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

High binding energy of localized Frenkel excitons (Eb) 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 Eb 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 Eb 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 Eb and accelerate charge generation kinetics. Consequently, Y6-EE-based HJ-OSCs achieve a remarkable short-circuit current density (JSC) of 10.62 mA/cm2 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.