Electron Injection Dynamics at the SILAR Deposited CdS Quantum Dot/TiO2 Interface

Semiconductor quantum dots with their tunable band gap energies offer new opportunities for controlling photoresponse and photoconversion efficiency of solar cells. Exciton dissociation, charge injection, and charge recombination are key steps for efficient interfacial electron transfer. Here, electron injection and carrier relaxation dynamics at the CdS quantum dot/TiO2 interface were investigated by using ultrafast transient absorption spectroscopy and global fitting analyses. The CdS/TiO2 composites were prepared by depositing CdS in the TiO2 nanocrystalline films by the successive ionic layer adsorption and reaction (SILAR) technique. Comparing the transient absorption spectra of CdS/TiO2 composites formed by different numbers of CdS coating cycles and CdS/Al2O3 revealed size-dependent electron injection from excited CdS into TiO2 nanoparticles, which occurred on a time scale of 1-9 ps. We also demonstrated that holes are trapped in a localized state with a time constant of 0.3 ps, which was faster than the electron trapping with a time constant of about 30 ps.