Charge transfer in the heterointerfaces of CdS/CdSe cosensitized TiO ₂ photoelectrode

One of the key issues affecting the performance of solar cells is the behavior of carrier transfer. In this work, the time-resolved photoluminescence (TRPL) technique was utilized to investigate the electron transfer at the CdS/CdSe, TiO ₂/CdS, and TiO ₂/CdSe heterointerfaces. By varying the excitation wavelengths, photoluminescence lifetimes of CdSe and CdS in TiO ₂/CdSe, TiO ₂/CdS, TiO ₂/CdS/CdSe, and TiO ₂/CdSe/CdS photoelectrodes were measured. The results show that, for the single sensitizer electrodes (TiO ₂/CdS, TiO ₂/CdSe), the average PL lifetime of CdS (0.69 ns) is shorter than CdSe (0.99 ns), suggesting that CdS has higher electron transfer rate toward TiO ₂ compared with CdSe. For the TiO ₂/CdSe/CdS electrode, the PL lifetime of CdSe exhibits an excitation-wavelength-dependent behavior. A shorter excitation wavelength leads to a longer PL lifetime of CdSe. This additional long lifetime is ascribed to the rapid carrier transfer from the photoexcited carriers in CdS layer into the CdSe layer. On the contrary, the PL lifetime of CdSe is independent of the excitation wavelength in the TiO ₂/CdS/CdSe electrode, indicating that the excited electrons in the CdS layer did not inject into the CdSe layer. This observation confirms that the charge transfer from the cosensitizers toward the TiO ₂ is much more efficient in the TiO ₂/CdS/CdSe electrode rather than in the TiO ₂/CdSe/CdS electrode.