Tuning Active Layer Morphology via Ternary Copolymerization with an Asymmetric Benzothiazole Unit as the Third Component to Enhance Photovoltaic Performance

Controlling the morphology and phase separation of the active layer is of great significance for the development of efficient organic solar cells. This study employs ternary copolymerization to optimize the phase separation and surface morphology of the active layer. Three terpolymers (PMz-5, PMz-10, and PMz-20) are synthesized by incorporating an asymmetric benzo[<i>d</i>]thiazole (BTz) unit as the third component to suppress the strong aggregation of the PM6:L8-BO blend film. Compared to the PM6 blend film, terpolymer blend films exhibit improved surface morphology, an appropriate phase separation scale, and, thus, an enhanced device performance. Furthermore, the incorporation of a BTz unit can lower the highest occupied molecular orbital levels of three terpolymers, which is conducive to charge transport. The champion device PMz-5:L8-BO system achieves an efficiency of 16.57%, featuring a high open-circuit voltage of 0.886 V, a high short-circuit current density of 25.49 mA cm^-2, and a remarkable fill factor of 73.35%. This work provides an efficient and straightforward ternary copolymerization strategy that can be used to enhance the device performance.