Effect of dithiophene-based π-spacer and auxiliary acceptor on the photovoltaic performance for DSSCs based on dithiafulvenyl and triphenylamine double donor dyes

Developing new small-molecule organic dyes is one of the effective ways to enhance the photovoltaic performance of dye-sensitized solar cells (DSSCs). Based on the described organic dye WD10, we have designed the organic dyes DTT and DT1–4 with a D1-D2-π-A structure where dithiafulvenyl (D1) and triphenylamine (D2) were used as electron donors, dithiophene and fused dithiophene as the electron acceptor and anchoring group (A). On this basis, benzo[c][1, 2, 5] thiadiazole was further introduced as an auxiliary acceptor unit(A’) between D2 and the π-bridge to construct organic dyes DTTA and DTA1–4 with a D1-D2-A’-π-A structure. Through DFT and TD-DFT calculations, compared with the reported dye WD10, the designed dyes in this paper show a smaller HOMO-LUMO energy gap, longer absorption maximum wavelength (λ_max), lower reorganization energy (λ_total), and higher light-harvesting efficiency (LHE), as well as a negative electron injection driving force (ΔG_inj). These characteristics are all conducive to improving the photoelectric conversion efficiency of DSSCs. Moreover, the introduction of the auxiliary acceptor unit (A’) further reduces the HOMO-LUMO energy gap of the dyes, broadens the absorption band, and causes a red shift of λ_max, but it also decreases the oscillator strength, thereby having a certain negative impact on the light-harvesting efficiency (LHE). Among the designed dyes, DT2 shows superior photoelectric properties compared with other dyes and is considered the most promising candidate for DSSC applications. In addition, this study also thoroughly investigated the electronic structure and optical properties of these dyes when adsorbed on the model semiconductor (TiO₂)₁₆, aiming to provide valuable insights into the adsorption behavior of dyes on the semiconductor surface and the electron transfer mechanism. According to the research, changing the structure of the π-bridge is a workable and effective way to improve DSSC performance.