Energy level modulation of ITIC derivatives: Effects on the photodegradation of conventional and inverted organic solar cells

Next-generation organic photovoltaic technology is currently geared towards non-fullerene organic solar cells. Among the non-fullerene small molecule acceptors, ITIC derivatives play a central role with power conversion efficiency above 15% in single junction cells. However, knowledge about the stability of these new types of devices is lagging behind, creating an efficiency-lifetime gap for commercial viability. Here, we study the photostability of three ITIC derivatives, namely ITIC together with IT-M and IT-F, representative of methylated and halogenated ITIC, the usual modification to this small molecule acceptor. While the best performing solar cell yields a PCE of 8.6%, we find that the photostability of the devices greatly depends on the structure of the acceptor and the configuration of the devices. The methylation of ITIC into IT-M improves V_oc but does not greatly affect the photostability of the devices. While fluorination generally decreases the efficiency of the cells, the stability of the fluorinated ITIC-based cells appears to depend on the device structure. Thus, the change from ITIC to IT-F may not in all cases necessarily be beneficial to the advancement of the technology. Subtle changes in molecular structure coupled with imbalance charge mobilities is at the origin of the observed differences in degradation.