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/TiO 2 interface. The CdS/TiO 2 nanostructures were prepared by depositing CdS in the TiO 2 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 CdS crystallinity size remains constant after reaching approximately 4 nm. Comparison of the transient absorption spectra between CdS/TiO 2 and CdS/Al 2 O 3 (Al 2 O 3 is employed as an insulator) suggests that an efficient electron injection occurs from CdS to a TiO 2 conduction band. Charge recombination kinetics for the CdS/TiO 2 appears to be multiexponential, being similar to the transient dynamics observed for dye-sensitized TiO 2 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 CdS 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 TiO 2 solar cells.