Side-chain engineering for efficient non-fullerene polymer solar cells based on a wide-bandgap polymer donor

Abstract

In this work, a new wide-bandgap polymer, PSBZ, based on thienyl substituted benzodithiophene (BDTT) as the donor unit and difluorobenzotriazole (BTz-2F) as the acceptor unit was synthesized for photovoltaic applications. Compared to the analogous polymer J61 with linear dodecylthio side chains in the BDTT unit and a long 2-hexyldecyl side chain in BTz-2F, PSBZ possesses branched 2-butyloctyl side chains to increase steric hindrance of the BDTT unit and a short 2-butyloctyl side chain to decrease steric hindrance of the BTz-2F unit for more efficient charge separation and transport in the devices. As a result, PSBZ exhibited stronger π–π interaction and smaller stacking spacing leading to a higher extinction coefficient of 1.48 × 10⁵ cm⁻¹ and a high hole mobility of 8.56 × 10⁻³ cm² V⁻¹ s⁻¹. Compared to the analogous polymer J61 with a power conversion efficiency (PCE) of 9.53% and a short-circuit current density (J_sc) of 17.43 mA cm⁻², the PSBZ:ITIC-based polymer solar cells yielded a higher PCE of 10.5% with a higher J_sc of 19.0 mA cm⁻². The results show that our design strategy is successful for improving photovoltaic performance by side chain engineering.