Low‐Order Molecular Packing of Y‐Series Acceptors for High‐Efficiency Organic Photovoltaics

ABSTRACT Ordered molecular packing is considered essential for optimizing organic optoelectronic properties. However, despite disordered morphologies with nanoscale phase‐separated structure, bulk heterojunction (BHJ) organic photovoltaics (OPVs) have recently achieved remarkable efficiencies exceeding 20% with Y6‐based non‐fullerene acceptors (NFAs). To elucidate the underlying mechanism and assess whether disorder imposes an intrinsic limitation to OPVs, we systematically investigate the impact of molecular packing order on the critical photon‐to‐electron conversion processes, i.e. the carrier recombination, photo‐current generation, and charge transport. Using the droplet‐pinned crystallization method, we fabricated high‐order ( ho ) thin films and compared their carrier dynamics with conventional low‐order ( lo ) films. Surprisingly, the lo ‐L8‐BO films exhibited suppressed non‐radiative charge recombination, and longer excited‐state lifetimes, attributed to reduced electron‐vibration coupling. These effects simultaneously contribute to a higher open‐circuit voltage ( V OC ) and longer exciton diffusion length. Building on these new insights, further optimization of NFA side chains minimizes V OC losses and enables an excellent efficiency of 21.0% (20.6% certified). Remarkably, device with an active area exceeding 0.5 cm 2 achieves certified efficiency greater than 20%, representing the highest certified value reported to date at this area. This work elucidates the influence of molecular packing on OPV performance and offers practical guidelines for future molecule design.