Molecularly Modified Quinoxaline-Based Unfused NonFullerene Acceptors for Efficient and Stable Organic Photovoltaics

Unfused nonfullerene acceptors (NFAs) have emerged as promising candidates for organic photovoltaics due to their inherent thermal stability, streamlined synthetic accessibility and capacity for high power conversion efficiency. In this study, we report the rational design of a series of W-shaped, fullerene-free NFAs based on a quinoxaline core, chosen for its inherently low electron density. A consistent central electron-withdrawing moiety, 2-(5,6-dichloro-2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile, is employed across all molecules to investigate the effect of terminal group modifications on optical, optoelectronic and photovoltaic behavior and molecular packing. The reference molecule R(UF-Qx-2Cl) and nine structurally varied derivatives (F1–F9) were systematically examined using density functional theory (DFT) and time-dependent (TD-DFT) characterizations. Comprehensive electronic analyses, including frontier molecular orbitals, density of states, reorganization energies, open-circuit voltage estimations and charge transfer characteristics, were conducted. Most derivatives (F2–F9) exhibited narrowed band gaps (2.00–2.09 eV) compared to the reference (2.14 eV), suggesting enhanced light-harvesting capabilities. UV–Vis-NIR spectroscopy in chloroform solution revealed significant bathochromic shifts in absorption maxima for F2–F9 (770.52–835.76 nm), as compared to the reference (751.64 nm), corroborating improved optical responses. Notably, F1 with fluorinated terminals demonstrated a relatively high predicted open-circuit voltage (1.43 V). Among all candidates, F5 stood out with the lowest band gap (2.00 eV) and strongest absorption in both gas (738.99 nm) and solvent (835.76 nm) phases, highlighting its potential as a high-performance acceptor material in next-generation organic solar cells.