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%).     

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.     

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.     

Depth profile analysis of CuInSe2 (CIS) thin films grown by the electrodeposition technique

In this study, we report the results obtained from the auger electron spectroscopy (AES) depth profiling of CIS thin films grown by the electrodeposition technique. This result enables one to do a comparison between the bulk and superficial elemental compositions. The AES result is also compared with that obtained by the inductively coupled plasma (ICP) spectroscopy. These results support our proposition that the electrodeposited CIS film has a Cu-rich bulk region and an In rich surface, which leads to the formation of an n-layer (CuIn₂Se_3.5) on the top of the p-type CIS (CuInSe₂) phase     

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.     

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.     

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.     

Measurement of CuInSe2 solar cell AC parameters

The AC parameters (cell capacitance and cell resistance) of Copper Indium Diselenide (CuInSe₂) solar cell are measured using time-domain technique. The cell capacitance is calculated from the open circuit voltage decay (OCVD) and cell resistance with solar cell I–V characteristics measured in dark. The solar cell exhibits high parallel resistance and low parallel capacitance. The doping concentration and built in voltage are derived from the 1/C_P² versus bias voltage graph. The built-in voltage of the solar cell shows good agreement with measurements published in the literature.     

Complexing agent effect on the stoichiometric ratio of the electrochemically prepared CuInSe2 thin films

The electrodeposition of CuInSe₂ is investigated to improve the stoichiometric properties of CuInSe₂ layers on indium tin oxide (ITO)-coated glass substrates and to develop one-step electrodeposition method for solar cell applications. XPS was utilized for the characterization of the surface properties of CuInSe₂ layers. The influence of the complexing agent, e.g. benzotriazole, bulk concentration of Cu and Se and deposition potentials on the stoichiometric properties, are discussed.     

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.     

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.     

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.     

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.     

Study of the sub-bandgap absorption and the optical transitions in CuInSe2 polycrystalline thin films

Optical transitions near the fundamental band edge are studied for CuInSe₂ films having various Cu/In ratios by analysing the variations of the absorption coefficient with incident photon energy. The results indicate different transitions depending upon the Cu/In ratio. There are sub-bandgap absorption for near stoichiometric and Cu-rich films. The results are compared to some literature data.     

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).     

Off-stoichiometry effect on the CuInSe2 dielectric function

The dielectric function of CuInSe₂ has been studied for various composition ratios of Cu/In. The crystals were cut from ingots of 10 mm diameter and 40 mm length, grown by the classical Bridgman method. Spectroscopic ellipsometry measurements have been performed at room temperature in the range of 1.5–5.5 eV. From the measured spectra of the imaginary part of the dielectric function ₂, the broadening effect of the Cu/In ratio has been examined in connection with photoluminescence measurements. All transition edges were found to broaden as the In content increases. The effect of copper d levels has been observed to dominate in the 2.5–3.5 eV range.     

Effect of bath temperature and annealing on the formation of CuInSe2

CuInSe₂ films of 2 μm thickness were electrodeposited potentiostatically, from aqueous solution containing thiocyanate as a complexing agent, on Mo substrates. For all the experiments, the potential of the potentiostatic deposition of the materials was chosen to be −1 V, whereas the bath temperature of electrolyte was varied from 20 to 80 °C. It was found that the electrodeposited CuInSe₂ was characterized by an amorphous layer and densely-packed nanometric grains with a good homogeneity. After vacuum annealing at 200 °C, glancing angle X-ray diffraction revealed the presence of the CuInSe₂ phase whereas annealing under selenium atmosphere lead to the growth of molybdenum selenide compound MoSe₂, in addition to a better crystallization of the copper indium diselenide compound. Scanning electron microscopic revealed that despite an increase in the grains dimensions, there was no significant change in the films surface morphology when the bath temperature was varied from 20 to 80 °C. At the same time, the composition of the electrodeposited Cu-In-Se layers becomes richer in copper. This increase in copper concentration is mainly compensated by a deficit in selenium atoms.     

An investigation into effect of cationic precursor solutions on formation of CuInSe2 thin films by SILAR method

SILAR deposition of CuInSe₂ films was performed by using Cu²⁺–TEAH₃ (cupric chloride and triethanolamine) and In³⁺–CitNa (indium chloride and sodium citrate) chelating solutions with weak basic pH as well as Na₂SeSO₃ solution at 70 °C. A separate mode and a mixed one of cationic precursor solutions were adopted to investigate effects of the immersion programs on crystallization, composition and morphology of the deposited CuInSe₂ films. Chelating chemistry in two solution modes was deducted based on IR measurement. The XRD, XPS and SEM results showed that well-crystallized, smoothly and distinctly particular CuInSe₂ films could be obtained after annealing in Ar at 400 °C for 1 h by using the mixed cationic solution mode.     

The formation of CuInSe2 thin film solar cell absorbers from electroplated precursors with varying selenium content

We present results from real-time X-ray diffraction experiments on the formation of CuInSe₂ solar cell absorbers by annealing precursors, produced by simultaneous electrodeposition of copper, indium and selenium. The investigations reveal, that a reduced amount of electrochemically deposited selenium is the decisive parameter in order to realise a chalcopyrite formation behaviour as observed for sputtered stacked elemental layer (SEL) precursors. A simultaneous electrodeposition of the elements copper, indium and selenium in the molar ratio 1:1:2 of the chalcopyrite CuInSe₂ leads to the formation of binary copper and indium selenides during the electrodeposition process. The existence of binary selenides besides the intermetallic phase Cu₁₁In₉ as initial phases leads to an unfavourable absorber morphology. This can be explained by the observed semiconductor formation mechanism. A reduction of the deposited amount of selenium favours the formation of the intermetallic compound Cu₁₁In₉ and reduces the amount of binary selenides. These precursors show a formation behaviour and resulting absorber morphology as known for sputtered SEL precursors.