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/TiO<inf>2</inf> interface were investigated by using ultrafast transient absorption spectroscopy and global fitting analyses. The CdS/TiO<inf>2</inf> composites were prepared by depositing CdS in the TiO<inf>2</inf> nanocrystalline films by the successive ionic layer adsorption and reaction (SILAR) technique. Comparing the transient absorption spectra of CdS/TiO<inf>2</inf> composites formed by different numbers of CdS coating cycles and CdS/Al<inf>2</inf>O<inf>3</inf> revealed size-dependent electron injection from excited CdS into TiO<inf>2</inf> 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.
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