Reduced Charge-Transfer Threshold in Dye-Sensitized Solar Cells with an Au@Ag/N3/n-TiO2 Structure as Revealed by Surface-Enhanced Raman Scattering

Interfacial information, such as the binding type and charge transfer (CT) processes, is critical for the investigation of dye-sensitized solar cells (DSSCs). Herein, Au@Ag core–shell nanoparticles (NPs) were employed for the fabrication of Au@Ag/N3/n-TiO2 systems. With the help of the CT degree (ρCT), surface-enhanced Raman scattering (SERS) spectra are utilized to evaluate the CT processes upon layer-by-layer addition of TiO2 to metal/N3/n-TiO2 systems based on Au@Ag, Ag, and Au. The layer-dependent SERS intensity and ρCT for Au@Ag/N3/n-TiO2 revealed a CT enhancement involving TiO2 at excitation wavelengths of 488, 514.5, 647, and 785 nm, whereas Ag/N3/n-TiO2 presented such enhancement only at excitation wavelengths of 488 and 514.5 nm. The mechanisms of CT process are proposed to explain such reduced CT energy threshold: In Au@Ag/N3/n-TiO2, an equivalent CT process involving TiO2 is first proposed, in which the electrons are directly transferred from the HOMO level of N3 to the much lower CB3 level of ...