A. A. Ramadan

Preparation and Properties of Electrochemically Deposited Tungsten Oxide Films

Tungsten oxide films of thicknesses 1095 nm have been cathodically deposited onto ITO coated glass substrates kept at room temperature, at a rate of 13 nm/min. The electrolyte solution has been prepared by adding 0.025 M tungsten (dissolved in 30% H 2 O 2 ) to 0.25 M H 2 SO 4 . XRD observations showed that all films are amorphous. The film resistivity at room temperature was found to be 9 × 10 6 Ω cm, and from the resistivity-temperature dependence, the thermal activation energy was obtained as 0.29 eV. The refractive index, n, and the extinction coefficient, k, have been computed from the corrected transmittance and reflectance over the spectral range 400 to 800 nm. Absorption versus photon energy curves give an indirect transition with optical energy gap of 3.25 and 3.36 eV for the as-deposited and coloured states, respectively. The electrochromic properties of the prepared films have been investigated in situ and both solar transmittance and optical density have been evaluated in the coloured and bleached states. The electrochemical coloration and bleaching were performed in an electrolyte of 0.4 M H 2 SO 4 solution. The coloration efficiency has been evaluated as 50 cm 2 /C at λ = 600 nm, which is comparable with the results obtained from films prepared by other methods.

The electrical properties of CdTe films of different preparation conditions in correlation with microstructure changes

Abstract Polycrystalline films of CdTe have been thermally deposited on glass substrates. Using the Van der Pauw technique, the dark resistivities have been measured at different temperature (298–523 K). The resistivity, ϱ, was found to decrease markedly in the range of thickness less than 400 nm and was then subjected to a slight decrease at larger thicknesses. The behaviour was found to fit properly with the Fuchs and Sondheimer relation with parameters ϱ = 4.39 × 10 5 Ω cm and mean free path, l = 734 nm. Raising the substrate temperature the decreasing the deposition rate were found to increase the resistivity monotonically. Annealing the prepared films caused a significant decrease in the resistivity particularly for films of small thicknesses (t

Thermal effects on structural characterization of evaporated CdTe films during and after deposition

Abstract CdTe films were thermally deposited onto amorphous substrates at different temperatures (25–250°C). Post-annealing under vacuum at 300°C for 2 h has been also carried out. Using X-ray diffraction, the structural characteristics (preferential orientation, stoichiometry, microstructural properties) have been studied. Due to the high degree of preferred orientation, Voigt analysis of single reflection was used to determine the microstructural properties (crystallite size and microstrain). Raising the substrate temperature was observed to lead to a decrease in both integrated intensity and degree of preferred orientation as well as an increase in crystallite size and internal microstrain associated with improving the film stoichiometry. Post-annealing was found to increase the integrated intensity, the crystallite size and the degree of preferred orientation. On the other hand, it resulted in a decrease of FWHM and microstrain. The relative change in such parameters decreases as the film thickness increased with a pronounced change in thinner films.

Optimisation of Preparative and Performance Parameters on Electrochromic Properties of Electrochemically Deposited Tungsten Oxide Films

Tungsten oxide films of 240–1080 nm thickness were deposited on indium tin oxide (ITO) coated glass substrates using an electrochemical deposition technique. All films were amorphous, as proved by X-ray diffraction (XRD), and had an electrical resistivity of 106 Ωcm and spectral transmittance exceeding 75% in the visible region. The electrochromic (EC) properties were measured in situ during colouration and bleaching cycles. The EC parameters, Tsolb, Tsolc, ΔTsol and Δ(OD)sol and the solar colouration efficiency ηsol were evaluated at different preparation and performance parameters. The results showed that at small film thickness, the solar colouration efficiency changes linearly and tends toward saturation at larger thickness. At colouration potentials ≥2 V, the solar colouration efficiency is almost constant whereas the active sites are transformed to colour centres. In contrast, the efficiency has an exponential dependence on electrolyte concentration. The optimum values are: film thickness = 1080 nm, colouration potential - 2 V and electrolyte concentration = 0.4 M. The corresponding EC parameters are: ΔTsol =0.458, Δ(OD)sol=0.632 and ηsol=34 cm2/C.

Thickness-dependence of stoichiometry and microstructure characteristics in correlation with conductivity type of CdTe films

CdTe films were prepared by physical vapour deposition on a substrate at room temperature (RT) as well as on a cold (LT) one using low deposition rate. The thickness-dependence of stoichiometry revealed an abrupt decrease in the Cd/Te ratio as the thickness increases. Change of thickness did not affect the type of observed (111) crystallographic texture, only the degree of preferred orientation is enhanced as the film grows. The internal strain was negligible while the crystallite size increased rapidly at small thickness (up to 400 nm), and less thickness dependence was observed with further film growth. However, thickness dependence of lattice parameters showed a minimum and a maximum at approximately 300 nm in the case of RT and LT, respectively. The observed change in conductivity from n- to p-type and its vital correlation with the stoichiometry and structural characteristics were presented. Based on thickness dependence of stoichiometry and lattice parameters as well as the conductivity type, formation and annihilation of lattice defects were considered.