The effect of alkylthio substituents on the photovoltaic properties of conjugated polymers

Alkylthio groups are effectively utilized in molecule design to improve the performance of polymer solar cells (PSCs). In this study, two conjugated polymers, P1 and P2, were designed and synthesized with alkyl and alkylthio side chains substituted on thiophene as π bridges, respectively. Owing to the twist of the backbone induced by the steric hindrance of the hexyl side chains, polymer P1 shows a low power conversion efficiency (PCE) of 2.83% in PSC devices with PC71BM as acceptor and a low hole mobility of 1.85 × 10−5 cm2 V−1 s−1 in the blend film. In contrast to the steric hindrance of the alkyl side chain, the alkylthio side chain can form S⋯S non-covalent interaction with an adjacent thiophene to maintain the molecular planarity and strengthen intermolecular interaction, which is designed in polymer P2 to improve the photovoltaic performance. As a result, the P2-based devices exhibit a higher PCE of 6.15% with an open-circuit voltage (VOC) of 0.71 V, a short-circuit current (JSC) of 14.55 mA cm−2 and a fill factor (FF) of 59.5%. The hole mobility is also increased to 2.20 × 10−4 cm2 V−1 s−1. Our results demonstrate that introducing S⋯S non-covalent interaction into conjugated polymers could be a useful strategy for building high performance photovoltaic materials.