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     

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.     

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.     

Growth and characterization of stepwise flash evaporated CuInTe2 thin films

CuInTe₂ films grown by stepwise flash evaporation onto glass and silicon substrates held at 573 K were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS) and Raman spectroscopy. XRD and TEM studies showed the formation of single-phase polycrystalline CuInTe₂. Results of the RBS measurements showed the films to be near-stoichiometric and negligible diffusion of elements across the CuInTe₂/Si interface. Various lattice vibrational modes identified by Raman measurements were found to match well with those reported for single-crystal CuInTe₂, confirming the crystalline quality of the CuInTe₂ thin films.     

Experimental study of graded bandgap Cu(InGa)(SeS)2 thin films grown on glass/molybdenum substrates by selenization and sulphidation

High-performance Cu(InGa)(SeS)₂ (CIGSS) thin film absorbers with an intentionally graded bandgap structure grown by a two-stage method have been studied. Materials obtained from Showa Shell Sekiyu K.K., Japan have been grown using selenization and sulphidation of the Mo/Cu–Ga/In stacked precursors. Full characterizations have been carried out using X-ray diffraction, Raman, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence, inductively coupled plasma mass spectroscopy, glow discharge optical emission spectroscopy (GDOES) and photoelectrochemical (PEC) techniques to study various properties. The material layers were found to be polycrystalline with the (1 1 2) preferred orientation, and the largest grains were about 2 μm. Raman measurements show the presence of at least five different phases within the material. XPS confirmed the copper depletion and the richness of sulphur at the top surface region. Although the PEC studies indicate the overall electrical conductivity of the layer as p-type, GDOES profiling reveals the segregation of different phases at different depths suggesting the possibility of having buried junctions within the material itself. The results are presented together with suggestions for further improvements of CIGSS solar cell material.     

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.     

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.     

Blue-photon modification of nonstandard diode barrier in CuInSe2 solar cells

The current–voltage curves of many ZnO/CdS/CuInSe₂ solar cells display significant distortion when only red light illumination is employed. This distortion generally disappears or partially disappears for a period of time following illumination with blue light. Similarly, the dark diode curve shortly after illumination containing blue light is shifted significantly from the equilibrium dark curve. This effect is more common than generally realized and indicates a mechanism that is potentially detrimental to photovoltaic efficiency. A model is presented that is based on a low free-electron concentration and a high concentration of deep levels in the CdS window layer. This model is consistent with observed variations in current–voltage, capacitance, and laser scan data with illumination wavelength and history.     

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.     

Formation of CuInAlSe2 film with double graded bandgap using Mo(Al) back contact

Using Al added Mo back electrode to provide Al source to form CuInAlSe₂ (CIAS) absorber with self-formed double graded bandgap (or Al concentration) is reported. The double Al grading is self-forming and requires no process tweaking or modification. A 15 at % Al in Mo(Al) film yielded 0.39 Al/(In+Al) ratio in the CIAS film with a bandgap of 1.54 eV at the surface. The benefits of doping Al into Mo film are: lower resistance of the Mo layer, improved Mo to glass adhesion, increased surface electric field or improved minority carrier collection from the graded Al content (graded bandgap), and supply Al to form a CIAS absorber layer.     

Surface sulfurization studies of Cu(InGa)Se2 thin film

Sulfurization of copper indium gallium diselenide (CIGS) thin films solar cell absorber has been used to enhance the open-circuit voltage of the device by increasing the band gap of the absorber near the interface. Sulfurization of a homogeneous co-evaporated Cu(InGa)Se₂ thin film was studied in hydrogen sulfide and in a mixture of hydrogen sulfide and hydrogen selenide gases with the inclusion of oxygen. The structural and compositional properties of the absorber layer were investigated by XRD, EDS and AES. Sulfurization in hydrogen sulfide gas forms a fully converted sulfide layer at the top of the absorber layer, which in turn forms a barrier for the current collection. Sulfurization in a mixture of hydrogen sulfide and hydrogen selenide gases forms a wide band gap Cu(InGa)(SeS)₂ layer at the surface, but at the same time there is Ga diffusion away from the surface with the inclusion of sulfur at the surface.     

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄ (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643-683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10⁴ cm⁻¹.     

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.     

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.     

Ex-situ and in-situ analyses on reaction mechanism of CuInSe2 (CIS) formed by selenization of sputter deposited CuIn precursor with Se vapor

Formation mechanism of CIS thin films by selenization of sputter deposited CuIn precursor with Se vapor was investigated by ex-situ and in-situ phase analysis tools. Precursor films were composed of In, CuIn and Cu₂In compounds, and their relative fractions were systematically changed with Cu/In ratios. Those films were found to have a double layered structure with nearly pure In particles (top layer) placed on the flat Cu-rich bottom layer, and the morphologies were also significantly affected by Cu/In ratio. At the initial stage of selenization, the outer In-rich layer reacted with Se vapor to form In-Se binary, which is the first selenide phase appeared, and inner Cu-rich phases acted as a Cu source to supply Cu to outer In-Se phase to form ordered vacancy compounds (OVC). As these reactions continues, in conjunction with Se incorporation into inner Cu-rich region, the films gradually changes from OVC to α-CIS, reflecting that the formation route of CIS is closely related to the elemental and phase distribution in precursor films. Selenized CIS films were further processed to fabricate CIS thin film solar cells, resulting in the best cell efficiency of 10.44%.     

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.     

AC and dielectric properties of thermally evaporated p-type (Sb2Te3)70 (Bi2Te3)30 thin films

Thin films of (Sb₂Te₃)₇₀ (Bi₂Te₃)₃₀ were prepared by thermal evaporation. The composition of the film was confirmed by energy dispersive analysis (EDAX). X-ray diffraction studies showed that the film was polycrystalline with grain size of 4.39 Å and with a preferred orientation in the (0 1 5) directions. Al/((Sb₂Te₃)₇₀ (Bi₂Te₃)₃₀)/Al (MSM) thin film capacitors are formed and its AC and dielectric studies were carried out using a digital LCR metre at various frequencies (12 Hz–100 kHz) and temperatures (303–483 K). The dielectric constant for a film of thickness 3000 Å was found to be 86 for 1 kHz at room temperature. The temperature coefficient of capacitance (TCC) and temperature coefficient of permittivity (TCP) were estimated as 684 and 1409 ppm/K for 10 kHz at 303 K, respectively. The activation energy was estimated as 1.190 eV for frequency of 100 kHz at 303 K. The AC conductivity of the films was found to be a hopping mechanism.     

Pyrite (FeS2) thin films prepared by spray method using FeSO4 and (NH4)2Sx

A simple spray method for the preparation of pyrite (FeS₂) thin films has been studied using FeSO₄ and (NH₄)₂S_x as precursors for Fe and S, respectively. Aqueous solutions of these precursors are sprayed alternately onto a substrate heated up to 120°C. Although Fe–S compounds including pyrite are formed on the substrate by the spraying, sulfurization of deposited films is needed to convert other phases such as FeS or marcasite into pyrite. A single-phase pyrite film is obtained after the sulfurization in a H₂S atmosphere at around 500°C for 30 min. All pyrite films prepared show p-type conduction. They have a carrier concentration (p) in the range 10¹⁶–10²⁰ cm⁻³ and a Hall mobility (μ_H) in the range 200–1 cm²/V s. The best electrical properties (p=7×10¹⁶ cm⁻³, μ_H=210 cm²/V s) for a pyrite film prepared here show the excellence of this method. The use of a lower concentration FeSO₄ solution is found to enhance grain growth of pyrite crystals and also to improve electrical properties of pyrite films.     

Electrodeposition of CuInSe2 layers using a two-electrode system for applications in multi-layer graded bandgap solar cells

In order to develop low-cost, large area multi-layer graded bandgap solar cell structures, CuInSe₂ layers were grown using a simplified two-electrode system. Atomic force microscopy (AFM) shows the layers grown consist of nano- and micro- size particles. Photoelectrochemical (PEC) cell, X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) measurements confirm that it is possible to grow CuInSe₂ layers with p-, i- and n-type electrical conduction, as pre-determined for applications in multi-layer device structures. XRF, XPS and PEC measurements show that Cu-richness provides p-type conduction and In-richness provides n-type conduction in electrodeposited CuInSe₂ layers. It is also possible to grow materials with different bandgaps in the range 1.00–1.90 eV. The combination of these two properties allows growth of multi-layer structures and preliminary work on these devices show good rectifying properties and exhibit photovoltaic activity. These new developments will be presented in this paper.