Electrical metastabilities in chalcogenide photovoltaic devices

Abstract

Electrical properties of thin film photovoltaic modules exhibit metastable behavior when exposed to light, when kept in the dark after light exposure and as a result of external bias. These effects vary in magnitude and expression and can mainly be observed in CdTe and chalcopyrites. The light-soaking induced effect in these technologies was analyzed for electrical module parameters such as open circuit voltage (Voc), short circuit current, fill factor (FF), maximum power point (Pmpp) and the current-voltage characteristic slope at Voc. The influence of the light-soaking irradiance and the duration of the illumination at constant temperature was investigated. Furthermore, light-soaking with additional external bias was examined. Overall, all modules showed metastable behavior with increases in the FF, the Voc and the Pmpp after light-soaking. The magnitude varied for different technologies and was largest for the investigated CdTe modules. Differences among the technologies could be observed for the change in Voc. While CdTe exhibited a significant increase in both FF and Voc, Cu(In,Ga)Se2 (CIGS) modules mainly showed an increase in the FF. Differences in metastable behavior were also observed for relaxation times in the dark. While CdTe modules returned to the initial levels in several hours, it took several weeks for the CIGS modules to relax. The presented experiments confirm that metastabilities reported on cell behavior also occur at the module level.