Comparative investigation of the Ti and Mo additives influence on the opto-electronic properties of the spray deposited WO3 thin films

Abstract

In this paper we compare the optical and electrical properties of the WO3 thin films containing 2, 5 and 10at.% of Ti and Mo additives, deposited by spray pyrolysis. The influence of the type and additive concentration on the nanostructure, topography and composition of the WO3 layers are mainly related to the surface tension energy changes, and further correlated with the (photo)electrical and optical properties. The FTO/WO3 junction through its characteristic, namely barrier height, ideality factor, flat band potential, and series resistance, served as a tool for associating the before mentioned characteristics.The morphology of the WO3 thin films densifies and the roughness is reduced with increasing Ti and Mo concentration, in good agreement with solution surface tension reduction. WO3 based films exhibit a p-type semiconducting behavior, as confirmed also by the Mott–Schottky analysis, with a lower p-type conductivity for the Ti–WO3 films, as higher number of oxygen vacancies are generated by Ti addition. Changes in conductivity are mainly attributed to the oxygen vacancies concentration evolution at the film surface due to oxygen/water adsorption. For heavily doped WO3 thin films the contribution of these surface processes to the overall conductivity is reduced since surface reactivity is lost by densification.As opposed to Ti-doping which has a detrimental effect on layers structure, Mo addition, even in high concentrations, has a positive effect on layers crystallinity; hence higher conductivity and ideality factors close to 1 are obtained for these films. Surprisingly, Ti and Mo–WO3 films contain cation additives also in lower oxidation states, 3+ and 5+, respectively, compared to the ones in the precursor salt. A 2at.% Ti concentration is enough to significantly improve the photoconductivity of the WO3 films, whereas for Mo addition higher levels are needed (10at.%). The Ti and Mo–WO3 films have high transparency with average transmission values of 85% and 75%, respectively. The thin films reflectance decreases with increasing doping concentration along with roughness diminution.