoz, L. Michaelson, T. Tyson, W. J. Dauksher, and S. Bowden, “Development of Cu plating for silicon heterojunction solar cells,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 1972–1975. https://doi.org/10.1109/PVSC.2016.7749972

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

This paper reports the results of the study comparing various patterning and plating methods for the deposition of Cu electrodes on transparent conductive oxides for silicon heterojunction solar cells. We compared direct electroplating of Cu on different metal seeds (Ag‚ Ni‚ Cr and Ti deposited on transparent conductive oxide by physical vapor deposition) to the light induced plating of Ni/Cu directly on transparent conductive oxide. Patterning was done either using photoresists (formed by spin-on‚ screen printing or lamination) or lift-off of the PECVD dielectric using screen printed resist. The geometry of the fingers‚ line resistance‚ contact resistance and adhesion were used as comparative parameters. We identified direct electroplating of Cu on the sputtered Ag seed to achieve the lowest contact resistance and the best adhesion. All photoresists were able to achieve less than 60 micron resolution and could produce the fingers with the sought height (some‚ however‚ having a characteristic mushroom shape). The best silicon heterojunction cell with Cu contacts directly electroplated on the sputtered Ag seed achieved 21.9% efficiency on 153 cm2 area.

J. Karas, L. Michaelson, M. L. Castillo, K. Munoz, M. Bailly, H. Jain, A. Akey, J. Rand, T. Tyson, T. Buonassisi, and S. Bowden, “Addressing adhesion and reliability concerns of copper-plated c-Si solar cells and modules,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 0899–0903. https://doi.org/10.1109/PVSC.2016.7749739

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Copper-plated contacts for front side crystalline silicon solar cells are a topic of considerable interest‚ with many recent publications presenting a variety of successful methods and impressive cell results. Several of the more obvious challenges yet to be proven relate to the durability and reliability of plated contacts‚ especially the adhesion of plated metal to solar cells and the long-term stability of the metals that could potentially result in gradual power degradation. In this work‚ we have fabricated copper plated cells using several different front side patterning methods. For a resist-based process‚ we have optimized plated cell processing to achieve adhesion comparable to screenprinted silver paste contacts. For laser-based patterning methods‚ greater understanding of the metal-silicon interface and microstructure effecting adhesion is still needed.

L. Ding, C. Zhang, T. U. N?rland, N. Faleev, C. Honsberg, and M. I. Bertoni, “On the source of silicon minority-carrier lifetime degradation during molecular beam heteroepitaxial growth of III-V materials,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 2048–2051. https://doi.org/10.1109/PVSC.2016.7749989

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

A major hindrance to the development of devices integrating III-V materials on silicon is the preservation of its electronic quality. In this contribution‚ we report on the severe decrease in silicon bulk minority-carrier lifetime after heteroepitaxial growth of gallium phosphide‚ in our molecular beam epitaxy (MBE) system. The drop in lifetime occurs after annealing silicon above 500°C; we assign the increased recombination rate to extrinsic defect originating from highly mobile impurities diffusing from the MBE chamber. We show that the contaminant can be gettered by phosphorous diffusion. We investigate two approaches to protect the Si bulk lifetime by containing the contaminant to a part of the silicon that can be removed by etching. This provides a path to successful III-V growth on silicon.

A. Augusto, K. Tyler, S. Y. Herasimenka, and S. G. Bowden, “Series connection front-to-front and back-to-back of silicon heterojunction solar cells,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 2631–2634. https://doi.org/10.1109/PVSC.2016.7750125

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Alternating cells with p- and n-type emitters enables direct series connection of equivalent sides‚ i.e. front-to-front and back-to-back connection of adjacent cells. The challenge is to match the current of cells with p- and n-type emitters. The electrical properties of silicon heterojunction solar cells with front and rear junctions are remarkably similar. The short-circuit current density mismatch between front and rear junction cells is as low as 0.1 mAcm−2. The cells are connected using thin indium coated wires. One-cell and two-cells modules were manufactured‚ and efficiencies up to 21.2% were reached for one-cell modules. Electroluminescence of the two-cells module is a good indication about the quality of the direct series connection between front and rear junction cells.

M. Cotton, S. Y. Herasimenka, W. J. Dauksher, E. Howard, M. Strnad, and S. Bowden, “Processing of ultrathin silicon heterojunction solar cells bonded to a glass carrier,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 0643–0644. https://doi.org/10.1109/PVSC.2016.7749678

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

A method of processing ultrathin silicon solar cells by bonding them to a glass carrier is described. The method allows processing large area solar cells on the wafers with down to 10 micron thickness. In the method the rear side of a solar cell is processed on a stand-alone wafer. The cell is then bonded to a glass carrier followed by chemical thinning and processing of the front side. Finally‚ the cell is de-bonded from the glass carrier. This work applied bonding process previously developed at Arizona State University Flexible Electronics and Display Center to a SHJ solar cell. It was found that bonding material can withstand wafer thinning and acidic cleans used in SHJ processing. We also show that bonding material doesn’t contaminate PECVD or sputtering chambers and doesn’t prevent achieving very good surface passivation.

J. Oh, S. Dahal, B. Dauksher, S. Bowden, G. Tamizhmani, and P. Hacke, “A novel technique for performing PID susceptibility screening during the solar cell fabrication process,” in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 0907–0910. https://doi.org/10.1109/PVSC.2016.7749741

2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Various characterization techniques have historically been developed in order to screen potential induced degradation (PID)-susceptible cells‚ but those techniques require final solar cells. We present a new characterization technique for screening PID-susceptible cells during the cell fabrication process. Illuminated Lock-In Thermography (ILIT) was used to image PID shunting of the cell without metallization and clearly showed PID-affected areas. PID-susceptible cells can be screened by ILIT‚ and the sample structure can advantageously be simplified as long as the sample has the silicon nitride antireflection coating and an aluminum back surface field.

Y. Kim, K.-Y. Ban, S. N. Dahal, H. McFelea, and C. B. Honsberg, “Detection of the third transition of InAs/GaAsSb quantum dots,” Journal of Ceramic Processing Research, vol. 17, no. 4, pp. 369–372, Apr. 2016.

00000 WOS:000378629100021

Journal of Ceramic Processing Research

J. Ceram. Process. Res.

Abstract

The optical properties of InAs quantum dots (QDs) embedded in GaAs0.92Sb0.08 barriers have been studied. The samples studied consist of 20 multiple layers of InAs QDs embedded in GaAs0.92Sb0.08‚ with each QD/barrier system separated by a 100 nm GaAs spacer. No appreciable changes in the QD properties‚ such as size‚ shape‚ and density‚ are observed by Scanning Transmission Electron Microscopy (STEM) images. The delta-doping plane beneath the InAs QDs allows the occupancy of the QD electronic sub-band states to be controlled. Low temperature (77 K) Fourier Transformation-Infrared Spectroscopy (FT-IR) results‚ using a multiple internal reflection (MIR) technique to enhance the optical path length‚ show intersubband absorption in the InAs QD area. The broad peak observed around 240 meV corresponds to the energy separation between the electron ground state and the continuum state of the QDs. Another broad peak around 440 meV is ascribed to a transition between a deep level and shallow donor level due to delta-doping as the signal increases as the doping density increases. Band structure calculations using an eight band k center dot p method are used to confirm the experimental results observed here.

Y. Kim, K.-Y. Ban, C. Zhang, J. O. Kim, S. J. Lee, and C. B. Honsberg, “Efficiency enhancement in InAs/GaAsSb quantum dot solar cells with GaP strain compensation layer,” Applied Physics Letters, vol. 108, no. 10, p. 103104, Mar. 2016 [Online]. https://doi.org/10.1063/1.4943182

00000

4M398J5P year__2016 type__CrossRef ""

Applied Physics Letters

A. Maros, N. Faleev, R. R. King, C. B. Honsberg, D. Convey, H. Xie, and F. A. Ponce, “Critical thickness investigation of MBE-grown GaInAs/GaAs and GaAsSb/GaAs heterostructures,” Journal of Vacuum Science & Technology B, vol. 34, no. 2, p. 02L113, Mar. 2016 [Online]. https://doi.org/10.1116/1.4942897

Journal of Vacuum Science & Technology B

Abstract

GaInAs/GaAs and GaAsSb/GaAs heterostructures were grown by molecular beam epitaxy with different In/Sb compositions and thicknesses in order to obtain samples with different amounts of initial strain. High resolution x-ray diffraction was used to extract the alloys composition‚ specify the presence of dislocations‚ and determine the extent of relaxation while transmission electron microscopy and x-ray topography were used to observe these dislocations and characterize their type and density. The onset for the formation of misfit dislocations was found to be in agreement with the equilibrium theory. However‚ the films remained coherently strained for thicknesses far beyond this value. The onset for strain relaxation was found by considering the kinetics of plastic deformation using the approach proposed by Tsao and coworkers [Phys. Rev. Lett. 59‚ 2455 (1987)]. The mechanism of extended defect creation leading to measurable strain relief is described as a multistage process related with the structural stability and metastability of the epitaxialfilms.

A. Maros, N. Faleev, R. R. King, and C. B. Honsberg, “Growth and characterization of GaAs1−x−ySbxNy/GaAs heterostructures for multijunction solar cell applications,” Journal of Vacuum Science & Technology B, vol. 34, no. 2, p. 02L106, Mar. 2016 [Online]. https://doi.org/10.1116/1.4941424

Journal of Vacuum Science & Technology B

Abstract

The GaAsSbN dilute-nitride alloy can be grown lattice-matched to GaAs with a bandgap of 1 eV‚ making it an ideal candidate for use in multijunction solar cells. In this work‚ using molecular beam epitaxy in conjunction with a radio-frequency nitrogen plasma source‚ the authors focus first on the growth optimization of the GaAsSb and GaAsN alloys in order to calibrate the Sb and N compositions independently of each other. After the optimum growth conditions to maintain two-dimensional growth were identified‚ the growth of GaAsSbN films was demonstrated. Both a GaAsSb0.076N0.018/GaAs heterostructure (100 nm thick) and a GaAsSb0.073N0.015/GaAs quantum well (11 nm thick) were grown.X-ray diffraction analysis reveals quite high crystal quality with a small lattice mismatch of 0.13%–0.16%. Secondary ion mass spectrometry profiling revealed that nitrogen was unintentionally incorporated in the GaAsbuffer layer during the plasma ignition and stabilization. Nevertheless‚ a low temperature photoluminescence peak energy of 1.06 eV was measured for the GaAsSbNheterostructure sample while the quantum well emitted photoluminescence at 1.09 eV‚ which demonstrates promise for realizing 1-eV solar cells.

C. A. M. Fabien, A. Maros, C. B. Honsberg, and W. A. Doolittle, “III-Nitride Double-Heterojunction Solar Cells With High In-Content InGaN Absorbing Layers: Comparison of Large-Area and Small-Area Devices,” IEEE Journal of Photovoltaics, vol. 6, no. 2, pp. 460–464, Mar. 2016. https://doi.org/10.1109/JPHOTOV.2015.2504790

IEEE Journal of Photovoltaics

Abstract

This paper investigates the molecular beam epitaxy (MBE) growth‚ material characterization‚ and performance testing of indium gallium nitride (InGaN)/GaN double-heterojunction solar cells. Structures with varying thicknesses and compositions of the InGaN absorbing layer are studied. The N-rich MBE growth at low temperatures enables the growth of thick 10% and 20% InGaN films with minimal strain relaxation and defect generation. The characteristics of both large- and small-area devices are compared. While leakage current and high ideality factors associated with the double-heterojunction structure remain issues as detected by I-V and concentration effect measurements‚ the double-heterojunction cell with a record-high In content of 22% shows a promising photovoltaic response.

S. Kurtz, H. Atwater, A. Rockett, T. Buonassisi, C. Honsberg, and J. Benner, “Solar research not finished,” Nature Photonics, vol. 10, no. 3, pp. 141–142, Mar. 2016 [Online]. https://doi.org/10.1038/nphoton.2016.16

Nature Photonics

Nat Photon

P. Muralidharan, S. Bowden, S. M. Goodnick, and D. Vasileska, “A Multiscale Modeling Approach to Study Transport in Silicon Heterojunction Solar Cells,” Additional Conferences (Device Packaging, HiTEC, HiTEN, & CICMT), vol. 2016, no. DPC, pp. 002095–002110, Jan. 2016 [Online]. https://doi.org/10.4071/2016DPC-THA33

Additional Conferences (Device Packaging, HiTEC, HiTEN, & CICMT)

Abstract

Single junction solar cells based on Silicon continue to be relevant and commercially successful in the market due to their high efficiencies and relatively low cost processing. Heterojunction solar cells based on crystalline (c-Si) and amorphous (a-Si) silicon (HIT Cells) have paved the way for devices with high VOC’s (>700 mV) and high efficiencies (>20%) [1]. Panasonic currently holds the world record efficiency of 25.6% for its trademark a-Si/c-Si HIT cell [2]. The novel structure of the device precludes the usage of traditional methods (such as drift diffusion) to accurately understand the nature of transport. Theoretical models used by commercial simulators make a variety of assumptions that simplifies the transport problem (assumes a Maxwellian distribution of carriers) and thus lacks the sophistication to study defect transport. In this work we utilize a combination of Ensemble Monte Carlo (EMC) simulations‚ Kinetic Monte Carlo (KMC) simulations and traditional drift - diffusion (DD) simulations to study transport in the heterojunction solar cell. The device performance of an amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell depends strongly on the interfacial transport properties of the device [3]. The energy of the photogenerated carriers at the barrier strongly depends on the strength of the inversion at the heterointerface and their collection requires interaction with the defects present in the intrinsic amorphous silicon buffer layer [4]. In this work we present a multiscale model which can bridge the gap in time scales between different microscopic processes to study the transport through the interface by coupling an ensemble Monte Carlo (EMC) and a kinetic Monte Carlo (KMC). The EMC studies carrier properties such as the energy distribution function (EDF) at the heterointerface whereas the KMC method allows us to simulate the interaction of discrete carriers with discrete defects [5]. This method allows us to study defect transport which takes place on a time scale which is too long for traditional ensemble Monte Carlo’s to analyze. We analyze the injection and extraction of carriers via defects by calculating transition rates for different processes. By using the principles of SRH recombination‚ this method can also be extended to study recombination processes at the interface and in the amorphous bulk which are crucial parameters for solar cell performance. Therefore‚ by using the multiscale approach all important processes can be studied rigorously to evaluate device performance. Our simulations indicate that a phonon assisted emission process from a defect is the most favored extraction mechanism and both Poole-Frenkel emission (<2%) and thermionic emission (<1%) were not significant. We extended our simulation methodology to study recombination at the interface and in the buffer layer of the device to find that the device performance is mainly interface recombination limited and that defect densities in the buffer layer have to be really high (>1018 cm-3) in order to degrade device performance.

P. Muralidharan, S. Bowden, S. M. Goodnick, and D. Vasileska, “A multiscale modeling approach to study transport in silicon heterojunction solar cells,” presented at the IMAPS 12th International Conference and Exhibition on Device Packaging, 2016.

IMAPS 12th International Conference and Exhibition on Device Packaging

Abstract

In recent years‚ silicon photovoltaic technologies utilizing amorphous silicon (a-Si) to form heterojunction solar cells with thin passivating layers have consistently demonstrated high efficiencies (world record of 25.6%)‚ high fill factor’s (FF) and high open circuit voltages (VOC). Further improvements in efficiency require a rigorous approach to better understand and improve device behavior. In this work we analyze the transport and device performance of heterojunction cell by applying a multiscale simulation methodology. Our multiscale solver consists of three primary domains‚ namely; the drift-diffusion (DD) domain‚ the ensemble Monte Carlo (EMC) and the kinetic Monte Carlo (KMC) domain. We investigate the role of midgap defects in the a-Si and interface defects at the crystalline silicon (c-Si) and a- Si heterointerface. Simulations indicate that recombination at the interface is a key limiting factor in device performance and contributes to the ’S’ shaped current voltage characteristic.

A. Augusto, P. Balaji, H. Jain, S. Y. Herasimenka, and S. G. Bowden, “Heterojunction solar cells on flexible silicon wafers,” MRS Advances, vol. FirstView, pp. 1–6, Jan. 2016 [Online]. https://doi.org/10.1557/adv.2016.8

MRS Advances

Abstract

ABSTRACT Current large-scale production of flexible solar devices delivers cells with low efficiency. In this paper we present an alternative path to organic or inorganic thin films. Our cells combine the remarkable surface passivation properties of the silicon heterojunction solar cells design‚ and the quality of n-type Cz wafers. The cells were manufactured on 50-70 µm-thick wafers. The cells have and efficiency of 17.8-19.2%‚ open-circuit voltages of 735-742 mV‚ short-circuit currents of 34.5-35.5 mA/cm2‚ and fill-factors of 72-75%. The cells are not as flexible as bare wafers. Thin cells are particular sensitive to the additional stress introduced by the busbars and the soldered ribbons. For radiuses of curvature over 8cm the cells efficiency remains the same‚ for radius equal to 6cm the cell efficiency drops less than 2%‚ and for radius equal to 4cm the drop is less than 3%. The broken fingers due to smaller bend radius lead to higher series resistance and subsequently lower field-factors.

H. N. JAIN, “Nickel Silicide Contact for Copper Plated Silicon Solar Cells,” Ph. D. Thesis, Arizona State University, 2016 [Online].

Abstract

Nickel-Copper metallization for silicon solar cells offers a cost effective alternative to traditional screen printed silver paste technology. The main objective of this work is to study the formation of nickel silicide contacts with and without native silicon dioxide SiO2. The effect of native SiO2 on the silicide formation has been studied using Raman spectroscopy‚ Rutherford backscattering spectrometry and sheet resistance measurements which shows that SiO 2 acts as a diffusion barrier for silicidation at low temperatures of 350°C. At 400°C the presence of SiO2 results in the increased formation of nickel mono-silicide phase with reduced thickness when compared to samples without any native oxide. Pre and post-anneal measurements of Suns Voc‚ photoluminescence and Illuminated lock in thermography show effect of annealing on electrical characteristics of the device. The presence of native oxide is found to prevent degradation of the solar cells when compared to cells without any native oxide. A process flow for fabricating silicon solar cells using light induced plating of nickel and copper with and without native oxide (SiO2) has been developed and cell results for devices fabricated on 156mm wafers have been discussed.

J. Williams, “Engineering III-N Alloys and Devices for Photovoltaic Progress,” Ph. D. Thesis, Arizona State University, 2016 [Online].

Abstract

The state of the solar industry has reached a point where significant advancements in efficiency will require new materials and device concepts. The material class broadly known as the III-N’s have a rich history as a commercially successful semiconductor. Since discovery in 2003 these materials have shown promise for the field of photovoltaic solar technologies. However‚ inherent material issues in crystal growth and the subsequent effects on device performance have hindered their development. This thesis explores new growth techniques for III-N materials in tandem with new device concepts that will either work around the previous hindrances or open pathways to device technologies with higher theoretical limits than much of current photovoltaics. These include a novel crystal growth reactor‚ efforts in production of better quality material at faster rates‚ and development of advanced photovoltaic devices: an inversion junction solar cell‚ material work for hot carrier solar cell‚ ground work for a selective carrier contact‚ and finally a refractory solar cell for operation at several hundred degrees Celsius.

J. Oh, “Elimination of Potential-Induced Degradation for Crystalline Silicon Solar Cells,” Ph. D. Thesis, Arizona State University, 2016 [Online].

Abstract

Potential-Induced Degradation (PID) is an extremely serious photovoltaic (PV) durability issue significantly observed in crystalline silicon PV modules due to its rapid power degradation‚ particularly when compared to other PV degradation modes. The focus of this dissertation is to understand PID mechanisms and to develop PID-free cells and modules. PID-affected modules have been claimed to be fully recovered by high temperature and reverse potential treatments. However‚ the results obtained in this work indicate that the near-full recovery of efficiency can be achieved only at high irradiance conditions‚ but the full recovery of efficiency at low irradiance levels‚ of shunt resistance‚ and of quantum efficiency (QE) at short wavelengths could not be achieved. The QE loss observed at short wavelengths was modeled by changing the front surface recombination velocity. The QE scaling error due to a measurement on a PID shunted cell was addressed by developing a very low input impedance accessory applicable to an existing QE system. The impacts of silicon nitride (SiNx) anti-reflection coating (ARC) refractive index (RI) and emitter sheet resistance on PID are presented. Low RI ARC cells (1.87) were observed to be PID-susceptible whereas high RI ARC cells (2.05) were determined to be PID-resistant using a method employing high dose corona charging followed by time-resolved measurement of surface voltage. It has been demonstrated that the PID could be prevented by deploying an emitter having a low sheet resistance (\textasciitilde 60 /sq) even if a PID-susceptible ARC is used in a cell. Secondary ion mass spectroscopy (SIMS) results suggest that a high phosphorous emitter layer hinders sodium transport‚ which is responsible for the PID. Cells can be screened for PID susceptibility by illuminated lock-in thermography (ILIT) during the cell fabrication process‚ and the sample structure for this can advantageously be simplified as long as the sample has the SiNx ARC and an aluminum back surface field. Finally‚ this dissertation presents a prospective method for eliminating or minimizing the PID issue either in the factory during manufacturing or in the field after system installation. The method uses commercially available‚ thin‚ and flexible Corning® Willow® Glass sheets or strips on the PV module glass superstrates‚ disrupting the current leakage path from the cells to the grounded frame.

Y. Kim, N. N. Faleev, K.-Y. Ban, J. O. Kim, S. J. Lee, and C. B. Honsberg, “Growth of highly dense InAs quantum dots with improved crystal quality embedded in an InGaAsSb quantum well,” Journal of Physics D: Applied Physics, vol. 49, no. 30, p. 305102, 2016 [Online]. https://doi.org/10.1088/0022-3727/49/30/305102

00000

Journal of Physics D: Applied Physics

J. Phys. D: Appl. Phys.

Abstract

K.-Y. Ban, Y. Kim, D. Kuciauskas, S. P. Bremner, and C. B. Honsberg, “Investigation of carrier dynamics in InAs/GaAsSb quantum dots with different silicon delta-doping levels,” Semiconductor Science and Technology, vol. 31, no. 12, p. 125010, 2016 [Online]. https://doi.org/10.1088/0268-1242/31/12/125010

00000

Semiconductor Science and Technology

Semicond. Sci. Technol.

Abstract

A. F. Routhier, “Using thermal energy storage to increase photovoltaic penetration at Arizona State University’s Tempe campus,” M.S., Arizona State University, United States -- Arizona, 2016 [Online].

Abstract

This thesis examines using thermal energy storage as a demand side management tool for air-conditioning loads with the goal of increasing photovoltaic penetration. It uses Arizona State University (ASU) as a case study. The analysis is completed with a modeling approach using typical meteorological year (TMY) data‚ along with ASU’s historical load data. Sustainability‚ greenhouse gas emissions‚ carbon neutrality‚ and photovoltaic (PV) penetration are all considered along with potential economic impacts. By extrapolating the air-conditioning load profile from the existing data sets‚ it can be ensured that cooling demands can be met at all times under the new management method. Using this cooling demand data‚ it is possible to determine how much energy is required to meet these needs. Then‚ modeling the PV arrays‚ the thermal energy storage (TES)‚ and the chillers‚ the maximum PV penetration in the future state can be determined. Using this approach‚ it has been determined that ASU can increase their solar PV resources by a factor of 3.460‚ which would amount to a PV penetration of approximately 48%.

J. Oh, G. TamizhMani, S. Bowden, and S. Garner, “Surface Disruption Method With Flexible Glass to Prevent Potential-Induced Degradation of the Shunting Type in PV Modules,” IEEE Journal of Photovoltaics, vol. PP, no. 99, pp. 1–6, 2016. https://doi.org/10.1109/JPHOTOV.2016.2618606

IEEE Journal of Photovoltaics

Abstract

Potential-induced degradation of the shunting type (PID-s) has recently been recognized by the industry as a critical photovoltaic (PV) module durability issue. Many methods to prevent PID-s have been developed at the cell and module levels in the factory and at the system level in the field. This paper presents a prospective method for eliminating or minimizing the PID-s issue either in the factory during manufacturing or in the field after system installation. The method uses commercially available‚ thin‚ and flexible Corning Willow Glass sheets or strips on the PV module glass superstrates‚ disrupting the current leakage path from the cells to the grounded frame.

Y. Kim, K. Y. Ban, S. N. Dahal, H. McFelea, and C. Honsberg, “Detection of the third transition of InAs/GaAsSb quantum dots,” Journal of Ceramic Processing Research, vol. 17, no. 4, 2016 [Online].

Journal of Ceramic Processing Research

2015

K. D. Tyler, “Wire Interconnections in Solar Modules,” Barrett Honors College Thesis, Arizona State University, 2015 [Online].

Abstract

Wire connected solar cells are a promising new technology that can increase the efficiency and reduce the cost of solar modules. The use of wire rather than ribbon bus bars can lead to reduced shading‚ better light trapping‚ and reduced material costs‚ all while eliminating the need for soldering. This research first analyzes the optimal wire gauge to reduce cracking and improve efficiency. Wire sizes between 20 AWG and 28 AWG were tested‚ with the optimal size being between 24 AWG and 26 AWG for the ethylene vinyl acetate (EVA) layer used in the module. A polyethylene sheet was then added between the wires and EVA layer to prevent the EVA from running underneath the wires during lamination‚ ultimately allowing for a more uniform contact and only a slight reduction in quantum efficiency. Then‚ a comparison between tinned copper wires and indium coated copper wires is shown. A mini-module efficiency of 20.0% has been achieved using tinned copper wires‚ while indium coated copper wires have produced a mini-module efficiency of 21.2%. Thus‚ tinned copper wires can be a viable alternative to indium coated copper wires‚ depending on the needs of the customers and the current price of indium. The module design throughout the research utilizes a planar assembly method‚ which improves the ease of manufacturing for wire interconnection technology. A two-cell base component is constructed and shown‚ with the intended future application of making large wire connected modules. Finally‚ wire applications in both single-cell and four-cell flexible modules are explored‚ with an efficiency of 18.65% achieved on a single-cell‚ flexible‚ heterojunction solar module using wire interconnections. A fully flexible four-cell string is developed‚ and future recommendations for related research are included.

H. C. Martin, “Solar Powered Quadcopter,” Barrett Honors College Thesis, Arizona State University, 2015 [Online].

Abstract

The purpose of the solar powered quadcopter is to join together the growing technologies of photovoltaics and quadcopters‚ creating a single unified device where the technologies harmonize to produce a new product with abilities beyond those of a traditional battery powered drone. Specifically‚ the goal is to take the battery-only flight time of a quadcopter loaded with a solar array and increase that flight time by 33% with additional power provided by solar cells. The major concepts explored throughout this project are quadcopter functionality and capability and solar cell power production. In order to combine these technologies‚ the solar power and quadcopter components were developed and analyzed individually before connecting the solar array to the quadcopter circuit and testing the design as a whole. Several solar copter models were initially developed‚ resulting in multiple unique quadcopter and solar cell array designs which underwent preliminary testing before settling on a finalized design which proved to be the most effective and underwent final timed flight tests. Results of these tests are showing that the technologies complement each other as anticipated and highlight promising results for future development in this area‚ in particular the development of a drone running on solar power alone. Applications for a product such as this are very promising in many fields‚ including the industries of power‚ defense‚ consumer goods and services‚ entertainment‚ marketing‚ and medical. Also‚ becoming a more popular device for UAV hobbyists‚ such developments would be very appealing for leisure flying and personal photography purposes as well.

C. Honsberg, S. Bowden, P. Balaji, Z. Kiefer, K. Tyler, H. Jain, A. Augusto, J. Lee, C. J. Tracy, W. J. Dauksher, and S. Y. Herasimenka, “Front- and Rear-Emitter Screen Printed Silicon Heterojunction Solar Cells with >20% Efficiency,” 31st European Photovoltaic Solar Energy Conference and Exhibition, pp. 761–764, Nov. 2015 [Online]. https://doi.org/10.4229/EUPVSEC20152015-2AV.3.18

31st European Photovoltaic Solar Energy Conference and Exhibition

Abstract

WAFER-BASED SILICON SOLAR CELLS AND MATERIALS TECHNOLOGY‚ Silicon Solar Cell Improvements‚ Front- and Rear-Emitter Screen Printed Silicon Heterojunction Solar Cells with >20% Efficiency

P. Muralidharan, D. Vasileska, S. M. Goodnick, and S. Bowden, “A kinetic Monte Carlo study of defect assisted transport in silicon heterojunction solar cells,” physica status solidi (c), vol. 12, no. 9–11, pp. 1198–1200, Nov. 2015 [Online]. https://doi.org/10.1002/pssc.201510071

physica status solidi (c)

Phys. Status Solidi C

Abstract

The device performance of an amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell depends strongly on the interfacial transport properties of the device. The energy of the photogenerated carriers at the barrier strongly depends on the strength of the inversion at the heterointerface and their collection requires interaction with the defects present in the intrinsic amorphous silicon buffer layer. In this work we present a theoretical model to study the defect assisted transport of photogenerated carriers through the intrinsic amorphous silicon barrier. We implement the kinetic Monte Carlo (KMC) method which allows us to simulate the interaction of discrete carriers with discrete defects. This method allows us to study defect transport which takes place on a time scale which is too long for traditional ensemble Monte Carlo’s to analyze. We analyze the injection and extraction of carriers via defects by calculating transition rates‚ i.e. probability of transition to defect states within the intrinsic amorphous silicon barrier. The KMC results allow us to quantitatively study the properties of the heterointerface barrier in terms of how it affects transport. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA‚ Weinheim)

J. Oh, S. Bowden, and G. TamizhMani, “Potential-Induced Degradation (PID): Incomplete Recovery of Shunt Resistance and Quantum Efficiency Losses,” IEEE Journal of Photovoltaics, vol. 5, no. 6, pp. 1540–1548, Nov. 2015. https://doi.org/10.1109/JPHOTOV.2015.2459919

IEEE Journal of Photovoltaics

PID

Abstract

Potential-induced degradation (PID)‚ specifically PID leading to shunts (PID-s)‚ has recently been identified as one of the major field durability issues of photovoltaic (PV) modules. The industry is attempting to address this issue at the module/cell production level by modifying the cell‚ glass‚ and/or encapsulant properties‚ as well as at the system level through the application of reverse potential at night. However‚ there is a lingering question on the full recovery of the cells through the reverse potential application technique. The results obtained in this study indicate that the near-full recovery of efficiency at high irradiance levels can be achieved‚ but the full recovery of efficiency at low irradiance levels‚ shunt resistance‚ and quantum efficiency (QE) at low wavelengths could not be achieved. The wavelength-dependent QE response after PID and recovery has been modeled based on experimental data. We address the challenge in measuring accurate QE of shunted cells and the input impedance of traditional QE test equipment. A new very low impedance method minimizes‚ but does not totally eliminate‚ the scaling error in the QE system data for solar cells that have very low shunt resistances. We also evaluate previously proposed models on the effects of sodium experimentally and through simulation.

Y. Kim, K.-Y. Ban, A. Boley, D. J. Smith, and C. B. Honsberg, “Effect of spacer layer thickness on structural and optical properties of multi-stack InAs/GaAsSb quantum dots,” Applied Physics Letters, vol. 107, no. 17, p. 173109, Oct. 2015 [Online]. https://doi.org/10.1063/1.4934695

Applied Physics Letters

Abstract

The structural and optical properties of ten-stack InAs/GaAsSb quantum dots (QDs) with different spacer layer thicknesses (ds = 2‚ 5‚ 10‚ and 15 nm) are reported. X-ray diffraction analysis reveals that the strain relaxation of the GaAsSb spacers increases linearly from 0% to 67% with larger ds due to higher elastic stress between the spacer and GaAs matrix. In addition‚ the dislocation density in the spacers with ds = 10 nm is lowest as a result of reduced residual strain. The photoluminescence peak energy from the QDs does not change monotonically with increasing ds due to the competing effects of decreased compressive strain and weak electronic coupling of stacked QD layers. The QD structure with ds = 10 nm is demonstrated to have improved luminescence properties and higher carrier thermal stability.

Y. Kim, K.-Y. Ban, C. Zhang, and C. B. Honsberg, “Material and device characteristics of InAs/GaAsSb sub-monolayer quantum dot solar cells,” Applied Physics Letters, vol. 107, no. 15, p. 153103, Oct. 2015 [Online]. https://doi.org/10.1063/1.4933272

Applied Physics Letters

Abstract

We have studied the material and photovoltaic characteristics of InAs/GaAsSb sub-monolayer quantum dot solar cells (QDSCs) with different Sb contents of 0%‚ 5%‚ 15%‚ and 20%. All QDSCs exhibit an extended external quantum efficiency (EQE) response in the wavelength range of 960–1000 nm that corresponds to sub-bandgap photon absorption. As Sb content increases from 5% to 20%‚ the cutoff wavelength in the EQE extends towards longer wavelength whilst the EQE in the wavelength region of 300–880 nm is lowered due to increased defect density. Compared to the QDSC (Sb 0%)‚ an Sb incorporation of 5% enhances the short-circuit current density from 20.65 to 22.15 mA/cm2 induced by Sb surfactant effect. Since the open-circuit voltage and fill factor of the QDSC (Sb 5%) are comparable to those of the QDSC (Sb 0%)‚ an enhancement in solar cell efficiency (10.5%) of the QDSC (Sb 5%) is observed. Further increasing Sb content to 15% and 20% results in the degradation of solar cell performance due to increased nonradiative recombination and large valence band offset in a type-II band line-up.

D. Tang, Y. Kim, N. Faleev, C. B. Honsberg, and D. J. Smith, “Investigation of single-layer/multilayer self-assembled InAs quantum dots on GaAs1-xSbx/GaAs composite substrates,” Journal of Applied Physics, vol. 118, no. 9, p. 094303, Sep. 2015 [Online]. https://doi.org/10.1063/1.4929639

Journal of Applied Physics

Abstract

The structure-performance properties of single-layered and multi-layered InAs/GaAs1−xSbx quantum dot (QD) system‚ grown by molecular beam epitaxy on GaAs (001) substrates‚ have been investigated as a function of Sb concentration. Electron microscopy observations showed no significant crystalline defects for the single-layered InAs QDs (Sb 20%). X-ray diffraction analysis revealed that the increase of Sb concentration from 7.3% to 10.2% for the multi-layered QDs increased the strain relaxation from 0% to ∼23% and the dislocation density of GaAsSb layers went up to 3.6 × 109 cm−2. The peak energy of QD luminescence was red-shifted with increasing Sb concentration due to reduced strain inside QDs. Moreover‚ the carrier lifetime of the QDs was highly improved from 1.7 to 36.7 ns due to weak hole confinement as the Sb concentration was increased from 7.3% to 10.2%. These structures should be highly promising as the basis for photovoltaic solar-cell applications. Finally‚ the increased Sb concentration increased the thermal activation energy of electrons confined in the QDs from 163.7 to 206.8 meV‚ which was indicative of the improved thermal stability with Sb concentration.

J. J. Williams, A. M. Fischer, T. L. Williamson, S. Gangam, N. N. Faleev, M. A. Hoffbauer, and C. B. Honsberg, “High growth speed of gallium nitride using ENABLE-MBE,” Journal of Crystal Growth, vol. 425, pp. 129–132, Sep. 2015 [Online]. https://doi.org/10.1016/j.jcrysgro.2015.04.007

Journal of Crystal Growth

Abstract

Films of gallium nitride were grown at varying growth speeds‚ while all other major variables were held constant. Films grown determine the material impact of the high flux capabilities of the unique nitrogen plasma source ENABLE. Growth rates ranged from 13 to near 60 nm/min. X-ray ω scans of GaN (0002) have FWHM in all samples less than 300 arc sec. Cathodoluminescence shows radiative recombination for all samples at the band edge. In general material quality overall is high with slight degradation as growth speeds increase to higher rates.

M. M. Karow, N. N. Faleev, A. Maros, and C. B. Honsberg, “Defect Creation in InGaAs/GaAs Multiple Quantum Wells – II. Optical Properties,” Journal of Crystal Growth, vol. 425, pp. 49–53, Sep. 2015 [Online]. https://doi.org/10.1016/j.jcrysgro.2015.03.048

Journal of Crystal Growth

Abstract

The optical properties of three sets of InGaAs/GaAs multiple quantum well (MQW) structures grown by molecular beam epitaxy and previously characterized by x-ray diffraction for crystal perfection were investigated. The correlations between growth conditions‚ crystal defects‚ and optical properties are discussed. Evaluation of the relative importance of non-radiative Shockley-Read-Hall (SRH) recombination was carried out according to a method presented herein. The optimal deposition temperature was determined based on both proper carrier confinement in the nanostructures and the least non-radiative recombination. Growing below this temperature increased SRH-recombination whereas higher growth temperatures led to carrier localization in local band edge minima. Varying the MQW periodicity and MQW period allowed the study of their effects on the strength of SRH-recombination. MQW periodicity results are explained in the frame of a cumulative deterioration effect with continued epitaxial growth‚ while MQW period data shows correlations between relaxation of the initial elastic stress and SRH-strength. Limitations of the underlying model for SRH-analysis are pointed out.

Stuart Bowden, T. Blake Jennings, Padmapriya Rangarajan, Sepehr Rostamzadeh, Varun Gupta, and Ronald A. Sinton, “Lilac: A Photovoltaic Module Tester,” presented at the 25th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes, Keystone, Colorado, USA, 2015.

25th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes

Abstract

We describe a simple photovoltaic module tester (named Lilac) based around the open source hardware Arduino microcontroller system. The tester fills the gap between current commercial systems costing thousands of dollars‚ and simple multimeters that do not provide the ability to sweep the photovoltaic curve and track through the maximum power point. Lilac enables module efficiency measurements on modules up to 300 W using readily available low cost parts. The open development environment enables further enhancements for both teaching and research purposes.

Kevin Tyler, Stanislau Herasimenka, Stuart Bowden, and Clarence Tracy, “Interconnection of Solar Cells in PV Modules using Copper Wires and Aluminum Foil,” presented at the 25th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes, Keystone, Colorado, USA., 2015.

25th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes

Abstract

Wire connected solar cells are a promising new technology‚ yet often require the use of expensive indium coated wires. Using only tinned copper wires‚ pressure contact on the front of the cell‚ and aluminum foil as the back contact‚ a current of 37.58 mA/cm2 and an efficiency of 20.04% was achieved in mini module silicon heterojunction 10x10 cm cells. The inclusion of a high density polyethylene film between the wires and the EVA layer dramatically increased the contact area. However‚ the quantum efficiency of the cell was slightly decreased due to this added layer. The wire connections are ideal for flexible solar cells‚ as conventional ribbon bus bars have a greater chance of cracking the cell. Also‚ for manufacturing purposes‚ the method used demonstrates the potential of a planar process to increase production speed and efficacy. Further development of this process is ongoing‚ along with future plans for reliability testing of wire connected silicon heterojunction cells.

Zachary A Kiefer, Stanislau Herasimenka, Tanmay Monga, Apoorva Srinivasa, André Augusto, Karl Noss, William Dauksher, and Stuart Bowden, “Student-Led Pilot Line Development at ASU: Heterojunction and Diffused Cell Fabrication through Education,” presented at the 25th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes, Keystone, Colorado, USA., 2015.

25th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes

Abstract

As part of the Arizona State University Solar Power Laboratory’s commitment to education‚ a baseline process for the fabrication and characterization of multiple solar cell structures is entirely led and operated by students under the mentorship of senior research personnel. Graduate‚ undergraduate‚ and high school students directly participate in the operation and management of the facility. The diffused line uses the industry-standard six-inch solar wafer form factor‚ allowing the development and transfer of new materials and processes to industry. This pilot line has baseline processes for fabrication of large-area silicon solar cells with efficiencies currently at 17.3%‚ and has been used to train over 25 undergraduate students and teachers per year during 10 week summer internship programs. In the search for higher efficiency cells‚ the HIT (Heterojunction with Intrinsic Thin layer) SHJ cell pilot line was created‚ producing a working 28 cell module and a mean cell efficiency of 19.45%. Due to these stable and repeatable baseline processes‚ researchers at the SPL are able to complete sensitive experiments with relative ease. One recent success has been the production of very high VOC test structures of silicon heterojunction cells on 50 µm wafers‚ which resulted in a VOC of over 760 mV for non-metallized structures and over 750 mV on metallized structures‚ confirmed by measurements at NREL. The student-led pilot line aims to continue to be a forum for education‚ research‚ and development‚ with an emphasis on industrially relevant high-efficiency silicon solar cells and modules.

Lynelle Whitehead, Jacob Markey, Danny Simonet, Michael Minjares, Mark Bailly, and Stuart Bowden, “Optimization of Intermediate Process Steps for Rear Laser Formed Contacts,” presented at the 25th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes, Keystone, Colorado, USA., 2015.

25th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes

Y. Kim, K.-Y. Ban, and C. B. Honsberg, “Multi-stacked InAs/GaAs quantum dots grown with different growth modes for quantum dot solar cells,” Applied Physics Letters, vol. 106, no. 22, p. 222104, Jun. 2015 [Online]. https://doi.org/10.1063/1.4922274

Applied Physics Letters

Abstract

We have studied the material properties and device performance of InAs/GaAs quantum dotsolar cells (QDSCs) made using three different QD growth modes: Stranski-Krastanov (S-K)‚ quasi-monolayer (QML)‚ and sub-monolayer (SML) growth modes. All QDSCs show an extended external quantum efficiency (EQE) at near infrared wavelengths of 950–1070 nm from the QDabsorption. Compared to the S-K and SML QDSCs‚ the QML QDSC with a higher strain exhibits a poor EQE response in the wavelength region of 300–880 nm due to increased non-radiative recombination. The conversion efficiency of the S-K and SML QDSCs exceeds that of the reference cell (13.4%) without QDs due to an enhanced photocurrent (>16% increase) produced by the silicon doped QD stacks. However‚ as expected from the EQE of the QML QDSC‚ the increase of strain-induced crystalline defects greatly degrades the photocurrent and open-circuit voltage‚ leading to the lowest conversion efficiency (8.9%).

P. Muralidharan, D. Vasileska, S. M. Goodnick, and S. Bowden, “A Kinetic Monte Carlo approach to study transport in amorphous silicon/crystalline silicon HIT cells,” in 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC), 2015, pp. 1–4. https://doi.org/10.1109/PVSC.2015.7356048

2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC)

PVSC

Abstract

The photogenerated carriers in a-Si/c-Si HIT cells must traverse the intrinsic amorphous silicon barrier in order to be collected. As this barrier region is amorphous in nature‚ it contains many defect states‚ and thus carrier transport is mainly described by defect assisted transport. The present work applies the Kinetic Monte Carlo (KMC) method for the defect assisted transport by analyzing the interactions between discrete defects and discrete carriers. We explore the ‘hopping’ nature of transport via defects by considering the effect of phonons. The addition of phonons allows us to study non-iso-energetic transitions for injection and extraction of carriers. Once the carriers ‘hop’ through the barrier‚ they are extracted by three main mechanisms‚ namely‚ thermionic emission‚ Poole-Frenkel emission and phonon assisted defect emission. Simulations indicate that Poole-Frenkel emission and thermionic emission are negligible whereas emission of carriers via phonon assisted hopping is the dominant mode of extraction. The effect of different defect distributions on transport is also analyzed.

A. Maros, S. Gangam, Y. Fang, J. Smith, D. Vasileska, S. Goodnick, M. I. Bertoni, and C. B. Honsberg, “High temperature characterization of GaAs single junction solar cells,” in 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC), 2015, pp. 1–5. https://doi.org/10.1109/PVSC.2015.7356338

2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC)

PVSC

Abstract

We report temperature-dependent characterization of the electrical and optical properties of GaAs single junction solar cells up to 450 °C. An external quantum efficiency (EQE) of 75% was maintained at temperatures up to 300 °C‚ with a corresponding increase in the absorption edge as a function of wavelength due to the decrease in band gap with temperature‚ in agreement with theory. Above 300 °C‚ the EQE decreased strongly as the temperature was further increased. This drop in EQE resulted in a corresponding decrease in short-circuit current‚ also observed in the I-V characteristics as a function of temperature. When cooled back to room temperature the short-circuit current nearly fully recovers whereas the open-circuit voltage is found to be irreversibly degraded. The origin of the degradation is discussed. Modeling is used to support the experimental results.

J. Oh, S. Bowden, G. TamizhMani, and P. Hacke, “Quantum efficiency loss after PID stress: Wavelength dependence on cell surface and cell edge,” in Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd, 2015, pp. 1–4. https://doi.org/10.1109/PVSC.2015.7355629

Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd

PVSC

Abstract

It is known that the potential induced degradation (PID) stress of conventional p-base solar cells affects power‚ shunt resistance‚ junction recombination‚ and quantum efficiency (QE). One of the primary solutions to address the PID issue is a modification of chemical and physical properties of antireflection coating (ARC) on the cell surface. Depending on the edge isolation method used during cell processing‚ the ARC layer near the edges may be uniformly or non-uniformly damaged. Therefore‚ the pathway for sodium migration from glass to the cell junction could be either through all of the ARC surface if surface and edge ARC have low quality or through the cell edge if surface ARC has high quality but edge ARC is defective due to certain edge isolation process. In this study‚ two PID susceptible cells from two different manufacturers have been investigated. The QE measurements of these cells before and after PID stress were performed at both surface and edge. We observed the wavelength dependent QE loss only in the first manufacturer’s cell but not in the second manufacturer’s cell. The first manufacturer’s cell appeared to have low quality ARC whereas the second manufacturer’s cell appeared to have high quality ARC with defective edge. To rapidly screen a large number of cells for PID stress testing‚ a new but simple test setup that does not require laminated cell coupon has been developed and is used in this investigation.

J. Oh, G. TamizhMani, S. Bowden, and S. Garner, “Application of flexible glass to prevent PID in PV modules,” in Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd, 2015, pp. 1–4. https://doi.org/10.1109/PVSC.2015.7355701

Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd

PVSC

Abstract

Potential induced degradation (PID) has recently been recognized by the industry as a critical PV module durability issue. Many methods to prevent PID have been developed at the cell and module levels in the factory and at the system level in the field. This paper presents a potential method for eliminating or minimizing the PID issue either in the factory during manufacturing or in the field after system installation. The method uses commercially available‚ thin and flexible Corning Willow™ glass sheets or strips on the glass superstrates of PV modules‚ disrupting the current leakage path from the cells to the grounded frame.

A. Augusto, L. Reboulet, S. Y. Herasimenka, and S. G. Bowden, “Flexible silicon heterojunctions solar cells and modules,” in Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd, 2015, pp. 1–3. https://doi.org/10.1109/PVSC.2015.7356295

Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd

PVSC

Abstract

In this manuscript is discussed the potential of using silicon heterojunction solar cells to obtain flexible silicon modules. Modules were manufactured using non-metalized silicon heterojunction solar cell with thicknesses down to 70 μm. The modules were rolled around curved surfaces with different diameters. Rolling diameters down to 5 cm were tested. Damages in the solar cells due to the rolling process were inspected using photoluminescence (PL). PL images after the rolling show promising results towards flexibility. Rolling reliability tests were also performed in thin passivated wafers (60-80 μm). The wafers were rolled down to 7 cm diameter. PL images show no evidences of mechanical damage in the wafers due to the lamination or after the rolling process.

J.-Y. Choi, T. L. Alford, and C. B. Honsberg, “Fabrication of Periodic Silicon Nanopillars in a Two-Dimensional Hexagonal Array with Enhanced Control on Structural Dimension and Period,” Langmuir, vol. 31, no. 13, pp. 4018–4023, Apr. 2015 [Online]. https://doi.org/10.1021/acs.langmuir.5b00128

Langmuir

Abstract

We present a method to fabricate well-controlled periodic silicon nanopillars (Si NPs) in hexagonal arrays using silica nanosphere (SNS) lithography (SNL) combined with metal-assisted chemical etching (MaCE). The period of the Si NPs is easily changed by using our silica nanosphere (SNS) spin-coating process‚ which provides excellent monolayer uniformity and coverage (>95%) over large surface areas. The size of the deposited SNS is adjusted by reactive ion etching (RIE) to produce a target diameter at a fixed period for control of the surface pattern size after a gold metal mask layer deposition. The Si NPs are etched with the MaCE technique following introduction of a Ni interfacial layer between the Si and Au catalyst layer for adhesion and improved lithographical accuracy. The result is a fast‚ convenient‚ and large-area applicable Si surface nanolithography technique for accurate and reproducible Si NP fabrication.

J.-Y. Choi, S. Das, N. D. Theodore, I. Kim, C. Honsberg, H. W. Choi, and T. L. Alford, “Advances in 2D/3D Printing of Functional Nanomaterials and Their Applications,” ECS Journal of Solid State Science and Technology, vol. 4, no. 4, pp. P3001–P3009, Jan. 2015 [Online]. https://doi.org/10.1149/2.0011504jss

ECS Journal of Solid State Science and Technology

ECS J. Solid State Sci. Technol.

Abstract

Advanced printing techniques include innovative and/or integrated processes that are used to produce an object with enhanced functionality and with a wide range of applications. This is done by realizing printing of functional materials such as ink‚ paste‚ polymer‚ ceramic powder‚ and organic materials. Unlike conventional manufacturing methods‚ a new technique is needed to manipulate small objects to fabricate desired parts‚ as materials are scaled down to the nanometer range. In this regard‚ the traditional subtractive production has been changed to a bottom-up approach as device structures have changed to multilayer structures which contain several functional nanomaterials. A powder is used to fabricate an object with a desired geometry. This reduces the material loss due to the bottom-up nature of the process. In this article‚ we review the current state of knowledge for advanced manufacturing using two methods‚ 3D printing and roll-to-roll manufacturing. Several applications of 3D printing and nanotechnological development are also being considered here.

M. M. Karow, N. N. Faleev, D. J. Smith, and C. B. Honsberg, “Defect creation in InGaAs/GaAs multiple quantum wells–I. Structural properties,” Journal of Crystal Growth, vol. 425, pp. 43–48, 2015 [Online]. https://doi.org/10.1016/j.jcrysgro.2015.03.051

4M398J5P year__2015 type__CrossRef ""

Journal of Crystal Growth

A. Srinivasa, “Front Grid Metallization of Silicon Solar Cells,” Masters Thesis, Arizona State University, 2015 [Online].

Abstract

In order to ensure higher penetration of photovoltaics in the energy market and have an immediate impact in addressing the challenges of energy crisis and climate change‚ this thesis research focusses on improving the efficiency of the diffused junction silicon solar cells of an already existing line with established processes. Thus‚ the baseline processes are first made stable and demonstrated as a pilot line at the Solar Power Lab at ASU‚ to be used as a backbone on which further improvements could be made. Of the several factors that affect the solar cell efficiency‚ improvement of short circuit current by reduction of the shading losses is chosen to achieve the improvement. The shading losses are reduced by lowering the finger width of the solar cell .This reduction of the front metal coverage causes an increase in the series resistance‚ thereby adversely affecting the fill factor and hence efficiency. To overcome this problem‚ double printing method is explored to be used for front grid metallization. Before its implementation‚ it is important to accurately understand the effect of reducing the finger width on the series resistance. Hence‚ series resistance models are modified from the existing generic model and developed to capture the effects of screen-printing. To have minimum power loss in the solar cell‚ finger spacing is optimized for the front grid design with each of the finger widths chosen‚ which are narrower than the baseline finger width. A commercial software package called Griddler is used to predict the results of the model developed to capture effects of screen-printing. The process for double printing with accurate alignment for finger width down to 50um is developed. After designing the screens for optimized front grid‚ solar cells are fabricated using both single printing and double printing methods and an improvement of efficiency from 17.2% to 17.8%‚ with peak efficiency of 18% is demonstrated.

T. Monga, “Development of Thin Heterojunction Solar Cells with High Open Circuit Voltage,” Masters Thesis, Arizona State University, 2015 [Online].

Abstract

The aim of this thesis research is the development of thin silicon heterojunction solar cells with high open circuit voltage (Voc). Heterojunction solar cells are higher in efficiency than diffused junction c-Si solar cells‚ and they are less vulnerable to light degradation. Furthermore‚ the low temperature processing of heterojunction cells favour a decrease in production costs and improve cell performance at the same time. Since about 30 % of the module cost is a result of substrate cost‚ thin solar cells are of economic advantage than their thicker counterparts. This lead to the research for development of thin heterojunction solar cells. For high cell efficiencies and performance‚ it is important for cells to have a high operating voltage and Voc. Development of heterojunction cells with high Voc required a stable and repeatable baseline process on which further improvements could be made. Therefore a baseline process for heterojunction solar cells was developed and demonstrated as a pilot line at the Solar Power Lab at ASU. All the processes involved in fabrication of cells with the baseline process were optimized to have a stable and repeatable process. The cells produced with the baseline process were 19-20% efficient. The baseline process was further used as a backbone to improve and develop thin cells with even higher Voc. The process recipe was optimized with an aim to explore the limits of Voc that could be achieved with this structure on a much thinner substrate than used for the baseline process. A record Voc greater than 760mV was recorded at SPL using Suns-Voc tester on a 50 microns thick heterojunction cell without metallization. Furthermore‚ Voc of 754.2 mV was measured on a 50 microns thick cell with metallization by National Renewable Energy Laboratory (NREL)‚ which is a record for Voc for heterojunction cells with metallization. High Voc corresponds to high cell efficiency and therefore‚ higher module voltage and power with using the same number of cells as compared to other c-Si solar cells.

Y. Kim, “Epitaxial Growth of High Quality InAs/GaAsSb Quantum Dots for Solar Cells,” Ph. D. Thesis, Arizona State University, 2015 [Online].

Abstract

The development of high efficiency III-V solar cells is needed to meet the demands of a promising renewable energy source. Intermediate band solar cells (IBSCs) using semiconductor quantum dots (QDs) have been proposed to exceed the Shockley-Queisser efficiency limit [1]. The introduction of an IB in the forbidden gap of host material generates two additional carrier transitions for sub-bandgap photon absorption‚ leading to increased photocurrent of IBSCs while simultaneously allowing an open-circuit voltage of the highest band gap. To realize a high efficiency IBSC‚ QD structures should have high crystal quality and optimized electronic properties. This dissertation focuses on the investigation and optimization of the structural and optical properties of InAs/GaAsSb QDs and the development of InAs/GaAsSb QD-based IBSCs. In the present dissertation‚ the interband optical transition and carrier lifetime of InAs/GaAsSb QDs with different silicon delta-doping densities have been first studied by time-integrated and time-resolved photoluminescence (PL). It is found that an optimized silicon delta-doping density in the QDs enables to fill the QD electronic states with electrons for sub-bandgap photon absorption and to improve carrier lifetime of the QDs. After that‚ the crystal quality and QD morphology of single- and multi-stack InAs/GaAsSb QDs with different Sb compositions have been investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The TEM studies reveal that QD morphology of single-stack QDs is affected by Sb composition due to strain reducing effect of Sb incorporation. The XRD studies confirm that the increase of Sb composition increases the lattice mismatch between GaAs matrix and GaAsSb spacers‚ resulting in increase of the strain relaxation in GaAsSb of the multi-stack QDs. Furthermore‚ the increase of Sb composition causes a PL redshift and increases carrier lifetime of QDs. Finally‚ the spacer layer thickness of multi-stack InAs/GaAsSb QDs is optimized for the growth of InAs/GaAsSb QD solar cells (QDSCs). The InAs/GaAsSb QDSCs with GaP strain compensating layer are grown and their device performances are characterized. The increase of GaP coverage is beneficial to improve the conversion efficiency of the QDSCs. However‚ the conversion efficiency is reduced when using a relatively large GaP coverage.

J. LeBeau, “Bulk Laser Material Modification: Towards a Kerfless Laser Wafering Process,” Ph. D. Thesis, Arizona State University, 2015 [Online].

Abstract

Due to the ever increasing relevance of finer machining control as well as necessary reduction in material waste by large area semiconductor device manufacturers‚ a novel bulk laser machining method was investigated. Because the cost of silicon and sapphire substrates are limiting to the reduction in cost of devices in both the light emitting diode (LED) and solar industries‚ and the present substrate wafering process results in >50% waste‚ the need for an improved ingot wafering technique exists. The focus of this work is the design and understanding of a novel semiconductor wafering technique that utilizes the nonlinear absorption properties of band-gapped materials to achieve bulk (subsurface) morphological changes in matter using highly focused laser light. A method and tool was designed and developed to form controlled damage regions in the bulk of a crystalline sapphire wafer leaving the surfaces unaltered. The controllability of the subsurface damage geometry was investigated‚ and the effect of numerical aperture of the focusing optic‚ energy per pulse‚ wavelength‚ and number of pulses was characterized for a nanosecond pulse length variable wavelength Nd:YAG OPO laser. A novel model was developed to describe the geometry of laser induced morphological changes in the bulk of semiconducting materials for nanosecond pulse lengths. The beam propagation aspect of the model was based on ray-optics‚ and the full Keldysh multiphoton photoionization theory in conjuncture with Thornber’s and Drude’s models for impact ionization were used to describe high fluence laser light absorption and carrier generation ultimately resulting in permanent material modification though strong electron-plasma absorption and plasma melting. Although the electron-plasma description of laser damage formation is usually reserved for extremely short laser pulses (<20 ps)‚ this work shows that it can be adapted for longer pulses of up to tens of nanoseconds. In addition to a model describing damage formation of sub-band gap energy laser light in semiconducting and transparent crystalline dielectrics‚ a novel nanosecond laser process was successfully realized to generate a thin plane of damage in the bulk of sapphire wafers. This was accomplished using high numerical aperture optics‚ a variable wavelength nanosecond laser source‚ and three-dimensional motorized precision stage control.

J. Choi, “Development of Nanosphere Lithography Technique with Enhanced Lithographical Accuracy on Periodic Si Nanostructure for Thin Si Solar Cell Application,” Ph. D. Thesis, Arizona State University, 2015 [Online].

Abstract

In this thesis‚ a novel silica nanosphere (SNS) lithography technique has been developed to offer a fast‚ cost-effective‚ and large area applicable nano-lithography approach. The SNS can be easily deposited with a simple spin-coating process after introducing a N‚N-dimethyl-formamide (DMF) solvent which can produce a highly close packed SNS monolayer over large silicon (Si) surface area‚ since DMF offers greatly improved wetting‚ capillary and convective forces in addition to slow solvent evaporation rate. Since the period and dimension of the surface pattern can be conveniently changed and controlled by introducing a desired size of SNS‚ and additional SNS size reduction with dry etching process‚ using SNS for lithography provides a highly effective nano-lithography approach for periodically arrayed nano-/micro-scale surface patterns with a desired dimension and period. Various Si nanostructures (i.e.‚ nanopillar‚ nanotip‚ inverted pyramid‚ nanohole) are successfully fabricated with the SNS nano-lithography technique by using different etching technique like anisotropic alkaline solution (i.e.‚ KOH) etching‚ reactive-ion etching (RIE)‚ and metal-assisted chemical etching (MaCE). In this research‚ computational optical modeling is also introduced to design the Si nanostructure‚ specifically nanopillars (NPs) with a desired period and dimension. The optical properties of Si NP are calculated with two different optical modeling techniques‚ which are the rigorous coupled wave analysis (RCWA) and finite-difference time-domain (FDTD) methods. By using these two different optical modeling techniques‚ the optical properties of Si NPs with different periods and dimensions have been investigated to design ideal Si NP which can be potentially used for thin c-Si solar cell applications. From the results of the computational and experimental work‚ it was observed that low aspect ratio Si NPs fabricated in a periodic hexagonal array can provide highly enhanced light absorption for the target spectral range (600 \textasciitilde 1100nm)‚ which is attributed to (1) the effective confinement of resonant scattering within the Si NP and (2) increased high order diffraction of transmitted light providing an extended absorption length. From the research‚ therefore‚ it is successfully demonstrated that the nano-fabrication process with SNS lithography can offer enhanced lithographical accuracy to fabricate desired Si nanostructures which can realize enhanced light absorption for thin Si solar cell.

N. Vulic, J. Y. Choi, C. B. Honsberg, and S. M. Goodnick, “Silica nanosphere lithography defined light trapping structures for ultra-thin si photovoltaics,” presented at the 2015 MRS Spring Meeting, 2015 [Online]. https://doi.org/10.1557/opl.2015.548

2015 MRS Spring Meeting

Y. Kim, K.-Y. Ban, D. Kuciauskas, P. C. Dippo, and C. B. Honsberg, “Impact of delta-doping position on photoluminescence in type-II InAs/GaAsSb quantum dots,” Semiconductor Science and Technology, vol. 30, no. 3, p. 035006, 2015 [Online]. https://doi.org/10.1088/0268-1242/30/3/035006

Semiconductor Science and Technology

Semicond. Sci. Technol.

Abstract

We studied the optical properties of InAs/GaAs 0.83 Sb 0.17 quantum dots (QDs)‚ with varying silicon delta-doping position (spatial distance‚ d = 0.5‚ 1‚ and 2 nm)‚ using photoluminescence (PL) measurements. Compared with the undoped QDs‚ the PL peak energies of the ground state (GS) emissions for the doped QDs with d = 0.5 and 2 nm were found to be greatly blueshifted by ∼31 meV‚ which was much larger than that for the doped QDs with d = 1 nm. The radiative recombination rate of the GS emissions for the doped QDs with d = 1 nm was estimated to be slower than that for the other doped QDs at 10 K. The doped QDs with d = 1 nm showed the fastest redshift of the GS peak energy with temperature and lowest thermal activation energy (151 meV) of electrons among the QD samples. Further‚ the time-resolved PL data revealed that the average carrier lifetime (6.3 ns) in the doped QDs with d = 1 nm was longer even than that in the undoped QDs (5.5 ns) because of the weakened electron-hole wavefunction overlap by the V-shaped potential barrier in the doped QDs.

2014

Y. Kim, K.-Y. Ban, D. Kuciauskas, P. C. Dippo, and C. B. Honsberg, “Effect of silicon delta-doping density on optical properties of type-II InAs/GaAsSb quantum dots,” Journal of Crystal Growth, vol. 406, pp. 68–71, Nov. 2014 [Online]. https://doi.org/10.1016/j.jcrysgro.2014.08.009

Journal of Crystal Growth

Abstract

We have investigated the optical properties of type-II InAs/GaAs0.83Sb0.17 quantum dots (QDs) with different silicon delta-doping densities of 5×1010‚ 5×1011‚ and 2×1012 cm−2 using photoluminescence (PL). The PL spectra of the QD ground state (GS) emission peaks for the samples are blueshifted at a slower rate with increasing the doping density due to the enhanced radiative recombination rate of the carriers. The PL intensity ratio of the GS emission to the first excited state emission increases with the doping density‚ which is indicative of the faster radiative recombination at the GS subbands with the doping density. The redshift rate of the GS emissions becomes faster at a high temperature (>130 K) as the doping density increases up to 5×1011 cm−2 resulting from the quantum confined Stark effect by the electric field of the ionized dopants‚ and decreases at an increased doping density of 2×1012 cm−2 due to the enhanced QD size uniformity. Time-resolved PL exhibits that the QD sample doped at 5×1010 cm−2 has a longer total radiative lifetime than the undoped sample‚ and a further increase in the doping density to 2×1012 cm−2 decreases the lifetime due to the enhancement of the radiative recombination through fast carrier relaxation.

J.-Y. Choi and C. B. Honsberg, “Silicon nano-fabrication by using silica nanosphere lithography technique for enhanced light management,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 2206–2208. https://doi.org/10.1109/PVSC.2014.6925363

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

We present a complete silicon (Si) nano-fabrication process to provide controlled shapes of nanostructures over large-scale Si surface area by combining our novel solvent controlled silica nanosphere (SNS) spin-coating method with reactive ion etching. Our novel spin-coating method shows that the introduction of N‚N-dimethyl-formamide solvent for SNS spin-coating can greatly enhance the uniformity of spin-coated 2-dimensional SNS layer and its coverage with significantly less sensitivity to deposition area. The enhanced quality and coverage of SNS provided excellent nano-patterning for diverse etching applications. With our SNS lithography‚ reactive ion etching (RIE) has been applied with fluorine (F) and chlorine (Cl) based gases to provide (1) controlled etching selectivity between SNS (SiO2) and Si substrate and (2) desired etching orientation depending on target shape of structure. Here we focus on the fabrication of Si nanopillar structures with various top diameters but fixed height which show significantly improved anti-reflection effect. In addition‚ computational optical modeling with rigorous coupled wave analysis (RCWA) shows that well-tapered nanocone structures can provide greatly reduced incident light angle dependence for surface reflection.

N. N. Faleev, D. J. Smith, and C. B. Honsberg, “Epitaxial growth: Phenomenological model of defect creation,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 0724–0728. https://doi.org/10.1109/PVSC.2014.6925022

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

Structural features‚ related to epitaxial growth and crystal defect creation in common III-V and Si-Ge materials were investigated by high-resolution x-ray diffraction and Transmission Electron Microscopy. Strong correlations between crystal perfection of epitaxial structures and growth conditions‚ specified by the initial elastic strain‚ deposition temperature and growth rate‚ elastic properties of epitaxial structures and thickness of epitaxial layers‚ were revealed. The investigations allowed suggest phenomenological model of defect creation in epitaxial structures‚ specify four different stages of defect creation‚ preferred crystalline defects‚ their density and spatial distribution in the volume. The main crystalline defect‚ responsible for deterioration of crystal perfection and physical properties of epitaxial structures were identified. Correct description of defect creation allows improved growth conditions and design of future devices to avoid/minimize deterioration of physical properties due to initially deteriorated growth conditions.

A. Maros, N. Faleev, and C. B. Honsberg, “Defect creation in low lattice-mismatched epitaxial structures,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 0499–0504. https://doi.org/10.1109/PVSC.2014.6924969

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

The formation of crystalline defects is studied as a function of the epitaxial layer thickness in InGaAs and GaAsSb material systems grown by molecular beam epitaxy on (001) GaAs wafers. The Sb and In composition is roughly 8% in both sets of samples while the nominal thicknesses are respectively 50‚ 125‚ 250nm and 500nm for the InGaAs structures and 100‚ 250 and 500nm for the GaAsSb structures. High-resolution x-ray diffraction results show that similar partial relaxation is obtained in both systems for nearly the same thickness. Consistent structural transformation of point defects into dislocation loops related to the thickness of ternary layers is revealed. This resulted in a partial relaxation of 42 and 46% in the 250 nm thick GaAsSb and InGaAs layers respectively due to a density of secondary 60° dislocation loops of 1 × 109 cm-2. The relaxation increased to 64% in the 500nm thick InGaAs and to 68% for the 500nm thick GaAsSb films even though the density of 60° dislocation loops in the volume was reduced due to intersections of these dislocation loops. Explanation of revealed structural features is suggested.

L. Ding, M. Boccard, J. Williams, A. Jeffries, S. Gangam, K. Ghosh, C. Honsberg, S. Bowden, Z. Holman, H. Atwater, T. Buonassisi, S. Bremner, M. Green, C. Ballif, and M. Bertoni, “Thin silicon solar cells: A path to 35% shockley-queisser limits: a DOE funded FPACE II project,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 2467–2470. https://doi.org/10.1109/PVSC.2014.6925429

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

Crystalline silicon technology is expected to remain the leading photovoltaic industry workhorse for decades. We present here the objectives and workplan of a recently launched project funded by the U.S. Department of Energy through the Foundational Program to Advance Cell Efficiency II (FPACE II)‚ which aims at leading crystalline silicon to an efficiency breakthrough. The project will tackle fundamental approach of materials design‚ defect engineering‚ device simulations and materials growth and characterization. Among the main novelties‚ the implementation of carrier selective contacts made of wide bandgap material or stack of materials is investigated for improved passivation‚ carrier extraction and carrier transport. Based on an initial selection of candidate materials‚ preliminary experiments are conducted to verify the suitability of their critical parameters as well as preservation of the silicon substrate surface and bulk properties. The target materials include III-V and metal-oxide materials.

S. Limpert, K. Ghosh, H. Wagner, S. Bowden, C. Honsberg, S. Goodnick, S. Bremner, A. Ho-Baillie, and M. Green, “Results from coupled optical and electrical sentaurus TCAD models of a gallium phosphide on silicon electron carrier selective contact solar cell,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 0836–0840. https://doi.org/10.1109/PVSC.2014.6925045

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

We report results from coupled optical and electrical Sentaurus TCAD models of a gallium phosphide (GaP) on silicon electron carrier selective contact (CSC) solar cell which show that Auger-limited open-circuit voltages up to 787 mV (on a 10 μm monocrystalline silicon substrate) and efficiencies up to 26.7% (on a 150 μm monocrystalline silicon substrate) may be possible for front-contacted devices which exhibit low interface recombination velocity (IRV) at the GaP/Si interface and which employ random pyramidal texturing‚ a detached silver reflector‚ rear locally diffused point contacts and a SiO2/Al2O3 rear oxide passivation stack.

Y. Zou, C. B. Honsberg, A. Freundlich, and S. M. Goodnick, “Simulation of electron escape from GaNAs/GaAs quantum well solar cells,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 2908–2911. https://doi.org/10.1109/PVSC.2014.6925540

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

Quantum wells embedded in the active region of single-bandgap solar cells have previously been shown to increase the absorption of photons of energies lower than the host bandgap‚ while maintaining the open-circuit voltage. Fast carrier escape from the quantum wells is essential to achieve such performance enhancements. In the present work‚ we use ensemble Monte Carlo simulation to model the escape time for photo-excited carriers in dilute nitride GaNAs/GaAs quantum wells to the continuum‚ for different well structures. The simulated electron escape rate due to polar optical phonon absorption and emission decreases exponentially with the well depth in agreement with thermionic emission theory.

J. J. Williams, A. M. Jeffries, L. Ding, S. Gangam, K. Ghosh, T. L. Williamson, M. I. Bertoni, and C. B. Honsberg, “Structural and optical investigations of GaN-Si interface for a heterojunction solar cell,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 0841–0843. https://doi.org/10.1109/PVSC.2014.6925046

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

In recent years the development of heterojunction silicon based solar cells has gained much attention‚ lead largely by the efforts of Panasonic’s HIT cell. The success of the HIT cell prompts the scientific exploration of other thin film layers‚ besides the industrially accepted amorphous silicon. The band gap‚ mobilities‚ and electron affinity of GaN make it an interesting candidate to solve problems of parasitic absorption while selectively extracting electrons. Using a novel MBE based growth technique‚ thin films of GaN have been deposited at temperature significantly lower than industry standards. Crystalline measurements and absorption data of GaN are presented. Additionally‚ effects of deposition on the silicon wafer lifetimes are presented.

N. Vulic, M. Patil, Y. Zou, S. H. Amilineni, C. B. Honsberg, and S. M. Goodnick, “Matching AC loads to solar peak production using thermal energy storage in building cooling systems - A case study at Arizona State University,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 1504–1509. https://doi.org/10.1109/PVSC.2014.6925200

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

This paper proposes the possibility of scaling up solar generation while shifting cooling load to the daytime. The focus is on buildings equipped with a water tank used to actively store cold water produced by a series of chillers. Water has the flexibility to be chilled and stored for later use. To lower the load demand during the peak hours of the day‚ the cooling loads are commonly shifted to the night hours through thermal storage. The present work studies the possibility of using solar power to meet the cooling demand by taking advantage of the fact that solar generation closely precedes the peak cooling demand. Also‚ in cases where solar capacity is scaled up‚ chiller storage tanks can store excess solar power generated‚ thus stabilizing the grid.

M. M. Karow, N. N. Faleev, C.-Z. Ning, D. J. Smith, and C. B. Honsberg, “InGaAs/GaAs MQWs: Correlation of crystal and physical properties,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 0660–0665. https://doi.org/10.1109/PVSC.2014.6925008

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

InGaAs/GaAs multiple quantum well structures were grown by molecular beam epitaxy with a variation in deposition temperature among the samples to change crystal and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties‚ whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature Tdep = 505°C was found for 20% In composition. The density of 60° dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection. Elevated deposition temperatures led to In decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering‚ as well as a growth surface instability against perturbations.

S. Y. Herasimenka, C. J. Tracy, W. J. Dauksher, C. B. Honsberg, and S. Bowden, “A simplified process flow for silicon heterojunction interdigitated back contact solar cells: Using shadow masks and tunnel junctions,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 2486–2490. https://doi.org/10.1109/PVSC.2014.6925434

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

A novel process flow‚ which can allow the formation of interdigitated p- and n-type a-Si strips and corresponding transparent conductive oxide (TCO) and metal layers for silicon heterojunction interdigitated back contact (SHJ-IBC) solar cells using only a single alignment step and without using any resist patterning is presented. The flow is based on the deposition of a-Si‚ TCO and metal layers through a stack of shadow masks. Three variation of the flow are described. Several key process components to include a-Si deposition and H2 plasma etch through the shadow mask are demonstrated and described.

S. Bowden, S. Herasimenka, W. Dauksher, C. Tracy, and C. Honsberg, “High open-circuit voltages on thin silicon solar cells,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 0577–0581. https://doi.org/10.1109/PVSC.2014.6924986

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

Silicon heterostructures solar cells have very low surface recombination and the resulting cells can achieve high open circuit voltages. We have demonstrated an open circuit voltage for a silicon solar cell at 753 mV. We show high lifetimes on textured substrates with an average of 3 ms using thin layers of doped and intrinsic amorphous silicon and that the lifetimes are consistent across batches. We show that for high measured lifetimes there is a close match between the Implied VOC (measured from photoconductance) and the Actual VOC (as measured on a final cell). Recent measurements with have produced Implied VOC’s of 761 mV on textured substrates.

J. Lee and C. B. Honsberg, “Limiting efficiencies of integrating single junction with intermediate band solar cells for multiphysics effects,” in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, 2014, pp. 1068–1072. https://doi.org/10.1109/PVSC.2014.6925098

Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th

PVSC

Abstract

The multi-physics aspects of hybrid solar cells has provided the numerous advantages. To overcome the disadvantage of tandem solar cells like three or four junctions‚ single junction and intermediate band solar cell (IBSC) integrated concept is useful alternatives to replace the tandem solar cells configuration. IBSC has triple carrier transitions so that integrating with single junction can show the similar characteristics of conventional four junction tandem solar cells. From the advantages of this hybrid solar cells‚ we investigates the theoretical approaches and the appropriate material selections.

J. Lee and C. B. Honsberg, “Limiting Efficiencies of Multijunction Solar Cells With Multiple Exciton Generation,” IEEE Journal of Photovoltaics, vol. 4, no. 3, pp. 874–880, May 2014. https://doi.org/10.1109/JPHOTOV.2014.2307156

IEEE Journal of Photovoltaics

Abstract

The introduction of multiple exciton generation (MEG) processes with a low maximum quantum yield (QY) (maximum 200%) into a multijunction solar cell provides an efficiency increase up to 4.2% absolute for a triple-junction solar cell under concentration. In addition‚ the efficiency contour plots show increased flexibility in material choices‚ even for series connected devices. Importantly‚ the MEG QYs necessary to achieve these advantages are moderate and within experimental measured values‚ requiring no more than the generation of two electron hole pairs from a photon.

V. Sharma, C. Tracy, D. Schroder, S. Herasimenka, W. Dauksher, and S. Bowden, “Manipulation of K center charge states in silicon nitride films to achieve excellent surface passivation for silicon solar cells,” Applied Physics Letters, vol. 104, no. 5, p. 053503, Feb. 2014 [Online]. https://doi.org/10.1063/1.4863829

Applied Physics Letters

Abstract

High quality surface passivation (Seff < 5 cm/s) was achieved on polished float zone and textured p- and n-type solar grade Czochralski silicon substrates by externally injecting and storing positive or negative charges (>±8 × 1012 cm−2) into a dual layer stack of Plasma Enhanced Chemical Vapor Deposition (PECVD) Silicon Nitride (SiNx)/PECVD Silicon Oxide (SiO2) films using a corona charging tool. We demonstrate long term stability and uniform charge distribution in the SiNx film by manipulating the charge on K center defects while negating the requirement of a high temperature thermal oxide step.

K. Nelson, “Misconceptions of Emergent Semiconductor Phenomena,” Arizona State University, 2014 [Online].

Abstract

The semiconductor field of Photovoltaics (PV) has experienced tremendous growth‚ requiring curricula to consider ways to promote student success. One major barrier to success students may face when learning PV is the development of misconceptions. The purpose of this work was to determine the presence and prevalence of misconceptions students may have for three PV semiconductor phenomena; Diffusion‚ Drift and Excitation. These phenomena are emergent‚ a class of phenomena that have certain characteristics. In emergent phenomena‚ the individual entities in the phenomena interact and aggregate to form a self-organizing pattern that can be observed at a higher level. Learners develop a different type of misconception for these phenomena‚ an emergent misconception. Participants (N=41) completed a written protocol. The pilot study utilized half of these protocols (n = 20) to determine the presence of both general and emergent misconceptions for the three phenomena. Once the presence of both general and emergent misconceptions was confirmed‚ all protocols (N=41) were analyzed to determine the presence and prevalence of general and emergent misconceptions‚ and to note any relationships among these misconceptions (full study). Through written protocol analysis of participants’ responses‚ numerous codes emerged from the data for both general and emergent misconceptions. General and emergent misconceptions were found in 80% and 55% of participants’ responses‚ respectively. General misconceptions indicated limited understandings of chemical bonding‚ electricity and magnetism‚ energy‚ and the nature of science. Participants also described the phenomena using teleological‚ predictable‚ and causal traits‚ indicating participants had misconceptions regarding the emergent aspects of the phenomena. For both general and emergent misconceptions‚ relationships were observed between similar misconceptions within and across the three phenomena‚ and differences in misconceptions were observed across the phenomena. Overall‚ the presence and prevalence of both general and emergent misconceptions indicates that learners have limited understandings of the physical and emergent mechanisms for the phenomena. Even though additional work is required‚ the identification of specific misconceptions can be utilized to enhance semiconductor and PV course content. Specifically‚ changes can be made to curriculum in order to limit the formation of misconceptions as well as promote conceptual change.

G. V. Pickett, “Development of a Diffused Junction Silicon Solar Cell Pilot Line,” Masters Thesis, Arizona State University, 2014 [Online].

Abstract

In the interest of expediting future pilot line start-ups for solar cell research‚ the development of Arizona State University’s student-led pilot line at the Solar Power Laboratory is discussed extensively within this work. Several experiments and characterization techniques used to formulate and optimize a series of processes for fabricating diffused-junction‚ screen-printed silicon solar cells are expounded upon. An experiment is conducted in which the thickness of a PECVD deposited anti-reflection coating (ARC) is varied across several samples and modeled as a function of deposition time. Using this statistical model in tandem with reflectance measurements for each sample‚ the ARC thickness is optimized to increase light trapping in the solar cells. A response surface model (RSM) experiment is conducted in which 3 process parameters are varied on the PECVD tool for the deposition of the ARCs on several samples. A contactless photoconductance decay (PCD) tool is used to measure the dark saturation currents of these samples. A statistical analysis is performed using JMP in which optimum deposition parameters are found. A separate experiment shows an increase in the passivation quality of the a-SiNx:H ARCs deposited on the solar cells made on the line using these optimum parameters. A RSM experiment is used to optimize the printing process for a particular silver paste in a similar fashion‚ the results of which are confirmed by analyzing the series resistance of subsequent cells fabricated on the line. An in-depth explanation of a more advanced analysis using JMP and PCD measurements on the passivation quality of 3 aluminum back-surface fields (BSF) is given. From this experiment‚ a comparison of the means is conducted in order to choose the most effective BSF paste for cells fabricated on the line. An experiment is conducted in parallel which confirms the results via Voc measurements. It is shown that in a period of 11 months‚ the pilot line went from producing a top cell efficiency of 11.5% to 17.6%. Many of these methods used for the development of this pilot line are equally applicable to other cell structures‚ and can easily be applied to other solar cell pilot lines.

C. Zhang, “High Efficiency GaAs-based Solar Cells Simulation and Fabrication,” Masters Thesis, Arizona State University, 2014 [Online].

Abstract

GaAs-based solar cells have attracted much interest because of their high conversion efficiencies of \textasciitilde28% under one sun illumination. The main carrier recombination mechanisms in the GaAs-based solar cells are surface recombination‚ radiative recombination and non-radiative recombination. Photon recycling reduces the effect of radiative recombination and is an approach to obtain the device performance described by detailed balance theory. The photon recycling model has been developed and was applied to investigate the loss mechanisms in the state-of-the-art GaAs-based solar cell structures using PC1D software. A standard fabrication process of the GaAs-based solar cells is as follows: wafer preparation‚ individual cell isolation by mesa‚ n- and p-type metallization‚ rapid thermal annealing (RTA)‚ cap layer etching‚ and anti-reflection coating (ARC). The growth rate for GaAs-based materials is one of critical factors to determine the cost for the growth of GaAs-based solar cells. The cost for fabricating GaAs-based solar cells can be reduced if the growth rate is increased without degrading the crystalline quality. The solar cell wafers grown at different growth rates of 14 μm/hour and 55 μm/hour were discussed in this work. The structural properties of the wafers were characterized by X-ray diffraction (XRD) to identify the crystalline quality‚ and then the as-grown wafers were fabricated into solar cell devices under the same process conditions. The optical and electrical properties such as surface reflection‚ external quantum efficiency (EQE)‚ dark I-V‚ Suns-Voc‚ and illuminated I-V under one sun using a solar simulator were measured to compare the performances of the solar cells with different growth rates. Some simulations in PC1D have been demonstrated to investigate the reasons of the different device performances between fast growth and slow growth structures. A further analysis of the minority carrier lifetime is needed to investigate into the difference in device performances.

J. Lee, “Advanced Hybrid Solar Cell Approaches for Future Generation Ultra-High Efficiency Photovoltaic Devices,” Ph. D. Thesis, Arizona State University, 2014 [Online].

Abstract

Increasing the conversion efficiencies of photovoltaic (PV) cells beyond the single junction theoretical limit is the driving force behind much of third generation solar cell research. Over the last half century‚ the experimental conversion efficiency of both single junction and tandem solar cells has plateaued as manufacturers and researchers have optimized various materials and structures. While existing materials and technologies have remarkably good conversion efficiencies‚ they are approaching their own limits. For example‚ tandem solar cells are currently well developed commercially but further improvements through increasing the number of junctions struggle with various issues related to material interfacial defects. Thus‚ there is a need for novel theoretical and experimental approaches leading to new third generation cell structures. Multiple exciton generation (MEG) and intermediate band (IB) solar cells have been proposed as third generation alternatives and theoretical modeling suggests they can surpass the detailed balance efficiency limits of single junction and tandem solar cells. MEG or IB solar cell has a variety of advantages enabling the use of low bandgap materials. Integrating MEG and IB with other cell types to make novel solar cells (such as MEG with tandem‚ IB with tandem or MEG with IB) potentially offers improvements by employing multi-physics effects in one device. This hybrid solar cell should improve the properties of conventional solar cells with a reduced number of junction‚ increased light-generated current and extended material selections. These multi-physics effects in hybrid solar cells can be achieved through the use of nanostructures taking advantage of the carrier confinement while using existing solar cell materials with excellent characteristics. This reduces the additional cost to develop novel materials and structures. In this dissertation‚ the author develops thermodynamic models for several novel types of solar cells and uses these models to optimize and compare their properties to those of existing PV cells. The results demonstrate multiple advantages from combining MEG and IB technology with existing solar cell structures.

Q. Yang, “Charged Silicon Nitride Films: Field-Effect Passivation of Silicon Solar Cells and a Novel Characterization Method through Lifetime Measurements,” Masters Thesis, Arizona State University, 2014 [Online].

Abstract

Silicon (Si) solar cells are the dominant technology used in the Photovoltaics industry. Field-effect passivation by means of electrostatic charges stored in an overlying insulator on a silicon solar cell has been proven to be a significantly efficient way to reduce effective surface recombination velocity and increase minority carrier lifetime. Silicon nitride (SiNx) films have been extensively used as passivation layers. The capability to store charges makes SiNx a promising material for excellent feild effect passivation. In this work‚ symmetrical Si/SiO2/SiNx stacks are developed to study the effect of charges in SiNx films. SiO2 films work as barrier layers. Corona charging technique showed the ability to inject charges into the SiNx films in a short time. Minority carrier lifetimes of the Czochralski (CZ) Si wafers increased significantly after either positive or negative charging. A fast and contactless method to characterize the charged overlying insulators on Si wafer through lifetime measurements is proposed and studied in this work‚ to overcome the drawbacks of capacitance-voltage (CV) measurements such as time consuming‚ induction of contanmination and hysteresis effect‚ etc. Analytical simulations showed behaviors of inverse lifetime (Auger corrected) vs. minority carrier density curves depend on insulator charge densities (Nf). From the curve behavior‚ the Si surface condition and region of Nf can be estimated. When the silicon surface is at high strong inversion or high accumulation‚ insulator charge density (Nf) or surface recombination velocity parameters (Sn0 and Sp0) can be determined from the slope of inverse lifetime curves‚ if the other variable is known. If Sn0 and Sp0 are unknown‚ Nf values of different samples can be compared as long as all have similar Sn0 and Sp0 values. Using the saturation current density (J0) and intercept fit extracted from the lifetime measurement‚ the bulk lifetime can be calculated. Therefore‚ this method is feasible and promising for charged insulator characterization.

J. J. Williams, T. L. Williamson, M. A. Hoffbauer, Y. Wei, N. N. Faleev, and C. Honsberg, “Growth of high crystal quality InN by ENABLE-MBE,” physica status solidi (c), vol. 11, no. 3–4, pp. 577–580, 2014 [Online]. https://doi.org/10.1002/pssc.201300693

physica status solidi (c)

Phys. Status Solidi C

Abstract

Indium nitride is of interest as a small band gap material for hot carrier solar cells and for alloying of III-Nitrides in conventional solar cells. Growth of photovoltaic device quality InN on cost effective sapphire wafers is challenging. Lattice mismatch between sapphire and indium nitride (∼25%) makes growth of epitaxial crystals with low crystalline imperfection problematic. InN films with promising XRD results were grown by MBE using a well-aligned AlN buffer layer to grow InN on sapphire substrates. Crystal quality‚ analyzed by HRXRD‚ gives evidence that films of both AlN on sapphire and InN on AlN are almost fully relaxed by edge dislocations created on the interfaces with a rather low density of closed dislocation loops‚ created subsequently in the film volume of both epitaxial layers. These films exhibit threading dislocation densities of 3 x 107 cm-2 and 2 × 108 cm-2 for AlN and InN. Additional metrics quantifying film quality of AlN and InN characterized by XRD and TEM will be presented. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA‚ Weinheim)

Y. Kim, N. Faleev, D. Tang, D. Smith, D. Kuciauskas, P. C. Dippo, and C. Honsberg, “Structural and optical properties of multi-stack InAs/GaAsSb quantum dots with different Sb composition,” in 40th IEEE Photovoltaic Specialist Conference, PVSC 2014, June 8, 2014 - June 13, 2014, 2014, pp. 1056–1058. https://doi.org/10.1109/PVSC.2014.6925095

Abstract

The impact of the Sb composition on the structural and optical properties of ten-stack InAs/GaAsSb quantum dots (QDs) were investigated using transmission electron microscopy (TEM)‚ high-resolution x-ray diffraction (XRD)‚ and photoluminescence (PL). The TEM images demonstrate that the Sb composition affects the change in the QD density and morphology. From analysis of XRD reciprocal space maps (RSMs) of the (224) asymmetrical reflection‚ it is found that as the Sb composition increases the relaxation of the initial elastic stress of the GaAsSb increases up to 23 %. In addition‚ the Sb composition influences the interband optical transitions such as the PL peak redshift and carrier lifetimes.

J.-Y. Choi, T. L. Alford, and C. B. Honsberg, “Solvent-Controlled Spin-Coating Method for Large-Scale Area Deposition of Two-Dimensional Silica Nanosphere Assembled Layers,” Langmuir, vol. 30, no. 20, pp. 5732–5738, 2014 [Online]. https://doi.org/10.1021/la5001842

Langmuir

Abstract

In this article‚ we show that introducing a N‚N-dimethyl-formamide (DMF) solvent for silica nanosphere (SNS) monolayer spin-coating can offer a low-cost and simple spin-coating approach for SNS monolayer deposition even on large-area silicon surfaces. From our method‚ more than 95% monolayer coverage for a 2 in round Si surface was achieved‚ which is one of the highest reported coverage by a spin-coating method. We prove that DMF offers highly enhanced wettability and slow solvent evaporation rate compared to a conventional solvent‚ water‚ in addition to excellent SNS dispersibility in solution preventing SNS cluster deposition on the surface and consequently produces a close-packed SNS monolayer with good uniformity over the surface. In addition‚ the benefits of DMF are retained as the deposition area increases indicating its high tolerance to spin-coating area. Better than 90% SNS monolayer coverage on a 4 in Si substrate was achieved with the DMF spin-coating method. Moreover‚ DMF has the advantage that SNS spin-coating can be done under common ambient laboratory conditions with 100% pure DMF unlike previous approaches which require humidity and temperature controls or additional surfactant additions to the solution.

2013

S. Herasimenka, B. Dauksher, C. Tracy, V. Sharma, K. Ghosh, M. Bailly, and S. Bowden, “Surface Preparation and Optimization of Amorphous Silicon Deposition for Silicon Heterojunction Solar Cells,” in 28th European Photovoltaic Solar Energy Conference and Exhibition, 2013, pp. 1943–1946 [Online]. https://doi.org/10.4229/28thEUPVSEC2013-2DV.3.39

Abstract

Design of Experiment (DOE) approach was used to optimize the characteristics of intrinsic a-Si and doped a-Si films for silicon heterojunction (SHJ) solar cells where effective lifetime and bulk resistivity were used as optimization parameters for intrinsic and doped a-Si respectively. The performance of the optimized films on alkaline etched wafers required additional optimization of post etch cleans. We have developed a cleaning cycle which allows 725-735 mV local Voc and 81- 83% local pFF on non-metallized SHJ solar cells with no reflectance losses. A batch of 16 wafers demonstrated good across wafer and wafer to wafer passivation uniformity with 2.3 ms mean effective lifetime and 0.6 ms standard deviation.

S. Y. Herasimenka, C. J. Tracy, V. Sharma, N. Vulic, W. J. Dauksher, and S. G. Bowden, “Surface passivation of n-type c-Si wafers by a-Si/SiO2/SiNx stack with <1 cm/s effective surface recombination velocity,” Applied Physics Letters, vol. 103, no. 18, p. 183903, Oct. 2013 [Online]. https://doi.org/10.1063/1.4827821

Applied Physics Letters

Abstract

The passivation quality of an a-Si/SiO2/SiNx (aSON) stack deposited by conventional PECVD at <250 °C with and without additional corona charging of SiNx is presented. <2 fA/cm2 surface dark saturation current density and <1 cm/s effective surface recombination velocity were demonstrated on both planar and textured n-type Czochralski (CZ) substrates. It was shown that very good passivation can be achieved using <5 nm a-Si layers to provide low parasitic absorption. We also report effective minority carrier lifetimes >60 ms on 5000 Ω-cm and 20.9 ms on 1.7 Ω-cm mirror polished float zone (FZ) material passivated with aSON stacks.

S. P. Bremner, K.-Y. Ban, N. N. Faleev, C. B. Honsberg, and D. J. Smith, “Impact of stress relaxation in GaAsSb cladding layers on quantum dot creation in InAs/GaAsSb structures grown on GaAs (001),” Journal of Applied Physics, vol. 114, no. 10, p. 103511, Sep. 2013 [Online]. https://doi.org/10.1063/1.4819962

Journal of Applied Physics

Abstract

We describe InAs quantum dot creation in InAs/GaAsSb barrier structures grown on GaAs (001) wafers by molecular beam epitaxy. The structures consist of 20-nm-thick GaAsSb barrier layers with Sb content of 8%‚ 13%‚ 15%‚ 16%‚ and 37% enclosing 2 monolayers of self-assembled InAs quantum dots. Transmission electron microscopy and X-ray diffraction results indicate the onset of relaxation of the GaAsSb layers at around 15% Sb content with intersected 60° dislocation semi-loops‚ and edge segments created within the volume of the epitaxial structures. 38% relaxation of initial elastic stress is seen for 37% Sb content‚ accompanied by the creation of a dense net of dislocations. The degradation of In surface migration by these dislocation trenches is so severe that quantum dot formation is completely suppressed. The results highlight the importance of understanding defect formation during stress relaxation for quantum dot structures particularly those with larger numbers of InAs quantum-dot layers‚ such as those proposed for realizing an intermediate band material.

S. Y. Herasimenka, W. J. Dauksher, and S. G. Bowden, “>750 mV open circuit voltage measured on 50 μm thick silicon heterojunction solar cell,” Applied Physics Letters, vol. 103, no. 5, pp. 053511-053511-4, Aug. 2013 [Online]. https://doi.org/doi:10.1063/1.4817723

4M398J5P year__2013 type__AIP ""

Applied Physics Letters

Abstract

This paper presents experimental evidence that silicon solar cells can achieve >750 mV open circuit voltage at 1 Sun illumination providing very good surface passivation is present. 753 mV local open circuit voltage was measured on a 50 μm thick non-metalized silicon heterojunction solar cell. The paper also considers a recombination model at open circuit based on the recent Auger and radiative recombination parameterization and the measured surface saturation current density. The loss mechanisms at open circuit and several practical pathways to achieve >760 mV open circuit voltage in silicon heterojunction solar cells are discussed.

S. Bowden, K. Ghosh, and C. Honsberg, “Solar cells without p-n junctions,” SPIE Newsroom, Jul. 2013 [Online]. https://doi.org/10.1117/2.1201307.004681

4M398J5P year__2013 type__CrossRef ""

SPIE Newsroom

K. G. Nelson, S. Brem, C. H. Foster, S. Bowden, J. Husman, and C. Honsberg, “Assessing the formation of misconceptions when students learn PV using current curricular tools,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 2383–2388. https://doi.org/10.1109/PVSC.2013.6744954

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

The purpose of this work is to discover the misconceptions students have related to PV content‚ in light of the way in which the content is portrayed curricular tools‚ namely through simulations. Undergraduate students were recruited from their first circuits courses (N=20) to participate in this study. Participants in the study were presented with PV related content displayed through a simulation previously developed by the PVCDROM. They observed these simulations and were asked associated questions‚ questions designed to uncover their misconceptions formed as a result of engaging with these simulations. Findings indicate that students hold misconceptions related to both of the simulations utilized; diffusion and drift. The results of this study indicate a need to conduct a deeper level of analysis of the participants’ responses‚ which could potentially provide future evidence related to misconception formation as a result of how simulations related to drift and diffusion are presented using curricular tools. Diffusion and drift should be presented such that it discourages the formation of misconceptions‚ and ultimately removes barriers to learning PV.

V. Sharma, C. Tracy, D. Schroder, M. Flores, B. Dauksher, and S. Bowden, “Study and manipulation of charges present in silicon nitride films,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 1288–1293. https://doi.org/10.1109/PVSC.2013.6744377

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

As crystalline silicon solar cells continue to get thinner‚ the surfaces of the cell play an ever important role in controlling the cell efficiency. One tool to minimize surface recombination is field effect passivation from the charges present in the thin films applied on the cell surfaces. Basic PC 1D simulations were carried to understand the relation between the amount and sign of charge on cell efficiencies with varying emitter-doping levels. Silicon nitride (SiNx) thin films are known to carry net positive fixed charges that originate from specific silicon nitrogen dangling bonds (•SiN3) known as K centers. The properties of fixed positive charges present in as-deposited SiNx films are studied by capacitance — voltage (CV) and electron spin resonance (ESR) techniques. We report that the as-deposited SiNx films also carry neutral defects (K0 centers) that can easily be manipulated to either positive (K+) or negative (K) charge states depending on the end application. Corona charging was used to change the net charge in the film to either positive or negative and high energy (sub-300 nm) UV light was used to neutralize or annihilate the charges. ESR measurements showed that the neutral K0 defects are distributed throughout the bulk of the nitride film. A high temperature annealing step decreases the amount of neutral defects possibly due to bonding of hydrogen with the K center. First order effects of both positive and negative nitride charges on test structures were studied by photoconductance measurements.

T. Reblitz, C. Tracy, B. Dauksher, S. Herasimenka, and S. Bowden, “p+ Emitters on n-type c-Si using rapid thermal annealing of PECVD a-Si films and aluminum metallization,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 2257–2262. https://doi.org/10.1109/PVSC.2013.6744927

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

We present the development and characterization of n-type mono-Si photovoltaic cells with p+ emitters formed by the rapid thermal annealing of PECVD boron-doped amorphous silicon (a-Si) films. Aluminum metallization was deposited from evaporator and sputtering tool sources. The process yielded emitters of excellent uniformity (average non-uniformity of 4.4% over 40 samples) with sheet resistance ranging from 53 Ω/□ to 249 Ω/□ inversely dependent on the thickness of p+ film annealed. A reasonably-low post-anneal series resistance of 0.602 Ω∗cm2 suggests that all-aluminum metallization is sufficient for silicon photovoltaic cells of high efficiency.

S. Gangam, A. Jeffries, D. P. Fenning, B. Lai, J. Maser, T. Buonassisi, C. Honsberg, and M. I. Bertoni, “In-situ stage development for high-temperature X-ray nanocharacterization of defects in solar cells,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 1394–1395. https://doi.org/10.1109/PVSC.2013.6744404

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

The vast majority of photovoltaic materials are highly sensitive to the presence of inhomogeneously distributed nanoscale defects‚ which commonly regulate the overall performance of the devices. The defects can take the form of impurities‚ stoichiometry variations‚ microstructural misalignments‚ and secondary phases — the majority of which are created during solar cell processing. Scientific understanding of these defects and development of defect-engineering techniques have the potential to significantly increase cell efficiencies‚ as well as provide a science-based approach to increase the competitiveness for the US PV industry on a dollar per installed kWh criterion. For the case of Cu(In‚ Ga)Se2 devices for example‚ the theoretically limit sits at 30.5% efficiency [1]‚ thus‚ surpassing DOE’s SunShot goals for cost-competitive solar power. However‚ to date‚ CIGS laboratory scale cells have been reported to achieve only 20.3% efficiencies and modules have not crossed the 15 % certified efficiency barrier. Recent reports have suggested that these record cells are limited by non-ideal recombination and‚ more specifically‚ by an increased saturation current that seems to originate from the particular defect chemistry at structural defects. In order to understand the severe efficiency limitations that currently affect solar cell materials‚ it is necessary to understand in detail the role of defects and their interactions under actual operating and processing conditions. In this work we propose to develop a high-temperature‚ in-situ stage for X-ray microscopes‚ with the capabilities of temperature and ambient control. Here‚ we provide insight into the design and preliminary testing at the Advanced Photon Source with beam sizes ≈100nm.

G. Pickett, S. Bowden, J. Husman, K. Ross, D. Shell, and K. Nelson, “Student-led solar cell fabrication pilot line: Engaging the next generation of PV engineers,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 2389–2391. https://doi.org/10.1109/PVSC.2013.6744955

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

A student-led solar cell fabrication pilot line has been formed with the goal of encouraging young engineering students to pursue PV related research careers‚ and to better prepare them for addressing our future energy needs. One example of its impact is a five week REU (Research Experience for Undergraduates) that was implemented on the pilot line. From evaluations administered at the start and end of the program‚ the most notable outcomes are the improvement of the participants’ self-efficacy resultant from hands-on experience and a general increase in PV related knowledge.

K. Ghosh, S. Herasimenka, B. Dauksher, and S. Bowden, “Correlation between lifetime curve and performance of amorphous silicon/ crystalline silicon heterostructure solar cell,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 1232–1237. https://doi.org/10.1109/PVSC.2013.6744363

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

Detailed balance calculations are prevalently used to calculate the thermodynamic limit of performance of photovoltaic devices. This work combines the detailed balance calculations with the lifetime curve to determine the limiting performance of silicon heterostructure solar cell. The detailed balance model uses the state-of-the-art values for the recombination coefficients. The calculation shows that for an undoped silicon with only intrinsic (radiative and auger) recombination mechanisms considered‚ the efficiency limit is 29.63 % at 26 μm. However‚ the value changes with the inclusion of the extrinsic recombination processes that occurs due to the presence of surfaces and defects within crystalline silicon.

S. Limpert, S. Goodnick, C. Honsberg, G. Conibeer, and S. Bremner, “A hot carrier solar cell device model using a coupled electron phonon energy balance model,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 1054–1059. https://doi.org/10.1109/PVSC.2013.6744322

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

Towards the goal of designing‚ building and optimizing an operational hot carrier solar cell‚ components of a hot carrier solar cell numerical device model have been developed and used to investigate the operation of the device. A coupled electron phonon energy balance model and an energy selective contact transport model have been written. An investigation using the energy selective contact transport model compared the performance of differing extraction barrier structures. A second investigation coupled the current calculating portion of the energy selective contact transport model to the electron phonon energy balance model to study the operation of these codependent device subsystems. In this paper‚ the construction of the energy selective contact transport model‚ and results from the two aforementioned investigations are given.

J.-Y. Choi and C. B. Honsberg, “Reactive ion etching surface texturing of c-Si using silica nanosphere lithography technique for solar cell application,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 1199–1202. https://doi.org/10.1109/PVSC.2013.6744355

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

A reactive ion etching (RIE) process has been applied to etch diverse shape of nanoscale surface texturing on crystalline silicon (c-Si) for solar cell application. In this work‚ silica nanospheres (NS) were used as a mask material to utilize selective etching between silicon surface and silica NS for texturing. For effective silica NS deposition‚ we also developed our own solvent-control spin-coating method showing great monolayer coverage under common laboratory environment which is possibly more suitable for low-cost fabrication compared to conventional approach (moisture and temperature controlled spin-coating or dipping coating method by Langmuir-Blodgett trough). In RIE process‚ the surface texturing was etched with various shapes to reduce the reflectivity from surface‚ and the spectral response measurement confirms the effectiveness of RIE texturing which showed phenomenal anti-reflection effect with less than 2% of light reflection below 1.0 um wavelength. In addition‚ experiments for Quinhydrone/Methanol (QHY/ME) surface passivation for RIE textured surface were proceeded to evaluate RIE texturing effect for surface recombination velocity and minority carrier lifetime.

J. J. Williams, K. Ghosh, N. N. Faleev, T. L. Williamson, and C. B. Honsberg, “Induced junction III-nitride solar cells for wide band gap solar cells: Modeling charge transport and band bending in polarized material,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 2144–2146. https://doi.org/10.1109/PVSC.2013.6744898

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

III-N alloys of aluminum nitride‚ gallium nitride‚ and indium nitride are of high interest for solar cells as they span the majority of the solar spectrum from 6.2eV to 0.7eV. There are however challenges in creating conventional cells from these materials. Issues include an inability to produce high quality p-type material and polarization effects that block carrier transport in standard heterojunctions. We propose using an induced junction‚ a form of heterojunction‚ to create band bending and thus an effective p-n junction solely within n-type material. In this paper‚ we discuss theoretical equilibrium‚ transport‚ generation and recombination mechanisms within a III-N induced junction device. Recent experimental work with III-N material and device architectures will be added to help the model’s accuracy.

J. Lee and C. B. Honsberg, “The impact of quantum yield through limiting efficiency for multiple exciton generation with intermediate band solar cells,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 1041–1045. https://doi.org/10.1109/PVSC.2013.6744319

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

We develop the hybrid thermodynamic limit model using the intermediate band solar cells assisted with multiple exciton generation under blackbody radiation. For this hybrid solar cell model‚ we manage the spectral splitting to maximize the generated number of electron and hole pairs (EHP). First‚ we have separated two areas to explain the carrier transition. For regarding of quantum yield and charge neutrality‚ the multiple EHPs are generated at barrier bandgap and one carrier generation is in quantum dot. Thus‚ to extract additional carrier in quantum dot‚ it is required additional absorption paths or more photon energy. After studying the procedure of carrier multiplication in intermediate band solar cells‚ we have calculated the theoretical conversion efficiencies with number of generated EHPs. Its maximum theoretical efficiencies are increased and optimum bandgap is lowered compared to conventional intermediate band solar cells. And‚ based on these results‚ we can also choose the suitable material for these hybrid solar cells.

J. Lee, S. M. Goodnick, and C. B. Honsberg, “Limiting efficiency of silicon based nanostructure solar cells for multiple exciton generation,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 1046–1049. https://doi.org/10.1109/PVSC.2013.6744320

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

The materials for multiple exciton generation (MEG) solar cells have often focused on colloidal systems using low band gap materials such as PbSe. However‚ detailed balance calculations with non-ideal quantum yield (QYs) lead to higher band gaps‚ with silicon close to the optimum value. We calculate the conversion efficiency of MEG processes including non-idealities for nanostructured silicon. We also boost efficiency of MEG solar cells using multijunction solar cell configurations. Incorporating MEG into multijunction solar cells leads to increased calculated efficiencies due to QYs greater than unity in each junction. Here we have simulated the possible MEG enhanced QY of each junction and the corresponding conversion efficiencies for double junction hybrid solar cells. This hybrid structure extends the opportunities to maximize the MEG effect and also to select the appropriate effective bandgaps using silicon nanostructures.

A. Jeffries, S. Bowden, C. Honsberg, and M. Bertoni, “Sensitivity analysis of materials availability for terawatt PV deployment,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 3353–3356. https://doi.org/10.1109/PVSC.2013.6745169

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

The road map for cost competitive large-scale PV deployment is ever changing and with PV grade poly silicon prices averaging 16 UD/kg and companies focusing on using less materials the cost effectiveness of readily available materials for solar cell applications should be revisited. In this work we analyze to which extent the extraction cost of the absorber layer material plays a role in the overall cost of generating electricity‚ taking into account the potential for light trapping and the non-power producing component costs. Our calculations show that nearly all presently used materials have fundamental cost and availability potential which are well below a level at which material availability is a dominant consideration. Instead‚ constraints on parameters such as the amount of copper for wiring or substrate material for module fabrication become dominant issues.

J. Lee and C. B. Honsberg, “Impact of threshold energy of multiple exciton generation solar cells,” in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, 2013, pp. 1050–1053. https://doi.org/10.1109/PVSC.2013.6744321

Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th

PVSC

Abstract

The threshold energy (Eth) over 100% quantum yield (QY) is a key factor to determine the performance of multiple exciton generation (MEG) solar cells. By investigating non-idealities of MEG models‚ it is critical to consider non-idealities in Eth for the onset of MEG processes. Detailed balance calculations show that despite a large experimental emphasis on the maximum quantum yield‚ the threshold energy has a substantial and significant effect. The first effect is that at threshold energies between 2 and 3Eg (for one sun) or 3 and 4Eg for maximum concentration‚ even theoretical benefits of the MEG process disappear. Since measured values are within this range‚ this is an important effect to consider. The second effect is that the inclusion of non-ideal threshold energies increases the optimum band gap‚ moving to values consistent with silicon.

N. N. Faleev, C. Honsberg, and V. I. Punegov, “Quantitative analysis of the quantum dot superlattice by high-resolution x-ray diffraction,” Journal of Applied Physics, vol. 113, no. 16, p. 163506, Apr. 2013 [Online]. https://doi.org/doi:10.1063/1.4802662

4M398J5P year__2013 type__AIP ""

Journal of Applied Physics

Abstract

A new high-resolution x-ray diffraction approach for quantitative analysis of superlattice structures (SLs) with self-assembled quantum dots (QDs) was developed. For numerical simulations of the 2D angular distribution of diffracted x-ray radiation‚ both the coherent and diffuse scattering components have been calculated. Direct comparison of simulated patterns and experimental results revealed good agreement of the calculated intensity distribution with experimental reciprocal space maps for the superlattice GaAs(001)-AlGaAs-\InAs QDs-GaAs\SL with 20 periods of quantum dots. The simulation procedure allows one to obtain data about the shape‚ average size‚ elastic strains around the QDs‚ average density of the QDs‚ the presence of short- or long-range order in the arrangement of QDs in the semiconducting matrix‚ the vertical and lateral correlation lengths of the ensemble of quantum dots‚ and the parameters of the intermediate GaAs and AlGaAs layers.

N. Faleev, N. Sustersic, N. Bhargava, J. Kolodzey, A. Y. Kazimirov, and C. Honsberg, “Structural investigations of SiGe epitaxial layers grown by molecular beam epitaxy on Si(0 0 1) and Ge(0 0 1) substrates: I—High-resolution x-ray diffraction and x-ray topography,” Journal of Crystal Growth, vol. 365, pp. 44–53, Feb. 2013 [Online]. https://doi.org/10.1016/j.jcrysgro.2012.12.002

Journal of Crystal Growth

Abstract

Epitaxial structures of different SiGe composition grown by molecular beam epitaxy on Si(0 0 1) and Ge(0 0 1) substrates have been studied by high-resolution x-ray diffraction and x-ray topography to establish correlations between epitaxial growth conditions and crystal perfection. It was confirmed that epitaxy under initial elastic stress inevitably led to the creation of extended crystal defects. The type of defects created and their density and spatial distribution‚ strongly depended on the value and sign of the initial elastic strain‚ the elastic constants of solid solutions‚ the temperature of deposition and growth rate‚ and the thickness of the epitaxial layers. All of the investigated structures were classified by their crystal perfection‚ using x-ray diffraction with the volume density of dislocation loops as a parameter. It was found that the accommodation and relaxation of initial elastic stress and creation of crystal defects were multistage “chain” processes‚ necessary to stabilize the crystal structure at a level corresponding to the particular growth conditions. Types‚ density and spatial distribution of crystal defects‚ related to each stage of defect creation and matched to structural features‚ as revealed by high resolution x-ray diffraction‚ were considered for explanation.

N. Faleev, N. Sustersic, N. Bhargava, J. Kolodzey, S. Magonov, D. J. Smith, and C. Honsberg, “Structural investigations of SiGe epitaxial layers grown by molecular beam epitaxy on Si(001) and Ge(001) substrates: II—Transmission electron microscopy and atomic force microscopy,” Journal of Crystal Growth, vol. 365, pp. 35–43, Feb. 2013 [Online]. https://doi.org/10.1016/j.jcrysgro.2012.11.067

Journal of Crystal Growth

Abstract

The creation of crystal defects during epitaxial growth‚ and their proper characterization and classification are among the most critical issues impacting epitaxial structures and device applications. Epitaxial layers of different SiGe composition grown by molecular beam epitaxy (MBE) on Si(001) and Ge(001) substrates have been studied by Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). The volumetric and surface structure of crystal defects revealed and characterized by TEM and AFM provided a detailed understanding of the major processes associated with defect creation and structural transformation during epitaxial growth. The main structural features were identified and correlations were made between crystal perfection and epitaxial growth conditions as also revealed by X-ray diffraction.

S. M. Goodnick, N. Faleev, and C. Honsberg, “Nanoscale Photovoltaics and the Terawatt Challenge,” in Nanoscale Applications for Information and Energy Systems, A. Korkin and D. J. Lockwood, Eds. Springer New York, 2013, pp. 77–116 [Online].

Abstract

Achieving a sustainable energy system providing terawatts (TWs) of electricity is one of the defining challenges of the coming decades. Photovoltaic technology provides the most likely path to realizing TW scale conversion of solar energy in the future and has been on a nearly 40% growth curve over the past two decades. In order to maintain this rapid level of growth‚ innovations in cell design and conversion efficiency are needed that are compatible with existing technology and can lead to improved performance and lower cost. Nanotechnology offers a number of advantages to realizing such innovation‚ by providing new materials and the implementation of advanced concepts that circumvent the current physical limits on efficiency. This chapter reviews several of the promising applications of nanotechnology to photovoltaic technologies and their prospects for the future.

K. Ghosh, S. Bowden, and C. Tracy, “Role of hot carriers in the interfacial transport in amorphous silicon/crystalline silicon heterostructure solar cells,” physica status solidi (a), vol. 210, no. 2, pp. 413–419, 2013 [Online]. https://doi.org/10.1002/pssa.201228277

physica status solidi (a)

Abstract

The transport of photogenerated minority carriers (photocarriers) across the heterointerface of amorphous silicon (a-Si) and crystalline silicon (c-Si) in a-Si/c-Si heterostructure solar cell is shown in this work to critically depend on the non-Maxwellian energy distribution function (EDF) of those carriers impinging on the heterointerface. A theoretical model is presented that integrates the effect of the high electric field inversion region upon EDF of the impinging carriers with the transmission probability of those carriers across the heterointerface. The transport of the photocarriers across the high electric field inversion region is simulated by the full solution of the Boltzmann transport equation by Monte Carlo (MC) technique while the transmission probability of carriers across the heterointerface is calculated through the percolation path technique. The results are discussed under two different condition of band bending; strongly inverted and weakly inverted c-Si surface. The results comparing different conditions of band bending show that the energy distribution of the carriers impinging on the heterointerface is non-Maxwellian and the integrated photocarrier collection increases with the strength of the inversion field since the carrier population is weighted toward higher energy where the transmission probability through the barrier is higher. Thus‚ we demonstrate that hot carriers play an important role in heterostructure cell operation.

S. Y. Herasimenka, “Large Area Ultrapassivated Silicon Solar Cells Using Heterojunction Carrier Collectors,” Arizona State University, 2013 [Online].

Abstract

Silicon solar cells with heterojunction carrier collectors based on a-Si/c-Si heterojunction (SHJ) have a potential to overcome the limitations of the conventional diffused junction solar cells and become the next industry standard manufacturing technology of solar cells. A brand feature of SHJ technology is ultrapassivated surfaces with already demonstrated 750 mV open circuit voltages (Voc) and 24.7% efficiency on large area solar cell. Despite very good results achieved in research and development‚ large volume manufacturing of high efficiency SHJ cells remains a fundamental challenge. The main objectives of this work were to develop a SHJ solar cell fabrication flow using industry compatible tools and processes in a pilot production environment‚ study the interactions between the used fabrication steps‚ identify the minimum set of optimization parameters and characterization techniques needed to achieve 20% baseline efficiency‚ and analyze the losses of power in fabricated SHJ cells by numerical and analytical modeling. This manuscript presents a detailed description of a SHJ solar cell fabrication flow developed at ASU Solar Power Laboratory (SPL) which allows large area solar cells with >750 mV Voc. SHJ cells on 135 um thick 153 cm2 area wafers with 19.5% efficiency were fabricated. Passivation quality of (i)a-Si:H film‚ bulk conductivity of doped a-Si films‚ bulk conductivity of ITO‚ transmission of ITO and the thickness of all films were identified as the minimum set of optimization parameters necessary to set up a baseline high efficiency SHJ fabrication flow. The preparation of randomly textured wafers to minimize the concentration of surface impurities and to avoid epitaxial growth of a-Si films was found to be a key challenge in achieving a repeatable and uniform passivation. This work resolved this issue by using a multi-step cleaning process based on sequential oxidation in nitric/acetic acids‚ Piranha and RCA-b solutions. The developed process allowed state of the art surface passivation with perfect repeatability and negligible reflectance losses. Two additional studies demonstrated 750 mV local Voc on 50 micron thick SHJ solar cell and < 1 cm/s effective surface recombination velocity on n-type wafers passivated by a-Si/SiO2/SiNx stack.

V. Sharma, “Study of Charges Present in Silicon Nitride Thin Films and Their Effect on Silicon Solar Cell Efficiencies,” Ph. D. Thesis, Arizona State University, 2013 [Online].

Abstract

As crystalline silicon solar cells continue to get thinner‚ the recombination of carriers at the surfaces of the cell plays an ever-important role in controlling the cell efficiency. One tool to minimize surface recombination is field effect passivation from the charges present in the thin films applied on the cell surfaces. The focus of this work is to understand the properties of charges present in the SiNx films and then to develop a mechanism to manipulate the polarity of charges to either negative or positive based on the end-application. Specific silicon-nitrogen dangling bonds (·Si-N)‚ known as K center defects‚ are the primary charge trapping defects present in the SiNx films. A custom built corona charging tool was used to externally inject positive or negative charges in the SiNx film. Detailed Capacitance-Voltage (C-V) measurements taken on corona charged SiNx samples confirmed the presence of a net positive or negative charge density‚ as high as +/- 8 x 1012 cm-2‚ present in the SiNx film. High-energy (\textasciitilde 4.9 eV) UV radiation was used to control and neutralize the charges in the SiNx films. Electron-Spin-Resonance (ESR) technique was used to detect and quantify the density of neutral K0 defects that are paramagnetically active. The density of the neutral K0 defects increased after UV treatment and decreased after high temperature annealing and charging treatments. Etch-back C-V measurements on SiNx films showed that the K centers are spread throughout the bulk of the SiNx film and not just near the SiNx-Si interface. It was also shown that the negative injected charges in the SiNx film were stable and present even after 1 year under indoor room-temperature conditions. Lastly‚ a stack of SiO2/SiNx dielectric layers applicable to standard commercial solar cells was developed using a low temperature (< 400 °C) PECVD process. Excellent surface passivation on FZ and CZ Si substrates for both n- and p-type samples was achieved by manipulating and controlling the charge in SiNx films.

J. J. Williams, T. L. Williamson, M. A. Hoffbauer, A. M. Fischer, S. M. Goodnick, N. N. Faleev, K. Ghosh, and C. B. Honsberg, “Inducing a junction in n-type InxGa(1-x)N,” Journal of Vacuum Science Technology B: Microelectronics and Nanometer Structures, vol. 31, no. 3, p. 03C127-03C127-6, 2013. https://doi.org/10.1116/1.4797489

Journal of Vacuum Science Technology B: Microelectronics and Nanometer Structures

Abstract

The pseudo-binary alloy of indium(x)gallium(1-x)nitride has a compositionally dependent bandgap ranging from 0.65 to 3.42 eV‚ making it desirable for light emitting diodes and solar cell devices. Through modeling and film growth‚ the authors investigate the use of InxGa1-xN as an active layer in an induced junction. In an induced junction‚ electrostatics are used to create strong band bending at the surface of a doped material and invert the bands. The authors report modeling results‚ as well as preliminary film quality experiments for an induced junction in InGaN by space charge effects of neighboring materials‚ piezoelectric effects‚ and spontaneous polarization.

S. Bowden, K. Ghosh, and C. Honsberg, “Non PN junction solar cells using carrier selective contacts,” 2013, vol. 8620, p. 86200T–86200T–6 [Online]. https://doi.org/10.1117/12.2004259

Abstract

A novel device concept utilizing the approach of selectively extracting carriers at the respective contacts is outlined in the work. The dominant silicon solar cell technology is based on a diffused‚ top-contacted p-n junction on a relatively thick silicon wafer for both commercial and laboratory solar cells. The VOC and hence the efficiency of a diffused p-n junction solar cell is limited by the emitter recombination current and a value of 720 mV is considered to be the upper limit. The value is more than 100 mV smaller than the thermodynamic limit of VOC as applicable for silicon based solar cells. Also‚ in diffused junction the use of thin wafers (< 50 um) are problematic because of the requirement of high temperature processing steps. But a number of roadmaps have identified solar cells manufactured on thinner silicon wafers to achieve lower cost and higher efficiency. The carrier selective contact device provides a novel alternative to diffused p-n junction solar cells by eliminating the need for complementary doping to form the emitter and hence it allows the solar cells to achieve a VOC of greater than 720 mV. Also‚ the complete device structure can be fabricated with low temperature thin film deposition or organic coating on silicon substrates and thus epitaxially grown silicon or kerfless silicon‚ in addition to standard silicon wafers can be utilized.

D. Lubyshev, J. M. Fastenau, Y. Qiu, A. W. K. Liu, E. J. Koerperick, J. T. Olesberg, D. Norton, N. N. Faleev, and C. B. Honsberg, “MBE growth of Sb-based nBn photodetectors on large diameter GaAs substrates,” 2013, vol. 8704, pp. 870412-870412–10 [Online]. https://doi.org/10.1117/12.2019039

Abstract

The GaSb-based family of materials and heterostructures provides rich bandgap engineering possibilities for a variety of infrared (IR) applications. Mid-wave and long-wave IR photodetectors are progressing toward commercial manufacturing applications‚ but to succeed they must move from research laboratory settings to general semiconductor production and they require larger diameter substrates than the current standard 2-inch and 3-inch GaSb. Substrate vendors are beginning production of 4-inch GaSb‚ but another alternative is growth on 6-inch GaAs substrates with appropriate metamorphic buffer layers. We have grown generic MWIR nBn photodetectors on large diameter‚ 6-inch GaAs substrates by molecular beam epitaxy. Multiple metamorphic buffer architectures‚ including bulk GaSb nucleation‚ AlAsSb superlattices‚ and graded GaAsSb and InAlSb ternary alloys‚ were employed to bridge the 7.8% mismatch gap from the GaAs substrates to the GaSb-based epilayers at 6.1 Å lattice-constant and beyond. Reaching \textasciitilde6.2 Å extends the nBn cutoff wavelength from 4.2 to <5 µm‚ thus broadening the application space. The metamorphic nBn epiwafers demonstrated unique surface morphologies and crystal properties‚ as revealed by AFM‚ high-resolution XRD‚ and cross-section TEM. GaSb nucleation resulted in island-like surface morphology while graded ternary buffers resulted in cross-hatched surface morphology‚ with low root-mean-square roughness values of \textasciitilde10 Å obtained. XRD determined dislocation densities as low as 2 × 107 cm-2. Device mesas were fabricated and dark currents of 1 × 10-6 A/cm2 at 150K were measured. This work demonstrates a promising path to satisfy the increasing demand for even larger area focal plane array detectors in a commercial production environment.

S. M. Goodnick, C. Honsberg, and Y. Zou, “Ultrafast carrier relaxation processes in advanced concept solar cells,” in Optics for Solar Energy, SOLAR 2013, November 3, 2013 - November 7, 2013, 2013.

Abstract

We discuss short time carrier relaxation in advanced concept solar cells conditions using ensemble Monte Carlo (EMC) simulation coupled with rate equation and thermodynamic models‚ to understand the limiting factors affecting solar cell performance. Renewable Energy and the Environment Congress. 2013.

2012

N. Vulic, “SOLAR POWER PURCHASE AGREEMENTS FOR 10MWP DISTRIBUTED GRID-TIED PHOTOVOLTAIC SYSTEMS AT THE ARIZONA STATE UNIVERSITY MAIN CAMPUS: ESTIMATED VS. ACTUAL ENERGY OUTPUT,” Barrett Honors College Thesis, Arizona State University, 2012 [Online].

Abstract

The majority of the 52 photovoltaic installations at ASU are governed by power purchase agreements (PPA) that set a fixed per kilowatt-hour rate at which ASU buys power from the system owner over the period of 15-20 years. PPAs require accurate predictions of the system output to determine the financial viability of the system installations as well as the purchase price. The research was conducted using PPAs and historical solar power production data from the ASU’s Energy Information System (EIS). The results indicate that most PPAs slightly underestimate the annual energy yield. However‚ the modeled power output from PVsyst indicates that higher energy outputs are possible with better system monitoring.

B. R. Jampana, C. R. Weiland, R. L. Opila, I. T. Ferguson, and C. B. Honsberg, “Optical absorption dependence on composition and thickness of InxGa1 − xN (0.05 < × < 0.22) grown on GaN/sapphire,” Thin Solid Films, vol. 520, no. 22, pp. 6807–6812, Sep. 2012 [Online]. https://doi.org/10.1016/j.tsf.2012.07.003

Thin Solid Films

Abstract

We report the change in optical absorption properties of InGaN epilayers around the critical layer thickness determined from X-ray diffraction. Detrimental sub-band gap absorption is observed in InGaN thin films grown beyond the critical layer thicknesses‚ and is caused by localized electric fields around extended crystalline defects and aided by V-defects through light channeling. The photoluminescence response from InGaN thin films‚ grown beyond the critical layer thickness‚ is reduced owing to absorption of the incident laser light by non-radiative recombination extended crystalline defects. The formation of V-defects is observed to occur beyond the critical layer thickness and continues to grow in areal coverage aiding in sub-band gap absorption. This optical behavior sets constraints to be incorporated in the design of InGaN solar cell and requirement for improvement in epitaxial growth techniques to reduce V-defects.

T. Reblitz, S. Bowden, and S. Herasimenka, “Web applications for: centralization of photovoltaic cell production and characterization data, and processing of Sentaurus simulation files,” in proceedings of 22nd Workshop on Crystalline Silicon Solar Cells & Modules, Vail Colorado, 2012.

Abstract

Abstract — We present the development‚ architecture‚ and usage of two web applications for photovoltaic cell research and fabrication on a pilot production line. The first tool‚ eTraveller‚ centralizes photovoltaic cell production data and cell measurements from a variety of disparate tools and stores the results in a centralized database. The data is stored alongside uploaded run sheets and reviewing past data is as straight forward as browsing a website. The second tool‚ Minotaur‚ provides a web based front end to the semiconductor simulation tool Sentaurus. Minotaur is specifically tailored to the simulation of solar cells and enables the use of sophisticated models in an easy to use package. Index Terms — photovoltaic cells‚ silicon‚ web applications‚ Sentaurus‚ Minotaur‚ eTraveller

J. Lee, S. N. Dahal, and C. B. Honsberg, “Theoretical analysis for intermediate band and tandem hybrid solar cell materials,” in 2012 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 000068–000072. https://doi.org/10.1109/PVSC.2012.6317570

2012 38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The efficiency limit of an intermediate band (IB) solar cell can be increased by a #x201C;tandem #x201D; configuration of multiple intermediate band devices. Thermodynamic models show that the efficiency of a two-stack tandem of IB devices achieves the efficiency of a six junction series connected solar cell. The efficiency of an IB in conjunction with a single or double stack tandem has similar efficiency advantages. Further‚ analysis of the materials which can be used to implement IB solar cells in a tandem configuration shows advantages relating to the ability to implement IB materials with quantum wells or quantum dots. For a single IB solar cell‚ a key difficulty is identifying materials for the barrier and the quantum well which have a small valence band offset and large conduction band offset (or the reverse). The use of an IB solar cell as the bottom solar cell of a tandem allows a larger range of materials with suitable barrier band gaps and a smaller ideal conduction band offset. A further theoretical advantage of such a structure is that it avoids the extremely low open circuit voltages achieved from pn junctions in low bandgap materials; for example‚ the thermodynamic optimum for a 6 junction tandem solar cell has its lowest bandgap below 0.4 eV. We present a thermodynamic model for IB hybrid tandem configurations which does not assume spectral selectivity among the different solar cells and predicts that a barrier/quantum dot structure can have an efficiency as high as 60 to 70 percent at 1000X blackbody radiation.

S. M. Goodnick, S. Limpert, C. Honsberg, and P. Lugli, “Simulation of carrier relaxation in hot carrier solar cells,” in 2012 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 001657–001662. https://doi.org/10.1109/PVSC.2012.6317914

2012 38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Hot carrier solar cells depend critically on the energy relaxation dynamics of photo-generated carriers in an absorber material‚ where hot carriers are extracted through energy selective contacts. Here we combine ensemble Monte Carlo (EMC) simulation with an energy balance equation approach‚ to simulate the microscopic carrier relaxation processes and corresponding electron and hole temperatures in semiconductor quantum well (QW) hot carrier solar cell structures‚ both under transient and steady state illumination. We include nonequilibrium optical phonons‚ in which a detailed balance of emission and absorption events is used to simulate the phonon population in time‚ with the anharmonic decay of the optical phonon population to acoustic phonons described using a phenomenological phonon lifetime. Simulation of femtosecond laser excitation in GaAs QWs show reduced cooling‚ depending on the optical phonon lifetime and excitation intensity. Steady state simulation under AM0 solar illumination shows a build-up of hot phonons over long times depending on the phonon lifetime‚ although they are not readily re-absorbed due to momentum and energy conservation considerations.

S. P. Bremner and C. B. Honsberg, “Intermediate band solar cell with non-ideal band structure under AM1.5 spectrum,” in 2012 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 000021–000024. https://doi.org/10.1109/PVSC.2012.6317559

2012 38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The limiting efficiency of an intermediate band solar cell with a non-ideal band structure is analyzed using the principle of detailed balance. Firstly‚ the impact of finite band width on the optimum band gap design for AM1.5 spectrum is examined. It is found that the band width may be a determining factor in the optimum band gap arrangement‚ but that the degradation in efficiency due to the band width up to 200 meV is moderate. It is also found that the band width can determine which band gap combination gives the highest global limiting efficiency. Further to the intermediate band width modification‚ the inclusion of band tails‚ analogous to those present in amorphous materials is discussed in terms of realizing an intermediate band using quantum dot arrays. In this model the worst case scenario is assumed‚ that of low absorption‚ but maximum emissivity at the band tail edge of the intermediate band. Results show that‚ with multiple band gap combinations giving several local peaks‚ the one giving the global maximum efficiency changes with band width. When band tails are included in a materials system proposed for implementing an intermediate band solar cell via quantum dot miniband formation significant drop off in efficiency is seen. The results taken together suggest the intermediate band width and any band tails should be considered in designs for intermediate band solar cells.

J. Lee and C. B. Honsberg, “Limiting efficiencies over 50% using multijunction solar cells with multiple exciton generation,” in 2012 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 000062–000067. https://doi.org/10.1109/PVSC.2012.6317569

2012 38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The achievement of solar cells over 50% is a critical goal for photovoltaics. Multijunction solar cells over 5 junctions allow such efficiencies‚ but are severely limited by material constraints and growth requirements for lattice matching. Nanostructured approaches such as multiple exciton generation (MEG) potentially offer a route to higher efficiency but still require high values of sunlight concentration and large quantum yields. We show an approach that allows for higher efficiencies based on including MEG in a multijunction solar cell. We also present a thermodynamic model for multijunction solar cells with MEG that demonstrates possible improvements.

S. Bowden and J. LeBeau, “Laser wafering,” in 2012 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 001826–001829. https://doi.org/10.1109/PVSC.2012.6317948

2012 38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

A new technique for the laser wafering of semiconductors is presented using sub surface laser engraving. In the laser wafering process‚ a high intensity laser is used to form sub-surface etch pits or defects at multiple depths in an ingot. This allows a rapid‚ controllable approach to the formation of wafers with thickness ranging from below 10 microns to over 100 microns. Laser wafering provides a way to cut a a brick of silicon into multiple wafers with minimal kerf loss and the possibility to dramatically lower the cost of silicon solar cell production. The techniques relies on the principle that many materials‚ including silicon and other semiconductors‚ have a non-linear absorption co-efficient with intensity‚ such that sub-band gap light is absorbed above a given intensity. This allows a high intensity laser to be focused at an arbitrary point below the surface‚ and allowing absorption of the high intensity light only at the focal point.

K. G. Nelson, R. Koselar, and J. Husman, “Work in Progress: Towards the development of a model for beneficial use of educational technology through a photovoltaics engineering website.,” presented at the American Society for Engineering Education Annual Conference, San Antonio, TX, 2012.

American Society for Engineering Education Annual Conference

S. Bremner, K. Ghosh, N. Ekins-Daukes, C. Honsberg, and S. Goodnick, “Issues in the physical measurement of the intermediate band effect,” in 2012 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 000641–000646. https://doi.org/10.1109/PVSC.2012.6317692

2012 38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The intermediate band (IB) solar cell offers a route to higher efficiency solar cells by inserting a band into the band gap of large band gap material. Theoretically‚ the open circuit voltage of the IB solar cell is close to that of the high band gap regions‚ and the current is increased through the addition of the intermediate band. However‚ experimental results have shown a decrease in the open circuit voltage‚ and a small increase in the short circuit current. We explain the decrease in Voc‚ as well as the small increase in Jsc though a thermodynamic and physical model. These models show that both these results are consistent with the absence of multiple quasi-Fermi levels. Because of the importance of realizing multiple quasi-Fermi levels‚ the definitive proof of this effect is critical in demonstrating the viability of the intermediate band‚ but show that demonstrated indicators of an intermediate band may be caused by other physical mechanisms. Instead‚ a key indicator of a quasi-Fermi level separation not explained by non-idealities are the features associated with the intermediate band quasi-Fermi level crossing the intermediate band energy level‚ demonstrated by shifting peak energies.

C. B. Honsberg and S. M. Goodnick, “Realizing Terawatt-Scale Solar Electricity: Nanotechnology-Enabled Physical Mechanisms and Material Properties,” IEEE Nanotechnology Magazine, vol. 6, no. 2, pp. 6–14, Jun. 2012. https://doi.org/10.1109/MNANO.2012.2192652

IEEE Nanotechnology Magazine

Abstract

In this article‚ we briefly review the importance of efficiency for PVs‚ the thermodynamic efficiency limits‚ and implications for PVs. Also‚ we have reviewed how nanostructures can improve existing devices and theoretically allow devices near the thermodynamic limits.

S. Limpert, “Solar Energy Technologies - A Comparative Study of Commercial Applications and Government Policies,” Arizona State University, 2012.

X. Wang, N. Waite, P. Murcia, K. Emery, M. Steiner, F. Kiamilev, K. Goossen, C. Honsberg, and A. Barnett, “Lateral spectrum splitting concentrator photovoltaics: direct measurement of component and submodule efficiency,” Progress in Photovoltaics: Research and Applications, vol. 20, no. 2, pp. 149–165, Mar. 2012 [Online]. https://doi.org/10.1002/pip.1194

Progress in Photovoltaics: Research and Applications

Prog. Photovolt: Res. Appl.

Abstract

To achieve high energy conversion efficiency‚ a solar module architecture called lateral spectrum splitting concentrator photovoltaics (LSSCPV) is being developed. LSSCPV can concentrate available sunlight and laterally split a single beam into bands with different spectra for absorption by different solar cells with band gaps matched to the split spectrum. Test assemblies of a sample LSSCPV architecture were constructed‚ each of which contains four p–n junctions and two optical pieces. Independent experiments or simulations had been implemented on the components but by using optimal assumptions. In order to examine the actual performances of all the components‚ which are dependent on each other and the light source‚ direct outdoor measurements were made. A set of self-consistent efficiency definitions was articulated and a test bed was developed to measure the parameters required by the efficiency calculation. By comparing the component efficiency items derived from the outdoor measurement and the expected values based on independent simulations‚ the potential opportunities for efficiency improvement are determined. In the outdoor measurement at the University of Delaware‚ the optical component demonstrated 89·1% efficiency. Additional assemblies were tested at the National Renewable Energy Laboratory. One assembly demonstrated 36·7% submodule efficiency‚ which compares favorably with the 32·6% previously reported verified submodule efficiency. Copyright © 2011 John Wiley & Sons‚ Ltd.

K. . LaRosa, “Photoluminescence Imaging of Silicon Wafers Using High Power Light Emitting Diode Arrays,” Arizona State University, 2012.

Abstract

Due to its ability to map the minority carrier lifetime of an entire cell within minutes‚ photoluminescence has been an increasingly prevalent new technology for solar cell characterization. It is capable of obtaining high spatial resolution with inexpensive Si-CCD cameras. However‚ it has focused primarily on near-infrared lasers as the excitation source. The development of a photoluminescence system which uses light emitting diode (LED) arrays instead of lasers allows for a cheaper‚ safer design while increasing the potential functionality. The low cost and versatility of LED arrays make them ideal for inline characterization while also allowing for characterization using a variety of wavelengths. Production of high lifetime solar cells has begun at the Solar Power Lab at Arizona State University‚ and spatial lifetime mapping is important for characterizing and improving new large area solar cells. Within the last eight months of research‚ a photoluminescence characterization device has been constructed and verified for high minority carrier lifetime materials. The system designed is an improvement over other systems because of its cost-effectiveness combined with the high resolution spatial variation mapping and short measurement times‚ making it ideal for the Solar Power Lab’s needs.

V. Sharma, A. Bailey, B. Dauksher, C. Tracy, S. Bowden, and B. O’Brien, “Characterization and comparison of silicon nitride films deposited using two novel processes,” J. Vac. Sci. Technol. A, vol. 30, no. 2, pp. 021201-1, Feb. 2012 [Online]. https://doi.org/10.1116/1.3687423

J. Vac. Sci. Technol. A

Abstract

Hydrogenated silicon nitride films (SiNx:H) deposited using a PECVD process enhance the performance of crystalline silicon solar cells by functioning as an efficient antireflection coating and passivating layer. In this paper‚ we compared two SiNx:H novel deposition processes using two different PECVD tools—one nontraditional in process regime and the other nontraditional in type—to determine their suitability to solar cell fabrication. The parameter space was explored by employing a design of experiment methodology followed by material characterization using variable angle spectroscopic ellipsometry‚ reflectance‚ FTIR‚ RBS and elastic recoil detection. The thickness and reflectance of Si-rich films changed dramatically after annealing. Further‚ FTIR results showed that the Si–H bond peak present at 2160 cm−1 in such films disappeared after a typical Al firing step. Therefore‚ the optimized films were deposited with a lower SiH4/NH3 ratio to minimize the changes in the film properties after annealing.

J. Lee and C. B. Honsberg, “The thermodynamic limits of tandem photovoltaic devices with intermediate band,” Proceedings of SPIE, vol. 8256, no. 1, p. 82560Q–82560Q–12, Feb. 2012 [Online]. https://doi.org/10.1117/12.910813

Proceedings of SPIE

Abstract

We present a hybrid thermodynamic model for multijunction solar cells with intermediate bands that demonstrates possible improvements to conventional multijunction photovoltaic systems. Applying this model to selected tandem cell structures shows that the performance of such hybrid solar cells is enhanced and that multiple transitions from intermediate bands can reduce the number of material stacks and boost overall efficiency. We demonstrate the results of detailed simulations for multiple numbers of stacks of hybrid multijunction solar cells. And‚ we can choose proper materials to compose intermediate band for each junction. Furthermore‚ we suggest other alternative hybrid solar cell systems to absorb moderate photon energy range and find appropriate materials for hybrid solar cells.

S. M. Goodnick and C. B. Honsberg, “Modeling carrier relaxation in hot carrier solar cells,” Proceedings of SPIE, vol. 8256, no. 1, p. 82560W–82560W–10, Feb. 2012 [Online]. https://doi.org/doi:10.1117/12.910858

Proceedings of SPIE

Abstract

K.-Y. Ban, S. P. Bremner, D. Kuciauskas, S. N. Dahal, and C. B. Honsberg, “The role of Sb compositions on the properties of InAs/GaAsSb quantum dots (QDs),” Proceedings of SPIE, vol. 8256, no. 1, p. 82561C–82561C–6, Feb. 2012 [Online]. https://doi.org/doi:10.1117/12.910834

Proceedings of SPIE

Abstract

QD size‚ uniformity and density in InAs/GaAsSb material system for increasing Sb content are studied using Atomic Force Microscopy (AFM). AFM results show that QD density and uniformity improve with Sb content increase. The improvement of QD uniformity is ensured by the narrowing of the analysis of AFM scans. To obtain minimum VBO‚ InAs/GaAsSb with various Sb compositions is investigated by PL and TRPL measurements. PL data shows a blue-shift as excitation power increases as evidence of a type II band structure. Since the PL peak of 8 and 13 % Sb samples did not shift while that of 15 % Sb sample is blue-shifted with increasing the excitation power it is concluded that InAs QDs/GaAs0.86Sb0.14 would have minimum valence band offset. This tendency is supported by the change of a carrier lifetime estimated from TRPL data

K. Ghosh, C. Tracy, and S. Bowden, “Experimental and theoretical verification of the presence of inversion region in a-Si/c-Si heterojunction solar cells with an intrinsic layer,” presented at the 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 001046–001048 [Online]. https://doi.org/10.1109/PVSC.2012.6317782

38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

S. Herasimenka, P. Altermatt, S. Bowden, and C. Honsberg, “Reassessment of classic recombination mechanisms in silicon point contact concentrator solar cell,” presented at the 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 001055–001058 [Online]. https://doi.org/10.1109/PVSC.2012.6317784

38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

V. Sharma and S. Bowden, “Peak load offset and the effect of dust storms on 10 MWp distributed grid tied photovoltaic systems installed at Arizona State University,” presented at the 38th IEEE Photovoltaic Specialists Conference, 2012, pp. 000590–000595 [Online]. https://doi.org/10.1109/PVSC.2012.6317682

38th IEEE Photovoltaic Specialists Conference

K. Ghosh, C. Tracy, S. Goodnick, and S. Bowden, “Effect of band bending and band offset in the transport of minority carriers across the ordered/disordered interface of a-Si/c-Si heterojunction solar cell,” presented at the 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 000221–000226 [Online]. https://doi.org/10.1109/PVSC.2012.6317605

38th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

K.-Y. Ban, D. Kuciauskas, S. P. Bremner, and C. B. Honsberg, “Observation of band alignment transition in InAs/GaAsSb quantum dots by photoluminescence,” Journal of Applied Physics, vol. 111, no. 10, pp. 104302-104302–4, 2012. https://doi.org/10.1063/1.4717766

Journal of Applied Physics

Abstract

The band alignment of InAs quantum dots (QDs) embedded in GaAsSb barriers with various Sb compositions is investigated by photoluminescence (PL) measurements. InAs/GaAsSb samples with 13% and 15% Sb compositions show distinct differences in emission spectra as the PL excitation power increases. Whilst no discernible shift is seen for the 13% sample‚ a blue-shift of PL spectra following a 1/3 exponent of the excitation power is observed for the 15% sample suggesting a transition from a type I to type II band alignment. Time-resolved PL data show a significant increase in carrier lifetime as the Sb composition increases between 13% and 15% implying that the transformation from a type I to type II band alignment occurs between 13% and 15% Sb compositions. These results taken together lead to the conclusion that a zero valence band offset (VBO) can be achieved for the InAs/GaAsSb system in the vicinity of 14% Sb composition.

X. Wang, N. Waite, P. Murcia, K. Emery, M. Steiner, F. Kiamilev, K. Goossen, C. Honsberg, and A. Barnett, “Lateral spectrum splitting concentrator photovoltaics: Direct measurement of component and submodule efficiency,” Progress in Photovoltaics: Research and Applications, vol. 20, no. 2, pp. 149–165, 2012. https://doi.org/10.1002/pip.1194

Progress in Photovoltaics: Research and Applications

Abstract

To achieve high energy conversion efficiency‚ a solar module architecture called lateral spectrum splitting concentrator photovoltaics (LSSCPV) is being developed. LSSCPV can concentrate available sunlight and laterally split a single beam into bands with different spectra for absorption by different solar cells with band gaps matched to the split spectrum. Test assemblies of a sample LSSCPV architecture were constructed‚ each of which contains four p-n junctions and two optical pieces. Independent experiments or simulations had been implemented on the components but by using optimal assumptions. In order to examine the actual performances of all the components‚ which are dependent on each other and the light source‚ direct outdoor measurements were made. A set of self-consistent efficiency definitions was articulated and a test bed was developed to measure the parameters required by the efficiency calculation. By comparing the component efficiency items derived from the outdoor measurement and the expected values based on independent simulations‚ the potential opportunities for efficiency improvement are determined. In the outdoor measurement at the University of Delaware‚ the optical component demonstrated 891% efficiency. Additional assemblies were tested at the National Renewable Energy Laboratory. One assembly demonstrated 367% submodule efficiency‚ which compares favorably with the 326% previously reported verified submodule efficiency. Copyright 2011 John Wiley Sons‚ Ltd.

N. N. Faleev, C. B. Honsberg, and D. J. Smith, “Four stages of defect creation in epitaxial structures: High resolution X-ray diffraction and transmission electron microscopy characterization,” in ISTFA 2012, 11-15 Nov. 2012, 2012, pp. 337–46.

ISTFA

Abstract

Different epitaxial structures have been studied by high-resolution x-ray diffraction and x-ray topography‚ Transmission Electron Microscopy and Atomic Force Microscopy to establish correlations between epitaxial growth conditions and crystal perfection. It was confirmed that epitaxial growth under initial elastic stress inevitably leads to the creation of extended crystal defects like dislocation loops and edge dislocations in the volume of epitaxial structures‚ which strongly affect crystal perfection and physical properties of future devices. It was found that the type of created defects‚ their density and spatial distribution strongly depended on growth conditions: the value and sign of the initial elastic strain‚ the elastic constants of solid solutions‚ the temperature of deposition and growth rate‚ and the thickness of the epitaxial layers. All of the investigated structures were classified by their crystal perfection‚ using the volume density of extended defects as a parameter. It was found that the accommodation and relaxation of initial elastic stress and creation of crystal defect were up to four stages "chain" processes‚ necessary to stabilize the crystal structure at a level corresponding to the deterioration power of particular growth conditions.

2011

K. G. Nelson, C. Foster, M. Crowder, J. Husman, and T. Ganesh, “The Solar Energy Challenge: Engineering Notebook,” in Arizona Science Teachers Association, Mesa, AZ, 2011.

N. Chandra, V. Sharma, G. Y. Chung, and D. K. Schroder, “Four-point probe characterization of 4H silicon carbide,” Solid-State Electronics, vol. 64, no. 1, pp. 73–77, Oct. 2011 [Online]. https://doi.org/10.1016/j.sse.2011.07.004

Solid-State Electronics

Abstract

We report on four-point probe measurements on SiC wafers as such measurements give erratic data. Current–voltage measurements on n-type SiC wafers doped to 3 × 1018 cm−3 are non-linear and single probe I–V measurements are symmetrical for positive and negative voltages. For comparison‚ similar measurements of p-type Si doped to 5 × 1014 cm−3 gave linear I–V‚ well-defined sheet resistance and the single probe I–V curves were asymmetrical indicating typical Schottky diode behavior. We believe that the reason for the non-linearity in four-point probe measurements on SiC is the high contact resistance. Calculations predict the contact resistance of SiC to be approximately 1012 Ω which is of the order of the input resistance of the voltmeter in our four-point probe measurements. There was almost no change in two-probe I–V curves when the spacing between the probes was changed from 10mm to 20cm‚ further supporting the idea that the I–V characteristics are dominated by the contact resistance.

S. M. Goodnick and C. B. Honsberg, “Ultrafast Carrier Relaxation and Nonequilibrium Phonons in Hot Carrier Solar Cells,” presented at the Conference Record of the 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA, 2011.

Conference Record of the 37th IEEE Photovoltaic Specialists Conference

Abstract

Third generation concepts in photovoltaic devices depend critically on the dynamics of ultrafast carrier relaxation and electron-phonon interactions on very short times scales in nanostructures. Hot carrier solar cells in particular depend on the reduction in the energy relaxation rate in an absorber material. Here we investigate the short time carrier relaxation in quantum well‚ hot electron solar cells under varying photoexcitation conditions using ensemble Monte Carlo (EMC) simulation coupled with rate equation models‚ to understand the limiting factors affecting cell performance. It has recently been argued that nonequilibrium ‘hot’ phonons may play a critical role in reducing carrier energy loss‚ and maintaining energy within the absorber system. Here we explicitly include nonequilibrium optical phonons‚ in which a detailed balance of emission and absorption events and the phonon population is simulated‚ and the anharmonic decay of the optical phonon population to acoustic phonons is described using a phenomenological phonon lifetime. We have simulated the average energy versus time of carriers in the quantum well absorber using GaAs effective mass parameters‚ but varying the effective bandgap to study the effect of this parameter on heating an carrier relaxation‚ relative to optical excitation at the mean solar spectrum energy at 1000 suns. Results with a 5 ps phonon LO optical phonon lifetime show that the after initial heating‚ the mean carrier temperature relaxes to relatively low excess temperature‚ with relatively little extraction through selective contacts‚ due to the cooling of the photoexcited population‚ which it turns out are able to effectively lose energy through electron-hole scattering‚ where the holes in turn have additional relaxation channels through TO modes. However‚ the inclusion of nonequilibrium phonon effects for all possible phonon modes is expected to reduce this channel for carrier cooling‚ and improve performance.

S. N. Dahal and C. B. Honsberg, “Effect of Elastic Strain on Band Edge Alignment and Position of Intermediate Band of Isolated and Coupled Quantum Dots,” presented at the Conference Record of the 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA, 2011.

Conference Record of the 37th IEEE Photovoltaic Specialists Conference

Abstract

Solar cells with quantum dot nanostructure absorbing medium have a potential to overcome single junction limit and achieve the solar energy conversion efficiency up to 63%. Self-assembled quantum dots that are grown by using molecular beam epitaxy(MBE) or metal organic chemical vapor deposition (MOCVD) have significant effect of strain on the band edge alignment and hence the confinement potential of electrons and holes. The energetic positions of confined energy states in quantum dots‚ which‚ act as intermediate state or band in intermediate band solar cells (IBSCs) are strongly affected by the strain in and around a quantum dot (QD). This work is focused on the calculation of strain distribution and its effect on band structure of QD array for its potential application in quantum dot intermediate band solar cells (QDIBSCs). Strain distribution in and around a QD is calculated using continuum theory of elasticity. When the inter-dot distance in the growth direction is sufficiently close‚ there is the interaction in strain distribution of QD layers .The strain distribution due to a vertically aligned QD array is calculated from superposition of the strain due to single quantum dot. The strain calculated this way is given as input for the calculation of band edge alignment and the position of QD confined states.

C. B. Honsberg, S. P. Bremner, J. Lee, A. J. Bailey, and S. N. Dahal, “Hyrbid Advanced Concept Solar Cells,” presented at the Conference Record of the 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA, 2011.

Conference Record of the 37th IEEE Photovoltaic Specialists Conference

Abstract

Advanced concept or third generation solar cells rely on modification to the basic process of absorption /recombination or collection to increase the efficiency above that from a singe junction solar cell. While there are a number of advanced concept approaches‚ a feature of theses is that they have focused primarily on inclusion of a single type of physical mechanism. Such single mechanism advanced concept solar cells are in general limited in efficiency to the equivalent tandem solar cell which has the same number of processes. This paper demonstrates that solar cells using combinations of different approaches have higher thermodynamic efficiencies. Some combinations demonstrate higher efficiencies that either of the constituent approaches‚ and‚ significantly‚ even exceed the efficiency of the “equivalent” tandem structure. Moreover‚ such advanced concept hybrid structures have benefits other than efficiency increases‚ including reduced sensitivity to materials or to physical processes which are difficult to implement‚ or to non-idealities in the solar cell structure.

J. Lee and C. B. Honsberg, “Limiting Efficiencies of Intermediate Band Solar Cell Assisted with Multiple Exciton Generation,” presented at the Conference Record of the 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA, 2011.

Conference Record of the 37th IEEE Photovoltaic Specialists Conference

Abstract

We propose and calculate the hybrid thermodynamic model using the intermediate band solar cells assisted with multiple exciton generation. We have been also studied this hybrid thermodynamic model under blackbody radiation and changing concentration to compare the conventional intermediate band solar cells. Because of multiple electron and hole pair on the conduction band edge‚ its maximum efficiency and optimum bandgap positions have been enhanced compared with conventional detailed balance model of intermediate band solar cells. The maximum efficiencies of this solar cell are both 47.31 percent for one sun and 65.07 percent under maximum concentration. And its corresponding overall bandgap energies are also reduced because of multiple electron and hole pairs.

K. Ghosh and C. B. Honsberg, “Absorption coefficient for multiple-quasi fermi level system in quantum well,” presented at the Conference Record of the 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA, 2011.

Conference Record of the 37th IEEE Photovoltaic Specialists Conference

N. Faleev, S. P. Bremner, K.-Y. Ban, D. Smith, and C. B. Honsberg, “Investigation of the Correlations of Structural and Physical Features of Inas Quantum Dots, Embedded Between Strain-Relief Gaassb Layers with Different Sb Compositions,” in Conference Record of the 37th IEEE Photovoltaic Specialists Conference, 2011.

Abstract

Strong correlations between crystal perfection of epitaxial structures and size‚ density and PL features of deposited InAs QDs have been found. Rise of Sb composition in GaAsSb strain relief layers from 8% to 37% significantly disturbed crystal perfection of epitaxial structures and increased the density of deposited QDs from 2 – 2.5 × 1010 cm-2 at 8% Sb structure to 7.5 – 9.5 × 1010 cm-2 at 16% Sb structure. At 37% Sb initial elastic stress were noticeably relaxed (≈ 40%) while creation of QDs were fully blocked. Relaxation of elastic stress strongly increased the density of dislocation loops in epitaxial layers and affected atoms migration on the growth front. Observed correlations between densities of dislocation loops and QDs are related with a diminution of the energy of migrated atoms‚ caused by dislocations. At definite density of dislocation loops reduced energy diminished accumulation of deposited atoms below the level critical for their transformation to QDs.

J. Lee and C. B. Honsberg, “Thermodynamic Limits of Hybrid Photovoltaic Systems Using Multiple Junction Solar Cells with Carrier Transitions at Intermediate Band,” presented at the Conference Record of the 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA, 2011.

Conference Record of the 37th IEEE Photovoltaic Specialists Conference

Abstract

We have proposed the hybrid thermodynamic approach for multiple junction solar cells introducing the intermediate band. Because of increasing number of transitions inside multiple junction solar cells‚ it give an effect that small number of stacks can be possible to realize increased number of multiple junction solar. The maximum efficiency of two stacks hybrid solar cells has been enhanced and its value is larger than over 4 stacks tandem solar cells. We have also found optimum bandgap of each stack and studied optimum transition levels of each stack both from valence band to intermediate band and intermediate band to conduction band.

K. Ghosh, N.-K. Song, M.-S. Oh, D.-S. Kim, and S. Bowden, “Extraction of recombination parameters of a-Si/c-Si solar cell from lifetime spectroscopy,” presented at the Conference Record of the 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA, 2011.

Conference Record of the 37th IEEE Photovoltaic Specialists Conference

Keun-Yong Ban, Woong-Ki Hong, S. P. Bremner, S. N. Dahal, H. McFelea, and C. B. Honsberg, “Controllability of the subband occupation of InAs quantum dots on a delta-doped GaAsSb barrier,” Journal of Applied Physics, vol. 109, no. 1, p. 014312 (6 pp.), Jan. 2011 [Online]. https://doi.org/10.1063/1.3527039

Journal of Applied Physics

USA

Abstract

Optical properties of InAs quantum dots (QDs) embedded in GaAsSb barriers with delta-doping levels equivalent to 0‚ 2‚ 4‚ and 6 electrons per dot (e/dot) are studied using time-integrated photoluminescence (PL). When the PL excitation power is increased the full width at half maximum (FWHM) of the 4 and 6 e/dot samples is found to increase at a much greater rate than the FWHMs for the 0 and 2 e/dot samples. PL spectra of the 4 e/dot sample show a high energy peak attributed to emission from the first excited states of the QDs‚ a result deduced to be due to preoccupation of states by electrons supplied by the delta-doping plane. When temperature dependent PL results are fitted using an Arrhenius function‚ the thermal activation energies for the 4 and 6 e/dot samples are similar and greater than the thermal activation energies for the 0 and 2 e/dot samples (which are similar to each other). This increased thermal activation energy is attributed to the enhanced Coulombic interaction in the InAs QD area by the delta-doping plane for higher doping levels. It is concluded that delta-doping of the barrier in QD systems is a feasible method for controlling the level of carrier occupation in a QD mediated intermediate band.

B. Chhabra, C. Weiland, R. L. Opila, and C. B. Honsberg, “Surface characterization of quinhydrone-methanol and iodine-methanol passivated silicon substrates using X-ray photoelectron spectroscopy,” Physica Status Solidi A, vol. 208, no. 1, pp. 91–5, Jan. 2011 [Online]. https://doi.org/10.1002/pssa.201026101

Physica Status Solidi A

Germany

Abstract

Hydrogen-terminated silicon substrates were passivated with quinhydrone-methanol (QHY/ME) and iodine-methanol (I2/ME)‚ and the chemical changes occurring at the surface were investigated using X-ray photoelectron spectroscopy (XPS). The XPS surface studies demonstrate that QHY/ME passivation provides reduced oxidation‚ less carbon contamination‚ and a chemically inert surface. Electrical characterization also demonstrates higher minority carrier lifetimes of QHY/ME passivated substrates as compared to I2/ME passivated substrates. The quality of surface treatment was also characterized using the contact angle measurement‚ which confirms the presence of a hydrophobic organic layer on the surface after QHY/ME passivation. Si 2p XPS spectra of the QHY/ME‚ I2/ME samples‚ and H-terminated silicon. With a peak at 102.9 eV for I2/ME‚ it is evident that it provides poor surface passivation than QHY/ME where no surface oxidation is observed.

S. N. Dahal, “Advanced Nanostructured Concepts in Solar Cells using III-V and Silicon-Based Materials,” Ph. D. Thesis, Arizona State University, 2011 [Online]. Retrieved from http://hdl.handle.net/2286/R.I.9450

Abstract

As existing solar cell technologies come closer to their theoretical efficiency‚ new concepts that overcome the Shockley-Queisser limit and exceed 50% efficiency need to be explored. New materials systems are often investigated to achieve this‚ but the use of existing solar cell materials in advanced concept approaches is compelling for multiple theoretical and practical reasons. In order to include advanced concept approaches into existing materials‚ nanostructures are used as they alter the physical properties of these materials. To explore advanced nanostructured concepts with existing materials such as III-V alloys‚ silicon and/or silicon/germanium and associated alloys‚ fundamental aspects of using these materials in advanced concept nanostructured solar cells must be understood. Chief among these is the determination and predication of optimum electronic band structures‚ including effects such as strain on the band structure‚ and the material’s opto-electronic properties. Nanostructures have a large impact on band structure and electronic properties through quantum confinement. An additional large effect is the change in band structure due to elastic strain caused by lattice mismatch between the barrier and nanostructured (usually self-assembled QDs) materials. To develop a material model for advanced concept solar cells‚ the band structure is calculated for single as well as vertical array of quantum dots with the realistic effects such as strain‚ associated with the epitaxial growth of these materials. The results show significant effect of strain in band structure. More importantly‚ the band diagram of a vertical array of QDs with different spacer layer thickness show significant change in band offsets‚ especially for heavy and light hole valence bands when the spacer layer thickness is reduced. These results‚ ultimately‚ have significance to develop a material model for advance concept solar cells that use the QD nanostructures as absorbing medium. The band structure calculations serve as the basis for multiple other calculations. Chief among these is that the model allows the design of a practical QD advanced concept solar cell‚ which meets key design criteria such as a negligible valence band offset between the QD/barrier materials and close to optimum band gaps‚ resulting in the predication of optimum material combinations.

K. Ghosh, “Heterojunction and Nanostructured Photovoltaic Device: Theory and Experiment,” Ph. D. Thesis, Arizona State University, 2011 [Online].

Abstract

A primary motivation of research in photovoltaic technology is to obtain higher efficiency photovoltaic devices at reduced cost of production so that solar electricity can be cost competitive. The majority of photovoltaic technologies are based on p-n junction‚ with efficiency potential being much lower than the thermodynamic limits of individual technologies and thereby providing substantial scope for further improvements in efficiency. The thesis explores photovoltaic devices using new physical processes that rely on thin layers and are capable of attaining the thermodynamic limit of photovoltaic technology. Silicon heterostructure is one of the candidate technologies in which thin films induce a minority carrier collecting junction in silicon and the devices can achieve efficiency close to the thermodynamic limits of silicon technology. The thesis proposes and experimentally establishes a new theory explaining the operation of silicon heterostructure solar cells. The theory will assist in identifying the optimum properties of thin film materials for silicon heterostructure and help in design and characterization of the devices‚ along with aiding in developing new devices based on this technology. The efficiency potential of silicon heterostructure is constrained by the thermodynamic limit (31%) of single junction solar cell and is considerably lower than the limit of photovoltaic conversion (\textasciitilde 80 %). A further improvement in photovoltaic conversion efficiency is possible by implementing a multiple quasi-fermi level system (MQFL). A MQFL allows the absorption of sub band gap photons with current being extracted at a higher band-gap‚ thereby allowing to overcome the efficiency limit of single junction devices. A MQFL can be realized either by thin epitaxial layers of alternating higher and lower band gap material with nearly lattice matched (quantum well) or highly lattice mismatched (quantum dot) structure. The thesis identifies the material combination for quantum well structure and calculates the absorption coefficient of a MQFl based on quantum well. GaAsSb (barrier)/InAs(dot) was identified as a candidate material for MQFL using quantum dot. The thesis explains the growth mechanism of GaAsSb and the optimization of GaAsSb and GaAs heterointerface.

S. N. Dahal and C. Honsberg, “Effect of elastic strain on band edge alignment and position of intermediate band of isolated and coupled quantum dots,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 002042–002046. https://doi.org/10.1109/PVSC.2011.6186354

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Solar cells with quantum dot nanostructure absorbing medium have a potential to overcome single junction limit and achieve the solar energy conversion efficiency up to 63%. Self-assembled quantum dots that are grown by using molecular beam epitaxy(MBE) or metal organic chemical vapor deposition (MOCVD) have significant effect of strain on the band edge alignment and hence the confinement potential of electrons and holes. The energetic positions of confined energy states in quantum dots‚ which‚ act as intermediate state or band in intermediate band solar cells (IBSCs) are strongly affected by the strain in and around a quantum dot (QD). This work is focused on the calculation of strain distribution and its effect on band structure of QD array for its potential application in quantum dot intermediate band solar cells (QDIBSCs). Strain distribution in and around a QD is calculated using continuum theory of elasticity. When the inter-dot distance in the growth direction is sufficiently close‚ there is the interaction in strain distribution of QD layers. The strain distribution due to a vertically aligned QD array is calculated from superposition of the strain due to single quantum dot. The strain calculated this way is given as input for the calculation of band edge alignment and the position of QD confined states.

S. M. Goodnick and C. Honsberg, “Ultrafast carrier relaxation and nonequilibrium phonons in hot carrier solar cells,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 002066–002070. https://doi.org/10.1109/PVSC.2011.6186359

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

K. G. Nelson, J. Husman, S. K. Brem, C. Honsberg, and S. Bowden, “Optimizing educational approaches for University Photovoltaics education,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 001211–001216. https://doi.org/10.1109/PVSC.2011.6186175

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The demands for increasing matriculation and retention in engineering are exceedingly high. To discover future innovations‚ the Nation needs many more engineers than it produces. This paper reviews the literature regarding some of the most significant barriers to student recruitment and retention in the physical sciences and engineering‚ especially with regard to underrepresented groups. Many students traditionally find it hard to be successful in engineering‚ not because they are not as successful in the key competencies (e.g. mathematics)‚ but because they have not been provided with experiences that will provide the motivation required to complete the degree. The focus of this paper is the need to support students’ utility value perceptions‚ Engineering is typically a rigorous and difficult curriculum for all students‚ not just those at risk. Therefore‚ educational tools and interventions are needed that aid in overcoming these challenges and enhance learning of material. We argue that an ideal case for supporting student learning and motivation would be to emphasize Photovoltaics (PV) in physical science and engineering curricula. Specifically‚ we argue that (a) students’ perceived value of learning PV is high given the current climate for the need to develop renewable energy technologies‚ and (b) PV educators and educational researchers need to work together to optimize the motivational opportunities.

A. J. Bailey and C. B. Honsberg, “Thermal-quantum solar cells,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 002036–002041. https://doi.org/10.1109/PVSC.2011.6186353

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The realization of a solar cell with a small number of bandgaps and physical processes which can theoretically attain the maximum possible thermodynamic limits is of substantial interest. Furthermore‚ achieving the maximum thermodynamic efficiency without concentration is particularly important because of the simpler system configuration. To approach the maximum one-sun efficiency of 68%‚ a thermal-quantum converter must be used.

K.-Y. Ban, S. P. Bremner, D. Kuciauskas, S. N. Dahal, and C. B. Honsberg, “Detection of the third transition of InAs/GaAsSb quantum dots,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 003503–003506. https://doi.org/10.1109/PVSC.2011.6186704

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

We have investigated InAs quantum dots (QDs) on GaAsSb barrier layers. Low temperature photoluminescence (PL) for InAs/GaAsSb with various δ-doping levels is performed to observe interband transitions. PL spectra of heavily doped QD samples show that the electrons injected from the δ-doping plane increase the intensity of the emission peak between the electron and hole first excited states‚ E1H1‚ not observed from undoped and lightly QD samples. Time resolved photoluminescence (TRPL) data as a function of δ-doping density reveal that the introduction of a δ-doping plane in the GaAsSb barrier decreases a carrier lifetime making recombination between ground states in QD area faster. As an evidence of carriers more injected from a δ-doping plane an Arrhenius fitting curve taken from temperature dependent PL indicates that the doped samples have the greater thermal activation energies than those of the lightly doped samples. Additionally‚ intersubband transitions of 20 multiple InAs QDs embedded in GaAsSb barriers are experimentally determined by low temperature (77K) Fourier Transformation-Infrared Spectroscopy (FT-IR) using a multiple internal reflection technique. It is noted that there is a broad peak around about 240meV corresponding to the energy separation between the electron ground state and the continuum state in the conduction band offset (CBO). The band structure based upon an eight band k.p method confirms the experimental results observed here. Furthermore‚ all related physical phenomena will be discussed as well.

K. Ghosh, Ghosh, N.-K. Song, M.-S. Oh, D.-S. Kim, and S. Bowden, “Extraction of recombination parameters of amorphous silicon/crystalline silicon solar cell from lifetime spectroscopy,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 001429–001432. https://doi.org/10.1109/PVSC.2011.6186225

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

A technique to measure the recombination parameter of a-Si/c-Si heterojunction solar cell is described in the work. In this methodology‚ the experimentally measured inverse lifetime by Sinton lifetime tester is fitted with A+BΔn+CΔn2 to determine the recombination parameters. The coefficients B and C are radiative and auger recombination coefficient while coefficient A depends on bulk lifetime and surface recombination velocity. The radiative and auger recombination coefficients determined from the work agrees well with previously published results while the surface recombination velocity extracted from coefficient A is typical of well passivated c-Si surface in a-Si/c-Si heterojunction solar cell.

J. Lee and C. Honsberg, “Limiting efficiencies of intermediate band solar cell assisted with multiple exciton generation,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 002074–002077. https://doi.org/10.1109/PVSC.2011.6186361

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

We propose the hybrid thermodynamic model using the intermediate band solar cells assisted with multiple exciton generation. We have calculated this thermodynamic model under blackbody radiation and standard AM1.5 spectrum and changed concentration to compare the conventional intermediate band solar cells. Because of multiple electron and hole pair at the conduction band edge‚ its maximum efficiencies and optimum bandgaps have been enhanced compared to conventional intermediate band solar cells. The maximum efficiencies of this solar cell are both 47.31 (Blackbody Radiation) and 49.85 (AM1.5G) percent for one sun and 65.07 (Blackbody Radiation) and 67.83 (AM1.5D) percent for maximum concentration. And its corresponding overall bandgap energies are also reduced because of multiple electron and hole pairs.

M. Lu, U. Das, S. Bowden, S. Hegedus, and R. Birkmire, “Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero-interface band structures while maintaining surface passivation,” Progress in Photovoltaics: Research and Applications, vol. 19, no. 3, pp. 326–338, 2011 [Online]. https://doi.org/10.1002/pip.1032

Progress in Photovoltaics: Research and Applications

Abstract

Interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells have the potential for high open circuit voltage (VOC) due to the surface passivation and heterojunction contacts‚ and high short circuit current density (JSC) due to all back contact design. Intrinsic amorphous silicon (a-Si:H) buffer layer at the rear surface improve the surface passivation hence VOC and JSC‚ but degrade fill factor (FF) from an “S” shape J–V curve. Two-dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero-interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness‚ (2) increased conductivity‚ and/or (3) reduced band gap. Experimental IBC-SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF‚ consistent with simulation. However‚ this has only marginal effect on efficiency since JSC and VOC also decrease due to poor surface passivation. A narrow band gap a-Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero-interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation‚ and 26% with light trapping. This work provides criterion to design IBC-SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons‚ Ltd.

K. Ghosh and C. Honsberg, “Absorption coefficient for Multiple-Quasi Fermi level system in quantum well,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 002622–002624. https://doi.org/10.1109/PVSC.2011.6186486

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The absorption coefficients are necessary to determine the rate of radiative transition. In this work‚ absorption coefficient of one of the transition (transition from valence band to confined state in the quantum well) for a Multiple Quasi-Fermi level system (MQFL system) is derived from Fermi’s golden rule while the other two absorption coefficients are calculated based on previously published work. The results are discussed with respect to AlInAs (barrier)/InAsP (well) quantum well system that is previously identified as one of the material combinations for a MQFL system. The results show the non-linear variation of absorption coefficients governing the transitions in the well (both from the confined states in the well to the continuum states and also from the valence band to the confined states) with the change in the quasi-Fermi level of the confined state. The results also show that the two absorption coefficients balance each other. As the carrier concentration in the confined states increase‚ the absorption coefficient for transition from the well to the continuum states increase while the absorption coefficient for transition from valence band to the confined states decrease and vice-versa. The results hence signify that provided the continuum states are coupled to the confined states only by radiative transition‚ it is possible to maintain a third quasi-Fermi level corresponding to the confined carriers and hence a MQFL system can be realized.

J. Lee and C. Honsberg, “Thermodynamic limits of hybrid photovoltaic systems using multiple junction solar cells with carrier transitions at intermediate band,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 002082–002086. https://doi.org/10.1109/PVSC.2011.6186363

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

We have proposed the first demonstration of hybrid thermodynamic approach between multiple junction solar cells and the intermediate band solar cells. Because of increasing number of transitions inside each multiple junction from intermediate band‚ reduced number of stacks in this hybrid solar cell can be possible to increase efficiency compared to more than four stacks of conventional multijunction solar cells. The maximum efficiency of two stacks hybrid solar cells has been enhanced and its value is larger than over 4 stacks of multijunction solar cells. We have also found optimum bandgap of each stack and studied optimum transition of each stack both from valence band to intermediate band and intermediate band to conduction band.

N. Faleev, K. Ban, S. Bremner, D. J. Smith, and C. Honsberg, “Investigation of the main correlations between structural and physical propertiess of InAs quantum dots, embedded between strain-relief GaAsSb layers,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 000474–000479. https://doi.org/10.1109/PVSC.2011.6185996

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Strong correlations between crystal perfection of epitaxial structures and size‚ density and PL features of deposited InAs QDs have been found. Increase of a Sb composition in GaAsSb strain relief layers from 8% to 37% significantly disturbed crystal perfection of epitaxial structures and increased the density of deposited QDs from 2-2.5 × 1010 cm-2 at 8% Sb structure to 7.5-9.5 × 1010 cm-2 at 16% Sb structure. At 37% Sb initial elastic stress was noticeably relaxed (≈ 40%) while creation of QDs was fully blocked. Relaxation of elastic stress strongly increased the density of dislocation loops in epitaxial layers and affected atomic migration on the growth front. Observed correlations between density of dislocation loops and QDs are related with a diminution of the energy of migrated atoms‚ caused by dislocations. At definite density of dislocation loops‚ reduced energy diminished accumulation of deposited atoms below the level critical for their transformation to QDs.

V. Sharma, A. Bailey, B. Dauksher, C. Tracy, S. Bowden, and B. O’Brien, “Characterization and comparison of silicon nitride films deposited using two novel processes,” in 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 2011, pp. 002206–002211. https://doi.org/10.1109/PVSC.2011.6186395

2011 37th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Hydrogenated silicon nitride films (SiNx:H) deposited using a PECVD process enhance the performance of crystalline silicon solar cells by functioning as an efficient anti-reflection coating and passivating layer. In this paper‚ we compared two novel SiNx:H deposition processes using two different PECVD tools - one non-traditional in process regime‚ and the other non-traditional in type‚ to determine their suitability to solar cell fabrication. The parameter space was explored by employing a design of experiment methodology followed by material characterization using VASE‚ reflectance‚ FTIR and RBS/ERD. The thickness and reflectance of Si-rich films changed dramatically after annealing. Further‚ FTIR results showed that the Si-H bond peak present at 2160 cm-1 in such films disappeared after a typical Al firing step. Therefore‚ the optimized films were deposited with a lower SiH4/NH3 ratio to minimize the changes in the film properties after annealing.

K.-Y. Ban, S. P. Bremner, S. N. Dahal, and C. B. Honsberg, “Investigation of Intersubband Transitions of InAs/GaAsSb Quantum Dots,” presented at the NAMBE Conference, San Diego, CA, 2011.

NAMBE Conference

2010

V. I. Punegov and N. N. Faleev, “Coherent and diffuse X-ray scattering from a multicomponent superlattice with quantum dots,” JETP Letters, vol. 92, no. 7, pp. 437–443, Dec. 2010 [Online]. https://doi.org/10.1134/S002136401019001X

JETP Letters

N. Faleev and I. Levin, “Strain and crystal defects in thin AlN/GaN structures on (0001) SiC,” Journal of Applied Physics, vol. 107, no. 11, pp. 113529-113529–7, Jun. 2010. https://doi.org/10.1063/1.3437632

Journal of Applied Physics

Abstract

High-resolution x-ray diffraction was used to compare strain relaxation and defect populations in thin GaN/AlN heterostructures (total thickness ≈480 nm) grown on (0001) SiC using metalorganic chemical vapor deposition (MOCVD) and hydride vapor epitaxy (HVPE) techniques. The results of high-resolution x-ray diffraction measurements (rocking curves and reciprocal space mapping) were corroborated using transmission electron microscopy. Differently grown films exhibited dissimilar strain relaxation and defect populations that were related to specific growth conditions. In the MOCVD films‚ grown under lower deposition rates‚ the elastic strain in the AlN and GaN layers was fully relaxed at the initial stages of the epitaxial growth yielding nearly similar densities of threading dislocation segments (TDS) in layer volumes. Additional‚ “secondary” elastic stresses in these layers were attributed to the excess of point defects. In the HVPE films‚ grown under higher (five to ten times) deposition rates‚ these layers were over relaxed and the density of TDS in the GaN layer was an order of magnitude larger than that in AlN. The MOCVD-grown sample was devoid of planar defects whereas the HVPE film contains significant densities of stacking faults in both GaN and AlN layers. Formation of “secondary” extended defects was interpreted in terms of creation and structural transformation of point defects during epitaxial growth. Differences in strain levels‚ types‚ and defect populations/distributions for the two heterostructures were attributed to the different growth rates for MOCVD and HVPE.

B. Jampana, Tianming Xu, A. Melton, M. Jamil, R. Opila, C. Honsberg, and I. Ferguson, “Realization of InGaN solar cells on (111) silicon substrate,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 2010, pp. 000457–000460. https://doi.org/10.1109/PVSC.2010.5616748

2010 35th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The III-nitride material system offers substantial potential to develop high-efficiency solar cells. Currently InGaN based solar cells have been demonstrated on sapphire substrate. This substrate expense adds up significantly to the cost of solar cells realization and further issues like sapphire substrate removal are of concern. Alternatively‚ InGaN epitaxial layers have been successfully grown on silicon substrate. An InGaN based quantum well solar cell structure is grown simultaneously by MOCVD on both GaN/sapphire and GaN/silicon substrates. The fabricated solar cells have comparable photo-response. The Voc of InGaN solar cell on sapphire is higher while the FF of InGaN solar cell on silicon is higher.

K. Ghosh, C. J. Tracy, S. Herasimenka, C. Honsberg, and S. Bowden, “Explanation of the device operation principle of amorphous silicon/ crystalline silicon heterojunction solar cell and role of the inversion of crystalline silicon surface,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 2010, pp. 001383–001386. https://doi.org/10.1109/PVSC.2010.5614387

2010 35th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The device operation principle of amorphous silicon/crystalline silicon heterojunction solar cell is discussed. The band diagram obtained by the computer model developed in the commercial simulator Sentaurus shows that the c-Si surface is inverted at the interface between a-Si and c-Si (heterointerface). A strong inversion gives a strong electric field at the c-Si surface‚ which in turn facilitates the transport of minority carriers across the heterointerface. A high performance device requires a strongly inverted c-Si surface. Calculations are performed to show that the doping of the doped a-Si layer‚ the thickness of the intrinsic layer‚ and the defect state density at the heterointerface all affect the inversion of the crystalline silicon surface. Unlike homojunction devices‚ the defects in heterojunction devices have a greater role in transport mechanism than in recombination mechanism. The results show that in devices with a large number of defects at the interface‚ the fill factor degrades with little change in open circuit voltage. This explains why it is relatively easy to obtain VOC’s approaching 700 mV with heterojunctions but often with low fill factors.

B. Chhabra, R. L. Opila, and C. B. Honsberg, “12.4% efficient freestanding 30µm ultra-thin silicon solar cell using a-Si/c-Si heterostructure,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 2010, pp. 001325–001329. https://doi.org/10.1109/PVSC.2010.5614352

2010 35th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The goal of this work is to demonstrate and analyze heterojunction devices on ultra-thin silicon wafers with imperfect surfaces and lower minority carrier lifetimes. Our previous results show intrinsic amorphous-Si passivation (i-a-Si) gives surface recombination velocity‚ S \textasciitilde 20 cm/sec on freestanding 35 μm Si wafers (Chhabra et al.‚ 2008)‚ and the work presented in this paper shows S \textasciitilde 84 cm/sec on chemically etched wafers. The degradation in the S values is assumed mainly due to the contamination in the PECVD deposition system. The simulated results for this work show that the dominant loss mechanism is the low absorption in the a-Si layer thickness. The device results show that even without light trapping‚ texturing‚ optimizations‚ and with minority carrier lifetimes \textasciitilde 18 μs‚ the structure can have realistic voltages \textasciitilde 613 mV and efficiencies \textasciitilde 12.4%.

S. Herasimenka, K. Ghosh, S. Bowden, and C. Honsberg, “2D modeling of Silicon Heterojunction Interdigitated Back Contact solar cells,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 2010, pp. 001390–001394. https://doi.org/10.1109/PVSC.2010.5614394

2010 35th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Silicon Heterojunction Interdigitated Back Contact (SHJ-IBC) solar cells were studied by two dimensional modeling using Sentaurus TCAD tools. It was shown that low fill factor caused by the S-shape behavior of experimental J-V curves of standard interdigitated back contact cells can be recovered by making small openings in the intrinsic buffer layer. The small openings in the buffer layer also substantially reduce the influence of the relative dimensions of the silicon strips as when compared to cells with a continuous buffer layer.

Jongwon Lee and C. Honsberg, “Detailed balance calculations of multiple exciton generation and Tandem Hybrid solar cells,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 2010, pp. 002932–002937. https://doi.org/10.1109/PVSC.2010.5615913

2010 35th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The detailed balance of multiple exciton generation and tandem hybrid photovoltaic devices are calculated for standard AM1.5G spectrum with one sun. We also obtain the efficiencies both unconstrained and constrained connection. The quantum yield is set to 2 to maximize the multiple exciton generation. The maximum efficiencies of two stacks unconstrained and constrained connection are 48.77% and 47.88% in AM1.5G. Its optimum bandgaps for unconstrained connection are Eg1: 0.7 eV and Eg2: 1.82eV as well as optimum bandgaps for constrained connection are Eg1: 0.57eV and Eg2: 1.38eV. For three stacks of unconstrained and constrained connection‚ the maximum efficiency is 52.78% and 51.78% respectively. The optimum bandgaps of unconstrained connection are Eg1: 0.69eV‚ Eg2: 1.64 eV and Eg3 2.25eV. The optimum bandgaps of constrained connection are Eg1: 0.7eV‚ Eg2: 1.2 eV and Eg3 1.8 eV.

K. Ghosh, C. J. Tracy, B. Dauksher, S. Herasimenka, C. Honsberg, and S. Bowden, “Determination of charged state density at the interface between amorphous silicon and crystalline silicon by lateral conductance,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 2010, pp. 002680–002683. https://doi.org/10.1109/PVSC.2010.5617086

2010 35th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The charged state density at the a-Si/c-Si interface is an important parameter in a heterojunction a cell. The extraction of the charged state density at the interface from measurements of lateral conductance is demonstrated by simulations. In a-Si/c-Si heterojunction an inversion layer is formed at the interface between a and c-Si (heterointerface). The lateral conductance of the inversion layer is much higher than the doped or intrinsic a-Si layer conductance and the current primarily flows through this path. The increase of the charged state density at the heterointerface weakens the invers hence lowers the lateral conductance of these devices This effect is studied in this work by applying a theoretical model developed in the commercial simulator Sentaurus. The simulation results based on this model have shown that in an optimized device structure the sensitivity of the measurement technique in determining the charged state density can be on the order of 1 × 1010/cm2.

S. N. Dahal, Keun-Yong Ban, and C. Honsberg, “Absorption coefficients of quantum dot intermediate band material with negligible valence band offsets,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 2010, pp. 001797–001799. https://doi.org/10.1109/PVSC.2010.5615908

2010 35th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Solar cells with quantum dot nanostructure absorbing medium have a potential to overcome single junction limit and achieve the solar energy conversion efficiency as high as 63%. The confined energy states in quantum dots can mediate the absorption of photons with energy lower than the band gap of the barrier material. Closely spaced array of quantum dots (QDs) can form a mini band due to electronic coupling of the confined states among the neighboring dots. Absorption properties of the quantum dot nanostructures are different from that of a bulk material. For the detailed balance efficiency calculations‚ the absorption coefficients of the QD nanostructures are required for realistic QD structures. After finding out material combinations with negligible valence band offset for quantum dot intermediate band solar cells(QDIBSCs)‚ present work is focused on the calculation of absorption coefficients of QD arrays. The confined electronic states are calculated with the effective mass theory for single and coupled quantum dots. The electronic coupling of the ground states of an array of quantum dots is calculated for negligible valence band offset material combinations (especially InAs dots in GaAs(0.84)Sb(0.16) matrix grown on [001] GaAs substrate). The intermediate bandwidth vs the vertical interdot separation is presented. For some suitable interedot separation‚ the absorption coefficients are calculated for valence band to intermediate band‚ Intermediate band to conduction band transitions.

B. Chhabra, R. Kamada, R. L. Opila, and C. B. Honsberg, “Effect of small continuous loads on system efficiency,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 2010, pp. 002327–002331. https://doi.org/10.1109/PVSC.2010.5614448

2010 35th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The paper focuses on constructing software program to investigate lighting systems followed by system implementation and measurement of inverter efficiency at part-load condition with the small continuous load. The idea here is to observe the behavior of inverter efficiency and to see its effect on system performance and cost. The stand-alone systems are badly affected as compared to grid connected systems in terms of system performance and cost.

S. N. Dahal, S. P. Bremner, and C. B. Honsberg, “Identification of candidate material systems for quantum dot solar cells including the effect of strain,” Progress in Photovoltaics, vol. 18, no. 4, pp. 233–239, Jun. 2010. https://doi.org/10.1002/pip.937

WOS:000277905100001

Progress in Photovoltaics

Prog. Photovoltaics

Abstract

Heterostructures that include self-assembled quantum dots (SAQDs) have been suggested as model systems for the realization of novel high efficiency solar cells such as those based on intermediate bands (IBs). The lattice mismatch in the epitaxial growth of these structures‚ necessary for the formation of SAQDs‚ introduces strain throughout the structure‚ making the selection of materials systems with appropriate physical parameters problematic. The model solid theory is used to calculate the energy band edge alignment at Gamma point of such quantum dot (QD) heterostructures including the effects of strain. With the modified band gaps due to strain‚ a materials search was performed for high efficiency QD solar cells among III-V binaries and ternaries with negligible valence band offsets. This requirement of the valence band offset along with the limited band gap ranges for optimum efficiency results in only a few feasible materials systems being identified. The optimum barrier/dot material system found was Al(0.57)In(0.43)As/InP(0.87)Sb(0.13) grown on lattice matched metamorphic buffer layer‚ but due to miscibility gap concerns it is suggested that the Al(0.50)In(0.50)As/InAs(0.41)P(0.59) fully strained system may be preferred. Copyright (C) 2010 John Wiley & Sons‚ Ltd.

A. Pancholi, S. P. Bremner, J. Boyle, V. G. Stoleru, and C. B. Honsberg, “Variability of heterostructure type with thickness of barriers and temperature in the InAs/GaAsSb quantum dot system RID G-1695-2011,” Solar Energy Materials and Solar Cells, vol. 94, no. 6, pp. 1025–1030, Jun. 2010. https://doi.org/10.1016/j.solmat.2010.02.002

WOS:000278206300015

Solar Energy Materials and Solar Cells

Sol. Energy Mater. Sol. Cells

Abstract

We report photoluminescence studies of InAs quantum dots embedded in GaAsSb barrier layers of various thicknesses. Time resolved photoluminescence results suggest that the thicknesses of the GaAsSb barrier layers determines whether the quantum dot system acts as a type I (thicker) or type II (thinner) heterostructure due to strain induced changes in the band energies. Temperature dependent photoluminescence reveal that the transition type is also dependent on the temperature with one sample seeming to transform from type II to type I heterostructure as the temperature is increased. These results support previous assertions that it is possible to obtain a zero valence band offset in the InAs/GaAsSb quantum dot system‚ making it a system of interest for realization of a novel photovoltaic structure‚ the intermediate band solar cell. The results highlight some of the inherent issues in designing structures with specific band offsets. Some of the implications of these results on the design methodology for quantum dot based solar cells are briefly discussed. (C) 2010 Elsevier B.V. All rights reserved.

S. P. Bremner, K. Ghosh, L. Nataraj, S. G. Cloutier, and C. B. Honsberg, “Influence of Sb/As soak times on the structural and optical properties of GaAsSb/GaAs interfaces,” Thin Solid Films, vol. 519, no. 1, pp. 64–8, Jun. 2010 [Online]. https://doi.org/10.1016/j.tsf.2010.07.060

Thin Solid Films

Switzerland

Abstract

The effects of Sb/As soak times‚ prior to growth of GaAsSb on GaAs were investigated by High Resolution X-Ray Diffraction (HRXRD) and photoluminescence (PL). Multiple quantum well samples with soak times of 0s‚ 30s and 60s were grown at 500C with nominally identical Sb and As fluxes. HRXRD results show that a 30s soak minimizes diffuse scattering seen around superlattice peaks in the reciprocal space maps‚ an effect attributed to corrugations in the GaAs-GaAsSb interface. An inferred band diagram calculated using a four band k.p model and modified taking into account the HRXRD results was used to explain PL spectra taken for each sample at 80K. It is concluded that an optimum soak time exists for GaASb growth on GaAs‚ determined by the growth conditions. [All rights reserved Elsevier].

K.-Y. Ban, S. N. Dahal, C. B. Honsberg, L. Nataraj, S. P. Bremner, and S. G. Cloutier, “Room temperature capacitance-voltage profile and photoluminescence for delta doped InGaAs single quantum well,” Journal of Vacuum Science & Technology B (Microelectronics and Nanometer Structures), vol. 28, no. 3, pp. 3–6, May 2010 [Online]. https://doi.org/10.1116/1.3268614

Journal of Vacuum Science & Technology B (Microelectronics and Nanometer Structures)

USA

Abstract

Room temperature capacitance-voltage (C-V) profile and photoluminescence (PL) studies of -doped single InGaAs quantum well samples are reported. The purpose was to obtain the confined carrier occupancy in the conduction band offset and observe any relevant phenomena. The results show that the peak intensity of the C-V profiles was almost linearly proportional to sheet carrier concentration and the full width at half maximum of the C-V profiles became narrower with increasing doping level in the barrier layer. This is interpreted as being due to improved confinement of electrons as a result of band bending induced by the -doping layer. This explanation was further supported by PL data that show the transition corresponding to the dominant peak changed with different -doping levels and that all of the transitions were redshifted. Finally‚ theoretical calculations of the band structure based on a four band kp method are presented to explain the observed results.

Keun-Yong Ban, S. P. Bremner, Guangming Liu, S. N. Dahal, P. C. Dippo, A. G. Norman, and C. B. Honsberg, “Use of a GaAsSb buffer layer for the formation of small, uniform, and dense InAs quantum dots,” Applied Physics Letters, vol. 96, no. 18, p. 183101 (3 pp.), May 2010 [Online]. https://doi.org/10.1063/1.3409691

Applied Physics Letters

USA

Abstract

InAs quantum dots grown on GaAsSb buffer layers with varying Sb content have been studied. Atomic force microscopy results show that the dot size is reduced as the Sb content increases with a concomitant increase in number density. Analysis of the size distribution indicates that the spread of dot sizes narrows with increasing Sb content. This is confirmed by photoluminescence measurements showing a significant narrowing of the dot emission peak for a GaAs0.77Sb0.23 buffer compared to a GaAs buffer. The results are attributed to the strained buffer reducing interactions between dots and the Sb acting as a surfactant.

B. R. Jampana, A. G. Melton, M. Jamil, N. N. Faleev, R. L. Opila, I. T. Ferguson, and C. B. Honsberg, “Design and Realization of Wide-Band-Gap (similar to 2.67 eV) InGaN p-n Junction Solar Cell,” Ieee Electron Device Letters, vol. 31, no. 1, pp. 32–34, Jan. 2010. https://doi.org/10.1109/LED.2009.2034280

WOS:000273090800012

Ieee Electron Device Letters

IEEE Electron Device Lett.

Abstract

The design of coherently strained InGaN epilayers for use in InGaN p-n junction solar cells is presented in this letter. The X-ray diffraction of the epitaxially grown device structure indicates two InGaN epilayers with indium compositions of 14.8% and 16.8%‚ which are confirmed by photoluminescence peaks observed at 2.72 and 2.67 eV‚ respectively. An open-circuit voltage of 1.73 V and a short-circuit current density of 0.91 mA/cm(2) are observed under concentrated AM 0 illumination from the fabricated solar cell. The photovoltaic response from the InGaN p-n junction is confirmed by using an ultraviolet filter. The solar cell performance is shown to be related to the crystalline defects in the device structure.

S. Bowden, C. Honsberg, and D. Schroder, “Moore’s Law of Photovoltaics,” Future Photovoltaics, no. 1, 2010.

Future Photovoltaics

K.-Y. Ban, S. N. Dahal, C. B. Honsberg, L. Nataraj, S. P. Bremner, and S. G. Cloutier, “Room temperature capacitance-voltage profile and photoluminescence for delta doped InGaAs single quantum well,” Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures, vol. 28, p. C3I6-C3I9, 2010 [Online]. https://doi.org/10.1116/1.3268614

Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures

Abstract

K.-Y. Ban, D. Kuciauskas, S. P. Bremner, and C. B. Honsberg, “Determination of a Sb composition in InAs/GaAsSb for negligible valence band offset,” in 2010 35th IEEE Photovoltaic Specialists Conference (PVSC), 20-25 June 2010, Piscataway, NJ, USA, 2010, pp. 003306-9 [Online]. https://doi.org/10.1109/PVSC.2010.5617048

PVSC

Abstract

InAs quantum dots (QDs) embedded in GaAsSb barriers with various Sb compositions was investigated by photoluminescence (PL). The peak position of 8% and 13% Sb sample does not shift while that of 15% Sb sample was blue-shifted with increasing the excitation power. In addition‚ time-resolved PL (TRPL) data also show that 15% Sb sample has a much longer PL decay time compared to that of 8% and 13% Sb sample‚ implying that the transformation from type I to II occurs between 13% and 15% Sb composition. It is noted that the improvement of QD uniformity was achieved by an increase of a Sb composition in the GaAsSb barrier due to a Sb surfactant effect.

B. Chhabra, S. Bowden, R. L. Opila, and C. B. Honsberg, “High effective minority carrier lifetime on silicon substrates using quinhydrone-methanol passivation,” Applied Physics Letters, vol. 96, no. 6, p. 063502, 2010 [Online]. https://doi.org/10.1063/1.3309595

Applied Physics Letters

Appl. Phys. Lett.

K.-Y. Ban, D. Kuciauskas, S. P. Bremner, S. N. Dahal, N. J. Ekins-Daukes, and C. B. Honsberg, “Controllability of the occupation of quantum dot electronic states and determination of minimum valence band offset in InAs/GaAsSb,” presented at the 25th EU PVSEC & WCPEC-5, Valencia, Spain, 2010.

25th EU PVSEC & WCPEC-5

K.-Y. Ban, W.-K. Hong, and C. B. Honsberg, “Photoluminescence study of InAs quantum dots on a δ-doped GaAsSb barrier,” presented at the MRS Spring Meeting, San Francisco, CA, USA, 2010.

MRS Spring Meeting

M. Y. Levy and C. Honsberg, “Absorption coefficients of intermediate-band media,” Journal of Applied Physics, vol. 106, no. 7, p. 073103 (12 pp.), Oct. 2009 [Online]. https://doi.org/10.1063/1.3213337

Journal of Applied Physics

USA

Abstract

This paper models the absorption coefficients of an intermediate-band (IB) absorbing medium. Equilibrium absorption coefficients are presented for several IB absorbers‚ each distinguished by their energy-wavevector dispersion and equilibrium temperature. Nonequilibrium absorption coefficients are also presented for solar cells implemented with IB absorbers. Several simplifying assumptions are made including that the energy-wavevector dispersions are parabolic. The model requires the absolute locations of three quasi-Fermi levels. This is made possible by using two balance equations. One of these‚ a charge-neutrality condition‚ necessitates the numerical computation of the carrier statistics in each band of the IB absorber. The use of the incomplete Fermi-Dirac functions makes this possible. The authors conclude that (i) if the concentration of intermediate states is greater than the concentration of carriers in the conduction band and greater than the concentration of carriers in the valence band‚ then the IB will be partially filled; (ii) an IB absorber may or may not absorb all photons with energies greater than the smallest bandgap in the system; (iii) an IB absorber may permit absorption overlap so that an absorbed photon would likely generate an electron-hole pair across a bandgap other than the largest bandgap less than the energy of the absorbed photon; (iv) as the temperature of the IB absorber approaches absolute zero‚ the absorption edges resulting from transitions at intermediate levels may blueshift.

A. Melton, B. Jampana, R. Opila, C. Honsberg, M. Jamil, and I. Ferguson, “Minimizing shadow losses in III-nitride solar cells,” in Proceedings of SPIE, 2009, vol. 7409, pp. 740916-740916–7 [Online]. https://doi.org/doi:10.1117/12.829264

Abstract

In this work InGa0.85N p-n homojunction solar cells were grown by MOCVD on GaN/sapphire substrates and fabricated using standard techniques. When illuminated from the backside‚ these devices showed 65.9% improvement in JSC and 4.4% improvement in VOC as compared to identical illumination from the front. These improvements arise from removal of the losses from electrical contact shading on the front of the devices (11.7% of active area)‚ as well as significant optical absorption by the top current spreading layer. These improvements can likely be further enhanced by utilizing double-side polished wafers‚ which would eliminate scattering losses on the back surface. In addition to improving electrical characteristics of single cells‚ backside illumination is necessary for the realization of monolithic tandem InGaN solar cells.

S. Bowden, “From the Valley of Death to the Golden Decade: Crystalline Silicon Solar Cells from 10 to 100 microns,” in 19th Workshop on Crystalline Silicon Solar Cells and Modules, Vail, Colorado, 2009, pp. 192–195.

N. Faleev, B. Jampana, O. Jani, H. Yu, R. Opila, I. Ferguson, and C. Honsberg, “Correlation of crystalline defects with photoluminescence of InGaN layers,” Applied Physics Letters, vol. 95, no. 5, pp. 051915-051915-3, Aug. 2009 [Online]. https://doi.org/doi:10.1063/1.3202409

Applied Physics Letters

Abstract

We report structural studies of InGaN epilayers of various thicknesses by x-ray diffraction‚ showing a strong dependence of the type and spatial distribution of extended crystalline defects on layer thickness. The photoluminescence intensity for the samples was observed to increase with thickness up to 200 nm and decrease for higher thicknesses‚ a result attributed to creation of dislocation loops within the epilayer. Correlation of physical properties with crystalline perfection open the way for optimized designs of InGaN solar cells‚ with controlled types and dislocation densities in the InGaN epilayers‚ a key requirement for realizing high photocurrent generation in InGaN.

Meijun Lu, U. Das, S. Bowden, S. Hegedus, and R. Birkmire, “Optimization of interdigitated back contact silicon heterojunction solar cells by two-dimensional numerical simulation,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009, pp. 001475–001480. https://doi.org/10.1109/PVSC.2009.5411332

2009 34th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

In this paper‚ two-dimensional (2D) simulation of interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells is presented using Sentaurus Device‚ a software package of Synopsys TCAD. A model is established incorporating a distribution of trap states of amorphous-silicon material and thermionic emission across the amorphous-silicon / crystalline-silicon hetero-interface. The 2D nature of IBC-SHJ device is evaluated and current density-voltage (J-V) curves are generated. Optimization of IBC-SHJ solar cells is then discussed through simulation. It is shown that the open circuit voltage (VOC) and short circuit current density (JSC) of IBC-SHJ solar cells increase with decreasing front surface recombination velocity. The JSC improves further with the increase of relative coverage of p-type emitter contacts‚ which is explained by the simulated and measured position dependent laser beam induced current (LBIC) line scan. The S-shaped J-V curves with low fill factor (FF) observed in experiments are also simulated‚ and three methods to improve FF by modifying the intrinsic a-Si buffer layer are suggested: (i) decreased thickness‚ (ii) increased conductivity‚ and (iii) reduced band gap. With all these optimizations‚ an efficiency of 26% for IBC-SHJ solar cells is potentially achievable.

S. Bremner, N. Faleev, L. Nataraj, S. Cloutier, S. Dahal, and C. Honsberg, “Passivation of InAs quantum dots for novel photovoltaics,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009, pp. 001306–001311. https://doi.org/10.1109/PVSC.2009.5411253

2009 34th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Novel photovoltaic device designs offer the possibility of limiting efficiencies well in excess of conventional limits [1‚2]. Many of the suggested devices rely on the inclusion of nanostructures in order to improve conventional energy conversion mechanisms or allow new mechanisms to be exploited. Because nanostructures rely on quantum confinement realized by heterojunctions‚ strain induced defects at hetero-interfaces‚ and the non-ideal recombination pathways they bring‚ need to be passivated‚ in order for the novel devices to operate as intended. We report attempts to passivate quantum dots in the much studied InAs/GaAs system using a Sb flux treatment immediately prior to capping of the quantum dots with GaAs. The photoluminescence results suggest an optimum exposure time to the Sb flux after which the performance degrades substantially. Temperature dependent photoluminescence results as well as X-ray diffraction results are also presented in order to explain the structure at the quantum dot - cap interface. The impact of these results in terms of the design of two novel photovoltaic devices‚ the intermediate band solar cell and the hot carrier solar cell is also discussed.

J. G. Mutitu, Shouyuan Shi, A. Barnett, C. Honsberg, and D. W. Prather, “Light trapping designs for thin silicon solar cells based on photonic crystal and metallic diffractive grating structures,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009, pp. 000579–000583. https://doi.org/10.1109/PVSC.2009.5411616

2009 34th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

We present novel light trapping designs applied to thin (5 micron) silicon solar cells. The design structures incorporate diffractive gratings to increase the optical path length of light within the solar cells. We incorporate a combination of dielectric and metallic materials to create the gratings. We form a one dimensional photonic crystal stack with the dielectric materials to which we then add the metallic layers. The combination of the two materials enhances the reflective properties of the gratings and thus increasing their effectiveness in light trapping. We use the particle swarm optimization and scattering matrix methods to realize the design structures.

B. Jampana, A. Melton, M. Jamil, I. Ferguson, R. Opila, and C. Honsberg, “InGaN solar cell design by surface inversion caused by piezoelectric polarization,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009, pp. 002175–002178. https://doi.org/10.1109/PVSC.2009.5411401

2009 34th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

The III-nitride material system offers substantial potential to develop high-efficiency solar cells. The solar cell operation requires the formation of a depletion region. Conventionally‚ this is achieved by a p-n junction. The piezoelectric polarization introduces a strong band bending at the hetero-junction interface and hence creating a depletion region. The growth of a thin AlN or GaN epi-layer on InGaN introduces the required piezoelectric polarization to create a depletion region. This paper presents the polarization-incorporated simulations in “Silense” showing the depletion region formation by GaN or AlN epilayers on p-InGaN. Three structures are then MOCVD grown and characterized for crystal quality and electrical properties. The fabricated devices demonstrated the diode characteristics with an open-circuit voltages ≫ 2.0 V.

Ruiying Hao, C. P. Murcia, T. Creazzo, T. Biegala, A. Lochtefeld, Ji-Soo Park, C. Honsberg, and A. Barnett, “Thin silicon solar cells using epitaxial lateral overgrowth structure,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009, pp. 000949–000953. https://doi.org/10.1109/PVSC.2009.5411132

2009 34th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Thin Si solar cell with epitaxial lateral overgrowth (ELO) structure described in this paper should demonstrate higher voltage. PC-1D program has been used to study the open circuit voltage and efficiency as a function of the thin Si thickness and light trapping. According to the simulation results‚ high voltage can be obtained even without light trapping on the backside of the thin Si layer. Thin n type silicon layer has been grown on p+ Si substrate using the method of epitaxial lateral overgrowth by CVD. The scanning electron microscopy (SEM) has been used to show the dimension of the pn junction region and light generation region after the n type Si growth.

K.-Y. Ban, S. N. Dahal, and C. B. Honsberg, “Optical properties of delta doped InAs/GaAs0.88Sb0.12 structure for novel concept solar cells,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009, pp. 001264–001267. https://doi.org/10.1109/PVSC.2009.5411245

2009 34th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

We have investigated optical properties of InAs quantum dots (QDs) on GaAsSb barrier layer with different doping levels. In order to precisely control occupancy of subband levels in conduction band offset (CBO) in the material system‚ delta doping layer in the barrier was incorporated as an efficient carrier supply. Interband transitions between ground states or excited states were detected by implementing time-integrated photoluminescence (PL) whereas intersubband transitions have shown via Fourier transform-infrared spectroscopy (FT-IR) at room temperature. The results are compared and discussed with theoretical calculation based on k.p. method.

Xiaoting Wang, N. Waite, P. Murcia, K. Emery, M. Steiner, F. Kiamilev, K. Goossen, C. Honsberg, and A. Barnett, “Improved outdoor measurements for Very High Efficiency Solar Cell sub-modules,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC), 2009, pp. 000409–000414. https://doi.org/10.1109/PVSC.2009.5411652

2009 34th IEEE Photovoltaic Specialists Conference (PVSC)

PVSC

Abstract

Very High Efficiency Solar Cell (VHESC) program is developing integrated optical/photovoltaic modules for portable applications that operate at 50 percent efficiency. Test sub-modules incorporating four p-n junctions and corresponding optics have been realized and are predicted to realize efficiency greater than 40%. Phased implementation requires corresponding measurement to inspect accomplished work and provide improvement direction for the next step. The comparison between the real performance of the four-junction test sub-module and the theoretical prediction of its efficiency is a significant indication of the realizability of the final VHESC module including six junctions which is designed to achieve 50% efficiency. For the sub-module measurement‚ a test bed was set up for outdoor test. Previous outdoor measurements of the VHESC test sub-modules resulted in a preliminary sub-module efficiency of 36.2%. As solar cells with better performance were fabricated‚ the measurement methodology was refined and corresponding improvements were made to the initial test bed. Three test sub-modules containing new solar cells were measured with the new test setup for three different concentration levels at University of Delaware (UD). One test sub-module demonstrated efficiency as high as 39.5%‚ coupled with 44.3% efficient solar cells and 89.1% efficient optics‚ at 30.48 X concentration. The measurements were taken when the direct light intensity was over 860 W/m2 and the Isc was not calibrated to 1000 W/m2. Another two test sub-modules including solar cells in the same batch as the ones tested at UD were taken to National Renewable Energy Laboratory (NREL). The Isc data of the two test sub-modules were recorded outdoors at NREL when the direct light intensity was over 970 W/m2. In addition‚ the Isc was calibrated to the standard spectrum condition using ASTM G173 direct data. Comparison of the results shows t- he difference between the test sub-module efficiency measured at UD and NREL is less than 4%.

A. Barnett, D. Kirkpatrick, C. Honsberg, D. Moore, M. Wanlass, K. Emery, R. Schwartz, D. Carlson, S. Bowden, D. Aiken, A. Gray, S. Kurtz, L. Kazmerski, M. Steiner, J. Gray, T. Davenport, R. Buelow, L. Takacs, N. Shatz, J. Bortz, O. Jani, K. Goossen, F. Kiamilev, A. Doolittle, I. Ferguson, B. Unger, G. Schmidt, E. Christensen, and D. Salzman, “Very High Efficiency Solar Cell Modules,” Progress in Photovoltaics, vol. 17, no. 1, pp. 75–83, Jan. 2009. https://doi.org/10.1002/pip.852

WOS:000262575900007

Progress in Photovoltaics

Prog. Photovoltaics

Abstract

The Very High Efficiency Solar Cell (VHESC) program is developing integrated optical system-PV modules for portable applications that operate at greater than 50% efficiency. We are integrating the optical design with the solar cell design‚ and have entered previously unoccupied design space. Our approach is driven by proven quantitative models for the solar cell design‚ the optical design‚ and the integration of these designs. Optical systems efficiency with an optical efficiency of 93% and solar cell device results under ideal dichroic splitting optics summing to 42.7 +/- 2.5% are described. Copyright (c) 2008 John Wiley & Sons‚ Ltd.

B. R. Jampana, I. T. Ferguson, R. L. Opila, and C. B. Honsberg, “Utilizing Polarization Induced Band Bending for InGaN Solar Cell Design,” in Materials Research Society Symposium. Compound Semiconductors for Energy Applications and Environmental Sustainability, 14-16 April 2009, Warrendale, PA, USA, 2009, pp. 3–8.

Abstract

Strong polarization effects observed in Ill-nitride materials can invert the surface carrier type. The corresponding band bending can be used to design InGaN solar cells. Similar surface inversion was observed in the past with silicon-based Schottky-barrier solar cells‚ but was limited by Fermi level pinning. The formation of two-dimensional electron gas by polarization fields in Ill-nitrides has been reported. Using a similar idea‚ the growth of a thin AlN capping layer on p-InGaN has resulted in band bending‚ hence depletion region‚ under the surface that can be used to separate any generated photo-carriers. Hall measurements at different depths on these structures confirm the inversion of surface carrier type. Solar cells based on this concept have resulted in an open circuit voltage of 2.15 V and short circuit current of 21.8 A.

B. R. Jampana, N. N. Faleev, I. T. Ferguson, R. L. Opila, and C. B. Honsberg, “Crystalline Perfection of Epitaxial Structure: Correlation With Composition, Thickness, and Elastic Strain of Epitaxial Layers,” in Materials Research Society Symposium. Compound Semiconductors for Energy Applications and Environmental Sustainability, 14-16 April 2009, Warrendale, PA, USA, 2009, pp. 71–6.

Abstract

Crystalline perfection of InGaN epi-layers is the missing design parameter for InGaN solar cells. Structural deterioration of InGaN epi-layers depends on the thickness‚ composition and growth conditions as well. Increasing the InGaN epi-layer thickness beyond a critical point introduces extended crystalline defects that hinder the optical absorption and electrical properties. Increasing the InGaN composition further reduces this critical layer thickness. The optical absorption band edge is sharp for Ill-nitride direct band gap materials. The band edge profile is deteriorated by creation of extended crystalline defects in the InGaN epitaxial material. The design of InGaN solar cells requires the growth of epi-layers where a trade off between crystalline perfection and optical absorption properties is reached.

C. P. Murcia, Ruiying Hao, T. Biegala, C. Honsberg, and A. Barnett, “High performance thin crystalline silicon solar cell grown on silicon-on-insulator,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC 2009), 7-12 June 2009, Piscataway, NJ, USA, 2009, p. 4 pp. [Online]. https://doi.org/10.1109/PVSC.2009.5411219

PVSC

Abstract

High open circuit voltage (VOC) is a potential benefit of thin silicon solar cells. A new thin silicon solar cell structure is proposed using silicon-on-insulator (SOI) technology that investigates the properties of high voltage in thin silicon designs with an epitaxial emitter. Key design parameters are low rear and front surface recombination‚ low dark current and efficient light trapping. We propose a patterned emitter area on a SOI substrate. The advantages of this design are the passivation properties embedded in the buried oxide and the reduced junction area. With a uniform epitaxial emitter‚ the top contact shadowing can be designed to be 0%. Preliminary results show Voc 525 mV and JSc 20 mA/cm2 with_anti-reflection coating. This represents a substantial increase from previous work by Danos et al. which reported Voc 500 mV and Jsc \textasciitilde 0.45 mA/cm2. This present design also demonstrates the effect of a smaller emitter area and reports higher performance parameters for reported silicon cells fabricated on SOI substrates.

B. Jampana, A. Melton, N. Faleev, O. Jani, M. Jamil, I. Ferguson, R. Opila, and C. B. Honsberg, “Correlation between crystalline perfection of InGaN epi-layers and wide-band gap InGaN solar cell performance,” in Proceedings of the 34th IEEE Photovoltaic Specialists Conference, Philadelphia, PA, USA, 2009.

Proceedings of the 34th IEEE Photovoltaic Specialists Conference

Abstract

We report the influence of crystalline quality of InGaN epitaxial layers grown by MOCVD on wide-band gap InGaN solar cell performance. The collection of photo-generated carriers is the primary challenge in InGaN solar cell design. This carrier collection is affected by the defects in the material‚ in particular the extended crystalline defects in III-nitrides epi-layers. The lattice-mismatch between InGaN and GaN results in creation of extended crystalline defects at the InGaN-GaN interface. The creation of these defects depends on the InGaN composition and thickness. An optimum thickness based on optical absorption properties and creation of extended crystalline defects needs to be established to successfully realize InGaN solar cells. This paper presents the spatial distribution of extended crystalline defects in InGaN and GaN epi-layers grown by MOCVD. The influence of these crystalline defects on homo-junction solar cells performance is presented. A critical thickness less than 100 nm for 15% InGaN epi-layers is observed‚ beyond which there is noticeable performance degradation in homo-junction solar cells. Extended design rules for InGaN solar cells based on optical absorption properties and crystalline quality are presented to be incorporated in future designs.

K.-Y. Ban, S. N. Dahal, S. P. Bremner, and C. B. Honsberg, “Room temperature capacitance-voltage profile and photoluminescence for delta doped InGaAs single quantum well,” presented at the NAMBE Conference, Princeton, NJ, USA, 2009.

NAMBE Conference

A. Melton, B. Jampana, N. Li, M. Jamil, T. Zaidi, W. Fenwick, R. Opila, C. Honsberg, and I. Ferguson, “High indium composition (20%) InGaN epi-layers on ZnO substrates for very high efficiency solar cells,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC 2009), 7-12 June 2009, Piscataway, NJ, USA, 2009, p. 4 pp. [Online]. https://doi.org/10.1109/PVSC.2009.5411216

PVSC

Abstract

In this report we present recent results for MOCVD growth of high indium content InGaN films on ZnO substrates. Growth was attempted on both bulk ZnO as well as ZnO epilayers grown on sapphire by MOCVD. ZnO is an attractive alternative substrate for III-Nitrides because of its superior lattice match: specifically ZnO is perfectly matched with In0.18Ga0.82N and low cost of substrates. Stable InGaN films with 18% indium were achieved on the bulk substrates and were characterized by HRXRD‚ PL‚ and optical transmission. Varying the growth parameters - primarily growth temperature and In/(In + Ga) flow ratio - was found to affect the optical and structural properties of the films. By growing on a better matched substrate the high indium composition InGaN epitaxial films experience less strain and can therefore be grown thicker without creating relaxation-induced extended crystal defects. This is important‚ as high indium content InGaN films cannot be grown on GaN thick enough for full above-bandgap absorption without introducing detrimental extended crystal defects. This limitation is thought to be a limiting factor in the achievable ISC in InGaN solar cells.

B. Chhabra, C. B. Honsberg, and R. L. Opila, “High open circuit voltages on 50 micron silicon substrates by amorphous-silicon (a-Si) and quinhydrone-methanol (QHY-ME) passivation,” in 2009 34th IEEE Photovoltaic Specialists Conference (PVSC 2009), 7-12 June 2009, Piscataway, NJ, USA, 2009, p. 4 pp. [Online]. https://doi.org/10.1109/PVSC.2009.5411398

PVSC

Abstract

Thin silicon solar cells offer the well-known advantages of cost reduction and higher efficiencies. A thinner solar cell may have a higher open circuit voltage than a thicker one assuming the surfaces are well passivated and the light trapping is included thus resulting in improved efficiencies. High open circuit voltage‚ Voc‚ of 720 mV or above has been achieved from several technologies on conventional thickness wafers and has approached 740 mV on thinner wafers. However‚ the theoretical limit from detailed balance calculations is between 830 mV and 850 mV (depending on spectrum). In order to achieve silicon solar cells which approach the detailed balance voltage limits‚ controlling the broad mechanisms that limit the open circuit voltage becomes very important and these are: (1) Auger recombination (e.g.‚ by controlling dopant concentration); (2) the thickness of the material; and (3) the surface passivation. While high open circuit voltages have been demonstrated on thicker silicon solar cells‚ achieving the necessary surface passivation for "thin" solar cells (i.e.‚ less than 50 micron thick wafers) to show increase in Voc has remained a challenge. The present research demonstrates via Implied-Voc measurements that amorphous-Si passivation as well as organic passivation based on quinhydrone-methanol has sufficient surface passivation such that open circuit voltage increases even at thickness of 35 microns. These results are also significant in demonstrating that non-ideal effects‚ such as high injection‚ play a significant role in determining Voc‚ but nevertheless experimentally still allow high open circuit voltages as the device is thinned.

K. Ghosh, S. Bowden, and C. Honsberg, “Devices and methods for providing carrier selective contact devices,” US20140137933 A122-May-2014 [Online].

U.S. Classification 136/256, 438/94; International Classification H01L31/20, H01L31/0248; Cooperative Classification Y02E10/50, H01L31/02167, H01L31/022425, H01L31/04, H01L31/035236, H01L31/0248, H01L31/20

4M398J5P year__ type__patent ""

Abstract

Devices comprising: an absorbing medium (AM) having first and second sides; a first membrane layer (ML) having first and second sides‚ wherein the first side of the first ML contacts the first side of the AM; a second ML having first and second sides‚ wherein the first side of the second ML contacts the second side of the AM; a first contact in contact with the second side of the first ML; and a second contact in contact with the second side of the second ML‚ wherein a first band alignment mismatch between the first contact and the AM causes a first surface of the AM on the first side of the AM to be in inversion‚ and wherein a second band alignment mismatch between the second contact and the AM causes a second surface of the AM on the second side of the AM to be under accumulation.

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