Site-Specific Carboxylate Functionalization in Conjugated Hole Transport Polymers Enables Efficient CsPbI2Br Single-Junction and Tandem Solar Cells.

Hole transport materials (HTMs) are crucial for achieving high-efficiency and stable all-inorganic CsPbI2Br perovskite solar cells (PSCs). While carboxylate-functionalized polymers have shown considerable promise, the influence of different carboxylate substitution positions on device performance remains poorly understood. In this work, we designed and synthesized two polymeric isomers (2TC-F and TTC-F) to systematically investigated the effects of connecting the carboxylate side chain at different sites of the thiophene (T) and thieno[3,4-b]thiophene (TT) units on the energy level structure, molecular packing behavior, hole mobility, and charge transfer properties at the perovskite/HTM interface. The study demonstrates that, compared to 2TC-F, TTC-F exhibits better energy level alignment, higher planarity, and more effective defect passivation. As a result, TTC-F-based CsPbI2Br PSCs achieved a power conversion efficiency (PCE) of 17.58%, significantly outperforming the 2TC-F-based device (14.54%), along with excellent stability under thermal aging and ambient storage. When integrated into perovskite/organic tandem solar cells (TSCs), the TTC-F-based device reached a PCE of 23.29%. This work highlights the importance of site-specific side-chain engineering in designing high-performance HTMs.