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Effect of annealing temperature on the optical parameters of CdS thin films prepared by thermal evaporation method
(2013-10-04)
Gaewdang, Thitinai
;
Wongcharoen, Ngamnit
;
Wongcharoen, Tiparatana
CdS thin films were prepared by thermal evaporation onto glass substrate in vacuum better than 5.5x10<sup>-5</sup> mbar. The obtained films were subsequently annealed in a pure nitrogen atmosphere at temperature between 100 to 500°C for 30 min. The crystal structure and surface morphology of the as-deposited and annealed films were investigated by XRD and SEM, respectively. Optical band gap and Urbach tail values of the films, determined from spectral transmission data, were found to be slightly varied in the range 2.36-2.40 eV and 110-160 meV, respectively, due to annealing temperature. The refractive index of the films was also evaluated from the spectral transmission data. The dependence of the refractive index on the wavelength obeys the single oscillator model, from which the important parameters such as refractive index, extinction coefficient, oscillator energy (E<inf>0</inf>) and dispersion energy (E<inf>d</inf>) of the films with different annealing temperatures were determined. From the experimental results, the optical parameters of the films are obviously influenced by annealing temperature. © (2013)Trans Tech Publications,Switzerland.
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Structural and thermoelectric properties of zn-doped cuprous aluminate delafossite
(2013-10-04)
Wongcharoen, Ngamnit
;
Gaewdang, Thitinai
;
Wongcharoen, Tiparatana
The polycrystalline Zn-doped CuAlO<inf>2</inf> ceramics were obtained by solid state reaction method. The mixture of high purity grade of CuO, Al<inf>2</inf>O<inf>3</inf> and Zn(CH<inf>3</inf>COO) <inf>2</inf>{dot operator}2H<inf>2</inf>O powders was ground and then pressed by using uniaxial pressure. The obtained pellet was sintered in air at 1,150 °C for 24 h. XRD patterns showed the crystal structure of the as-sintered CuAl<inf>1-x</inf>Zn<inf>x</inf>O<inf>2</inf> (0≤x≤0.05) belonging to rhombohedral, space group R3m along with the CuO and CuAl2O4 phases. From SEM micrographs, the grain size slightly decreases when the amount of Zn in CuAl<inf>1-x</inf>Zn<inf>x</inf>O<inf>2</inf> samples increases. Hall mobility and hole concentration of the as-sintered samples were obtained from Hall effect measurements in van der Pauw configuration at room temperature. The electrical resistivity measurements as a function of temperature were performed. The variation of Seebeck coefficient and power factor as a function of temperature was also investigated. In the present study, the Zndoped CuAlO<inf>2</inf> ceramics probably have advantages for use in high temperature thermoelectric devices. © (2013)Trans Tech Publications,Switzerland.
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Electrical properties of Al-doped CdO thin films prepared by thermal evaporation in vacuum
(2012-05-08)
Wongcharoen, Ngamnit
;
Gaewdang, Thitinai
;
Wongcharoen, Tiparatana
In this work, thin films of undoped and Al-doped CdO with 3, 5, 7 and 9 wt.% were prepared by thermal evaporation in vacuum on glass substrate. From XRD patterns, doping CdO films with Al up to 7 % causes small reduction in the intensity of the (200) plane, while small increase is observed in the intensity of (111) plane. However, intensity of all peaks rapidly decreases for the films with high Al content (9 %). The formation of tubular nanostructures of undoped and Al doped-CdO films was observed by SEM technique. Energy gap value of the films was evaluated from the optical transmission spectra in the spectral region 300-1000 nm. The best values of electrical conductivity, Hall mobility and electron concentration were obtained in the films with 5% Al doping. Temperature dependent resistivity measurements of the films doped with Al at 3, 5 and 7 % showed the metal-semiconductor transition around 100, 150 and 205 K respectively, which is rationalised by localisation of degenerate electrons in a weak-localisation regime. It was also found that the transition temperature is dependent on the Al concentration and is related to the increase in disorder induced by dopant addition. © 2011 Published by Elsevier Ltd.
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Characterisation of CdS/CdTe heterojunction solar cells by current-voltage measurements at various temperatures under illumination
(2012-05-08)
Gaewdang, Thitinai
;
Wongcharoen, Ngamnit
;
Wongcharoen, Tiparatana
Heterojunction CdS/CdTe thin film solar cells were fabricated with a superstate structure consisting of the successive layers: soda lime glass/ITO/CdS/CdTe/back contact. ZnTe:Cu films were deposited on the back surface of the CdTe layer presenting as ohmic back contact. The substrate was soda lime glass coated with ITO films by rf magnetron sputtering serving as the transparent front contact. A thin layer of CdS with thickness about 80 nm was applied by chemical bath deposition. Close-spaced sublimation of the CdTe films was accomplished by placing a CdTe source in a close proximity (6 mm) to the substrate in vacuum chamber with low pressure about 3×10 <sup>-2</sup> mbar. The source was heated to 550 °C and the substrate to 450 °C. This arrangement causes Cd and Te to sublime from source and diffuse to the substrate. The fabricated cells were investigated using current-voltage (I-V) in the temperature range 20-300 K under a standard AM1.5 illumination in order to define the transport mechanism in the heterojunction. Tunnelling enhanced interface recombination has been found to dominate carrier transport mechanism in the junction at all investigated temperatures. © 2011 Published by Elsevier Ltd.

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