Innovative pathways to efficiency in organic solar cells: a DFT perspective on small donors

Innovative small molecule donors (SMDs) in organic solar cells (OSCs) have gained attention due to their high absorbance and tunable band gaps, enabling improved efficiency and performance. In this study, three novel SMDs (M1, M2, and M3) were proposed by modifying terminal hydrogen atoms with fluorine (M1), methyl (M2), and methoxy (M3) groups. These substitutions were systematically analyzed for their effects on structural, electronic, and optical properties using density functional theory (DFT). The HOMO–LUMO energy gaps were found to be 2.03 eV (M1), 2.02 eV (M2), and 2.00 eV (M3). M3 also exhibited the highest absorption wavelength (λ_max) of 743 nm, the lowest excitation energy (1.67 eV), and the highest light-harvesting efficiency (LHE = 0.9996). Charge transfer analyses showed that M3 had the lowest electron reorganization energy (λ_e = 0.0046 eV), indicating superior charge mobility. These findings suggest that M3 is the most promising candidate for efficient OSC applications. Computations were performed using the Gaussian 09 suite, employing the B3LYP functional with the 6-31G(d,p) basis set and TD-DFT for excited state calculations. Solvent effects were considered using the PCM model, and CAM-B3LYP was used for excitation energy validation.