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

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.