Photoinduced Charge Separation in Retinoic Acid on TiO₂: Comparison of Three Anchoring Modes

A complete understanding of the interaction between the dye and the semiconductor is essential for the design of a dye-sensitized solar cell (DSC) because the chemical interactions and the electron coupling between semiconductor and dye directly impact electron transfer and the performance of the dye-sensitized photoanode. Three anchoring modes of retinoic acid (RA), which contains a carboxylic acid group adsorbed on TiO2, were studied by DFT calculations. Of the three anchoring modes, the mode involving surface Ti–OH groups has the highest driving force for photoinduced electron transfer. The mixing of the RA’s lowest unoccupied molecular orbital (LUMO) with the conduction band states of TiO2 is also the highest. The calculated results are supported by EPR measurements, which show enhanced light-driven charge separation from increasing the density of the Ti–OH groups on the surface of TiO2. The anchoring modes affect the bond length, degree of conjugation, energies of the highest occupied molecular orbital (HOMO) and LUMO, and the mixing of the dye’s excited states with the conduction band states of a semiconductor. These conclusions are important in the design of a DSC.