Insights into the relationship between molecular and order-dependent photostability of ITIC derivatives for the production of photochemically stable blends

Abstract

We present a systematic study of the intrinsic photostability of ITIC derivative acceptors, namely ITIC, ITIC-Th and ITIC-4F, in solution, in layers and in blends with donor polymers such as PBDB-T (also known as PCE12) or its fluorinated derivative PBDB-T-2F (known as PM6). We followed the evolution of the absorption spectra of ITIC derivative acceptors and blends under constant irradiation using different light sources (AM1.5 solar simulator, SUNTest or LED). Any interaction with oxygen under illumination was avoided by placing the solutions and thin films under inert conditions, i.e. nitrogen or vacuum. While donor polymers are highly photostable, all three NFAs show a common photodegradation process with the formation of photoproducts and molecular structure modifications under light exposure. Compared to the solutions where the degradation kinetics are very fast, the degradation in films is significantly slowed down. The corresponding photodegradation process including a cis–trans isomerisation of end groups is found to be common for all ITIC derivatives and independent of the light source. In blends, photodegradation is directly attributable to the acceptor, but varies according to the derivative. In fact, ITIC-4F based blends will benefit from a stabilizing mechanism due to a favorable molecular packing inside the blend. Thereafter, we have studied the photostability of blend layers post-annealed at different temperatures, and we show that the thermally induced packing and crystallinity of the ITIC derivatives dominate the photostability of the blends. We finally investigate the photostability of organic solar cells based on thermally annealed PM6:ITIC-4F blend layers.