"Side-Chain Engineering of Isoindigo-Containing Conjugated Polymers Using Polystyrene for High-Performance Bulk Heterojunction Solar Cells"

Lei Fang, Yan Zhou: Yu-Xing Yao, Ying Diao, Wen-Ya Lee, Anthony L. Appleton, Ranulfo Allen, Julia Reinspach, Stefan C. B. Mannsfeld, and Zhenan Bao; Chemistry of Materials, 12/03/13.

Additional Authors: Yu-Xing Yao, Ying Diao, Wen-Ya Lee, Anthony L. Appleton, Ranulfo Allen, Julia Reinspach, Stefan C. B. Mannsfeld, and Zhenan Bao

Abstract:

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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 (Mn = 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 (VOC) ≥ 0.95 V], compared to the highest reported performance (PCE = 6.3% and VOC = 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.