Mitigating Nonradiative Recombination Loss Using Alkoxy‐Substituted Third‐Component Acceptor for 20.1%‐Efficiency Organic Solar Cell

Abstract Organic solar cells (OSCs) offer an efficient solution for renewable energy applications. However, nonradiative recombination loss (Δ E non‐rad ) remains a key critical issue limiting further improvements in their efficiency. Herein, OBO‐4F, derived from a highly efficient L8‐BO system, is proposed. By replacing the alkyl chain at the β ‐position of the thienothiophene unit with an alkoxy chain, most of the original physicochemical properties of L8‐BO are retained in OBO‐4F, with a simultaneous modulation of the molecular packing. The introduction of OBO‐4F as the third component into the D18/L8‐BO binary system yields a diluted‐ternary device (D18+OBO‐4F/L8‐BO+OBO‐4F), achieving a power conversion efficiency (PCE) of 20.1%, outperforming previously developed binary systems. Notably, compared to the D18/L8‐BO binary counterpart, Δ E non‐rad is considerably reduced in diluted‐ternary devices due to the suppression of exciton–vibration coupling, leading to its high open‐circuit voltage of 0.927 V. Theoretical and experimental analyses demonstrate that the addition of OBO‐4F into L8‐BO enhances molecular packing and reduces reorganization energy, thereby increasing the exciton diffusion length and promoting efficient charge transport. This study demonstrates that using a rational third‐component molecular design, Δ E non‐rad can be effectively mitigated while further augmenting already exceptional PCE.