Analytic description of the impact of grain boundaries on Voc

Abstract

The impact of grain boundaries on the performance of polycrsytalline photovoltaics remains an open question. We present a simplified description of dark grain boundary recombination current. The dark current takes the form of a diode equation, and the model provides closed form expressions for the reverse saturation current and ideality factor in terms of grain boundary and system parameters. This model applies under conditions relevant for thin film photovoltaics such as CdTe, namely for p-type absorbers with reasonably high bulk hole mobility, positively charged grain boundaries with high defect density, and grains which are not fully depleted. The dark recombination current can be used to predict the open circuit voltage for a given short circuit density, providing a simple closed form expression which shows how grain boundaries impact $V_{\mathrm{o}\mathrm{c}}$ .