Investigation of Charge Transfer in Ag/N719/TiO2 Interface by Surface-Enhanced Raman Spectroscopy

The interfaces of metal dye molecule semiconductor sandwich structure are very important in the investigation of dye sensitized solar cells (DSSCs) where metals are used to enhance absorption. In this work, we first designed and synthesized Ag/N719 and Ag/N719/TiO2 sandwich systems to investigate the chemical binding type at the interfaces of Ag/N719/TiO2. The results of the Raman spectra under the laser excitations of 532, 633, and 785 nm clarified that the SCN groups adsorbed on the Ag surface via the S terminal and the TiO2 layer possibly bound to Ag/N719 via the ester linkage (-O-C=O) of the COOH group in N719. Then, we optimized the Ag substrate as an SERS detection platform and selected the Ag sol film as the substrate. Last, the relationship between the "degree of CT (rho(CT))" in the SERS spectra and the charge transfer (CT) process was investigated by tuning the contribution from the chemical effect. We found that, owing to the introduction of TiO2, the intensity and rho(CT) first increased (n = 0-2) and then decreased (n = 2-3) with the increase of the number of TiO2 layers under 532 and 633 nm laser excitations. However, the intensity decreased (n = 0-3) with the increase of the number of TiO2 layers under the laser excitation of 785 nm, and there was no obvious change about rho(CT). Meanwhile rho(CT) became higher with the increase of the laser excitation energy at the interfaces with the same TiO2 layer number. In order to explain these variations about rho(CT), we utilize ultraviolet photoelectron spectroscopy (UPS) and UV-vis spectra to calculate energy levels for better understanding the charge transfer (CT) process, and the calculation result is in accordance with the variation tendency of rho(CT).