Site‐Specific Carboxylate Functionalization in Conjugated Hole Transport Polymers Enables Efficient CsPbI ₂ Br Single‐Junction and Tandem Solar Cells

ABSTRACT Hole transport materials (HTMs) are crucial for achieving high‐efficiency and stable all‐inorganic CsPbI ₂ Br 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 CsPbI ₂ Br 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.