3D Conjugated Nonflat Biphenyl Side Chains: Their Exclusive Role in Inducing Negative Electrostatic Potential in Efficient Organic Solar Cells

Abstract In nonfullerene acceptor (NFA) (so‐called Y‐series)‐based organic solar cells (OSCs), reducing energy loss, particularly nonradiative energy loss, is critical for achieving high power conversion efficiency (PCE). However, at present, molecular design strategies for controlling and/or optimizing energy loss involve multiple complexities. Therefore, this study demonstrates that the introduction of simple, 3D conjugated nonflat biphenyl side chains into a Y‐series NFA (yielding BPY) can improve high dipole moments‐induced intermolecular interactions and negatively charged surface electrostatic potential driven by partially isolated negative charges. When BPY is blended with a PM6 donor, the resulting BPY‐based OSC exceeds that of the control Y6‐based OSC. This result is attributed to the minimized nonradiative energy loss via weakened π–π stacking and efficient charge dynamics. The addition of BPY to PM6:BTP‐eC9 blends yields ternary OSCs that exhibit a high PCE of 19.23% owing to significant improvements in both the open‐circuit voltage and fill factor. These findings not only highlight the influence of biphenyl side chains on the molecular properties of NFAs but also provide an effective material avenue to optimize nonradiative energy loss for high‐performance OSCs.