Allylrhodanine-processed all-small-molecule organic solar cell achieves an 18.43% efficiency breakthrough

Solid additives (SAs) improve organic solar cells (OSCs) morphology and performance, yet, their efficiency in all-small-molecule (ASM) OSCs is limited by strong donor-acceptor crystallinity, and the underlying mechanisms remain unclear. Herein, structurally resembling the MPhS-C2 donor, 3-allylrhodanine (3-AR) as a SA enables precise control over the active layer morphology. Interesting, combined theoretical calculations and thin-film morphology analyses reveal 3-AR preferentially interacts with acceptor N3, thereby directing morphology evolution. Moreover, 3-AR effectively narrows the film formation window, suppressing lateral over-aggregation and crystallization, while promoting vertical donor–acceptor homogeneity. This optimized morphology synergistically facilitates efficient charge dissociation and collection, suppresses both bimolecular and trap-assisted recombination, and extends the lifetimes of excitons and charge carriers. Therefore, the 3-AR-treated MPhS-C2:N3 device achieves a remarkable power conversion efficiency (PCE) of 18.43% (certified 18.16%)—the highest reported for binary ASM-OSCs. This work shows additive efficacy is governed by balanced selective interactions, not by simplistic structural similarity. Solid additives are typically used for improving morphology and efficiency in organic solar cells (OSC). Here, the authors present 3-AR, which optimizes the active layer morphology in all-small-molecule OSCs through selective interactions, achieving a PCE of 18.43% (certified 18.16%).