Influence of Annealing Temperature on Characteristics of Bismuth Doped Zinc Oxide Films

In this study, Bismuth (Bi) doped ZnO thin films were deposited on quartz substrates by a sol-gel spin coating method and annealed at different annealing temperatures of 200, 300, 400, 500, 600 and 700°C, respectively. Structural and optical properties of nanocrystalline Bi-doped ZnO film on quartz were investigated by using X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and UV-VIS spectrophotometer. The high annealing temperature of 700°C as a critical temperature causes the crystallographic reorientation plane in ZnO:Bi nanostructure mostly due to the initial formation of the polycrystalline phase with the inter-grain segregation of Bi dopant atoms. Bi-incorporating ZnO films with an increase in annealing temperature resulted in a blue wavelength shift of the photon absorption edge. The optical band gap of the films was increased from 3.27 eV to 3.34 eV. By decreasing the annealing temperatures from 700 to 200°C, the grain size of Bi-doped ZnO decreased from 18 nm to 8 nm. The effect of the annealing temperature on the electrical conductivity had been considered. The low electrical conductivity of 0.9 (Ω.cm)^-1 was obtained for ZnO:0.2 film annealed at 600°C with good nano-crystallization. However, the Bi-doped ZnO films prepared by cost-effective spin coating technique provided to have a very high photon absorption coefficient (10⁴-10⁵ cm^-1) and did not appreciably affect the optical transparency. ZnO films doped with 0.2% at. Bi can be used as a high resistive buffer layer for solar cell application.