Side‐Chain Controlled Exciton Binding Energy in Y‐Acceptors Toward Homo‐Junction Organic Solar Cells With 5.07% Efficiency

ABSTRACT High binding energy of localized Frenkel excitons ( E _b ) in organic semiconductors has fundamentally limited the development of homo‐junction organic solar cells (HJ‐OSCs). Although non‐fullerene acceptors (e.g., Y‐acceptors) exhibit intrinsically low E _b due to their unique molecular quadrupole moments and packing motifs—enabling an internal electrostatic field for exciton dissociation—their inefficient exciton dissociation and unbalanced charge transport have constrained HJ‐OSC efficiencies to approximately 4%. Herein, we report a record power conversion efficiency (PCE) of 5.07% in Y‐acceptor‐based HJ‐OSCs achieved through side‐chain engineering that effectively reduces E _b value. By systematically modulating the alkyl side chains of Y‐acceptors into three derivatives—Y6‐EE, Y6‐EH (pristine Y6), and Y6‐BO—we demonstrate that the compact 2‐ethylethyl chain in Y6‐EE induces denser molecular packing, enhances molecular polarization, and increases dielectric permittivity. These synergistic effects collectively reduce E _b and accelerate charge generation kinetics. Consequently, Y6‐EE‐based HJ‐OSCs achieve a remarkable short‐circuit current density ( J _SC ) of 10.62 mA/cm ² and a record PCE of 5.07%, substantially outperforming devices based on Y6‐EH and Y6‐BO. This work highlights the critical role of side‐chain topology in regulating exciton thermodynamics and charge generation efficiency, providing a rational molecular design strategy for high‐performance HJ‐OSCs.