Evolution of the properties of CuAlSe2 thin films with the oxygen content

CuAlSe₂ thin films have been synthesized by annealing, under argon flow, a multilayer structure of thin Cu, Al and Se layers sequentially deposited by evaporation under vacuum. It is shown that the oxygen content depends not only on the evaporation content but also on the argon flow. The properties of the thin films are modified by this oxygen percentage: when no more than 4–5 at % of oxygen is present in the films, their optical properties are very similar to that of single crystals. The inter-band transitions A, B, C typical of such chalcopyrite structure are clearly visible on the absorption spectra. The variation of the conductivity of these films in the high temperature domain is as that expected in CuAlSe₂, while in the low temperature, grain boundaries are dominant.     

Influence of CdCl2 treatment on structural and optical properties of vacuum evaporated CdTe thin films

In this article we have discussed the structural, optical properties of vacuum evaporated CdTe thin films before and after CdCl₂ treatment. The CdTe thin films were prepared by vacuum evaporation. Films were prepared under the vacuum of 10⁻⁶ Torr. The structural studies have been performed by the X-ray diffraction (XRD) technique. The XRD analysis of vacuum evaporated CdTe films reveals that the structure of films is polycrystalline in nature. However, the crystallinity has been improved after the CdCl₂ treatment as shown by an increase of the diffraction peak intensities. This is due to the enhancement in the atomic mobility of CdTe. The optical properties of the CdTe thin films have been studied by the spectrophotometer in the 300–800 nm wavelength range. It is observed that the optical band gap energy is highly dependent on CdCl₂ treatments. The optical transitions in these films are found to be direct and allowed.     

p-Type CuSbS2 thin films by thermal diffusion of copper into Sb2S3

We report the preparation of copper antimony sulfide (CuSbS₂) thin films by heating Sb₂S₃/Cu multilayer in vacuum. Sb₂S₃ thin film was prepared from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on well cleaned glass substrates. A copper thin film was deposited on Sb₂S₃ film by thermal evaporation and Sb₂S₃/Cu layers were subjected to annealing at different conditions. Structure, morphology, optical and electrical properties of the thin films formed by varying Cu layer thickness and heating conditions were analyzed using different characterization techniques. XRD analysis showed that the thin films formed at 300 and 380 °C consist of CuSbS₂ with chalcostibite structure. These thin films showed p-type conductivity and the conductivity value increased with increase in copper content. The optical band gap of CuSbS₂ was evaluated as nearly 1.5 eV.     

A comparative study of optical, electrical and structural properties of CuGaSe2 and CuGaTe2 thin films

The growth conditions, the composition and the structural, optical and electrical properties of thin films of CuGaSe₂ and CuGaTe₂ have been studied using “flash” and “slow” evaporation in vacuum. Single phase films, when analyzing the absorption coefficient, present several energy gaps. For CuGaSe₂, they are 1.59, 1.66, 2.03 and 2.11 eV, for CuGaTe₂ 1.23 and 1.89 eV. Both the CuGaSe₂ and CuGaTe₂ evaporated films are p-type; the resistivities, carrier densities and mobilities are appropriate for thin film solar cells.     

Optical characterization of In2S3 solar cell buffer layers grown by chemical bath and physical vapor deposition

In this paper, we study the optical properties of indium sulfide thin films to establish the best conditions to obtain a good solar cell buffer layer. The In₂S₃ buffer layers have been prepared by chemical bath deposition (CBD) and thermal evaporation (PVD). Optical behavior differences have been found between CBD and PVD In₂S₃ thin films that have been explained as due to structural, morphological and compositional differences observed in the films prepared by both methods. The resultant refractive index difference has to be attributed to the lower density of the CBD films, which can be related to the presence of oxygen. Its higher refractive index makes PVD film better suited to reduce overall reflectance in a typical CIGS solar cell.     

Synthesis and characterization of β-FeSi2 grown by thermal annealing of Fe/Si bilayers for photovoltaic applications

In this article we have reported the synthesis and characterization of FeSi₂ thin films. The Fe/Si thin films were obtained by electron beam evaporation. Thermal annealing was carried out at 650°C for 1 h. The formation of the β-FeSi₂ layers were characterized by the X-ray diffraction method and found to be polycrystalline in nature. The structural parameters were evaluated from the XRD pattern. The possible optical transition in these films is found to be direct and allowed.     

Electrochromic behaviour of Nb2O5 thin films with different morphologies obtained by spray pyrolysis

Two different procedures to stabilize the precursor NbCl₅ have been applied to obtain Nb₂O₅ thin films by spray pyrolysis. Depending on the procedure used, determined by the way in which the precursor solution was injected into the air stream of the spray nozzle, niobium oxide thin films with different surface morphologies can be obtained. The structural properties of the Nb₂O₅ thin films depend on the post-annealing temperature because as-deposited films are amorphous, independently of the synthesis procedure used. The electrochromic behaviour has been estimated for all films, where monochromatic colouration efficiency (at 660 nm) of 25.5 cm²/C and a cathodic charge density close to 24 mC/cm² were found to give the best results to date for niobium oxide thin films obtained by spray pyrolysis.     

Precursor modification for preparation of CIS films by selenization technique

CuInSe₂ films have been prepared using the selenization technique. Preparation of the precursor as well as selenization were carried out by the vacuum evaporation technique. The sequence of copper and Indium layer deposition during precursor preparation affects the morphological and structural properties of precursor which directly have effects on the properties of selenized CIS films. A thin layer of amorphous selenium at the substrate/film interface has been used to improve the adherence of the film. The effect of the Se under-layer has been studied on the layers of copper, indium, CuIn precursors and CIS films, using structural, morphological and optical properties. The surface morphology of a single layer of copper and indium, with and without the Selenium under-layer, are quite different and drastically affect the properties of the precursor and selenized films. The Se under-layer does not take part in the chemical reaction of CIS formation during the selenization process. The modified CIS films are uniform, single phase, polycrystalline, chalcopyrite with (1 1 2) preferred orientation showing an energy band gap of 0.99 eV and an absorption coefficient of 10⁵ cm⁻¹, and have good adherence to the substrate for the scotch tape test.     

Cu–Ga–Se thin films prepared by a combination of electrodeposition and evaporation techniques

Cu–Ga–Se thin films were prepared using a combination of electrodeposition and evaporation techniques. A Cu–Se/Mo/glass precursor thin film was first prepared by galvanostatic electrodeposition. On top of this film three different thicknesses of Ga were deposited by evaporation. The Cu–Ga–Se thin films were formed by annealing the Ga/Cu–Se/Mo/glass thin film configuration in a tubular chamber with Se powder, at different temperatures. Thin films were characterized by X-ray diffraction (XRD), photocurrent spectroscopy (PS), inductively coupled plasma (ICP) analysis, and scanning electron microscopy (SEM). The detailed analysis from X-ray reveals that after annealing at 550 °C the CuGaSe₂ phase is formed when the thickness of Ga is 0.25 μm, however at 0.5 μm and 1.0 μm Ga the formation of CuGa₃Se₅ and CuGa₅Se₈ phases is observed respectively. Band gap values were obtained using photocurrent spectroscopy.     

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.     

Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor

Surface sulfurization of Cu(In,Ga)Se₂ (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H₂S gas; a sequential evaporation of In₂S₃ after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga) (S,Se)₂ thin layer was grown on the surface of the CIGS thin film after sulfurization using In₂S₃, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques.     

Growth, structural and optical properties of copper indium diselenide thin films deposited by thermal evaporation method

Copper indium diselenide (CuInSe₂) compound was synthesized by reacting its constituent’s elements copper, indium and selenium in near stoichiometric proportions (i.e. 1:1:2 with 5% excess selenium) in an evacuated quartz ampoule. Synthesized pulverized compound material was used as an evaporant material to deposit thin films of CuInSe₂ onto organically cleaned sodalime glass substrates, held at different temperatures (300-573 K), by means of single source thermal evaporation method. The phase structure and the composition of chemical constituents present in the synthesized compound and thin films have been investigated using X-ray diffraction and energy dispersive X-ray analysis, respectively. The investigations show that CuInSe₂ thin films grown above 423 K are single phase, having preferred orientation of grains along the (112) direction, and having near stoichiometric composition of elements. The surface morphology of CuInSe₂ films, deposited at different substrate temperatures, has been studied using the atomic force microscopy to estimate its surface roughness. An analysis of the transmission spectra of CuInSe₂ films, recorded in the wavelength range of 500-1500 nm, revealed that the optical absorption coefficient and the energy band gap for CuInSe₂ films, deposited at different substrate temperatures, are ∼10⁴ cm⁻¹ and 1.01-1.06 eV, respectively. The transmission spectrum was analyzed using iterative method to calculate the refractive index and the extinction coefficient of CuInSe₂ thin film deposited at 523 K. The Hall effect measurements and the temperature dependence of the electrical conductivity of CuInSe₂ thin films, deposited at different substrate temperatures, revealed that the films had electrical resistivity in the range of 0.15-20 ohm cm, and the activation energy 82-42 meV, both being influenced by the substrate temperature.     

CuInSe2 thin film preparation through a new selenisation process using chemical bath deposited selenium

Copper indium selenide thin films were prepared through a novel and an eco-friendly selenisation process. In this method, selenium film required for selenisation was prepared using chemical bath deposition technique, at room temperature. Thus, totally avoided usage of highly toxic H₂Se or selenium vapour. Here, the process involved annealing the Stacked layer, Se/In/Cu in which Cu and In were deposited using vacuum evaporation technique. Investigations on the solid-state reaction between the layers were done by analysing structural and optical properties of films formed at different annealing temperatures. Optimum annealing condition for the formation of copper indium selenide thin film was found to be 673 K for 1 h in high vacuum. Compositional dependence of the growth process was also studied using various Cu/In ratios. Optical band gap was decreased with increase in Cu/In ratio. Carrier concentration and hence conductivity were found to be increased with increase in Cu/In ratio. The films obtained were p-type and highly Cu-rich films were degenerate.     

Crystal structure and composition of polycrystalline CuInSe2

This work presents and analyses the X-ray diffraction data for the semiconducting compound CuInSe₂, synthesized by the vertical Bridgman method. The (Cu/In) ratio was varied to produce a stoichiometric CuInSe₂ ingot. Structure factors (F_hkl) equations for the (hkl) reflections, which are available for the ternary chalcopyrite-structured semiconductors, were deduced analytically and have been used to calculate the relative peak intensities for CuInSe₂ diffraction planes.CuInSe₂ thin films were also prepared by flash evaporation of a stoichiometric CuInSe₂ powder, onto different substrates. Structural characterization of these films was carried out by X-ray diffraction and scanning electron microscopy studies. The composition of the different samples has been determined by energy dispersive spectrometry. The results obtained indicate the presence of the chalcopyrite phase and nearly stoichiometric compositions.     

The impact of etching during microfabrication on the microstructure and the electrical conductivity of gadolinia-doped ceria thin films

Gadolinia-doped ceria, Ce_0.8Gd_0.2O_1.9−x (CGO), thin films deposited by spray pyrolysis and annealed to different degree of crystallinity between 0% and 95% are exposed to different etchants and etching methods. The attack of the etchants on the CGO thin films is analyzed with respect to changes in microstructure and in-plane electrical conductivity. It is found that amorphous CGO films are dissolved in hydrochloric acid after elongated etching times. Hydrofluoric acid severely attacks CGO thin films after already short times of exposure (1 min), more intense the less crystalline the thin film is. Ar ion etching smoothens the surface of the CGO thin films without considerable removal of material. No microstructural attack of NaOH, CHF₃/O₂ and SF₆/Ar is found. The electrical conductivity is in general only affected when microstructural changes are severe. Therefore, it is concluded that CGO thin films can be well used as functional layers in micro-fabricated devices and that micro-fabrication is, with the exception of hydrofluoric, not harmful for the electrical properties of crystalline CGO thin films.     

Photoelectrical properties of ZnS thin films deposited from aqueous solution using pulsed electrochemical deposition

ZnS is an n-type semiconductor with a wide direct band gap (3.7 eV at room temperature), and it is very suitable as a window layer in heterojunction photovoltaic solar cells. We deposited ZnS thin films on Sn-doped In₂O₃-coated glass substrate using pulsed electrochemical deposition (ECD) from aqueous solutions containing Na₂S₂O₃ and ZnSO₄ with two different compositions, the first group grown from ZnSO₄-rich solution, and the second grown from Na₂S₂O₃-rich solution. We investigated electrical properties of the ZnS thin films and properties of contacts with different metals evaporated on the surfaces. We found that Au and In contacts have Ohmic-like characteristics to ZnS. Furthermore, we observed photoconductivity of the ZnS thin films by means of photoelectrochemical (PEC) measurements. We found that for both the groups of ZnS thin films, the as-deposited film shows weak photosensitivity and after annealing at 300 °C the photosensitivity improved.     

Thin films of CuInSe2 prepared by RF sputtering from various compositional powder targets

CuInSe₂ thin films were prepared in the temperature range of 300–500°C by RF sputtering from powder targets, which were previously synthesized by reacting Cu, In, and Se in various ratios. The peaks from X-ray diffraction analyses were assigned to the planes of the CuInSe₂ chalcopyrite structure. The full width at half maximum of the (112) diffraction peak decreased with an increase in Cu/In ratio in the thin films. The photoelectron energies of the prepared thin films agreed with those reported for single crystalline CuInSe₂ from X-ray photoelectron spectroscopy measurements. The electronic conduction type and optical properties were found to change according to the Cu/In ratio in the thin films.     

Solar cells based on CuIn(Se, S)2

In this contribution we report on three- and four source evaporation of CuIn(Se, S)₂ thin films. The existence of secondary phases as well as texture effects are discussed for these thin films. The quaternary system CuIn(Se, S)₂ will be considered regarding incorporation of selenium versus sulphur. Results of heterojunctions completed with (Zn, Cd)S or a composite window consisting of a thin CdS buffer and a highly conductive ZnO window layer will be reported and analyzed. A main feature will be the influence of CuS on the heterojunction and the methods to overcome this problem. Devices with an active area efficiency exceeding 10% could be obtained.     

Structure, morphology and photoelectrochemical activity of CuInSe2 thin films as determined by the characteristics of evaporated metallic precursors

CuInSe₂ thin films have been obtained by the sequential evaporation of Cu and In layers, and subsequent reaction at 400°C with elemental selenium vapor. The individual metallic film thickness and the substrate temperature during evaporation have been varied in order to promote intermixing and alloy formation before the selenization. The structure, morphology and photoelectrochemical activity of the CuInSe₂ films have been determined by the characteristics of the evaporated metallic precursors. An improvement in the CuInSe₂ quantum efficiency, related mainly to the increased homogeneity and smoothing of the sample surface, can be gained by using as precursors multiple stacked Cu–In bilayers evaporated onto unheated substrates.     

Compositional and optoelectronic properties of CIS and CIGS thin films formed by electrodeposition

CuInSe₂ (CIS) and Cu(In,Ga)Se₂ (CIGS) thin films were prepared by electrodeposition and processing. The influence of film deposition parameters such as bath composition, pH, deposition potential and material purity on film properties was studied. The structural, morphological, compositional and opto-electronic properties of electrodeposited and selenized CIS and CIGS thin films were characterized using various techniques. As-deposited as well as selenized films exhibited a compact or a granular morphology depending on the composition. The film stoichiometry was improved after selenization at 550°C in a tubular furnace. The films are formed with a mixed phase composition of CuInSe₂ and CuIn₂Se_3.5 ternary phases.