Accelerated Stress Testing and Diagnostic Analysis of Degradation in CdTe Solar Cells

Solar cell module reliability is inextricably linked to cell-level reliability. This is particularly so with thin-film technologies. In CdTe, reliability issues historically associate with back contact stability and the use of Cu as an extrinsic dopant. Using a simple approach by which identical cells are heated under open-circuit bias and I-sun illumination, degradation activation energies of 0.63 and 2.94 eV in laboratory-scale CdS/CdTe devices were identified in the accelerated stress temperature range of 60 to 120 degrees C. At lower stress temperatures, cell performance changes were linearly correlated with changes in both fill factor (FF) and short-circuit current (J(sc)). At higher stress temperatures, changes in efficiency were correlated with changes in FF and open-circuit voltage (V-oc). The measured activation energy of 0.63 is associated with Cu-diffusion. During the early stage of stress testing, which may provide additional back contact annealing, improvements in FF were due to Cu-diffusion. Decreased performance observed at longer stress times (decreased FF and V-oc), according to a two-diode Pspice model, were due to both increased space-charge recombination (near the junction) and decreased recombination in the bulk. Kirkendall void formation (S-outdiffusion) at the CdS/CdTe interface is given as responsible for the 2.9 eV degradation mechanism.