(TD)DFT investigation of donor influence on the electronic and photovoltaic properties of D-π-A-π-A quinoxaline dyes

Abstract Context The design of efficient metal-free organic dyes is crucial for improving the performance of dye-sensitized solar cells (DSSCs). In this work, we investigated the influence of different donor groups on the electronic, photophysical, and photovoltaic properties of two D- π -A- π -A quinoxaline-based dyes, QX-DMA and QX-TPA. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations with solvation in tetrahydrofuran (THF) were employed to examine structure–property relationships. The optimized geometries exhibit high planarity along the conjugated backbone, favoring π -conjugation and intramolecular charge transfer. The results indicate that QX-TPA exhibits superior photovoltaic parameters, including a higher electron injection efficiency, greater regeneration driving force, and lower charge recombination, which are attributed to the extended electronic delocalization promoted by the triphenylamine (TPA) group. Electronic analysis shows that the lowest singlet excited state ( $${S}_{1}$$ S 1 ) is a bright HOMO → LUMO transition with pronounced electron–hole separation. Owing to its electron-donating group and more extended π -delocalization, QX-TPA displays enhanced charge-transfer character, larger reorganization energy, and a significantly longer excited-state lifetime, indicating improved charge stabilization and slower recombination rate compared with QX-DMA. These molecular properties suggest enhanced photovoltaic performance, with QX-TPA-based DSSCs achieving nearly twice the power conversion efficiency of QX-DMA devices, mainly due to a higher short-circuit current density. Methods The geometrical and electronic properties of all systems were investigated using density functional theory with the B3LYP exchange–correlation functional combined with the 6-311G(d,p) basis set. The functionals B3LYP-D3(BJ), CAM-B3LYP, ω B97X, and M06-2X were also tested. Time-dependent DFT calculations were performed at the same level of theory to describe the first ten singlet excited states ( $${S}_{1} - {S}_{10}$$ S 1 - S 10 ). THF solvation effects were accounted for using the conductor-like polarizable continuum model (CPCM). All calculations were carried out using the ORCA 6.0 program package. Charge-transfer properties were analyzed using the TheoDORE program, whereas molecular orbital fragmentation analysis employed the Multiwfn (version 3.8) program package.