Rational Design and Theoretical Investigation of Quinoxaline-Based Small-Molecule Functional Materials for High-Performance Organic Solar Cells

Small organic molecules with promising optoelectronic properties have been widely adopted in organic solar cells (OSCs) due to their straightforward synthesis, purification, and well-defined structures. In this work, five quinoxaline-based small molecules (TQA1–TQA5) are designed and theoretically investigated as potential donors in OSCs. Density functional theory (DFT) and time-dependent DFT are employed at the MPW1PW91/6-31G (d,p) level to evaluate their electronic, optical, and photovoltaic properties. The results indicate that each TQA derivative exhibits deeper highest occupied molecular orbital levels and a reduced energy gap, with strong absorption in the visible region. Furthermore, the calculated frontier orbitals suggest pronounced intramolecular charge transfer from the donor (triphenylamine) segment to the quinoxaline–acceptor moieties, enhancing exciton dissociation. The estimated open-circuit voltage (V_oc) values calculated against [6,6]-Phenyl-C₆₁-butyric acid methyl ester (PC₆₁ BM) coupled cluster with single and double excitations range from 0.82 to 1.03 V, surpassing that of the reference molecule TQ2R (0.66 V). These theoretical findings highlight the potential of TQA1–TQA5 as high-performance donor materials for future OSC applications.