Benzobisthiazole-substituted terpolymers for non-halogenated solvent-processed polymer solar cells with enhanced efficiency, thermal stability and mechanical robustness

Abstract

High power conversion efficiency (PCE), eco-friendly processing, and long-term stability are essential for the commercialization of polymer solar cells (PSCs). In this study, we develop PM6-based terpolymer donors (PM6-DTzBX, where X = 5–20), by substituting the benzodithiophene-dione (BDD) unit with the benzobisthiazole (DTzB) unit, which aims to tune the crystalline properties of the polymers as well as achieving a blend morphology with sufficient intermixed domains. The DTzB-incorporating polymer donors (P_Ds) demonstrate stronger intermolecular interactions with a Y6-BO small molecule acceptor (SMA) and exhibit more pronounced crystalline properties than the reference PM6 P_D. Consequently, PM6-DTzB10 P_D-based PSCs achieve a higher PCE of 16.8% compared to that of PM6-based PSCs (15.6%) when processed in a non-halogenated ortho-xylene solvent. Furthermore, under thermal stress at 100 °C for 100 h, PM6-DTzB10-based PSCs maintain 88% of the initial PCE and exhibit enhanced thermal stability compared to PM6-based PSCs, which retain 72% of the initial PCE. Additionally, the PM6-DTzB10:Y6-BO blend films demonstrate a 7-fold increase in stretchability with a crack onset strain (COS) of 11.7%, compared to the PM6:Y6-BO blend films (COS = 1.7%). These enhancements in the PCE, thermal stability, and mechanical robustness can be mainly ascribed to the formation of a well-intermixed P_D:SMA blend morphology and enhanced crystalline properties of PM6-DTzB P_Ds. This study highlights the potential of the terpolymer strategy in developing efficient, thermally stable, and mechanically robust PSCs.