X-ray fluorescence investigation of the Ga distribution in Cu(In,Ga)Se2 thin films

The efficiencies of Cu(In,Ga)Se₂/CdS/ZnO solar cell devices in which the absorbers are produced by classical two-step processes are significantly lower that those in which co-evaporated absorbers are used. A significant problem related to two-step growth processes is the reported segregation of Ga towards the Mo back contact, resulting in separate CuInSe₂ and CuGaSe₂ phases. Furthermore, it is often reported that material losses (especially In and Ga) occur during high-temperature selenization of metallic precursors. In this study, X-ray fluorescence (XRF) analysis was used to study the diffusion behaviour of the chalcopyrite elements in single-stage and two-stage processed Cu(In,Ga)Se₂ thin films. This relatively simple characterization technique proved to be very reliable in determining the degree of selenium incorporation, possible material losses and the in-depth compositional uniformity of samples at different stages of processing. This information is especially important in the case of two-stage growth processes, involving high-temperature selenization steps of metallic precursors. Device quality Cu(In,Ga)Se₂ thin films were prepared by a relatively simple and reproducible two-step growth process in which all the metals were evaporated from one single crucible in a selenium-containing environment. The precursors were finally treated in an H₂Se/Ar atmosphere to produce fully reacted films. XRF measurement indicated no loss of In or Ga during this final selenization step, but a significant degree of element diffusion which depended on the reaction temperature. It was also possible to produce Cu(In,Ga)Se₂ thin films with an appreciable amount of Ga in the near-surface region without separated CuInSe₂ and CuGaSe₂ phases.     

Characterization of co-sputtered Cu-In alloy precursors for CuInSe2 thin films fabrication by close-spaced selenization

Sputtering technique for Cu–In precursor films fabrication using different Cu and In layer sequences have been widely investigated for CuInSe₂ production. But the CuInSe₂ films fabricated from these precursors using H₂Se or Se vapour selenization mostly exhibited poor microstructural properties. The co-sputtering technique for producing Cu–In alloy films and selenization within a close-spaced graphite box resulting in quality CuInSe₂ films was developed. All films were analysed using SEM, EDX, XRD and four-point probe measurements. Alloy films with a broad range of compositions were fabricated and XRD showed mainly In, CuIn₂ and Cu₁₁In₉ phases which were found to vary in intensities as the composition changes. Different morphological properties were displayed as the alloy composition changes. The selenized CuInSe₂ films exhibited different microstructural properties. Very In-rich films yielded the ODC compound with small crystal sizes whilst slightly In-rich or Cu-rich alloys yielded single phase CuInSe₂ films with dense crystals and sizes of about 5 μm. Film resistivities varied from 10⁻²–10⁸ Ω cm. The films had compositions with Cu/In of 0.40–2.3 and Se/(Cu+In) of 0.74–1.35. All CuInSe₂ films with the exception of very Cu-rich ones contained high amount of Se (>50%).     

Investigation of the chalcogen interdiffusion in CuIn(TeSe)2 thin films

Graded thin films of CuInSe₂ on CuInTe₂ have been obtained by annealing of precursor structures containing Se and Te separated in depth. The depth profile of the phases in the film was investigated using X-ray diffraction with grazing incidence of the primary beam. Quasi-epitaxial growth of CuInSe₂ on a CuInTe₂ film next to the Mo back-electrode was observed after annealing at 450°C in vacuum. Annealing at higher temperature lead to chalcogen interdiffusion resulting in quaternary films. However, heat treatments of already reacted films did not result in any detectable interdiffusion. From these results the mechanisms governing the growth of films from precursors containing the chalcogens Se and Te separated in depth are discussed with respect to their application for thin film solar cells.     

A comparative study of CuInSe2 and CuIn3Se5 films using transmission electron microscopy, optical absorption and Rutherford backscattering spectrometry

CuInSe₂ and CuIn₃Se₅ films were grown by stepwise flash evaporation onto glass and Si substrates held at different temperatures. Transmission electron microscopy (TEM) studies revealed that the films grown above 370 K were polycrystalline, with CuInSe₂ films exhibiting larger average grain size than CuIn₃Se₅. Optical absorption studies yielded band gaps of 0.97±0.02 and 1.26±0.02 eV for CuInSe₂ and CuIn₃Se₅, respectively. Rutherford backscattering spectrometry (RBS) study of the films on Si showed that CuInSe₂/Si structures included an inhomogeneous interface region consisting of Cu and Si, whereas CuIn₃Se₅/Si structures presented sharp interface.     

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.     

Preparation and study of structural and optical properties of CSVT deposited CuInSe2 thin films

A simple close-spaced vapour transport (CSVT) system has been designed and fabricated. Copper indium diselenide (CuInSe₂) thin films of wide range of thickness (4000–60000 Å) have been prepared using the fabricated CSVT system at source temperatures 713, 758 and 843 K. A detailed study on the deposition temperature has been made and the temperature profile along with the reaction kinetics is reported. The composition of the chemical constituents of the films has been determined by energy dispersive X-ray analysis. The structural characterization of the as-deposited CuInSe₂ films of various thicknesses has been carried out by X-ray diffraction method. The diffractogram revealed that the CuInSe₂ films are polycrystalline in nature with chalcopyrite structure. The structural parameters such as lattice constants, axial ratio, tetragonal distortion, crystallite size, dislocation density and strain have been evaluated and the results are discussed. The surface morphology of the as-deposited CuInSe₂ thin films has been studied using scanning electron microscope. The transmittance characteristics of the CuInSe₂ films have been studied using double beam spectrophotometer in the wavelength range 4000–15000 Å and the optical constants n and k are evaluated. The absorption coefficient has been found to be very high and is of the order of 10⁵–10⁶ m⁻¹. CuInSe₂ films are found to have a direct allowed transition and the optical band gap is found to be in the range 0.85–1.05 eV.     

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.     

Effect of selenization pressure on CuInSe2 thin films selenized using co-sputtered Cu-In precursors

CuInSe₂ thin films were formed from the selenization of co-sputtered Cu–In alloy layers. These layers consisted of only two phases, CuIn₂ and Cu₁₁In₉, over broad Cu–In composition ratio. The concentration of Cu₁₁In₉ phase increased by varying the composition from In-rich to Cu-rich. The composition of co-sputtered Cu–In alloy layers was linearly dependent on the sputtering power of Cu and In targets. The metallic layers were selenized either at a low pressure of 10 mTorr or at 1 atm Ar. A small number of Cu–Se and In–Se compounds were observed during the early stage of selenization and single-phase CuInSe₂ was more easily formed in vacuum than at 1 atm Ar. Therefore, CuInSe₂ films selenized in vacuum showed smoother surface and denser microstructure than those selenized at 1 atm. The results showed that CuInSe₂ films selenized in vacuum had good properties suitable for a solar cell.     

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.     

Real-time investigations on the formation of CuInSe2 thin film solar cell absorbers from electrodeposited precursors

In this article, we present results of a detailed real-time X-ray diffraction (XRD) study on the formation of CuInSe₂ from electroplated precursors. The solid-state reactions observed during the selenisation of three different types of precursors are presented. The first type of precursors (I) consists of the nanocrystalline phases Cu_2−xSe and InSe at room temperature, which react to CuInSe₂ starting at 470 K. The second type of precursor (II) shows an inhibited CuInSe₂ formation out of the initial phases Cu_2−xSe and γ-In₂Se₃ starting at 400 K. The third precursor type (III) shows completely different selenisation behaviour. Starting from the intermetallic compound Cu₁₁In₉ and amorphous selenium, the formation of the binary selenides In₄Se₃ and CuSe is observed after the melting point of selenium at 494 K. After selenium transfer reactions, the compound semiconductor CuInSe₂ is formed out of Cu_2−xSe and InSe. This type (III) reaction path is well known for the selenisation of SEL precursors (stacked elemental layers of sputtered copper and indium and thermally evaporated selenium).     

Sub-micrometer thick CuInSe2 films for solar cells using sequential elemental evaporation

CuInSe₂ thin films were prepared using sequential vacuum evaporation of In, Se and Cu at moderately low substrate temperatures, avoiding any treatment using toxic H₂Se gas. The samples were annealed at 400 °C at a pressure of 10⁻⁵ mbar to form CuInSe₂. Structural, optical, electrical, compositional and morphological characterizations were carried out on these films. We could obtain highly stoichiometric film, using this simple method, without opting for co-evaporation or high substrate temperature for deposition.     

Analysis of the optical absorption characteristics of CuInSe2 thin films

Thin films of compound CuInSe₂ have been developed onto glass substrates by in situ thermal annealing of the stack of successively evaporated elemental layers in vacuum. The atomic compositions and the optical properties of the films have been determined by proton-induced X-ray emission (PIXE) method and spectrophotometry in the photon wavelength range of 300–2500 nm, respectively. The typical optical absorption characteristic of the films has been critically analysed. The absorption coefficients vary from 10³ to 10⁵ cm⁻¹ in the measured wavelength range. The films have more than one type of fundamental electronic transitions. Direct allowed and direct forbidden transitions vary between 0.947 to 0.989 eV and 1.099 to 1.204 eV, respectively, depending on the composition of the films. The former transition varies inversely with the Cu/In ratio while the latter shows no such dependence. Valence band splittings due to spin–orbit coupling converge towards the single-crystal value for the near-stoichiometric (NS) and Cu-rich films.     

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.     

The influence of the vacuum annealing process on electrodeposited CuInSe2 films

CuInSe₂ films were electrodeposited on mechanical polished Mo substrates. The applied potential was adjusted to get a stoichiometric composition. The as-deposited films were annealed in a high vacuum system for a short time. The films have been characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, Auger electron spectroscopy. The results indicate that the crystallization of the films was greatly improved by the short time vacuum annealing process without significant change in composition. The capacitance–voltage measurement shows characteristic p-type behaviors. This annealing process after electrodeposition was proved to be a useful method to prepare the polycrystalline CuInSe₂ films for solar cell application.     

Band-gap engineering in CuIn(Se,S)2 absorbers for solar cells

Thin films based on CuInSe₂ have become very successful as absorber layers for solar cells. It is only in the recent past that gallium (Ga) and sulfur (S) were incorporated into CuInSe₂ in order to increase the energy band gap of the film to an optimum value with the ultimate aim of producing more efficient devices. This paper focuses on the incorporation of S into partly selenized CuInSe₂ films in order to produce CuIn(Se,S)₂ films with varying S/Se+S ratios, resulting in different band-gap energies. This was achieved by varying the conditions when selenizing Cu/In alloys in H₂Se/Ar, and then exposing these various partly selenized films to H₂S/Ar under identical conditions.     

SEM analysis and selenization of Cu-In alloy films produced by co-sputtering of metals

Co-sputtered copper-indium (Cu-In) alloy layers were investigated as precursors for CuInSe₂ (CIS) formation. Results of scanning electron microscopy (SEM), EDS and X-ray diffraction (XRD) studies reveal the inhomogeneity of the films composition. The films have a rough surface structure with well-defined islands crystallized within the film matrix. The elemental composition of the island-type crystals corresponds to the compound CuIn₂ and the composition of the matrix area corresponds to the Cu11In₉ phase. The influence of heating temperature, time and Se pressure on the morphology and composition of films is studied using SEM, XRD and Raman spectroscopies. Thereby optimal technological parameters for the production of single-phase CIS layers are determined.     

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.     

Observation of the growth process of CuInSe2 thin films

In order to observe the growth process of CuInSe₂ thin films by a usual vacuum evaporation method with two sources, thin films of the compound were grown on different substrates: glass slides, SnO₂, Al₂O₃, and molybdenum. During deposition, the shutter was moved stepwise to obtain layers for different evaporation times, and crystallographical, compositional, optical and electrical properties were investigated. In general, in the initial stage of evaporation, the films are In-rich, and then move toward the stoichiometrical composition. The grain size grows larger, and the film quality becomes better with time. The detailed variations of the properties of the films are presented.     

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.     

Study of CuInSe2 thin films prepared by electrodeposition

Thin films of p-type CuInSe₂ prepared by a one-step electrodeposition method have been studied by constructing CdS/CuInSe2 junctions. After the electrodeposition, the CuInSe₂ films were treated either in vacuum or in Ar. Cells of the form CdS (high σ)/CdS (low σ)/CuInSe₂ were then fabricated for studying the electrodeposited films. Measurements were specifically carried out to determine the diffusion length of minority carriers in the p-type CuInSe₂. It was found that the minority carrier diffusion length in CuInSe₂ films treated in Ar was generally greater than that for films treated in vacuum under similar conditions. A small area cell (active area 0.11 cm²) with a conversion efficiency of about 7% (under 125 mW/cm² illumination) has been fabricated.