Positive and negative charges in silicon nitride.
The surfaces of silicon solar cells are highly active and a major loss mechanism unless they are made electrical inactive by passivation of the surface defects. Silicon nitride is commonly used to passivate the front surface of solar cells and gives them their blue color. The positive charge in the nitride lowers the electrical activity at the surface of n-type material by repelling minority carrier holes. Recently we demonstrated a techniue to change the charge in silicon nitride from positive to negative. By flipping the charge we will be able to use silicon nitride to passivate p-type. The whole process is done at low temperature and relies on changing a metastable defect.
The results are published in 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. http://dx.doi.org/10.1063/1.4863829