Preparation of CuGaS2 thin films by two-stage MOCVD method

Copper gallium disulfide (CuGaS₂; CGS) films were deposited on glass or ITO glass by two-stage metal-organic chemical vapor deposition (MOCVD) method, using Cu- and Ga/S-containing precursors without toxic H₂S gas. First, pure Cu thin films were prepared on glass substrates by using a single-source precursor, bis(ethylbutyrylacetato)copper(II) or bis(ethylisobutyrylacetato)copper(II). Second, the resulting Cu films were processed using tris(N,N-ethylbutyldithiocarbamato)gallium(III) at 410-470 °C to produce CuGaS₂ films. The optical band gap of the CGS film grown at 440 °C was about 2.53 eV. In addition, it was found that the elemental ratio of Cu and Ga elements of the CGS films can be elaborately adjusted by controlling deposition conditions on demand.     

Electrical and luminescent properties of CuGaSe2 crystals and thin films

CuGaSe₂ thin films with thicknesses of about 2 μm were prepared by flash and single source evaporation onto mica and (1 1 0)-oriented ZnSe substrates in the substrate temperature range 150–450°C. The obtained polycrystalline CuGaSe₂ films had the chalcopyrite structure with the predominant growth direction 2 2 1. Hall effect, conductivity and luminescence measurements have been carried out on CuGaSe₂ thin films and source materials: CuGaSe₂ single crystals grown by Bridgman technique and by chemical vapour transport using I₂ as transport agent. All films and crystals are p-type. Two acceptor levels with ionization energies E_A150–56 meV and E_A2130–150 meV have been identified as due to Ga vacancy and presence of Se atoms on interstitial sites respectively.     

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.     

Carbon-doped Sb2S3 thin films: Structural, optical and electrical properties

We report the modification of electrical properties of chemical-bath-deposited antimony sulphide (Sb₂S₃) thin films by thermal diffusion of carbon. Sb₂S₃ thin films were obtained from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on glass substrates. A carbon thin film was deposited on Sb₂S₃ film by arc vacuum evaporation and the Sb₂S₃-C layer was subjected to heating at 300 °C in nitrogen atmosphere or in low vacuum for 30 min. The value of resistivity of Sb₂S₃ thin films was substantially reduced from 10⁸ Ω cm for undoped condition to 10² Ω cm for doped thin films. The doped films, Sb₂S₃:C, retained the orthogonal stibnite structure and the optical band gap energy in comparison with that of undoped Sb₂S₃ thin films. By varying the carbon content (wt%) the electrical resistivity of Sb₂S₃ can be controlled in order to make it suitable for various opto-electronic applications.     

Electrical and optical characterisation of CuInS2 crystals and polycrystalline coevaporated thin films

Single crystals CuInS₂ were grown by iodine vapour transport method, whereas polycrystalline thin films were obtained by coevaporation technique from three sources. The temperature dependence of the hole mobility in valence band is analysed by taking into account contributions from several scattering mechanisms of the charge carriers. To account for the temperature dependant conductivity of polycrystalline CuInS₂ thin films, grainboundary conduction process was suggested. In the low temperature region, we interpret the data in terms of the Mott law and the analysis is very consistent with the variable range hopping. However, thermionic emission is predominant at high temperatures. Photoluminescence measurements have been performed on CuInS₂ crystals and the analysis has revealed that the emission is mainly due to free-to-bound and donor–acceptor pair transitions. The band gap of that compound is derived from the excitonic emission line at 1.53 eV.     

Characterization of CuInS2 thin films prepared by electrodeposition and sulfurization with photoluminescence spectroscopy

Potentiostatic electrodeposition and sulfurization techniques were used to prepare polycrystalline CuInS₂ thin films. X-ray diffraction and photoresponse measurements in a photoelectrochemical cell (PEC) revealed that photoactive polycrystalline CuInS₂ films can be deposited on Ti substrate. Photoluminescence (PL) spectroscopy was used to investigate the prepared thin films and optically characterize them. PL spectra revealed the defect structure of the samples with an acceptor energy level at 109 meV above the valance band and a donor energy level at 71 meV below the conduction band. The CuInS₂ thin films prepared in this investigation are observed to be In-rich material with n-type electrical conductivity.     

Photoluminescence properties of sodium incorporation in CuInSe2 and CuIn3Se5 thin films

CuInSe₂ and CuIn₃Se₅ thin films have been deposited using sodium compounds such as Na₂Se and Na₂S onto Corning 7059 glass substrates by the two-stage co-evaporation method. Enhanced grain growth and preferred (1 1 2) grain orientation as well as a decrease in resistivity with respect to undoped films were observed with sodium incorporation. A clear correlation between the photoluminescence spectra and the resistivity of the films was found by comparing the properties of films with and without Na incorporation. These observations suggest that compensation is reduced due to the suppression of donor-type defects by the presence of Na.     

Structural and optical properties of CuInTe2 films prepared by thermal vacuum evaporation from a single source

The CuInTe₂ thin films were prepared by thermal vacuum evaporation from a single source. The effects of heat treatment on both structural an optical properties of CuInTe₂ films were studied. X-ray diffraction studies reveal that the films prepared by the present method showed formation of single phase CuInTe₂ at heating treatment temperature higher than 300°C. The calculated lattice constants for CuInTe₂ films annealed at 300°C were found to be and . The value of the grain size obtained in these films was of the order of 50 nm. The value of optical energy gap (0.97 eV) and the dispersive refractive index n(λ) for CuInTe₂ film annealed at 300°C were evaluated by optical absorption measurement.     

Structural and optical properties of sulfur-annealed CuInS2 thin films

CuInS₂ thin films were deposited by single source vacuum thermal evaporation method on substrates submitted to longitudinal thermal gradient. Some of these films were annealed in sulfur atmosphere and converted into CuInS₂ homogenous layers. Both of the as-deposited and sulfurated films were characterized by X-ray diffraction, optical transmission and reflection measurements. The optical band gap of films after sulfurization was 1.50 eV which is near the optimum value for photovoltaic energy conversion.     

Effect of Cl ion implantation on electrical properties of CuInSe2 thin films

The effects of Cl ion implantation on the properties of CuInSe₂ epitaxial thin films have been investigated. Using five kinds of accelerating energies, the doped layer with a constant profile of Cl concentration along the depth direction was fabricated. From the results of reflection of high-energy electron diffraction, the damages due to implantation were removed by annealing at 400°C in N₂. The conductivity type in all implanted films was n-type, and the carrier concentration was increased with increasing Cl concentration in the thin films. Consequently, it is considered that Cl acts as a donor in CuInSe₂.     

Fabrication of CuInSe2 films and solar cells by the sequential evaporation of In2Se3 and Cu2Se binary compounds

Cu₂Se/In_xSe(x≈1) double layers were prepared by sequentially evaporating In₂Se₃ and Cu₂Se binary compounds at room temperature on glass or Mo-coated glass substrates and CuInSe₂ films were formed by annealing them in a Se atmosphere at 550°C in the same vacuum chamber. The In_xSe thickness was fixed at 1 μm and the Cu₂Se thickness was varied from 0.2 to 0.5 μm. The CuInSe₂ films were single phase and the compositions were Cu-rich when the Cu₂Se thickness was above 0.35 μm. And then, a thin CuIn₃Se₅ layer was formed on the top of the CuInSe₂ film by co-evaporating In₂Se₃ and Se at 550°C. When the thickness of CuIn₃Se₅ layer was about 150 nm, the CuInSe₂ cell showed the active area efficiency of 5.4% with V_oc=286 mV, J_sc=36 mA/cm² and FF=0.52. As the CuIn₃Se₅ thickness increased further, the efficiency decreased.     

Modeling the optical properties of WO3 and WO3-SiO2 thin films

The optical properties and surface morphology of sol-gel spin coated WO₃ and WO₃-SiO₂ composite films annealed at 250 and are investigated. For the purpose of extracting the optical parameters of the films, a novel form for the dielectric function is introduced, consisting of two Tauc-Lorentz oscillators and an Urbach tail component, which is suited for amorphous multi-transition materials with substantial subgap absorption. The evolution of the refractive indices, transmittances, and band gaps with doping is marked by sizable shifts at 2.0-2.5% SiO₂ doping for the films, and 4.0-4.5% doping for the films. In addition, pronounced changes in the surface roughness of the films occur at these doping values.     

Optical and structural properties of TiO2 thin films prepared by sol-gel spin coating

Hydrogen is a renewable and non-polluting fuel. Its production from water using renewable energy is an attractive challenge. In this work we report some results on the preparation of titanium oxide TiO₂ thin films for environmental applications such as water photosplitting. TiO₂ thin films have been prepared by spin coating technique of sol precursor onto glass substrates. The deposited films were annealed at different temperatures in air. The X-ray diffraction (XRD) experiments show that the two well-known anatase and rutile phases were observed depending both on the conditions of deposition and on the temperature of annealing. The best conditions of crystallization were found to be around 400 °C in air. The influence of the number of deposited layer on the crystalline quality of the films was investigated. The surface morphology of the deposited film was characterized by atomic force microscopy (AFM) and scanning electronic microscopy (SEM). The UV-Vis-NIR spectroscopy shows that the film exhibits a high transmission around 90%. The best layers were obtained when concentrated (HCl) was added to the sol solutions. The direct band gap of the films was found to be around 3.7 eV, and their refractive index was found to vary from 2 to 2.4.     

Photoelectrochemical properties of spray deposited n-ZnIn2Se4 thin films

Zinc indium selenide (ZnIn₂Se₄) thin films have been prepared by spraying a mixture of an equimolar aqueous solution of zinc sulphate (ZnSO₄), indium trichloride (InCl₃), and selenourea (CH₄N₂Se), onto preheated fluorine-doped tin oxide (FTO)-coated glass substrates at optimized conditions of substrate temperature and a solution concentration. The photoelectrochemical (PEC) cell configuration of n-ZnIn₂Se₄/1 M (NaOH+Na₂S+S)/C has been used for studying the current voltage (I–V), spectral response, and capacitance voltage (C–V) characteristics of the films. The PEC study shows that the ZnIn₂Se₄ thin films exhibited n-type conductivity. The junction quality factor in dark (n_d) and light (n_l), series and shunt resistance (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. The measured (FF) and η of the cell are, respectively, found to be 0.435% and 1.47%.     

Sol-gel SiO2/TiO2 bilayer films with self-cleaning and antireflection properties

We report here that a facile sol-gel dip-coating technique can be used to fabricate a SiO₂/TiO₂ bilayer film with self-cleaning and antireflection properties. The bottom SiO₂ layer acts as an antireflection coating due to its lower refractive index; the top TiO₂ layer acts as a self-cleaning coating generated from its photocatalysis and photo-induced superhydrophilicity. The maximal transmittance of SiO₂/TiO₂ bilayer film at normally incident light can be reached 96.7%, independent of the high refractive index and coverage of TiO₂ nanoparticles. However, the photocatalytic activity of the bilayer film shows a close dependence on coverage of TiO₂ nanoparticles. After illuminated by ultraviolet light, the SiO₂/TiO₂ bilayer films are superhydrophilic with water contact angle less than 2°, which favors greatly the self-cleaning function of the films.     

TiO2 and TiO2–SiO2 thin films and powders by one-step soft-solution method: Synthesis and characterizations

Simple soft-solution method has been developed to synthesize films and powders of TiO₂ and mixed TiO₂–SiO₂ at relatively low temperatures. This method is simple and inexpensive. Furthermore, reactor can be designed for large-scale applications as well as to produce large quantities of composite powders in a single step. For the preparation of TiO₂, we used aqueous acidic medium containing TiOSO₄ and H₂O₂, which results in a peroxo-titanium precursor while colloidal SiO₂ has been added to the precursor for the formation of TiO₂–SiO₂. Post annealing at 500 °C is necessary to have anatase structure. Resulting films and powders were characterized by different techniques. TiO₂ (anatase) phase with (1 0 1) preferred orientation has been obtained. Also in TiO₂–SiO₂ mixed films and powders, TiO₂ (anatase) phase was found. Fourier transform infrared spectroscopy (FTIR) results for TiO₂ and mixed TiO₂–SiO₂ films have been presented and discussed. The method developed in this paper allowed obtaining compact and homogeneous TiO₂ films. These compact films are highly photoactive when TiO₂ is used as photo anode in an photoelectrochemical cell. Nanoporous morphology is obtained when SiO₂ colloids are added into the solution.     

Formation and metal-to-insulator transition properties of VO2-ZrV2O7 composite films by polymer-assisted deposition

VO₂-ZrV₂O₇ composite films were prepared on silica glass substrates by polymer-assisted deposition using a V-Zr-O solution. The coexistence of ZrV₂O₇ and VO₂ was confirmed by Raman spectroscopy, glance angle X-ray diffraction, and X-ray photoelectron spectroscopy. The composite films with similar thickness of about 95 nm showed decreased particle sizes and significantly enhanced luminous transmittances (from 32.3% at Zr/V=0 to 53.4% at Zr/V=0.12) with increasing Zr/V rations. The enhancement in the luminous transmittance was ascribed to the absorption-edge changes in the composite films. This feature benefits the application of VO₂ to smart windows.     

Optical and electrical properties of flash-evaporated amorphous CuInSe2 films

Amorphous films of CuInSe₂ were deposited on glass substrate by flash evaporation of source materials. The films were found to be p-type semiconductors. The direct optical band-gap energy was obtained to be 1.21–1.41 eV. The film DC conductivity ranged from 1.2–5.7 S cm⁻¹ at 285 K for different film thickness with corresponding activation energy of 55.5–301 meV. From temperature dependence of conductivity, the carrier transport was interpreted to be due to band conduction above 270 K.     

Structural, morphological and electrochemical properties of nanocrystalline V2O5 thin films deposited by means of radiofrequency magnetron sputtering

Due to easiness of preparation and high energy density, V₂O₅ nanocrystalline thin films are particularly attractive as cathode materials for all-solid-state rechargeable lithium microbatteries. However, their electrochemical performances are strictly related to the film microstructure, which, in turn, is related to the nature and parameters of the deposition technique. For this reason, the preparation of thin films with reproducible electrochemical properties is still an open problem.Here, we report on the deposition of V₂O₅ crystalline thin films by means of reactive radiofrequency (r.f.) magnetron sputtering, using vanadium metal as the target. Different deposition times and substrate temperatures were adopted. X-ray powder diffraction (XRD) and atomic force microscopy were used to investigate the structural and morphological features of the films. In particular, XRD analysis revealed that the deposition parameters affect the crystallographic orientation of the films. A h 0 0 orientation is observed in case of thin samples (about 100 nm) prepared at 300 °C, whereas a 1 1 0 preferential growth is obtained for thicker films. Films deposited at 500 °C display a 0 0 1 orientation irrespective on the deposition time.Reversible Li intercalation/deintercalation processes and high specific capacity are observed for the h 0 0-oriented V₂O₅ thinner films, with the ab plane arranged perpendicular to the substrate. In this case, the cycling behaviour is very promising, and a stable capacity higher than 300 mAh g⁻¹ was delivered in the potential range 3.8-1.5 V at 1C rate over at least 70 cycles.     

Pyrite (FeS2) thin films prepared by spray method using FeSO4 and (NH4)2Sx

A simple spray method for the preparation of pyrite (FeS₂) thin films has been studied using FeSO₄ and (NH₄)₂S_x as precursors for Fe and S, respectively. Aqueous solutions of these precursors are sprayed alternately onto a substrate heated up to 120°C. Although Fe–S compounds including pyrite are formed on the substrate by the spraying, sulfurization of deposited films is needed to convert other phases such as FeS or marcasite into pyrite. A single-phase pyrite film is obtained after the sulfurization in a H₂S atmosphere at around 500°C for 30 min. All pyrite films prepared show p-type conduction. They have a carrier concentration (p) in the range 10¹⁶–10²⁰ cm⁻³ and a Hall mobility (μ_H) in the range 200–1 cm²/V s. The best electrical properties (p=7×10¹⁶ cm⁻³, μ_H=210 cm²/V s) for a pyrite film prepared here show the excellence of this method. The use of a lower concentration FeSO₄ solution is found to enhance grain growth of pyrite crystals and also to improve electrical properties of pyrite films.