Physical properties of Bi doped CdTe thin films grown by the CSVT method

A study of the physical properties of CdTe thin films doped with Bi is presented. CdTe:Bi thin films were deposited by the close space vapor transport (CSVT) technique using powdered CdTe:Bi crystals grown by the vertical Bridgman method. CdTe:Bi crystals were obtained with nominal Bi doping concentrations varying in the 1×10¹⁷–8×10¹⁸ cm⁻³ range. The physical properties of CdTe:Bi thin films were studied performing photoluminescence, X-ray, SEM, photoacoustic spectroscopy and resistivity measurements. We observed a decrease of the resistivity values of CdTe:Bi films with the Bi content as low as 6×10⁵ Ω-cm for Bi concentrations of 8×10¹⁸ cm⁻³. These are meaningful results for CdTe-based solar cells.     

Cadmium sulphide prepared from cadmium oxide thin films

Undoped and doped CdS thin films have conveniently been prepared by conversion of cadmium oxide thin films using a chalcogen vapour. Dopants can be incorporated during the oxide formation stage, and the effects of three different dopants on the carrier transport properties are compared. Four X-ray diffraction peaks of the thin film polycrystalline material could be associated with the hexagonal (wurtzite) phase, two of the peaks are also consistent with the cubic (zincblende) phase. The reaction time required for the quantitative conversion of CdO to CdS decreased as the temperature increased, and quantitative conversion occurred in 30 min at 400°C to give a Cd/S atomic ratio of 1.01. The absorption spectrum indicated a direct transition with a band gap of 2.42 eV. The surface of the sample appeared uniform and densely packed with crystallites that are substantially smaller than 1 μm. This preparation technique can conveniently provide CdS films having a carrier concentration over the whole range of 10¹¹ to 10¹⁷ cm⁻³. Likewise, the resistivities can be conveniently controlled with values ranging from 5 Ω cm to 5 × 10⁷ Ω cm. In addition, this material has been prepared by several other procedures described in the literature, and the properties of the films are compared.     

Highly conducting transparent nanocrystalline Cd1−xSnxS thin film synthesized by RF magnetron sputtering and studies on its optical, electrical and field emission properties

Transparent conducting Cd_1−xSn_xS thin films have been synthesized by radio frequency magnetron sputtering technique on glass and Si substrates for various tin concentrations in the films. X-ray diffraction studies showed broadening of peaks due to smaller crystal size of the Cd_1−xSn_xS films, and SEM micrographs showed fine particles with average size of 100 nm. Sn concentration in the films was varied from 0% to 12.6% as determined from energy-dispersive X-ray analysis. The room-temperature electrical conductivity was found to vary from 8.086 to 939.7 S cm⁻¹ and corresponding activation energy varied from 0.226 to 0.076 eV. The optimum Sn concentration for obtaining maximum conductivity was found to be 9.3%. The corresponding electrical conductivity was found to be 939.7 S cm⁻¹, and the mobility 49.7 cm² V⁻¹ s⁻¹. Hall measurement showed very high carrier concentrations in the films lying in the range of 8.0218×10¹⁸–1.7225×10²⁰ cm⁻³. The conducting Cd_1−xSn_xS thin films also showed good field emission properties with a turn on field 4.74–7.86 V μm⁻¹ with variation of electrode distance 60–100 μm. UV–Vis–NIR spectrophotometric studies of the films showed not needed the optical band gap energy increased from 2.62 to 2.80 eV with increase of Sn concentration in the range 0–12.6%. The optical band gap was Burstein–Moss shifted, and the corresponding carrier concentration obtained from the shift also well matched with that obtained from Hall measurement.     

Electrochemical deposition of Zn3P2 thin film semiconductors on tin oxide substrates

Zn₃P₂ semiconductor thin films were prepared by electrodeposition technique form aqueous solutions. The deposition mechanism was investigated by cyclic voltammetry technique. Crystal structure, morphology and composition of as deposited and annealed Zn₃P₂ thin films grown on SnO₂/glass substrates were determined by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis. X-ray diffraction data indicated the formation of Zn₃P₂ as the predominant phase for both as-deposited and annealed films. The compositions of the deposited films were controlled by the bath temperature, deposition potential and Zn/P ratio in the solution.The dark current–voltage measurements of SnO₂/Zn₃P₂/C devices indicated a rectifying behavior and a reverse saturation current density of 1.7×10⁻⁷ A/cm², which is in good accordance with that obtained from films prepared using vacuum technique. Also, the capacitance–voltage measurements showed that the number of interface states and the built in potential are in the order of 5×10⁻⁹ cm⁻³ and 0.85 V, respectively. These preliminary results for Zn₃P₂ thin films reveal that, this semiconductor material can be used for solar cell applications.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Effect of 8 MeV electron irradiation on electrical properties of CuInSe2 thin films

Radiation damages due to 8 MeV electron irradiation in electrical properties of CuInSe₂ thin films have been investigated. The n-type CuInSe₂ films in which the carrier concentration was about 3×10¹⁶ cm⁻³, were epitaxially grown on a GaAs(0 0 1) substrate by RF diode sputtering. No significant change in the electrical properties was observed under the electron fluence <3×10¹⁶ e cm⁻². As the electron fluence exceeded 10¹⁷ e cm⁻², both the carrier concentration and Hall mobility slightly decreased. The carrier removal rate was estimated to be about 0.8 cm⁻¹, which is slightly lower than that of III–V compound materials.     

Effect of proton irradiation on electrical properties of CuInSe2 thin films

High-energy proton irradiation (380 keV and 1 MeV) on the electrical properties of CuInSe₂ (CIS) thin films has been investigated. The samples were epitaxially grown on GaAs (0 0 1) substrates by Radio Frequency sputtering. As the proton fluence exceeded 1×10¹³ cm⁻², the carrier concentration and mobility of the CIS thin films were decreased. The carrier removal rate with proton fluence was estimated to be about 1000 cm⁻¹. The electrical properties of CIS thin films before and after irradiation were studied between 80 and 300 K. From the temperature dependence of the carrier concentration in CIS thin films, we found N_D=9.5×10¹⁶ cm⁻³, N_A=3.7×10¹⁶ cm⁻³ and E_D=21 meV from the fitting to the experimental data on the basis of the charge balance equation. After irradiation, a defect level was created, and N_T=1×10¹⁷ cm⁻³ for a fluence of 3×10¹³ cm⁻², N_T=5.7×10¹⁷ cm⁻³ for a fluence of 1×10¹⁴ cm⁻² and E_T=95 meV were also obtained from the same fitting. The new defect, which acted as an electron trap, was due to proton irradiation, and the defect density was increased with proton fluence.     

Porous alumina templates and nanostructured CdS for thin film solar cell applications

Nanostructured CdS was grown by electrodeposition of cadmium sulfide inside a porous alumina template. Uniform pore size and spacing in the template was achieved when the starting material for the template was aluminum foil. Typical pore size was 45 nm. Nanostructured CdS was also deposited by electrodeposition on indium tin oxide (ITO)-coated glass and by solution growth on ITO-coated glass. Schottky diodes were formed on nanocrystalline CdS and the analysis of their current–voltage characteristics yielded a diode ideality factor (n) of 2.6 and a reverse saturation current density (J_S) of 1.00×10⁻⁵ A/cm². Corresponding values for the Schottky diode on polycrystalline CdS were 3.4 and 1.93×10⁻⁶ A/cm².     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

ZnO:F thin films deposited by chemical spray: effect of the fluorine concentration in the starting solution

Conductive and transparent fluorine-doped zinc oxide, ZnO:F, thin films were deposited at 500°C on glass substrates by the chemical spray technique. Two sets of samples were deposited, based on a fresh and a two-day old starting solution prepared from zinc acetyl acetonate and ammonium fluoride dissolved in a mixture of water and alcohol. The electrical, structural, morphological and optical characteristics of the films as a function of the fluorine concentration in the starting solution were studied. Films with a resistivity as low as 2×10⁻² Ω cm, mobility up to 5 cm²/(V s), carrier concentration in the range 1.5–5.7×10²⁰ cm⁻³ and a transmittance in the range 75–90% were achieved with the old solution. Films were polycrystalline, growing preferentially along the (0 0 2) and (1 0 0) directions, depending on the fluorine concentration in the starting solution.     

Effect of the substrate temperature and acidity of the spray solution on the physical properties of F-doped ZnO thin films deposited by chemical spray

F-doped ZnO thin films were prepared by using the spray pyrolysis technique. The dependence of the electrical, optical, structural and morphological properties on the substrate temperature and spray solution acidity was studied. Additionally, aging of the spray solution presents a clear effect on the resistivity of ZnO thin films. The best films obtained show a resistivity, mobility and carrier concentration of the order of 1.5×10⁻² Ω cm, 6 cm²/V s and 2×10¹⁹ cm⁻³, respectively. Wurtzite hexagonal structure, with a preferential growth along the [0 0 2] direction for all substrate temperatures and acidities used, was obtained. From scanning electron microscopy and atomic force microscopy analysis, it was determined that the grain size of the films decreases and its homogeneity increases when the acidity of the starting solution is increased. High optical transmittances, in the order of 90%, were obtained in all the cases.     

Hydrogen desorption properties of Mg thin films at room temperature

Pd–Mg–Pd thin films prepared by magnetron sputtering could absorb hydrogen entirely at room temperature and dehydrogenate completely and rapidly in ambient air. Our investigations of the structural, optical and electrical properties gave a detailed insight into the desorption mechanism. The overall activation energy and the hydrogen diffusion coefficient were deduced to be 48 kJ mol⁻¹ and 8.0 × 10⁻¹⁵ cm² s⁻¹ based on optical and electrochemical measurements, respectively. The desorption process followed the nucleation and growth mechanism by modeling and simulating the resistance data. The small activation energy and remarkable diffusion kinetics highlighted the applicability as on-board hydrogen storage systems.     

Electronic and photovoltaic properties of Al/p-Si/copper phthalocyanine photodiode junction barrier

Al/p-Si/copper phthalocyanine photovoltaic device has been fabricated and characterised by current–voltage and capacitance–voltage measurements. Electrical properties of the device were determined by current–voltage characterizations under dark and illumination conditions. The density distribution of the interface states of the photodiode was found to vary from 8.88×10¹² eV⁻¹ cm⁻² in E_ss-0.54 eV to 4.51×10¹² eV⁻¹ cm⁻² in E_ss-0.61 eV. The device shows a photovoltaic behaviour with a maximum open circuit voltage Voc of 0.16 V and short-circuits current I_sc of 0.45 μA under 3500 lux light intensity.     

A note on the Hall mobility and carrier concentration in pyrite thin films

This note presents results on Hall effect characteristics (mobility and density of majority charge carriers) of undoped p-type and n-doped pyrite thin films. Carrier mobilities between 200 and 0.07 cm²/Vs have been measured in undoped p-type films. Doped n-type films present values which vary from 200 to 0.1 cm²/Vs. The corresponding carrier densities change from 2×10¹⁸ to 10²² cm⁻³ in p-type films and from 6×10¹⁷ to 10²¹ cm⁻³ in n-type films. These results reinforce the physical basis and conclusions of the modelling of pyrite solar cells accomplished by Altermatt et al. (Sol. Energy Mater. Sol. Cells 71 (2002) 181).     

Effects of water in phthalocyanine layers

The molecular water concentration inside zinc phthalocyanine (ZnPc) thin films was measured. After exposure to air, gas effusion experiments show that the ZnPc layers contain (1.7±0.4)×10²⁰ water molecules per cm³, which corresponds to 1 H₂O per 10 ZnPc units. We can distinguish a mobile and an immobilized population of H₂O in ZnPc films. The mobile part effuses out at room temperature when exposing the films to a low pressure of 10⁻² mbar, whereas temperature activation is needed to reach a complete out-diffusion of water. The effusion process was observed to proceed with a diffusion coefficient D_H2O of (1.3±0.3)×10⁻¹⁰ cm² s⁻¹ at 296 K. The rate of water effusion directly correlates with the timescale of the decrease of surface conductivity when exposing the layers to an equally low pressure. This indicates the existence of an electrically active surface layer of water molecules, which is refilled from the bulk of water molecules during the effusion process.     

Preparation and characterisation of electrodeposited n-CdS/p-CdTe thin film solar cells

Cadmium sulphide and cadmium telluride films have been electrodeposited for n-CdS/p-CdTe solar cells. Cell efficiency varied considerably from 9.5% to 11.5% for each deposition set. The reverse saturation currents of 9.5% and 11.5% cells at 298 K were 25 and 6.7 nA cm⁻², respectively. The cells with higher efficiency has a lower number of interface states than the less efficient cells. The 11.5% cell had interface states (N_IS) of 3× 10¹⁰ cm⁻² eV⁻¹ at zero volt bias in dark and when it was illuminated with 35 mW cm⁻² light at zero volt bias N_IS increased by two orders to 1.2×10¹² cm⁻² eV⁻¹. At higher frequency the large voltage intercept of the Mott-Schottky plot indicates the existence of the near intrinsic layer of the polycrystalline heterojunction.     

Reduction of defects of polycrystalline silicon thin films by heat treatment with high-pressure H2O vapor

An improvement of electrical properties of pulsed laser crystalllized silicon films was achieved by simple heat treatment with high-pressure H₂O vapor. The electrical conductivity of 7.4×10¹⁷ cm⁻³ phosphorus-doped 50-nm-thick pulsed laser crystallized silicon films was markedly increased from 1.6×10⁻⁵ S/cm (as crystallized) to 2 S/cm by heat treatment at 270°C for 3 h with 1.25×10⁶ Pa H₂O vapor because of reduction of density of defect states localized at grain boundaries. Spin density was reduced from 1.7×10¹⁸ cm⁻³ (as crystallized) to 1.2×10¹⁷ cm⁻³ by heat treatment at 310°C for 3 h with 1.25×10⁶ Pa H₂O vapor.     

Low-temperature deposition of polycrystalline silicon thin films by hot-wire CVD

Polycrystalline silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430°C. The material properties have been optimized for photovoltaic applications by varying the hydrogen dilution of the silane feedstock gas, the gas pressure and the wire temperature. The optimized material has 95% crystalline volume fraction and an average grain size of 70 nm. The grains have a preferential orientation along the (2 2 0) direction. The optical band gap calculated from optical absorption by photothermal deflection spectroscopy (PDS) showed a value of 1.1 eV, equal to crystalline silicon. An activation energy of 0.54 eV for the electrical transport confirmed the intrinsic nature of the films. The material has a low dangling bond-defect density of 10¹⁷ cm³. A photo conductivity of 1.9 × 10⁻⁵ Ω⁻¹cm⁻¹ and a photoresponse (σ_ph/σ_d) of 1.4 × 10² were achieved. A high minority-carrier diffusion length of 334 nm as measured by the steady-state photocarrier grating technique (SSPG) and a large majority-carrier mobility-lifetime (μτ) product of 7.1 × 10⁻⁷cm²V⁻¹ from steady-state photoconductivity measurement ensure that the poly-Si : H films possess device quality. A single junction n---i---p cell made in the configuration n⁺-c-Si/i-poly-Si: H/p-μc-Si : H/ITO yielded 3.15% efficiency under 100 mW/cm² AM 1.5 illumination.     

Physical properties of ZnO:F obtained from a fresh and aged solution of zinc acetate and zinc acetylacetonate

Fluorine-doped zinc oxide thin films, ZnO:F, were deposited by the spray pyrolysis technique on sodocalcic glass substrates. Two different zinc precursors were used separately, namely, zinc acetate and zinc pentanedionate. The effect of the zinc precursor type, the aging of the starting solution, the substrate temperature and a vacuum-annealing treatment on the electrical, morphological, structural and optical properties was studied, in order to obtain conductive and transparent ZnO:F thin films. The resistivity values of ZnO:F thin films deposited from aged solutions were lower than those films obtained from fresh solutions. The lowest resistivity values of as-grown films deposited at 500 °C, using a two-day aged starting solution of zinc acetate and zinc pentanedionate, were 1.4×10⁻² and 1.8×10⁻² Ω cm, respectively. After a vacuum annealing treatment performed at 400 °C for 30 min a decrease in the resistivity was obtained, reaching a minimum value of 6.5×10⁻³ Ω cm for films deposited from an aged solution of zinc acetate. The films were polycrystalline, with a (0 0 2) preferential growth orientation in all the cases. Micrographs obtained by SEM show a uniform surface covers by rounded grains. No evident change in the surface morphology was observed with the different precursors used. The transmittance of films in the visible region was higher than 80%.     

Oxygen- and hydrogen-permeation measurements on-mixed conducting SrFeCo0.5Oy ceramic membrane material

The SrFeCo_0.5O_y system combines high electronic/ionic conductivity with appreciable oxygen permeability at elevated temperatures. This system has potential use in high-temperature electrochemical applications such as solid oxide fuel cells, batteries, sensors, and oxygen separation membranes. Dense ceramic membranes of SrFeCo_0.5O_y are prepared by pressing a ceramic powder prepared by using a sol–gel combustion technique. Oxygen and hydrogen permeation at high temperature on this material are studied. Measurements are conducted using a time-dependent permeation method at the temperature in the range of 1073–1273 K with oxygen- and hydrogen-driving pressures in the range (3×10²)–(1×10⁵) Pa (300–1000 mbar). The maximum oxygen-permeated flux at 1273 K is 6.5×10⁻³ mol m⁻² s⁻¹. The activation energies for the O₂-permeation fluxes and diffusivities are 240 and 194 kJ/mol, respectively. Due to the high fragility, the high temperature for the measurements and the high oxygen permeation through such material, a special membrane holder, and compression sealing system have been designed and realized for the permeation apparatus.