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.     

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.     

The formation of CuInSe2 thin-film solar cell absorbers by laser annealing of electrodeposited precursors

The formation of the compound semiconductor CuInSe₂ by laser annealing of electroplated precursor films in inert gas atmosphere represents an entire non-vacuum production process of thin-film solar cell absorbers. Besides this technological aspect, the impact of extreme annealing rates on structural properties of the resulting semiconductor is interesting from a fundamental research point of view. For this reason, we compared absorbers processed by laser annealing with absorbers annealed with moderate heating rates in the range of 1 K/s by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). All absorbers processed with laser or furnace annealing consist of crystalline CuInSe₂ in the chalcopyrite crystal structure with a high degree of cation disorder. We show that laser annealing does not lead to unintentional selenium loss during the semiconductor formation process.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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%).     

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.     

Effects of annealing on CuInSe2 films grown by molecular beam epitaxy

CuInSe₂ (CIS) thin films with a range of Cu/In ratios were grown by molecular beam epitaxy on GaAs (0 0 1) at substrate temperatures of T_s = 450–500°C and the effects of annealing under various atmospheres have been investigated. Photoluminescence spectra obtained from an ex-situ vacuum annealed CIS film at a temperature of T_A = 350°C showed a red-shift and a broadening of an emission peak (peak c) which originally appeared at 0.970 eV before annealing and the red-shifted peak c was found to consist of two overlapping peaks. The excitation power dependence of these overlapping peaks indicated the radiative recombination processes associated with the emissions to be a conduction band to acceptor transition (peak at 0.970 eV) and a transition due to donor-acceptor pairs (peak at 0.959 eV), indicating the formation of a shallow donor-type defect during the vacuum annealing process. The origin of this defect has tentatively been attributed to Se vacancies. On the other hand, the molar fraction of oxygen increased with increasing annealing temperature in dry-air. An epitaxially grown In₂O₃ phase was found both in Cu-rich and In-rich films annealed at T_A 350°C, which was not observed in the films annealed in Ar atmosphere. Thermodynamic calculations based on the Cu---In---Se---O---N system showed In₂O₃ to be the most stable phase in good agreement with the experimental results.     

The formation of CuInSe2 thin films by rapid thermal processing

Formation of polycrystalline thin film CuInSe₂ was achieved by the rapid thermal processing of vacuum-deposited copper, indium, and selenium. Films were fabricated and characterized in three composition regions: copper-poor (approximately 20 at.% Cu). stoichiometric (25 at.% Cu) and copper-rich (approximately 28 at.% Cu). Characterization results including X-ray diffraction analysis, electron probe for microanalysis, scanning electron microscopy, and optical reflection and transmission measurements are presented. The results show that nearly single-phase material has been formed from co-deposited precursors with a post-deposition annealing time of less than 2 min. The films have smooth morphologies amenable for photovoltaic device fabrication, optical absorption coefficients in the high 10⁴ cm⁻¹ range, and an optical band gap of 1.0 eV.     

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.     

Electrodeposited and selenized (CuInSe2) (CIS) thin films for photovoltaic applications

In the present communication, the authors report results on the characterization of electrodeposited and selenized (CuInSe₂) (CIS) thin films. The selenization process was carried out using a technique called chemical vapor transport by gas (CVTG). The precursors as well as selenized films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron microprobe analysis (EPMA). The film stoichiometry improved after selenization at 550°C. The films were formed with a mixed composition of the binary as well as the ternary phases.     

Electrodeposition of CuInSe2 from ethylene glycol at 150 °C

Polycrystalline chalcopyrite thin films were potentiostatically electrodeposited from ethylene glycol solution onto SnO₂-coated glass substrates at 150 °C. The thickness of the layers was estimated using talysurf at 1.0 μm after deposition for 60 min. X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were used to identify and characterise compounds formed at different potentials. It was found that Cu_1.75Se formation was dominant at −0.80 V vs Se and indium assimilation increased at more negative voltages forming a mixture of compounds including numerous Cu-Se binary phases and copper indium diselenide (CuInSe₂) at the cathode. As-deposited materials showed poor crystallinity and therefore films were annealed in Ar/5%H₂ in the presence of Se to improve the material quality for all investigations. Although the films were deposited at 150 °C, no noticeable improvement of the CuInSe₂ was observed, suggesting growth from aqueous media at room temperature to be preferable.     

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.     

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.     

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.