Heat transfer in binary and ternary bulk heterojunction solar cells

Ternary strategy is one of the most commonly used methods to boost the performance of organic solar cells (OSCs) from a binary blend of donor and acceptor. Fullerene derivatives are popular choices for the ternary component as they could benefit the electrical property. However, the ternary component could also affect other physical properties of the bulk-heterojunction (BHJ). Among these properties, heat transfer has rarely been reported, despite its relevance for thermal durability of OSCs. Here, we employ scanning photothermal deflection technique to study thermal diffusion properties of binary PM6:Y7 and ternary PM6:Y7:X BHJs, where X = PC71BM, ICBA, and N2200. It is found that fullerene derivatives deteriorate the thermal diffusivity (D) of blend films and the device thermal durability, despite enhancing the electrical and device performance. In contrast, when an n-type conjugated polymer N2200 is used as the ternary component, both the electrical and thermal properties are enhanced, with improved power conversion efficiency and prolonged device thermal durability. These results offer a perspective on how to choose a favorable third component. Fullerene derivatives are not necessarily the optimal choice for ternary component for BHJ cells because of the inferior thermal properties.