Achieving Near 21% Efficiency Organic Solar Cells through Controlled Packing of Fused-Ring Electron Acceptors.

Non-radiative energy loss remains a challenge in organic solar cells. Here we show that introducing bulky chlorobenzene moieties at the end of inner side chains yields two acceptor materials, Y6-2Cl and Y6-4Cl. The introduction of chlorobenzene groups enhances the solubility, while also boosting the photoluminescence quantum yield and inducing a distinct molecular packing mode in Y6-2Cl and Y6-4Cl. Rather than following the conventional three-dimensional network structure observed in Y-series acceptors, these materials form a highly ordered, array-like arrangement while maintaining excellent crystalline characteristics. This unique structural feature not only preserves high charge-transport properties but also optimizes aggregation behavior and morphology of the active layers. The D18:Y6-2Cl binary device demonstrates a PCE of 20.0%. The ternary device composed of D18:L8-BO:Y6-2Cl achieves a PCE of 20.92%, accompanied by a fill factor of 82.93%. Overall, the side-chain functionalization strategy proposed in this study offers a direction for developing high-performance non-fullerene acceptors.