Modeling the degradation/recovery of open-circuit voltage in perovskite and thin film solar cells

Developing theories behind the degradation/recovery of polycrystalline perovskite and thin film solar cells is essential in promoting the stability of such devices under stress conditions. Here we propose a novel approach to model the variation of open-circuit voltage by time, V_oc(t) for perovskite, CdTe, CIGS and CZTS-based solar cells. Several time-dependent equations have been derived and fitted with the experimental data on degradation/recovery of V_oc under stress of light, bias, moisture and temperature. Any stress condition will change the defect density across the absorber layer of a solar cell. This will in turn effect on the saturation current density, depletion width and finally on the V_oc of a device. A good fit has been obtained between these proposed models and the data reported in the literature on degradation rate of V_oc. In some cases more than one model is required to fit with the data as urges by the non-crystalline nature of perovskite and thin film materials and specific influence of every stress condition (i.e. prolonged irradiation, elevated temperature or air humidity) on materials properties and defect profile across the device. Therefore, the distribution of defects may change by time depend on the intensity, type of the stress or recovery process. Different than previous believes, we showed that V_oc, as a measure of recombination rate and carrier collection efficiency of a device, may follow 8 different time-dependent models.