Understanding the role of interfacial layers in the photostability of PM6:Y7-based organic solar cells under different degradation conditions

Abstract

Organic solar cells (OSCs) have reached an efficiency near 20%; however, their low long-term stability is the main limitation to their industrialization. In this work, we investigated the degradation of bulk heterojunction non-fullerene solar cells (NFA-OSCs) based on PM6:Y7 with an efficiency of 17.5%. The degradation analysis was carried out following the established ISOS-D-1 protocol under different degradation conditions: N2 atmosphere (H2O < 0.1 ppm and O2 < 0.1 ppm) and encapsulated devices and non-encapsulated devices exposed to ambient conditions (60 ± 5% relative humidity). The evolution of the current density-voltage (J-V) and impedance spectroscopy (IS) measurements were used to analyse the degradation process during 1000 h and its relationship with physical mechanisms. The degradation of encapsulated and non-encapsulated devices is mainly caused by the drop in the open circuit voltage (VOC). For devices exposed to the N2 atmosphere, the fill factor (FF) was the most affected parameter. The dependence of short circuit current density (JSC) versus light intensity study reveals that the efficiency of non-encapsulated devices decreases faster due to a higher bimolecular recombination degree. The devices under a N2 atmosphere and those encapsulated showed T80 lifetimes of 1000 h and 336 h, respectively, whereas the non-encapsulated devices have a short T80 lifetime of less than 24 h. The analysis of the efficiency decay was used to identify the different degradation mechanisms (by diffused environmental water or oxygen or by intrinsic polymer chemical reactions) under different conditions. The degradation origin of the active layer and interlayers was investigated through impedance spectroscopy measurements.