Direct analysis of the current-voltage curves of outdoor-degrading modules

We apply a phenomenological four-parameter equation to fit and analyze regularly measured current density-voltage JV curves of various technologies. The physically meaningful four parameters, short-circuit current density J_sc, open-circuit voltage V_oc, and differential resistances Rsc and Roc are determined for one-week data intervals of an exemplarily chosen CdTe module during 2.5 years of outdoor operation. For the chosen thin-film module, the fill factor FF degradation outweighs the degradation of J_sc and V_oc. Interestingly, the V_oc vs. log(J_sc) curves of the module at low irradiation unveil an increasing influence of a double-exponential diode behavior. The data hint at an increasing voltage-dependent charge-carrier collection in CdTe.