Voltage dependent photocurrent collection in CdTe/CdS solar cells

The voltage dependence of the photocurrent J(L)(V) of CdTe/CdS solar cells has been characterized by separating the forward current from the photocurrent at several illumination intensities. J(L)(V) reduces the fill factor (FF) of typical cells by 1015 points, the open circuit voltage (V-OC) by 20-50 mV, and the efficiency by 2-4 points. Eliminating the effect of JL(V) establishes superposition between light and dark J(V) curves for some cells. Two models for voltage dependent collection give reasonable fits to the data: (1) a single carrier Hecht model developed for drift collection in p-i-n solar cells in which fitting yields a parameter consistent with lifetimes of 10(-9) s as measured by others; or (2) the standard depletion region and bulk diffusion length model fits almost as well. The simple Hecht-like drift collection model for photocurrent gives very good agreement to J(V) curves measured under AM1.5 light on CdTe/CdS solar cells with FF from 53% to 70%, CdTe thickness from 1.8 to 7.0 mu m, in initial and stressed states. Accelerated thermal and bias stressing increases JL(V) losses as does insufficient Cu. This method provides a new metric for tracking device performance, characterizes transport in the high field depletion region, and quantifies a significant FF loss in CdTe solar cells. Copyright (C) 2007 John Wiley & Sons, Ltd.