Morphology, optical, and photoelectrochemical properties of electrodeposited nanocrystalline ZnO films sensitized with Cd x Zn1-x S nanoparticles

Nanocrystalline ITO/ZnO films formed by porous zinc oxide microplatelets 1-3 mu m in size and 100-200 nm in thickness, which consist of 30-50 nm ZnO crystallites, were sensitized to visible light by Cd (x) Zn1-x S nanocrystals deposited using the method of successive ionic layer adsorption and reaction (SILAR). The composition of Cd (x) Zn1-x S nanocrystals as well as the dependence between molar Cd(II) fraction in the films and the ratio of cadmium and zinc nitrate concentrations in solutions used for the SILAR procedure were determined by a combination of electron, Raman, and energy-dispersive X-ray spectroscopies. The photovoltage observed at illumination of the ITO/ZnO/Cd (x) Zn1-x S heterostructures by white light (lambda > 400 nm) in aqueous Na2S solution increases with a decrease of Cd(II) content proportionally to an increment in the conduction band potential of the Cd (x) Zn1-x S nanocrystals. The photocurrent density normalized to the light absorbance of the ITO/ZnO/Cd (x) Zn1-x S films increases by a factor of around four when the conduction band potential of Cd (x) Zn1-x S nanocrystals grows by 220 mV as a result of Cd(II) fraction changing from 1.0 to 0.62-0.67. The results show that Cd (x) Zn1-x S solid solutions are more advantageous sensitizers for the short-wavelength part of the sensitivity window of the liquid-junction solar cells (400-450 nm) than conventionally used cadmium sulfide.