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.     

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).     

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.     

Microstructure evolution of CuInSe2 thin films prepared by single-bath electrodeposition

The composition and the microstructure evolutions of CuInSe₂ thin films under single-bath electrodeposition processes were investigated. It was found that the film composition was mainly determined by the [Se⁴⁺]/[Cu²⁺] ratios in solution, but the film microstructure is strongly dependent on the initial concentrations of Se⁴⁺, Cu²⁺, and In³⁺ precursors. Higher initial concentrations of Cu²⁺ and In³⁺ in solution are beneficial for the fabrication of compact CuInSe₂ thin films with highly crystallized and large grain sized chalcopyrite phase. The microstructure evolution suggests that prior adsorption and reduction of Cu²⁺ ions and the formation of Cu₂Se compound on the substrate can promote the nucleation, growth, and coarsening of CuInSe₂ crystal to form a high quality thin film during the electrodeposition processes.     

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%).     

The phase relations in the Cu,In,Se system and the growth of CuInSe2 single crystals

The phase relations in the Cu,In,Se system are reviewed with particular attention to compositions in the vicinity of CuInSe₂. The pseudobinary Cu₂Se-In₂Se₃ is re-evaluated and the liquidus-solidus relationship of the δ phase is discussed. The growth of CuInSe₂ single crystals is described using the methods of gradient freeze, Bridgman, chemical vapor transport, zone leveling and liquid encapsulated Czochralski pulling.     

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.     

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.     

Thermal diffusion of Cu into In2Se3: A better approach to SEL technique in the deposition of CuInSe2 thin films

This paper reports the modifications made in the preparation techniques of getting CuInSe₂ thin films starting with chemical bath deposited (CBD) selenium films. In the present study, CBD Se film was converted into CuInSe₂ by stacked elemental layer (SEL) technique and also by thermal diffusion of Cu into In₂Se₃. In both the cases CBD Se films were used to avoid toxic Se vapor and H₂Se gas. Improvements were made in these techniques through a detailed study, varying the composition of the films over a wide range by changing the Cu/In ratio. Structural, optical and electrical characterizations were performed. On comparing the material properties of CuInSe₂ deposited by these two techniques, it was found that photosensitivity was better for samples prepared by thermal diffusion of Cu into In₂Se₃. So the technique of thermal diffusion of Cu into In₂Se₃ was found to be better than SEL technique in the preparation of CuInSe₂ using CBD Se. Cu-rich, In-rich and nearly stoichiometric samples could be prepared by thermal diffusion of Cu into In₂Se₃. These samples were analyzed using energy dispersive spectroscopy, Raman spectroscopy and atomic force microscopy also.     

Comparison of point defects in CuInSe2 and CuGaSe2 single crystals

Hall-effect and photoluminescence measurements have been carried out on as-grown and In/Ga-annealed CuInSe₂ and CuGaSe₂ single crystals grown by chemical vapor transport. Various defect levels in these related compounds have been identified and compared. V_Cu and V_Se show similar properties and activation energies in both materials. A tremendous difference is observed in the behavior of III_Cu antisite defects. Ga_Cu levels in CuGaSe₂ are much deeper than In_Cu in CuInSe₂, and furthermore, the formation of In_Cu is much easier compared to Ga_Cu. This is related to the higher formation energy of Ga_Cu in CuGaSe₂. Due to this difference in the defect chemistry of both compounds, it has not been possible until now, to prepare n-type CuGaSe₂ crystals, whereas CuInSe₂ is easily transformed from p- to n-type by annealing in vacuum or In-atmosphere.     

Study of phases formed during production of copper indium diselenide by reacting copper, indium and selenium layers

CuInSe₂ films were grown by reacting stacked layers of Cu, In and Se in an atmosphere of Se vapor. Incremental growth of the various phases was followed at different temperatures until a single phase CuInSe₂ film was formed. Conventional X-ray diffraction was used in identifying the different phases formed. Along with the knowledge of different phases formed at increasing reaction temperatures, it was concluded that CuInSe₂ is formed at temperatures as low as 235°C, although a single phase film is obtained only at higher temperatures (≈350°C).     

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.     

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.     

Selenization of Cu and In thin films for the preparation of selenide photo-absorber layers in solar cells using Se vapour source

Gas phase selenization of vacuum deposited Cu and In thin films employing an elemental Se vapour source is demonstrated as an essential first step in the search for optimized process parameters for the formation of single phase CuInSe₂ materials suitable for solar cell applications. The selenization was accomplished in Se vapour, derived from an elemental Se source, held at 240–260°C. This source was placed in a flow of nitrogen gas at 500 Torr to transport the Se vapour to the metal films. The selenization reaction readily occurs at Cu and In films kept at 340–400°C. Lower selenization temperatures invariably lead to the formation of Cu and In selenides with well defined crystalline microstructures. Hexagonal CuSe with an excess of Se in the matrix is the equilibrium growth phase, while the cubic Cu_2−xSe phase evolves under conditions of excess Se flux. Selenization of the In films consistently led to the formation of the β-form of hexagonal In₂Se₃. At high selenization temperatures (400°C), while the β-form still emerges as the major component, traces of the α-form of In₂Se₃ are also detected. Detailed X-ray diffraction, electron probe analysis and microstructure data are presented.     

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.     

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.     

Enhanced electrical properties of pulsed laser-deposited CuIn0.7Ga0.3Se2 thin films via processing control

Polycrystalline CuIn_0.7Ga_0.3Se₂ thin films were prepared on soda-lime glass substrates using pulsed laser deposition (PLD) with various process parameters such as laser energy, repetition rate and substrate temperature. It was confirmed that there existed a limited laser energy, i.e. less than 300 mJ, to get phase pure CIGS thin films at room temperature. Particularly, even at room temperature, distinct crystalline CIGS phase was observed in the films. Crystallinity of the films improved with increasing substrate temperature as evidenced by the decrease of FWHM from 0.65° to 0.54°. Slightly Cu-rich surface with Cu_2−xSe phase was confirmed to exist by Raman spectra, depending on substrate temperature. Improved electrical properties, i.e., carrier concentration of ∼10¹⁸ cm⁻³ and resistivity of 10⁻¹ Ω cm at higher substrate temperature for the optimal CIGS films are assumed to be induced by the potential contributions from highly crystallized thin films, existence of Cu_2−xSe phase and diffusion of Na from substrates to films.     

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.     

Study of electron irradiation-induced defects in and by electron spin resonance

Electron irradiation effects in CuInSe₂ and CuInGaSe₂ were investigated using electron spin resonance (ESR) method. ESR signal caused by electron irradiation was found in irradiated samples. The ESR spectra of irradiated samples were well reproduced by a computer simulation of powder spectrum assuming Cu²⁺ ion. Electron irradiation effects in CuInSe₂ and CuInGaSe₂ are discussed based on present ESR study together with previous deep level transient spectroscopy (DLTS) and Hall effect measurements.     

Liquid phase epitaxial growth and electrical characterization of CuInSe2

A CuInSe₂ layer was grown on a ZnSe substrate by the liquid phase epitaxy (LPE) method from a Bi solution. The optimum growth conditions, namely a maximum temperature of 600°C and a cooling rate of 0.5°C min⁻¹, resulted in the over-growth layer being of thickness 6 μm and the mixed crystal layer of thickness 4.5 μm. The flatness and crystalline quality of the layer decreased in the order of the orientations (111)_Se, (100), (111)_Zn and (110). The electrical properties of the layer were measured by the Hall effect after heat treatment in either Se or Zn atmospheres. The LPE wafer was fabricated into p-CuInSe₂/n-ZnSe and n-CuInSe₂/n-ZnSe heterodiodes, which were characterized using I–V and C–V characteristics, photoresponse and electron beam induced current measurements. The n-CuInSe₂/n-ZnSe heterodiode in particular showed a high photoresponse. The effects of diffusion of the component elements Cu, In and Bi into the substrate and the CuInSe₂ layer were investigated by means of cathodoluminescence, Hall-effect measurements and electron-acoustic microscopy. Bi diffused in CuInSe₂ acts as a high mobility donor.