Exploring the effect of aromatic π-spacers on the photophysical properties of triphenylamine and indoline dyes in DSSCs

Dye-sensitized solar cells (DSSCs) are positioned as a promising technology to address the rising global demand for clean energy, providing a sustainable option to meet the expanding requirements for eco-friendly energy outputs. The strategic selection of electron-donating/accepting groups, along with the π-bridge, in the design of light-harvesting dyes for DSSCs systematically enhances their photophysical properties to meet efficiency criteria and optimize DSSC performance. This study examines model dyes derived from triphenylamine (TPA) and indoline (IND), designated as TPA1, TPA2, TPA3, IND1, IND2, and IND3, each containing donor, bridge, and acceptor molecular units. The research aims to assess how different π-spacer configurations affect the optical, electronic, and photovoltaic properties critical for DSSCs. Using DFT and TDDFT methods, the study demonstrates that extending π-conjugation (in a D-π-π-A structure) causes a red shift in the absorption spectra by reducing the HOMO–LUMO gap. TPA2 outperformed the other TPA dyes with the highest short-circuit current density (J_sc) of 1.75 mA cm⁻², while IND2 led the IND series with a Jsc of 0.77 mA cm⁻². Additionally, TPA2 showed superior regeneration kinetics, with the fastest regeneration rate (ΔG_reg = 0.37 eV) and an open-circuit voltage of 0.88 eV. IND2, though achieving a competitive regeneration rate (ΔG_reg = 0.67 eV), recorded the highest open-circuit voltage within the IND series at 1.47 eV. The strong correlation between computational predictions and experimental data for both TPA and IND dyes confirms the reliability of the computational methodologies employed. These findings endorse TPA2 and IND2 as promising candidates for advancing the DSSCs efficiency.