Preserving Polymorphic Crystalline Structures in Bulk Heterojunctions for Efficient Organic Solar Cells

ABSTRACT The crystal structure of a material profoundly influences its physical, optical, and electronic properties. However, precise control over polymorph formation of non‐fullerene acceptors (NFAs) in bulk heterojunction active layers remains challenging for optimizing organic solar cell (OSC) performance. Here, we employ a non‐halogenated mixed‐solvent system comprising carbon disulfide (CS 2 ) and o‐xylene (OXY) to selectively regulate the crystallization of the high‐performance acceptor BTP‐eC9. A specific polymorph (Form II) is preferentially induced and consistently identified in powder, neat‐film, and bulk heterojunction samples by correlating wide‐angle x‐ray scattering (WAXS) and grazing‐incidence wide‐angle x‐ray scattering (GIWAXS) patterns with simulations derived from the corresponding single‐crystal structures. Compared with Form I, Form II exhibits improved optoelectronic properties, including extended exciton diffusion length, balanced charge mobility, and suppressed non‐radiative recombination. Consequently, Form II‐based devices achieve a power conversion efficiency (PCE) of 20.61% with a fill factor of 82.85%, representing a 27.0% relative improvement over Form I devices (PCE = 16.23%). Moreover, this method enables scalable manufacturing, with flexible OSCs (1 cm 2 active area) fabricated by slot‐die coating achieving a PCE of 17.93%. This work establishes a practical pathway for solution‐processed polymorph control and reveals the relationship among single‐crystal packing, acceptor molecular packing, and device performance.