Suppressing electron-phonon coupling in organic photovoltaics for high-efficiency power conversion

Reducing non-radiative energy loss is critical to improving power conversion efficiency in organic solar cells. Jiang et al. show that alkyl side-chain engineering in acceptors reduces the free volume ratio, lowering non-radiative decay, to achieve 18.6% efficiency in bulk-heterojunction binary cells. The nonradiative energy loss ( increment E-nr) is a critical factor to limit the efficiency of organic solar cells. Generally, strong electron-phonon coupling induced by molecular motion generates fast nonradiative decay and causes high increment E-nr. How to restrict molecular motion and achieve a low increment E-nr is a sticking point. Herein, the free volume ratio (FVR) is proposed as an indicator to evaluate molecular motion, providing new molecular design rationale to suppress nonradiative decay. Theoretical and experimental results indicate proper proliferation of alkyl side-chain can decrease FVR and restrict molecular motion, leading to reduced electron-phonon coupling while maintaining ideal nanomorphology. The reduced FVR and favorable morphology are simultaneously obtained in AQx-6 with pinpoint alkyl chain proliferation, achieving a high PCE of 18.6% with optimized V-OC, J(SC) and FF. Our study discovered aggregation-state regulation is of great importance to the reduction of electron-phonon coupling, which paves the way to high-efficiency OSCs.