Factors limiting the open-circuit voltage in microcrystalline silicon solar cells

In studying photovoltaic devices made with silicon thin films and considering them according to their grain size, it is curious that as the crystalline fraction increases, the open-circuit voltage (Voc) – rather than approaching that of the single-crystal case – shows a decline. To gain an insight into this behavior, observed in hydrogenated microcrystalline silicon (μc-Si:H) solar cells prepared under a variety of deposition conditions, we have used a detailed electrical-optical computer modeling program, ASDMP. Two typical μc-Si:H cells with low (~79%) and higher (~93%) crystalline volume fractions (Fc), deposited in our laboratory and showing this general trend, were modeled. From the parameters extracted by simulation of their experimental current density – voltage and quantum efficiency characteristics, it was inferred that the higher Fc cell has both a higher band gap defect density as well as a lower band gap energy. Our calculations reveal that the proximity of the quasi-Fermi levels to the energy bands in cells based on highly crystallized μc-Si:H (assumed to have a lower band gap), results in both higher free and trapped carrier densities. The trapped hole population, that is particularly high near the P/I interface, results in a strong interface field, a collapse of the field in the volume, and hence a lower open-circuit voltage. Interestingly enough, we were able to fabricate fluorinated μc-Si:H:F cells having 100% crystalline fraction as well as very large grains, that violate the general trend and show a higher Voc. Modeling indicates that this is possible for the latter case, as also for a crystalline silicon PN cell, in spite of a sharply reduced band gap, because the lower effective density of states at the band edges and a sharply reduced gap defect density overcome the effect of the lower band gap