Don’t fluorinate there! The Impact of Fluorination Position on Polymer Photostability and its Effect on Photovoltaic Device Stability

Abstract

Despite the excellent progress in organic photovoltaic (OPV) efficiencies, they still suffer from poor operational stability. This is especially true in ambient conditions, with degradation often driven by intrinsic material instabilities. Fluorination of the constituent organic semiconductors, which deepens frontier orbitals and improves organic semiconducting packing, is often utilised to improve photostability. Here, fluorinated analogues of the high-performance workhorse polymer PBDB-T are synthesised and their photostability characterised. Device stability, with both Y6 and IT-4F, is found to be critically dependent on the intrinsic polymer photostability, with fluorination of the benzodithiophene (BDT) being particularly ruinous for stability. The carbonyl group in these unstable BDT-fluorinated analogues is found to be highly unstable towards illumination. This instability arises from a disruption of non-covalent interactions along the polymer backbone, where the fluorine on BDT interferes with intramolecular S-O interactions between the thiophene and benzodithiophene-dione (BDD) carbonyl. This leads to increased backbone disorder and a more vulnerable carbonyl environment. These results challenge the conventional belief that fluorination universally improves OPV stability and underscore the crucial role of non-covalent interactions in governing material stability.