Side-chain engineering of isoindigo-containing conjugated polymers using polystyrene for high-performance bulk heterojunction solar cells

Developing organic photovoltaic systems that possess high efficiency, high reproducibility, and low cost remains a topic of keen investigation. From a molecular design perspective, developing a "multicomponent" copolymerization synthetic approach could potentially afford macromolecular materials encompassing all of the aforementioned desired parameters. Herein, we describe the synthesis of a series of poly(isoindigo-dithiophene)-based conjugated polymers with varying amounts of low molecular weight polystyrene (PS) side chains (M_n = 1300 g/mol) via random copolymerization. We observed better solubility with polymers containing the PS side chains (when compared to their non-PS-side-chain counterparts), hence leading to better batch-to-batch reproducibility in terms of molecular weights. Furthermore, the PS-side-chain-decorated copolymers also demonstrated better thin film processability, without affecting the electronic and optical properties, when the molar percentage of the PS-containing repeating units were ≤10%. Bulk heterojunction solar cell devices fabricated with these PS-containing copolymers demonstrated significantly improved performances [maximum power conversion efficiencies (PCE) > 7% and open circuit voltages (V_OC) ≥ 0.95 V], compared to the highest reported performance (PCE = 6.3% and V_OC = 0.70) based on similar isoindigo-containing polymers. Taken together, the synthesis, processing, and device performances of PS-containing copolymers represent a new approach in molecular engineering to achieve a balance between the optical/electronic properties and solubility/processability of reproducible polymeric systems.