Fill Factor Limitation of Silicon Heterojunction Solar Cells by Junction Recombination

Abstract

In this work we experimentally investigate the influence of the chemical and field effect passivation of a-Si:H films on the junction recombination at maximum power point conditions of silicon heterojunction solar cells. By simulating the charge of doped a-Si:H films with external corona charges, chemical and field effect passivation are investigated independent of each other. If we apply a negative corona charge to a lifetime test structure inversion conditions are obtained which corresponds to a p/n junction. With positive charges we can create accumulation i.e. a high/low junction (for an n-type wafer). From injection level dependent lifetime measurements the implied solar cell parameters iVoc and iFF are extracted. Especially the implied fill factor is influenced by the recombination characteristics at the injection level at maximum power point conditions. It is shown that under inversion conditions the implied solar cell parameters are lower compared to accumulation conditions. The main reason for this seems to be the asymmetry in defect capture cross sections for electrons and holes. As this defect characteristic at the c-Si/a-Si:H interface cannot be altered, a considerable reduction of the defect density is required to further decrease junction recombination at M PP conditions and thus increase the FF of SHJ cells.