Theoretical study of benzene/thiophene based photosensitizers for dye sensitized solar cells (DSSCs)

Complex organic compounds with benzene/thiophene as pi-segments are inspected as photosensitizers for applications in dye sensitized solar cells. To better understand the charge transport process involved in the dye sensitized solar cells, we used the results of Kohn-Sham density functional theory and time-dependent density functional theory (DFT) studies of benzene/thiophene based sensitizers as well as the dye bound to a TiO₂ nano cluster. We investigated the electronic structures and UV-Vis spectra of the sensitizers alone and linked to the cluster. We also showed energy level diagrams, the major transitions of molecular orbitals and free energy calculation of the electron transfer from the sensitizer to the conduction band of the TiO₂. The results show that LUMO of the dyes is greater than the conduction band of TiO₂ indicating that a full charge transfer from dyes to the conduction band of TiO₂ is thermodynamically allowed. The calculated results also indicate that D3 is the most plausible sensitizer due to the most negative ΔG_inject (0.91 eV) and a larger LHE value (0.95), which results in a higher IPCE.