The interface of semiconductor−dye−metal system is a crucial issue for investigating dye-sensitized solar cells (DSSCs), where the electron transfer takes place. In this work, a series of assemblies of TiO 2 /N3 (cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′dicarboxylato)ruthenium(II)) and TiO 2 /N3/Ag have been fabricated, which were employed for the investigation of the adsorption configuration and conformational change of N3 molecules. We plot degree of charge transfer (CT) (ρ CT ) as a function of excitation wavelength of TiO 2 /N3 and TiO 2 /N3/Ag assemblies, which contributes to the understanding of the CT process in the series of N3 assemblies. According to the variation tendency of ρ CT , when laser energy exceeds the CT energy threshold 2.071 eV, ρ CT shows an obvious increasing trend with the increasing laser energy. In the case of TiO 2 /N3/Ag assembly, when the laser energy exceeds the CT energy threshold 1.877 eV, ρ CT becomes lager with the increase in the laser energy, until asymptotic behavior appears under higher laser energy. To explain the variation tendency of ρ CT and the shift of CT energy threshold, we have proposed two models about the energy level scheme of TiO 2 /N3 and TiO 2 /N3/Ag assemblies. Furthermore, we investigated the influence of crystal structure of TiO 2 NPs on the CT process by the fabrication TiO 2 /N3/Ag assemblies based on anatase and rutile TiO 2 NPs. It is noted that the TiO 2 /N3/Ag assembly based on TiO 2 NPs calcinated at 450 °C with highest ρ CT and lowest CT energy threshold is most in favor of CT process. Besides the specific chemical binding mode in the TiO 2 /N3/Ag system, this study also found the relationship between the ρ CT and the CT process, which is of considerable importance and relevance to solar energy conversion.
A number of recent studies have focused on improving the performance of dye-sensitized solar cells (DSSCs). Cells with a ZnO-TiO/N3/Ag structure have attracted particular attention because of their excellent power conversion efficiencies. Using a dendritic crystal ZnO-TiO composite semiconductor and Ag in conjunction leads to different charge-transfer (CT) processes, and this is the main theoretical basis for the improvement of DSSC performances. Thus, in the present study, TiO/N3, ZnO/N3, ZnO-TiO/N3, TiO/N3/Ag, ZnO/N3/Ag, and ZnO-TiO/N3/Ag assemblies have been fabricated and their CT processes have been monitored by using surface-enhanced Raman scattering (SERS) spectra, with particular focus on the differences caused by the synergistic effect of the ZnO-TiO component. The dye loading capacity of the dendritic crystal ZnO-TiO is much larger than that of TiO. There are extra enhancements in the SERS intensity and degree of CT (ρ) in ZnO-TiO/N3 compared to ZnO + TiO/N3 (based on a simulation curve for the physically mixed TiO and ZnO semiconductors) with 476.5 nm excitation due to the synergistic effect of the ZnO-TiO component. And these enhancements in ZnO-TiO/N3/Ag compared to ZnO + TiO/N3/Ag appear with 476.5 and 532 nm excitation, which are particularly large with 532 nm excitation. Accordingly, the participation of Ag in this synergistic effect can reduce its energy threshold, which will make it easier to appear. Finally, to rationalize these extra enhancements, the models describing the CT mechanism have been proposed. Thus, the use of the dendritic crystal ZnO-TiO composite semiconductor in the semiconductor/N3/Ag system can improve the adsorption capacity of N3 compared to that with TiO. Meanwhile, the synergistic effect of ZnO-TiO and Ag can promote the CT process, demonstrating that ZnO-TiO/N3/Ag is an excellent structure for DSSCs.
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