The use of 2nd and 3rd level correlation analysis for studying degradation in polycrystalline thin-film solar cells

The correlation of stress-induced changes in the performance of laboratory-made CdTe solar cells with various 2^nd and 3^rd level metrics is discussed. The overall behavior of aggregated data showing how cell efficiency changes as a function of open-circuit voltage (V_oc), short-circuit current density (J_sc), and fill factor (FF) is explained using a two-diode, PSpice model in which degradation is simulated by systematically changing model parameters. FF shows the highest correlation with performance during stress, and is subsequently shown to be most affected by shunt resistance, recombination and in some cases voltage-dependent collection. Large decreases in J_sc as well as increasing rates of V_oc degradation are related to voltage-dependent collection effects and catastrophic shunting respectively. Large decreases in V_oc in the absence of catastrophic shunting are attributed to increased recombination. The relevance of capacitance-derived data correlated with both V_oc and FF is discussed.