Degradation of CdTe Solar Cells: Simulation and Experiment

Time-dependent numerical modeling is employed in conjunction with experimental data to investigate degradation mechanisms in cadmium telluride (CdTe) solar cells. Two mechanisms are tested against the data: 1) defect generation in the junction region caused by excess charge carriers and reactant defects and 2) back barrier increase. Junction effects result in stable J_sc with significant losses in V_oc and FF, in accordance with typical data for the type of light-soak stress conditions considered here. The back barrier increase causes additional FF loss. The results suggest that both mid-gap recombination centers and shallow acceptor-type defects form near the main junction as degradation proceeds. Our data reaffirm that the inclusion of copper in the back contact is associated with better initial performance and more rapid degradation. Correlations were observed between degradation rates and apparent doping hysteresis obtained from bidirectional capacitance-voltage (C-V) scans. Our time-resolved photoluminescence (PL) results show no correlation between light-soak-induced V_oc loss and PL lifetime.