Regulating H‐/J‐Aggregation and Suppressing Exciton‐Vibration Coupling of Fused Ring Electron Acceptors Enable 20.51% Efficiency of Organic Solar Cells

Abstract The limited exciton diffusion length ( L D ) of organic photovoltaic materials puts organic solar cells (OSCs) into a harsh position to compromise exciton diffusion and charge carrier transport, impeding the further improvement of power conversion efficiencies (PCEs). The exciton behavior of organic semiconductors is strongly related to the molecular alignment. Herein, an analogue of L8‐BO, L8‐OPh with benzyl‐alkyl inner side chains is developed to blend with L8‐BO and successfully regulate its molecular alignment. Incorporating L8‐OPh into L8‐BO enhances the H‐aggregation tendency, weakens exciton‐vibration coupling, and enforces the intermolecular interaction of L8‐BO:L8‐OPh blend films. Such endows the L8‐BO:L8‐OPh blend film with improved exciton lifetime and larger exciton diffusion coefficient to prolong the L D up to 22.1 nm, compared to L8‐BO (16.5 nm). Besides, L8‐BO:L8‐OPh blend films acquire increased crystallinity and better crystal quality to afford lower trap density, prolonged charge carrier lifetime and higher mobilities in D18:L8‐BO:L8‐OPh ternary devices. Therefore, D18:L8‐BO:L8‐OPh ternary OSCs achieve the remarkable PCE of 20.51% (certified as 20.43%), which is among the highest PCEs in OSCs so far. It highlights the importance of finely regulating molecular alignment of organic photovoltaic materials to prolong the L D and boost PCEs of OSCs.