Synergistic side-chain engineering of quinoxaline-based non-fullerene acceptors for high efficiency organic solar cells

Abstract

Non-fullerene acceptors (NFAs) based on quinoxaline (Qx) cores provide versatile platforms for tailoring molecular packing and optoelectronic properties through halogen and alkoxy substitutions. In this study, we report three Qx-based A-D-A '-D-A type NFAs Qx-6F, Qx-2F4Cl, and Qx-6Cl featuring meta-ethylhexyloxyphenyl side chains and systematic fluorine/chlorine substitution patterns. Optical and electrochemical characterizations revealed only minor variations in absorption coefficients and energy levels among the three NFAs; however, their solid-state morphologies differed significantly. Pristine Qx-6F films exhibited strong crystallinity with multidirectional molecular packing, whereas Qx-6Cl formed smooth and homogeneous films with a predominant face-on orientation and a converged it-it stacking motif. These intrinsic packing tendencies were largely preserved in blend films with PM6, as confirmed by atomic force microscopy (AFM) and grazing-incidence wideangle X-ray scattering (GIWAXS). Polymer solar cells (PSCs) employing Qx-6Cl achieved a power conversion efficiency (PCE) of 17.02%, outperforming devices with Qx-2F4Cl (16.55%) and Qx-6F (15.74%). The superior performance of Qx-6Cl is attributed to its uniform molecular packing, enhanced charge-collection efficiency, and balanced carrier mobilities. In contrast, excessive aggregation in Qx-6F disrupted transport continuity, leading to reduced hole transport and suppressed photocurrent generation. These findings demonstrated that strategic halogen substitution effectively regulates molecular packing and charge transport behavior, providing important design principles for high-performance Qx-based NFAs.