Structural, optical, and surface properties of WO3 thin films for solar cells

Transparent back contacts can be used in thin film solar cells facilitating their potential application in tandem cells, bifacial devices and solar windows. In this study, tungsten oxide (WO3) thin films were deposited by Radio Frequency (RF) reactive sputtering in Ar + O2 ambient. The effects of post deposition anneals in air on the structural, optical, and surface properties of the deposited films were investigated using X-ray diffraction, UV/Vis/NIR spectrophotometry, and X-ray photoelectron spectroscopy, respectively. As-deposited films exhibited amorphous structures with no change after annealing at 300 °C. Samples annealed at 400 and 500 °C were crystallized and identified as pure monoclinic WO3 phase with (2 0 0) and (0 0 2) preferred orientation, respectively, determined by XRD fiber texture analysis. Scherrer analysis of excess broadening indicated a coherency length of 50 and 65 nm for the 400 and 500 °C annealed films, respectively. High resolution XPS studies showed the presence of W6+ (WO3) oxidation states at the surface of the as-deposited and the 300 °C annealed films. Annealing at 400-500 °C led to an oxygen deficient surface with a sub-stoichiometric WO3−x phase. UV/Vis/NIR spectrophotometry revealed that post processing decreased the optical bandgap from 3.30 eV for the as-deposited films to 3.17 eV for the 500 °C annealed films. Results are compared with MoO3 films deposited by a similar technique.