Low‐Volatility Fused‐Ring Solid Additive Engineering for Synergistically Elongating Exciton Lifetime and Mitigating Trap Density Toward Organic Solar Cells of 20.5% Efficiency

Abstract Volatile solid additives (VSAs) with single or fused‐ring structures have attracted much attention for enhancing power conversion efficiencies (PCEs) of organic solar cells (OSCs). While the working mechanisms of high‐volatility single‐ring additives have been well studied, the influence of low‐volatility fused‐ring VSAs on molecular aggregations and exciton/carrier dynamics remains still unclear. Herein, 3,6‐dibromothieno[3,2‐b]thiophene (3,6TTBr) is selected as a representative low‐volatility fused‐ring VSA to elucidate its working mechanism. Via the theoretical and experimental joint investigation, it is found that rigid and planar 3,6TTBr molecules adsorb onto the terminal units of L8‐BO (acceptor), inducing loose space for adjacent molecules. The low‐volatility 3,6TTBr thus favors the L8‐BO center‐terminal packing with a larger interfragment distance, which relieves the L8‐BO over‐aggregation and induces the ordered packing. Consequently, the 3,6TTBr treatment reduces aggregation‐caused quenching, enhancing the photoluminescence quantum yield and exciton lifetime of L8‐BO film. The combination of the above properties with the reduced trap density and improved carrier transport in the 3,6TTBr‐treated devices contributed to PCE of 20.1%. To validate the broad applicability of the findings, 1,5‐dibromonaphthalene (1,5‐BN), another low‐volatility fused‐ring solid, is explored. The devices with 1,5‐BN achieved an impressive PCE of 20.5%, verifying the validity of the low‐volatility fused‐ring VSA strategy for boosting OSC performances.