Abstract
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.
Highlights
In a role of anti-reflection coating layer for gap of 3.2-3.3 eV at room temperature
The results show that these films exhibit a relatively high-preferential orientation along the c-axis and imply to the effectiveness of Bi incorporation
By using a sol-gel spin coating process, interstitial Bi impurities incorporated in the Zinc Oxide (ZnO) nanostructure were investigated under the different annealing temperatures (200-700°C)
Summary
In a role of anti-reflection coating layer for gap of 3.2-3.3 eV at room temperature. Due to its photovoltaic application, nanostructured ZnO exhibits an features such as good optical transparency in the visible excellent surface texture and can improve the solar cell wavelength range, the values of electrical resistance in the wide range of 10−4 -1012 Ω.cm. Bi-doped ZnO films (denoted as ZnO:Bi) were deposited by using RF magnetron sputtering (Jiang et al, 2009) They reported that the film annealed at 400°C had the lowest electrical resistivity (1.89×10−3 Ω.cm), high carrier density (3.45×1020 cm−3) and good transmittance in visible wavelength region. ZnO doped with 0.2-1.0 at% Bi films which were highly textured along the c-axis have been successfully deposited on quartz substrates by sol-gel spin-coating technique (Krongarrom et al, 2012). The effect of annealing temperatures on the structural, optical and electrical properties of Bi-doped ZnO films was investigated. The current-voltage characteristic of ZnO:Bi films and its dependence on the strain along the c-axis and crystal quality are discussed
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