Asymmetric small-molecule acceptor enables suppressed electron-vibration coupling and minimized driving force for organic solar cells

Minimizing the energy loss, particularly the non-radiative energy loss (ΔE_nr), without sacrificing the charge collection efficiency, is the key to further improve the photovoltaic performance of organic solar cells (OSCs). Herein, we proposed an asymmetric molecular design strategy, via developing alkyl/thienyl hybrid side chain based asymmetric small molecule acceptors (SMAs) BTP-C11-TBO and BTP-BO-TBO, to manipulate the intermolecular interactions to realize enhanced luminescence efficiency and reduced energy loss. Theoretical and experimental results indicate that compared to the three symmetric SMAs BTP-DC11, BTP-DTBO and BTP-DBO, the asymmetric SMAs BTP-C11-TBO and BTP-BO-TBO exhibit repressed electron-vibration coupling and reduced ΔE_nr. Moreover, the asymmetric nature of BTP-BO-TBO allows the formation of multiple D:A interfacial conformations and interfacial energies, which have made the charge-transfer state energies closer to that of the strongly absorbing (and emitting) local-exciton state, thus gaining the low ΔE_nr while maintaining efficient exciton dissociation. Consequently, the PM6:BTP-BO-TBO-based OSCs achieve a higher power conversion efficiency of 19.76%, with a high open circuit voltage of 0.913 V and an efficient fill factor of 81.17%, profiting from the more improved and balanced charge mobility and longer carrier lifetime. This work provides molecular design ideas to suppress nonradiative decay and paves the way to obtain high-performance OSCs.