Intrinsic Planarity in Partially Fused Electron Acceptors Enabled by Furan Thiophene Linkage Design
Electron acceptors containing single-bond-linked building blocks offer attractive advantages for organic solar cells owing to their synthetic simplicity and structural modularity. However, achieving backbone planarity without compromising electronic compatibility remains a persistent challenge. Conventional conformational locking strategies based on alkoxy substitution can effectively suppress torsional freedom but often elevate the highest occupied molecular orbital energy level, limiting compatibility with widely used donor polymers. Here, we report a partially fused electron acceptor design that achieves intrinsic backbone planarity through heterocycle selection rather than side-chain-assisted conformational locking. By incorporating a benzodifuran core and furan-thiophene linkages, the resulting acceptors exhibit a near-coplanar backbone geometry as revealed by density functional theory calculations, without the need for electronically perturbing alkoxy groups. Devices based on the optimized acceptor (BDF-1) deliver a binary power conversion efficiency of 12.2%, and further improvement to 19.5% is achieved in a ternary blend with PM6 and BTP-eC9. The enhanced performance is accompanied by favorable morphology, balanced charge transport, and suppressed recombination losses. This work provides molecular-level insight into partially fused acceptor design and demonstrates that heteroatom-guided conformational locking offers a viable strategy for expanding the design space of acceptors with single-bond-linked building blocks while maintaining compatibility with mainstream donor systems.