Effect of donor groups on the photovoltaic performance and TiO2 adsorption of thiophene-based D–π–A dyes for DSSCs: A first-principles study

This study employs Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) to systematically investigate the influence of different donor groups on the performance of six thiophene-based D-π-A organic dyes for Dye-Sensitized Solar Cells (DSSCs). With the bithiophene π-spacer and 2-cyanoacrylic acid acceptor fixed, we varied the donor moieties and analyzed key structural, electronic, optical, and photovoltaic properties relevant to the dye performance. Adsorption onto the TiO2 (101) anatase surface was modeled using DFT with a Hubbard U term (DFT + U) and D3 dispersion to correct Ti 3d self-interaction and reproduce the TiO2 band gap, enabling a realistic description of interfacial electronic structure and charge injection. This approach was used to evaluate adsorption energies, interfacial charge transfer via charge density difference analysis, and electronic structure modifications through Density of States (DOS) calculations. All dyes demonstrated exothermic adsorption via bidentate bridging and favorable electronic coupling with the TiO2 conduction band, essential for efficient electron injection. The results show that donor selection significantly impacts planarity, energy level alignment, and light-harvesting capabilities (predicted λmax range 443–470 nm in THF). The TPA donor (D2) exhibited the strongest ΔGinject (-1.36 eV) and the most stable adsorption (−1.54 eV), while the 2VCz donor (D1) yielded the highest predicted VOC (0.80 V). The mixed S/O donor 2TF (D5) presented a good balance of strong absorption, favorable energetics, and strong adsorption (−1.54 eV). This computational investigation highlights the structure-property relationships governing dye performance, suggesting that electron-rich donors like TPA and heteroatom-containing donors like 2TF, paired with a bithiophene spacer, are promising design strategies for efficient DSSC sensitizers.