Composition-Tuned Benzo[ d ][1,2,3]thiadiazole-Based Terpolymer Guest Donors for Efficient Ternary Organic Solar Cells

Ternary organic solar cells (T-OSCs) can mitigate the spectral and morphological limitations of binary blends, but the molecular requirements for effective polymeric guest donors remain insufficiently defined. Here, three composition-tuned, benzo[d][1,2,3]thiadiazole-containing wide-band gap terpolymers (F1–F3) were synthesized and evaluated as guest donors in PM6:Y6 devices. Varying the relative contents of the benzo[d][1,2,3]thiadiazole- and benzodithiophene-dione-derived segments modulates the absorption, frontier molecular orbitals (FMO), and blend compatibility. Among the series, F2 provides the most favorable combination of a deep highest occupied molecular orbital (HOMO) level, efficient exciton dissociation (98.8%), and balanced hole/electron transport (mobility ratio = 1.15). Its surface energy (22.7 mN m–1) lies between those of PM6 (20.6 mN m–1) and Y6 (27.4 mN m–1), while its comparatively low Flory–Huggins interaction parameter with Y6 (χ = 0.221) is consistent with improved interfacial mixing. Transmission electron microscopy and atomic force microscopy reveal a finer and more uniform blend morphology, supporting reduced recombination and improved charge collection. Consequently, PM6:Y6:F2 devices deliver a power conversion efficiency of 17.45%, compared with 16.19% for the binary PM6:Y6 control, through simultaneous increases in open-circuit voltage, short-circuit current density, and fill factor. These results demonstrate that terpolymer composition can be used to co-optimize electronic structure and interfacial compatibility, providing a practical route to polymeric guest donors for multicomponent organic solar cells (OSCs).