Structure dependent photostability of ITIC and ITIC-4F

Strong synthetic and engineering efforts have taken the efficiency of non-fullerene acceptor (NFA) based organic solar cells above 18% in a few years. Nonetheless, a deep understanding of the fundamental properties of this class of molecules is still missing. Here, we systematically investigated the morphological properties of two high efficient indacenodithienothiophene-based NFAs-namely ITIC and ITIC-4F-in order to correlate the hydrogen/fluorination substitutions with the materials structural and stability properties. We confirm that each NFA structurally evolves with increasing temperature into several polymorphs, identifying through spectroscopy their corresponding narrow temperature ranges. We demonstrate that the materials' response to accelerated stress tests (ASTs) is both substitution and polymorph dependent. ASTs underlined that the most vulnerable molecular segment corresponds to the thienothiophene CC bond along the central backbone, together with the CC linkage between the electron-rich donor and the electron-deficient acceptor moieties, with a degradation process triggered by oxygen and light. ITIC-4F showed lower oxidation capability and a higher bond strength retaining effect compared to ITIC. Lastly, the AST approach employed here allowed for the extrapolation of morphological and stability-related features within a high-throughput framework, and can be considered as a valuable methodological tool for future stability-related studies.