The effect of the introduction of internal acceptor and the variation of π-spacer groups in carbazole-based organic dyes on the photovoltaic performance of dye-sensitized solar cells: a DFT study

To examine the relationship between chemical structure modification and photovoltaic performance, we designed six metal-free organic molecules (M₂-M₇) of structure D-π₁-A-π₂-A based on the base molecule M₁ of structure D-π₁-π₂-A, introducing a new auxiliary acceptor (A) and varying the π₂-spacer at the same time, to study their performance in dye-sensitized solar cells. Using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, several parameters were calculated and discussed, including LUMO and HOMO energy levels of the HOMO–LUMO energy gap, frontier molecular orbitals, absorption properties, nonlinear optical properties (NLO), light-harvesting efficiency (LHE), electron injection driving force (ΔG_inj), dye regeneration driving force (ΔG_reg), reorganization energy (λ_total), and vertical dipole moment (μ_normal) of the studied dyes (M₁–M₇). The results of the proposed dyes proved promising for use as sensitizers in the DSSC device due to their lower energy gap, red- and NIR-shifted absorption band, higher LHE and NLO properties, lower ΔG_reg and λ_total values, negative ΔG_inj value, and higher μ_normal value reflecting higher V_OC compared to the reference dye M₁. The results of these proposed dyes (M₂-M₇) will be considered favorable indicators for experimenters to synthesize effective dyes for use in the DSSC device.