Charge Recombination Kinetics at an in Situ Chemical Bath-Deposited CdS/Nanocrystalline TiO2 Interface

We have conducted submicrosecond to millisecond transient absorption studies to elucidate the parameters controlling charge recombination kinetics at an in situ chemical bath-deposited CdS/TiO2 interface. The CdS/TiO2 nanostructures were prepared by depositing CdS in the TiO2 nanocrystalline films via the SILAR, successive ionic layer adsorption and reaction, technique. Steady-state absorption spectroscopy and XRD measurements of the films indicated that the CdS amount increases as a function of the CdS deposition cycles. In contrast, the US crystallinity size remains constant after reaching approximately 4 nm. Comparison of the transient absorption spectra between CdS/TiO2 and CdS/Al2O3 (Al2O3 is employed as an insulator) suggests that an efficient electron injection occurs from US to a TiO2 conduction band. Charge recombination kinetics for the CdS/TiO2 appears to be multiexponential, being similar to the transient dynamics observed for dye-sensitized TiO2 films. A detailed analysis of the charge recombination dynamics with nonadiabatic electron transfer theory revealed that the recombination half-lifetime, t(50%), correlates closely with the US crystallinity size, resulting in recombination retardation by a factor of 100 with increase in the crystallinity radius from 0.8 to 2.1 nm. This correlation is discussed in relation to the function of CdS quantum dots-sensitized TiO2 solar cells.