Preparation and characterization of semiconducting Zn1−xCdxSe thin films

The electrodeposition of Zn_1−xCd_xSe polycrystalline semiconducting thin films from aqueous acidic bath without any additives onto tin oxide-coated conducting glass and titanium substrates are described. The influence of deposition parameters on the film formation and deposition mechanism based on cyclic voltammetry is discussed. X-ray diffraction studies showed the polycrystalline wurtzite nature for all the films deposited under the proposed conditions. The optical studies revealed the band gap values in the range between 2.82 and 1.72 eV as the film composition changes from ZnSe to CdSe. It has been observed that the concentration of cadmium salt plays an essential role on the alloy formation. The surface morphological studies and composition analysis were carried out and the results are discussed.     

Electrodeposition of CuInxGa1−xSe2 thin films

CuIn_xGa_1−xSe₂ (CIGS) polycrystalline thin films with various Ga to In ratios were grown using a new two-step electrodeposition process. This process involves the electrodeposition of a Cu–Ga precursor film onto a molybdenum substrate, followed by the electrodeposition of a Cu–In–Se thin film. The resulting CuGa/CuInSe bilayer is then annealed at 600°C for 60 min in flowing Argon to form a CIGS thin film. The individual precursor films and subsequent CIGS films were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and Auger electron spectroscopy. The as-deposited precursor films were found to be crystalline with a crystal structure matching that of CuGa₂. The annealed bi-layers were found to have the same basic chalcopyrite structure of CuInSe₂, but with peak shifts due to the Ga incorporation. Energy dispersive spectroscopy results show that the observed shifts correlate to the composition of the films.     

Investigations on ZnxCd1−xO thin films obtained by spray pyrolysis

Zn_xCd_1−xO thin films were prepared on glass substrates by spray pyrolysis technique. The precursor solutions were obtained by varying the concentration of Zn(NO₃)₂·6H₂O and Cd(NO₃)₂·4H₂O in bi-distilled water. The structural properties have been studied using X-ray diffraction spectra. All the structures include the basic compounds, i.e. ZnO and CdO. The orientation and the crystalline phases of the deposited films were specified. With the addition of Zn to the precursor solution, we can observe the preferential orientation of the CdO in the [2 0 0] direction. The electrical measurements were performed using method of four contacts. Thin films transmittances, in the 1.5–4.3 eV range, for different compositions have been measured and the optical gaps have been determined. The variations are explained considering the gaps of the two pure films. The influence of increased Cd concentration in the films on the structural, electrical and optical properties is investigated in this study.     

Composition, surface topography, structure, Raman, and electrochemical/photoelectrochemical characterisation of CdxHg1−xTe films

Cd-rich Cd_xHg_{1 − x}Te films have been electrodeposited under potentiostatic conditions on conducting glass and Ti substrates from an acidic solution containing the respective ions as Cd²⁺:Hg²⁺:HTeO₂⁺ = 100:1:2. Six films one after another have been prepared from a single electrochemical cell. EDAX analysis of the air annealed films show decreasing Hg content in the deposit as the number of film preparation increases. SEM analysis indicate undulatory surface with Hg-rich clusters at the top surface. XRD analysis indicate the presence of Cd_xHg_{1 − x}Te along with . The Cd_xHg_{1 − x}Te alloy formation have been confirmed from Raman shift measurements which change with composition, x. The as-deposited films are n-type but converts to p-type after air annealing. Spectral response measurements gave band gap values that change with Hg content in the deposit. Band gap values ranging from 1.1 eV to 1.45 eV have been estimated. Photoelectrochemical solar cells using polysulphide electrolyte have been fabricated which gave an open-circuit photovoltage and short-circuit photocurrent, respectively, as 325 mV and 5.5 mA/cm² under 60 mW/cm² intensity of illumination.     

Investigation of non-aqueous electrodeposited CdS/Cd1−xZnxTe heterojunction solar cells

Polycrystalline Cd_1−xZn_xTe solar cells with efficiency of 8.3% were grown by cathodic electrodeposition on glass/ITO/CdS substrates using non-aqueous ethylene glycol bath. The deposit is characterised versus the process conditions by XRD and found to possess a preferred (1 1 1) orientation on Sb doping in the electroplating bath. The surface morphology of the deposit is studied using atomic force microscope. The average RMS roughness for the ternary film was higher than that for the binary CdTe. Optical properties of the films were carried out to study the band gap and calculation of molar concentration ‘x’. The effects of Sb doping in CdS/Cd_1−xZn_xTe heterojunctions have been studied. The short circuit current density (c) was found to improve and series resistance (R_s) reduced drastically upon Sb doping. This improvement in J_sc is attributed to an increase in quantum efficiency. The evaluation of solar cell parameters was also carried out using the current–voltage characteristics in dark and illumination. The best results were obtained when 2×10⁻³ M ZnCl₂ along with antimony were present in the deposition bath. Under AM 1.5 conditions the open circuit voltage, short circuit current density, and fill factor of our best cell were V_oc=600 mV, J_sc=26.66 mA/cm², FF=0.42 and efficiency, η=8.3%. The carrier concentration and built-in potential of Cd_1−xZn_xTe calculated from Mott–Schottky plot was 2.72×10¹⁷ cm⁻³ and 1.02 eV.     

Electrical and photoluminescence properties of evaporated CuIn1−xGaxTe2 thin films

Polycrystalline thin films of CuIn_1−xGa_xTe₂ have been deposited by flash evaporation on Corning glass 7059 substrates at T_s=200°C. Hall and resistivity measurements have been carried out down to 77 K. These films are p-type and the variation of the resistivity may be linked to defects, disorder of the material or grain boundaries. The PL spectra of these films after annealing in argon atmosphere at T_a=450°C have showed a broad band emission between 0.98 and 1.12 eV in which the main peak appears at 1.05 eV (at 4.2 K).     

Properties of CuIn1−xGaxSe2

Polycrystalline bulk samples of CuIn_1−xGa_xSe₂ weregrown with nominal x = 0.15, 0.25 and 0.5. Mobility, conductivity and band gap were measured at room and low temperatures. Mobilities for x = 0.21 were several hundred cm² V⁻¹s⁻¹ at room temperature and for x = 0.15 were 10³ cm² V⁻¹ s⁻¹, all n type. The band gaps were estimated from the spectra of photoelectrochemical cells at room temperature (with 8.5 K photoluminescence estimates shown in brackets), as 1.10 eV (1.14) for x = 0.21, and 1.07 eV (1.093) for x = 0.15. Crystal mechanical properties as regards cracks were not as good as for CuInSe₂, using similar growth techniques.     

Preparation of CuIn1−xGaxSe2 thin films by sputtering and selenization process

CuIn_1−xGa_xSe₂ polycrystalline thin films were prepared by a two-step method. The metal precursors were deposited either sequentially or simultaneously using Cu–Ga (23 at%) alloy and In targets by DC magnetron sputtering. The Cu–In–Ga alloy precursor was deposited on glass or on Mo/glass substrates at either room temperature or 150°C. These metallic precursors were then selenized with Se pellets in a vacuum furnace. The CuIn_1−xGa_xSe₂ films had a smooth surface morphology and a single chalcopyrite phase.     

Structural and optical characterization of hot wall deposited CdSexTe1−x films

CdSe_0.3Te_0.7 alloy was prepared from the individual components and its composition and structural analysis were done. Films were prepared by hot wall deposition technique using 0.15 m length tube under a vacuum of 5×10⁻⁵ Torr on well cleaned glass substrates. The composition, structural, morphological, and optical properties of hot wall deposited films were investigated. The XRD analysis revealed that the films are like amorphous in nature for lower thicknesses but with increasing thickness a more preferred orientation along (1 0 1) direction was observed. The crystallite size (D), dislocation density (δ) and strain () were evaluated. From the EDX composition analysis, the individual concentrations of Se and Te in the films were estimated. An analysis of optical measurements shows that all the films have fairly good transparency above 850 nm. The optical band gap was found to be around 1.55 eV and decreases with increasing thickness. Also comparison of band gap with corresponding values for CdSe and CdTe are made.     

CuIn1−xGaxSe2-based photovoltaic cells from electrodeposited precursor films

We have developed an electrodeposition bath based on a buffer solution so that the stability of the electrodeposition process is enhanced and no metal oxides or hydroxides precipitate out of solution. The buffer-solution-based bath also deposits more gallium in the precursor films. As-deposited precursors are stoichiometric or slightly Cu-rich CuIn_1−xGa_xSe₂. Only a minimal amount of indium was added to the electrodeposited precursor films by physical vapor deposition to obtain a 9.4%-efficient device.     

Preparation of Culn(SxSe1−x)2 thin films by sulfurization and selenization

A CuIn(S_xSe_1−x)₂ alloy thin-film was prepared by selenization of CuInS₂: its composition ratio x can be controlled by the number of selenization cycles implemented. Crystallinity of the films was improved by annealing in vacuum. The resistivity of the film was about 1 Ω cm and increased by one to two orders of magnitude after KCN treatment. An 8.1 % efficiency solar cell was obtained by using this annealed alloy thin-film.     

Solar cells with Cu(In1−xGax)S2 thin films prepared by sulfurization

By rapid thermal processing of Cu/In/GaS precursors, good-quality CuIn_1–xGa_xS₂ films are synthesized. By suppressing the formation of In-rich hillocks, we could obtain homogeneous CuIn_1–xGa_xS₂ surfaces. A conversion efficiency of 12% has been achieved using a relatively low (1.2) Cu/In ratio.     

La1−xSrxMyFe1−yO3−δ perovskites as oxygen-carrier materials for chemical-looping reforming

The use of perovskites with the general formula La_1−xSr_xM_yFe_1−yO₃ (M = Ni, Co, Cr, Cu) as oxygen carriers for syngas generation from methane by Chemical Looping Reforming is investigated in the present work. The experimental study concerns the oxidation of a fuel, using the oxygen from a solid oxygen carrier, instead of oxygen from air. Subsequent oxidation of the reduced solid is performed either with gaseous oxygen or with water. In the latter case additional hydrogen is produced, which is very pure and therefore appropriate to be used in fuel cell applications. The performance of the candidate materials is ranked by taking into account the hydrogen and carbon monoxide yields during the fuel oxidation step as well as the amount of oxygen per mole solid (δ) that can be delivered reversibly to the fuel. The effect of the materials composition and of NiO addition is examined. The best performance was obtained with the La_0.7Sr_0.3Cr_0.1Fe_0.9O₃ sample mixed with 5% NiO. The H₂ yield was up to 90%.     

Characterization and electrochromic properties of CuxNi1−xO films prepared by sol–gel dip-coating

Cu_xNi_1−xO electrochromic thin films were prepared by sol–gel dip coating and characterized by XRD, UV–vis absorption and electrochromic test. XRD results show that the structure of the Cu_x Ni_1−xO thin films is still in cubic NiO structure. UV–vis absorption spectra show that the absorption edges of the Cu_xNi_1−xO films can be tuned from 335 nm (x = 0) to 550 nm (x = 0.3), and the transmittance of the colored films decrease as the content of Cu increases. Cu_xNi_1−xO films show good electrochromic behavior, both the coloring and bleaching time for a Cu_0.2Ni_0.8O film were less than 1 s, with a variation of transmittance up to 75% at the wavelength of 632.8 nm.     

Properties of BaCe1−xTixO3 materials for hydrogen electrochemical separators

In this work the structural, microstructural and electrical properties of BaCe_1−xTi_xO₃ materials were investigated. The series of materials with different titanium concentrations x (0–0.3) were prepared by solid-state reaction method. The structural studies by X-ray diffraction have shown that undoped material crystallizes in orthorhombic phase, while the increasing concentration of Ti dopant up to x = 0.2 leads to the ordering of the structure to phases with higher symmetries (tetragonal and even cubic). The estimated solubility limit was found to be not higher than 20 at.% of Ti. Microstructure observations by scanning electron microscopy and linear contraction determination have shown the strong influence of Ti dopant on microstructure and an improvement of sinterability. The DC four-probe electrical conductivity measurements accompanied by the potentiometric EMF measurements of solid-state electrochemical cells in controlled gas atmospheres (containing H₂, O₂ and H₂O) and temperatures (500–800 °C) allowed determination of the total and partial electrical conductivities of selected materials. It was found that the introduction of Ti dopant leads to a decrease in total electrical conductivity by ca. one order of magnitude compared to the undoped material, almost independently of Ti concentration. Also, the modification of transport properties after doping with titanium was determined.     

Compositional dependence of the optical properties and the H local bonding in a-Si1−xGex: H (x<0.4) thin films

A study on hydrogenated amorphous silicon-germanium alloys is presented. Amorphous thin films are prepared by RF magnetron co-sputtering. The dependence of the optical properties and parameters and the local hydrogen bonding on the composition of a-Si_1−xGe_x: H films (x < 0.4) has been investigated. It is very important to take into account the variation of hydrogen concentration when the influence of Ge content on optical properties and parameters is considered.     

Optical, electrical and structural investigations on Cd1−xZnxSe sintered films for photovoltaic applications

II–VI polycrystalline semiconducting materials have come under increased scrutiny because of their wide use in the cost reduction of devices for photovoltaic applications. Cd_1−xZn_xSe is an important semiconducting alloy because of the tunability of its physical parameters such as band gap and lattice parameters by controlling its stoichiometry. Many more material characteristics of it would be altered and excellently controlled by controlling system composition x.Polycrystalline thin films of Cd_1−xZn_xSe with variable composition (0x1) have been deposited onto ultra-clean glass substrates by sintering process. The optical, structural and electrical transport properties of Cd_1−xZn_xSe thin films have been examined. The optical band gap and optical constants of these films were determined by using double beam spectrophotometer. The DC conductivity and activation energy of the films were measured in vacuum by two-probe technique. The Schottky junction of Cd_1−xZn_xSe with indium was made and the barrier height and ideality factor were determined using current–voltage characteristics. The nature of sample, crystal structure and lattice parameters were determined from X-ray diffraction patterns. The films were polycrystalline in nature having cubic zinc-blende structure over the whole range studied.Sintering is very simple and viable compared to other cost intensive methods. The results of the present investigation will be useful in characterizing the material, Cd_1−xZn_xSe, for its applications in photovoltaics.     

The performance of CuIn1−xGaxSe2-based photovoltaic cells prepared from low-cost precursor films

In this paper we report the 15.4%- and 13.4%-efficient CuIn_1−xGa_xSe₂ (CIGS)-based devices from electrodeposited (ED) and electroless deposited (EL) precursors. The efficiency of the device prepared from electroless precursor film has been improved from 12.4% to 13.4%. The dependence of quantum efficiencies on reverse-bias voltage has been measured for a 15.4%-efficient ED device, 18.8%-efficient physical-vapor-deposited device, and 14.2%-efficient Cd-free device. The purpose of this work is to explore and improve the current collection mechanism.     

Photoelectrochemical characteristics of brush plated tin sulfide thin films

Thin films of tin sulfide find wide applications in optoelectronic devices and window materials for heterojunction solar cells. Thin films of p-SnS were brush plated onto tin oxide coated glass substrates from aqueous solution containing SnCl₂ and Na₂S₂O₃. Deposits have been characterized with XRD and SEM for structural analysis. Hot probe method showed invariably p-type nature for all the brush plated SnS films. The variation of space charge capacitance, C_sc, with applied potential, V, was recorded for the PEC cell with p-SnS/Fe³⁺, Fe²⁺/Pt system. The spectral response of the PEC cell formed with SnS photoelectrode was studied and reported.     

The study on high efficient AlxGa1−xAs/GaAs solar cells

The investigation of Al_xGa_1−xAs/GaAs solar cells is carried out by means of both metalorganic chemical vapor deposition (MOCVD) and liquid-phase epitaxial (LPE) technique. The measurements of illuminated I–V characteristics, dark I–V characteristics and quantum efficiencies were performed for the GaAs solar cells made in author's laboratory. The measuring results revealed that the quality of materials in GaAs solar cell's structures is the key factor for getting high-efficient GaAs solar cells, but the effect of post-growth technology on the performances of GaAs solar cells is also very strong. The 21.95% (AM0, 2×27 cm², 25°C) high conversion efficiency in a typical GaAs solar cell has been achieved owing to improving the quality of materials as well as optimizing the post-growth technology of devices.