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 2nd and 3rd level metrics is discussed. The overall behavior of aggregated data showing how cell efficiency changes as a function of open-circuit voltage (Voc), short-circuit current density (Jsc), 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 Jsc as well as increasing rates of Voc degradation are related to voltage-dependent collection effects and catastrophic shunting respectively. Large decreases in Voc in the absence of catastrophic shunting are attributed to increased recombination. The relevance of capacitance-derived data correlated with both Voc and FF is discussed.