Crucial Role of the Electron Transport Layer and UV Light on the Open-Circuit Voltage Loss in Inverted Organic Solar Cells

Understanding the degradation mechanisms in organic photovoltaics is crucial in order to develop stable organic semiconductors and robust device architectures. The rapid loss of efficiency, referred to as burn-in, is a major issue to be addressed. This study reports on the influence of the electron transport layer (ETLs) and UV light on the drop-of open-circuit voltage (V-oc) for P3HT:PC60 BM-based devices. The results show that V-oc loss is induced by the UV and, more importantly, that the ETL can amplify it, with TiOx yielding a stronger drop than ZnO. Using impedance spectroscopy (IS) and X-ray photoelectron spectroscopy (XPS), different degradation mechanisms were identified according to whether the ETL is TiOx or ZnO. For TiOx-based devices, the formation of an interface dipole was identified, resulting, in a loss of the flat-band potential (V-oc,) and, thus, of the V-oc . For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer which impacts the V-oc. This study highlights the role of the architecture and, more specifically, of the ETL in the severity of burn-in and degradation pathways.