Due to the world's rapidly expanding population and industrial sector, which raises the demand for energy, solar cells can address both global energy and environmental needs. With the aim to enhance the dye-sensitized solar cell (DSSC) efficiency, we designed four metal-free BPA and TPA-based dyes (RK2-RK5) by increasing the donor strength (substituting different groups such as biphenylamine (BPA) and triphenylamine (TPA) in the donor side of the dye) based on the reference dye (RK1) as such dye showed improved DSSC’s power conversion efficiency. Density functional theory (DFT) was applied at the B3LYP/6-31G** level to calculate the ground-state (S₀) optimized geometries of (RK1-RK5). Time-dependent DFT (TD-DFT) was utilized to compute the absorption spectra utilizing four functionals (B3LYP, CAM-B3LYP, PBE1PBE and BHandHLYP) in gas phase and solvent such as dichloromethane (DCM) and ethanol. The comprehensive analysis of RK1-RK5 as well as dyes@TiO₂ was performed, and light was shed on the optoelectronic properties. Frontier molecular orbitals' (FMOs') charge density distribution revealed the sensitizers' intramolecular charge transfer (ICT) from the donor to the acceptor moiety. After adsorption charge transfer was noticed from sensitizer to the TiO₂ semiconductor's surface in dyes@TiO₂. Dyes adsorption on the TiO₂ cluster would be stable, as revealed by the dyes@TiO₂ cluster's negative binding energy. Additionally, it was found that double donor raises the electronic coupling and electron injection constants in RK4 and RK5, indicating that the charge injection in these newly designed dyes would be superior. As a result, the DSSC efficiency in newly designed derivatives has been improved to 8.05% for RK5 by substituting TPA unit at R1 and R2 position in parent compound. These well-established correlations between structure-property relationship, and performance provide profound insight into how improving the donor moiety strength in organic sensitizers affects device performance. It boosted photovoltaic performance through enhanced short-circuit current density, and light-harvesting efficiency. For high-efficiency in DSSCs, it can be a useful rational molecular designing strategy for D-π -A organic sensitizers.

All the calculations were performed in Gaussian16. The coordinates of optimized TPA dyes and dyes adsorbed on TiO2 surfaces has been given here.