The sensitization effect and microscopic essence of different additives on the electronic structure of nanocrystalline TiO₂ in dye-sensitized solar cell

The reliable periodic density functional theory calculations have been carried out to investigate the diverse sensitization effects of five different additives, Guanidinium thiocyanate (GNCS), 3-Methoxypropionitrile (MPN), N-Butylbenzimidazole (NBB), N-Methylbenzimidazole (NMB) and 4-tert-butylpyridine (TBP), on the electronic structures of TiO₂ anatase (1 0 1), (1 0 0) and (0 0 1) surfaces in vacuum and acetonitrile conditions, respectively. The complex microscopic essence of sensitization effect induced by additive can be revealed successfully by an in-depth interpretation on the inherent relations of the open-circuit photovoltage with fundamental electronic properties, i.e. Fermi energy, molecular orbital, dipole moment, electrostatic potential and work function. There is an explicit inverse correlation between the change of energy gap (E_{△(LUMO-HOMO)}) and Fermi energy shift (E_△Fermi) through the comparison among different additives adsorption under the same conditions of surface and solvent, namely, the smaller E_{△(LUMO-HOMO)} corresponds to the bigger E_△Fermi. The more positive E_△Fermi (i.e. negative potential shift) results in the higher open-circuit photovoltage of dye-sensitized solar cells. Moreover, the E_{△(LUMO-HOMO)} is in the order TiO₂ (1 0 1) < (1 0 0) < (0 0 1) for every additive adsorption, exactly opposite to the order of surface stability. Total dipole moment is more suitable than its component normal to the TiO₂ surface for measuring the sensitization performances of different species of additives. The smaller work function contributes to the bigger E_△Fermi of the sensitized nanocrystalline TiO₂. The influences of acetonitrile on the sensitization performances of five additives are diversified, better for MPN, NBB and NMB, general for GNCS, and the worst for TBP.