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

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.