Side-Chain Engineering on Benzo[1,2- <i>b</i> :4,5- <i>b</i> ′]Difuran-Based Polymer Donors Optimizes Film Formation Dynamics for High-Performance and Mechanically Robust All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) featuring high efficiency and excellent mechanical durability show great practical application prospects. However, the development of high-performance polymer donors has received less attention than that of polymer acceptors, which narrows the diversity of materials as well as device processing and thus hinders our understanding of the structure–performance relationship in all-PSCs. Herein, the combination of benzo[1,2-b:4,5-b′]difuran (BDF) as the electron-sufficient unit and dithieno[3,2-f:2′,3′-h]quinoxaline (DTQx) as the electron-deficient unit is proposed to construct the polymer backbone, and two polymer donors, PFQ-p and PFQ-m, are further synthesized through side-chain engineering of optimizing the alkyl substitution sites. When PY-IT is applied as the acceptor, the device based on PFQ-p obtains an excellent power conversion efficiency of 17.11%, which is significantly higher than that of the PFQ-m-based device (9.12%). The significant efficiency difference has been elucidated through film formation kinetics and morphological characterization. By further device optimization, the efficiency of the PFQ-p:PY-IT device can be improved to 18.32%, which bears comparison with that of most high-performance binary all-PSCs. Furthermore, both PFQ-p:PY-IT and PFQ-m:PY-IT-based flexible devices exhibit excellent mechanical stability. The results demonstrate that through rational molecular design BDF-based polymer donors are strong competitors in mechanically durable, high-performance all-PSCs.