Rhodanine Substitution of Asymmetric Nonfullerene Acceptors for High-Performance Organic Solar Cells

Asymmetric substitution is acknowledged as a straightforward yet potent approach for the optimization of small molecule acceptors (SMAs), thereby enhancing the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we have successfully engineered and synthesized a novel asymmetric SMA, designated as Y6-R, which features a rhodanine-terminated inner side-chain. In devices with PM6 as the polymer donor, the asymmetric Y6-R demonstrated an impressive PCE of 18.62% with an open-circuit voltage (V_oc) of 0.863 V, a short-circuit current (J_sc) of 27.89 mA cm^-2, and a fill factor (FF) of 77.35%, which is much higher than that of the symmetric SMA Y6-based devices (16.84%). The superior performance for PM6:Y6-R devices can be attributed to a combination of factors, including the upshifted LUMO energy levels, more desired exciton dissociation, collection, and extraction capability, as well as reduced recombination and suppressed E_loss. In addition, the Y6-R molecules promote the more desired coaggregation behaviors along with the donor, endowing the stronger and more ordered crystallinity of blend films. Our findings underscore the effectiveness of rhodanine-substitution, as a representative of "A" units, of the asymmetric SMAs in fine-tuning the coaggregation behavior with the donor, thereby enhancing crystallinity and efficiency.