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

Abstract

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_inj > 0.2 eV) and dye regeneration (ΔG_reg > 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.