Strategic control of electron donation in D-A-π-A dyes: Insights from DFT calculations for enhanced DSSC and NLO performance.

Dye-sensitized solar cells (DSSCs) are cost-effective and environmentally sustainable alternatives to traditional solar cells. In this study, two groups of novel metal-free organic (MFO) sensitizers (A1 and A2) were designed by modifying the experimentally tested WS-9 dye (E)(E)(E)-2-cyano-3-(3'-hexyl-5'-(7-(4-phenyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)benzo[c][1,2,5]thiadiazol-4-yl)-[2,2'-bithiophen]-5-yl)acrylic acid), which has a D-A-π-A structure and a power conversion efficiency (PCE) of 9.02 %. The designed dyes incorporated two distinct donor cores -indoline (D1) and 2-diphenylaminothiophene (D2)- and a range of electron-donating groups (J to O), resulting in 12 novel dyes with enhanced electron-donating abilities. Their geometrical, optical, electronic, and electrochemical properties were studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, combined with the Conductor-like Polarizable Continuum Model (CPCM) to simulate solvent effects (dichloromethane). Additionally, the adsorption behavior of the dyes on TiO₂ clusters was investigated by calculating adsorption energies and analyzing UV-Vis spectra. The results show significant improvements in the dyes' intramolecular charge transfer (ICT) properties compared to the reference WS-9 dye. A maximum red-shift of 109 nm in the absorption spectrum, an extended excited-state lifetime of 4.95 ns, and lower chemical hardness were observed, accompanied by enhanced electron injection (ΔG > 0.2 eV) and dye regeneration (ΔG > 0.15 eV) efficiencies. Furthermore, the dyes exhibited large Stokes shifts (231.64-177.62 nm) and superior nonlinear optical (NLO) properties. These findings suggest that the newly designed dyes are highly promising candidates for DSSC applications, offering enhanced light-harvesting capabilities and improved photoelectrical performance.