Unveiling Energy Loss Mechanisms to Empower Ternary Organic Solar Cells with over 20% Efficiency: A Systematic Oligomeric Approach

Abstract

Abstract In organic solar cells (OSCs), the ternary strategy is a mainstream approach to obtaining highly efficient OSCs. A deeper understanding of working mechanisms and the material selection criteria for boosting open‐circuit voltage ( V OC ) is essential for further OSC breakthrough. Through a modular design principle, a series of oligomeric donors – 5BDD, 5BDD‐F, 5BDT‐F, and 5BDT‐Cl – with similar molecular configurations but varying HOMO levels is systematically designed. These findings reveal that the HOMO levels of these oligomers have a negligible impact on the V OC of the ternary OSCs. Instead, their excellent compatibility with acceptors played a pivotal role in enhancing V OC . The oligomers effectively suppressed excessive acceptor aggregation and achieved Aggregation‐Caused Quenching Suppression (ACQS), strengthening the external electroluminescence quantum efficiency (EQE EL ) and reducing non‐radiative recombination energy losses. Simultaneously, oligomers fine‐tuned and optimized the morphology of the blend films, leading to a higher fill factor (FF) and improved performance. Notably, the 5BDT‐F‐ and 5BDT‐Cl‐based ternary OSCs achieved impressive power conversion efficiencies (PCEs) of 19.8% and 20.1% (certified 19.76%), with FFs of 80.9% and 80.7%, respectively. This work elucidates the unusual role of the third component energy levels on the V OC in ternary OSCs and offers valuable guidance for future OSC design.