Strategic fluorination of polymers and fullerenes improves photostability of organic photovoltaic blends

The photobleaching dynamics of a series of three organic photovoltaic (OPV) donor polymer blends with five different fullerenes are presented. The fullerenes studied include PC60BM and four perfluoroalkylfullerenes with relatively large electron affinities, namely C-60(CF3)(2), C-60(i-C3F7)(2), C-60(CF3)4(,) and C-60(CF3)(8). The donor polymers were all based on cyclopentadithiophene (CPDT) and thienopyrrolodione (TPD), but the TPD side chains were designed to include alkyl, partially fluorinated alkyl, and fluorinated phenyl groups to improve miscibility of the active layer components. Exciton harvesting was probed with photoluminescence quenching measurements. Accelerated photodegradation studies of polymer:fullerene blends were then carried out under white light illumination at similar to 1.2 suns in air. A strong correlation was observed between the polymer donor photobleaching rate and the electron affinity of the fullerene. The most dramatic effect was observed for a blend of C-60(CF3)(8) with the donor containing fluorinated phenyl groups: the blend required 150 times the dosing of photons to bleach to 80% of its initial optical density than an analagous blend of PC60BM and non-fluorinated donor polymer. These results ultimately suggest that appropriate fluorination strategies applied to both the donor and acceptor can be a viable route toward a new paradigm of intrinsically photo- and phase-stable OPV active layers.