19.3% Efficiency ternary organic solar cells enabled by the alkyl side-chain effect of guest non-fullerene acceptors

The effect of side-chain engineering of conjugated molecules on the morphology and device performance in binary organic solar cells has been widely investigated. However, this relationship has hardly been studied in the guest components of ternary organic solar cells. In this study, a family of non-fullerene guest acceptors, namely XY-3, XY-5 and XY-7, with hydrogen substituent, straight and branched alkyl chains on the bithiophene units, respectively, were designed and synthesized to understand their effects on aggregation properties and device performance. The straight and branched alkyl chains on the bithiophene units result in sightly blue-shifted absorption compared to the hydrogen substituent and the XY-7 demonstrates the most appropriate phase separation scale and the most balanced charge transport. Consequently, the OSCs based on D18:eC9:XY-7 achieve a high short-circuit current density (J_SC) and fill factor (FF), while maintaining the enhancement of the open-circuit voltage (V_OC) achieving an efficiency of 19.32%, exceeding those of D18:eC9, D18:eC9:XY-3, D18:eC9:XY-5 (PCE:18.28%, 19.04%, 18.75%, respectively). These results highlight that the side-chain engineering of Y series non-fullerene acceptors as the guest acceptors has great potential in optimizing morphology properties and promoting photovoltaic performance.