Physical properties of SnO2/WO3 bilayers prepared by reactive DC sputtering

Abstract

The combination of transparent and conductive SnO2 with colored WO3 thin films has interesting uses in electrochromic windows, photovoltaic cells and photocatalytic systems, where the SnO2/WO3 bilayer can improve the device efficiency by increasing the charge separation and extending the energy range of photoexcitation. In this work, SnO2 and WO3 thin films were prepared by reactive DC sputtering from Sn and W targets, respectively. Single layers and bilayers deposited on glass substrates have been analyzed by X-ray diffraction, atomic force microscopy, spectrophotometry, and electrical measurements. SnO2 crystallizes in the cassiterite structure, whereas amorphous WO3 is obtained on bare and SnO2-coated glasses, showing higher surface roughness on the SnO2 layer. Different oxygen vacancy defects have been identified in WO3 by analyzing photoconductivity transients. The oxygen vacancy defects are responsible for the sub-bandgap absorption that causes coloration in the WO3 films. Regarding SnO2, it shows a high transmittance of about 90% in the visible and near-infrared spectral ranges.