An enhanced degree of charge transfer in dye-sensitized solar cells with a ZnO-TiO2/N3/Ag structure as revealed by surface-enhanced Raman scattering
A number of recent studies have focused on improving the performance of dye-sensitized solar cells (DSSCs). Cells with a ZnO-TiO2/N3/Ag structure have attracted particular attention because of their excellent power conversion efficiencies. Using a dendritic crystal ZnO-TiO2 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, TiO2/N3, ZnO/N3, ZnO-TiO2/N3, TiO2/N3/Ag, ZnO/N3/Ag, and ZnO-TiO2/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-TiO2 component. The dye loading capacity of the dendritic crystal ZnO-TiO2 is much larger than that of TiO2. There are extra enhancements in the SERS intensity and degree of CT (ρCT) in ZnO-TiO2/N3 compared to ZnO + TiO2/N3 (based on a simulation curve for the physically mixed TiO2 and ZnO semiconductors) with 476.5 nm excitation due to the synergistic effect of the ZnO-TiO2 component. And these enhancements in ZnO-TiO2/N3/Ag compared to ZnO + TiO2/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-TiO2 composite semiconductor in the semiconductor/N3/Ag system can improve the adsorption capacity of N3 compared to that with TiO2. Meanwhile, the synergistic effect of ZnO-TiO2 and Ag can promote the CT process, demonstrating that ZnO-TiO2/N3/Ag is an excellent structure for DSSCs.