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

Highlights: • Ti and Mo–WO₃ thin films were obtained by spray pyrolysis. • Morphology and structure changes related to solution surface tension modification. • FTO/WO₃ junction parameters dependent on WO₃ layer morphology and structure. • Flat band potential and series resistance varied with cation doping. • Ad(de)sorption of oxygen/water during annealing and cooling influence conductivity. - Abstract: In this paper we compare the optical and electrical properties of the WO₃ thin films containing 2, 5 and 10 at.% 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 2 at.% Ti concentration is enough to significantly improve the photoconductivity of the WO₃ films, whereas for Mo addition higher levels are needed (10 at.%). 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