Dependence of recombination in protocrystalline a-Si:H films and cells on their different light induced gap states

The light induced gap states with the key role in the instability of hydrogen diluted protocrystalline a-Si:H, similar to that currently used in high performance cells, have been identified and found not to be the midgap states. Such states, which have been associated with the creation of dangling bonds, have for a long time been considered to be solely responsible for the Staebler-Wronski Effect and carrier recombination in a-Si:H films and cells. These new insights have been achieved by direct correlation of photoconductive carrier recombination in films with the Shockley Reed Hall carrier recombination in the corresponding solar cells under forward bias dark current. These results were then further correlated with subgap absorption results on films with the presence of multiple gap states being taken into account. Three Gaussian distributions of states A, B and C have been identified, with A 0.05 eV above midgap and B 0.095 eV and C 0.39 eV below midgap with distinctly different creation kinetics. It is found that the midgap states are efficient electron recombination centers, with A being the “fast” states. The C states on the other hand are very efficient hole recombination centers and are thus key to the instability of the 1 sun Fill Factors. The energy positions of the gap state distributions are consistent with those for not fully hydrogen passivated divacancies thus offering new insights into identifying the possible mechanisms for the creation of light induced defects and their possible control.