High‐Molecular‐Weight Polymer Donors Based on Bithiophene Imide for High‐Efficiency and Durable All‐Polymer Solar Cells

Abstract High‐molecular‐weight (high‐MW) polymer semiconductors are pivotal in advancing all‐polymer solar cells (all‐PSCs), known for their excellent device stability and mechanical resilience. However, the development of high‐MW polymer donors (P_Ds) faces challenges due to the scarcity of suitable polymer backbones that ensure both high MW and optimal solubility, along with well‐controlled miscibility with polymer acceptors (P_As). Herein, a series of bithiophene imide (BTI)‐based P_Ds with high number‐average molecular weights ranging from 120.3 to 145.5 kDa are developed, offering improved molecular aggregation property and optimized P_D:P_A miscibility. Notably, polymer PBTI‐OD, featuring a 2‐octadecyldodecyl side chain on the BTI moiety, exhibited superior blending character and optimal morphology with P_As, outperforming its 2‐hexyldecyl and 2‐decyltetradecyl analogues (PBTI‐HD and PBTI‐DT). As a result, PBTI‐OD achieved impressive power conversion efficiencies of 18.47% and 19.20% in binary and ternary devices, respectively. Furthermore, a progressive enhancement in device stability and mechanical robustness is realized from PBTI‐HD to PBTI‐OD and to PBTI‐DT, enabled by the longer alkyl chain and higher polymer MW. These results underscore the great potential of BTI‐based molecular backbones to construct high‐MW P_Ds for highly efficient and durable all‐PSCs, paving the way for next‐generation solar energy technologies.