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

Abstract

ABSTRACT Ingenious molecular engineering of small‐molecule acceptors (SMAs) with low nonradiative energy loss (Δ E 3 ) 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 Δ E 3 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 Δ E 3 of binary OSCs with asymmetric terminal‐based SMAs.