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

• Published in: Journal of alloys and compounds 2014-12, Vol.617 (C), p.609-615
• Main Authors: Simchi, H., McCandless, B.E., Meng, T., Shafarman, W.N.
• Format: Article
• Language: English
• Subjects: Applied sciences; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Energy; Energy storage materials; Exact sciences and technology; Natural energy; Optical properties; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of specific thin films; Oxide materials; Photovoltaic conversion; Physics; Semiconductors; Solar cells. Photoelectrochemical cells; Solar energy; Thin films
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2014.08.047

•WO3 films were deposited by RF reactive sputtering, and annealed in different temperatures.•As deposited films were amorphous while >400°C annealed films were crystallized as pure monoclinic WO3 phase.•Annealing at 400–500°C led to an oxygen deficient surface with a sub-stoichiometric WO3−x phase.•Optical bandgap decreased from 3.35eV to 3.17eV for the films annealed at 500°C.•Results are compared with MoO3 films deposited by a similar technique. 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 (200) and (002) preferred orientation, respectively, determined by XRD fiber texture analysis. Scherrer analysis of excess broadening indicated a coherency length of 50 and 65nm 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 3.30eV for the as-deposited films to 3.17eV for the 500°C annealed films. Results are compared with MoO3 films deposited by a similar technique.