Rational Asymmetric Acceptor Engineering via Unidirectional Terminal π-Extension and Optimizing Alkyl Branching Sites Affords a Binary Photovoltaic Efficiency of 20.7% by Suppressed Nonradiative Energy Loss

Ingenious molecular engineering of small-molecule acceptors (SMAs) with low nonradiative energy loss (ΔE₃) and enhanced exciton diffusion length (L_D) to overcome the efficiency bottleneck of binary organic solar cells (OSCs) remains a critical challenge. Herein, a series of symmetric SMAs (TC1-F to TC4-F) with progressively outward-shifted branching sites and asymmetric/symmetric counterparts (A-TC3-F and TC3-NF) incorporating unidirectional/bidirectional naphthyl-based terminals, are synthesized for efficient binary OSCs. The optimal 3ʳ^d carbon branching site induces a distinct triclinic crystallographic system with closer π-π stacking. Unidirectional naphthyl terminal-based single-crystal creates an unprecedented 2D lamellar network/3D interpenetrated packing that provides multidimensional charge-transport pathways, which enabled an improved L_D and electron mobility in A-TC3-F neat film. The A-TC3-F-based blends optimize film formation kinetics and exhibit superior ordered molecular stacking morphology, yielding faster charge transport. Consequently, the optimized A-TC3-F-based binary OSCs achieve a champion PCE of 20.70% and an ultralow ΔE₃ of 0.191 eV, setting a new benchmark for binary OSCs with asymmetric terminal-based SMAs. Our systematic work highlights an innovative pathway for precisely tailoring the side-chain branching position and a unidirectional terminal π-extension strategy to optimize molecular packing, mitigate trade-offs of device parameters, and boost benchmark PCE and minimal ΔE₃ of binary OSCs with asymmetric terminal-based SMAs.