Time-Dependent Density Functional Theory Investigation of Coumarin-Linked Dihydropyrimidinone and Indoxyl-Based Dyes for DSSC Applications

The growing demand for sustainable energy technologies continues to motivate the development of dye-sensitised solar cells (DSSCs) based on efficient organic sensitisers. This study investigates two structurally distinct dyes, a coumarin-linked dihydropyrimidinone-based dye (D1) and an indoxyl-based dye (D2), using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The molecular structures were optimised through a cascade approach using the PM6 semi-empirical method followed by DFT at the B3LYP/6-31+G* level without constraints. HOMO and LUMO energies were obtained at B3LYP/6-31+G* in the gaseous phase, whereas excited-state energies, absorption spectra, and oscillator strengths were calculated using TD-DFT at the B3LYP/6-311G level as implemented in Gaussian 09. The calculated HOMO/LUMO energies were -5.639/-2.337 eV for D1 and -5.337/-2.876 eV for D2, corresponding to energy gaps of 3.302 and 2.460 eV, respectively. The longest-wavelength calculated transitions were 413.00 nm for D1 and 541.29 nm for D2. D1 showed the most negative electron-injection free energy (-1.9397 eV) and the higher calculated open-circuit voltage (1.6631), whereas D2 showed a higher light-harvesting efficiency (0.4800). These results indicate different relative advantages for D1 and D2 across the evaluated optoelectronic and photovoltaic descriptors and provide a consistent theoretical comparison of the two dye systems for potential DSSC sensitisation.