Probing Crystallization Effects when Processing Bulk-Heterojunction Active Layers: Comparing Fullerene and Nonfullerene Acceptors

Control of polymer aggregation by selective introduction of backbone disorder via random copolymerization has proven to be effective for optimizing the coating of fullerene-based bulk-heterojunction photovoltaics based on the strongly aggregating donor polymer PffBT4T-2OD. In this report, we apply the approach to a nonfullerene acceptor: IDTBR. We determine that a new terpolymer composition, PffBT4T80-co-3T20-2OD (80–20), is well behaved with respect to its processability and solid-state performance and is the best candidate for processing IDTBR blend films without using an excessively hot stage and/or processing solution during blade coating, achieving power conversion efficiencies (PCEs) approaching 9%. However, the PCEs of IDTBR devices are lower than those of PC71BM devices because of the reduced fill factor, which we attribute to IDTBR’s tendency to disrupt polymer crystallization during processing. Using real-time characterization methods, we discover that both acceptors affect the kinetics and extent of polymer crystallization during processing. Although the fullerene blends have increased polymer crystallinity, IDTBR delays the onset of polymer solidification, and the overall crystallinity in the blend of those otherwise ordered materials is compromised. Delayed polymer solidification with IDTBR suggests that the latter partially solubilizes the polymer, which does not form a favorable morphology for optimum photovoltaic performance when processing via spin or blade coating.