Unsaturated Vinyl Branching: A Molecular‐Tailoring Strategy for Synergistic Stacking Regulation and Energy‐Loss Suppression Toward 20%‐Efficiency Organic Solar Cells

ABSTRACT The performance of organic solar cells (OSCs) is critically constrained by energy losses arising from disordered molecular packing and suboptimal blend morphology in non‐fullerene acceptor (NFA) systems. Achieving precise control over NFA aggregation during film formation is thus essential to realize optimal molecular orientation and phase separation. Herein, we propose an unsaturated vinyl branching (UVB) strategy that enables synergistic morphology control through the concurrent minimization of steric hindrance and enhancement of non‐covalent interactions. By incorporating UVB units into the inner side chains of the Y6‐OD backbone, we create a novel acceptor, BTP‐vinyl, which demonstrates optimized aggregation kinetics and facilitates compact 3D molecular packing. When blended with polymer donor D18, the BTP‐vinyl‐based system exhibits retarded crystallization, optimized domain size, and highly ordered face‐on orientation, collectively promoting efficient exciton dissociation and charge transport. Consequently, OSCs based on BTP‐vinyl display significantly reduced energetic disorder and suppressed non‐radiative energy loss compared to Y6‐OD‐based devices. The binary devices achieve a remarkable power conversion efficiency (PCE) of 19.6%, substantially outperforming the Y6‐OD‐based devices (18.1%). Furthermore, ternary devices incorporating L8‐BO attain a champion PCE of 20.06%, crossing the significant 20% threshold. This work establishes UVB as an effective molecular‐design paradigm for achieving ideal blend morphology through precise aggregation control in high‐performance OSCs.