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

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.