Bandgap bowing in Ta-W-O system for efficient solar energy conversion: Insights from density functional theory and X-ray diffraction

The electronic properties of Ta-W-O system have been studied using density functional theory with PBE0 hybrid functional. The bandgap calculations showed a large and composition-dependent bowing parameter. While the large bowing parameter at low W content (<25%) is related to the charge exchange between the localized W 5d state and the extended states in the conduction band, the volume change is considered the main reason for the large bowing parameter at high W content (>25%). The electron excitation from the Ta2O5 valence band to WO3 conduction band at high W content may contribute to the pronounced decrease in the conduction band energy. The staggered bandgap type between Ta2O5 and WO3, as revealed from the energy band diagram, resulted in efficient charge carriers' separation. The minimum hole effective mass occurs along the y-direction and decreases monotonically with increasing W content. Combining the wide tunability of bandgap with the small effective mass, it is suggested that Ta-W-O alloys can replace the various III-V materials for high efficiency solar cells as well as other optoelectronic devices.