The influence of sodium on the properties of CuInS2 thin films and solar cells

The controlled incorporation of sodium into the absorber layer of CuInS₂ solar cells improved cell performance remarkably. Without toxic KCN treatment, conversion efficiencies of over 6% were achieved by sulfurization of sodium-containing precursors. We also investigated the characteristics of the sodium-incorporated CuInS₂ films by intentional addition and diffusion from a soda-lime glass. The ternary compound semiconductor of NaInS₂ was found to form mainly on the surface of each of the CuInS₂ films.     

Electrical and optical characterisation of CuInS2 crystals and polycrystalline coevaporated thin films

Single crystals CuInS₂ were grown by iodine vapour transport method, whereas polycrystalline thin films were obtained by coevaporation technique from three sources. The temperature dependence of the hole mobility in valence band is analysed by taking into account contributions from several scattering mechanisms of the charge carriers. To account for the temperature dependant conductivity of polycrystalline CuInS₂ thin films, grainboundary conduction process was suggested. In the low temperature region, we interpret the data in terms of the Mott law and the analysis is very consistent with the variable range hopping. However, thermionic emission is predominant at high temperatures. Photoluminescence measurements have been performed on CuInS₂ crystals and the analysis has revealed that the emission is mainly due to free-to-bound and donor–acceptor pair transitions. The band gap of that compound is derived from the excitonic emission line at 1.53 eV.     

Electrical characterization of Al, Ag and In contacts on CuInS2 thin films deposited by spray pyrolysis

The specific contact resistivity (ρ_C) for aluminum (Al), silver (Ag) and indium (In) metallic contacts on CuInS₂ thin films was determined from I-V measurements, with the purpose of having the most appropriate ohmic contact for TCO/CdS/CuInS₂ solar cells; ρ_C was measured using the transmission line method (TLM) for the metallic contacts evaporated on CuInS₂ thin films deposited by spray pyrolysis with ratios x=[Cu]/[In]=1.0, 1.1, 1.3 and 1.5 in the spray solution. The results show that In contacts have the lowest ρ_C values for CuInS₂ samples grown with x=1.5. The minimum ρ_C was 0.26 Ω cm² for the In contacts. This value, although not very low, will allow the fabrication of CuInS₂ solar cells with a small series resistance.     

Carbon-doped Sb2S3 thin films: Structural, optical and electrical properties

We report the modification of electrical properties of chemical-bath-deposited antimony sulphide (Sb₂S₃) thin films by thermal diffusion of carbon. Sb₂S₃ thin films were obtained from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on glass substrates. A carbon thin film was deposited on Sb₂S₃ film by arc vacuum evaporation and the Sb₂S₃-C layer was subjected to heating at 300 °C in nitrogen atmosphere or in low vacuum for 30 min. The value of resistivity of Sb₂S₃ thin films was substantially reduced from 10⁸ Ω cm for undoped condition to 10² Ω cm for doped thin films. The doped films, Sb₂S₃:C, retained the orthogonal stibnite structure and the optical band gap energy in comparison with that of undoped Sb₂S₃ thin films. By varying the carbon content (wt%) the electrical resistivity of Sb₂S₃ can be controlled in order to make it suitable for various opto-electronic applications.     

Characterization of CuInS2 thin films prepared by electrodeposition and sulfurization with photoluminescence spectroscopy

Potentiostatic electrodeposition and sulfurization techniques were used to prepare polycrystalline CuInS₂ thin films. X-ray diffraction and photoresponse measurements in a photoelectrochemical cell (PEC) revealed that photoactive polycrystalline CuInS₂ films can be deposited on Ti substrate. Photoluminescence (PL) spectroscopy was used to investigate the prepared thin films and optically characterize them. PL spectra revealed the defect structure of the samples with an acceptor energy level at 109 meV above the valance band and a donor energy level at 71 meV below the conduction band. The CuInS₂ thin films prepared in this investigation are observed to be In-rich material with n-type electrical conductivity.     

Effect of Ga incorporation in sequentially prepared CuInS2 thin film absorbers

Thin film CuInS₂:Ga solar cell absorber films were prepared by sequential evaporation of Cu–In–Ga precursors and sulfurization in sulfur vapor. The depth distribution of Ga was found to be highly inhomogeneous caused by CuGaS₂ phase segregation at the back contact. Depending on overall Ga content and sulfurization temperature a quaternary CuGa_xIn_1−xS₂ compound formed exhibiting a shift in absorber lattice constant and band gap. Micro Raman measurements showed that crystal quality was also affected by Ga. Open-circuit voltages well above 800 mV were achieved while sustaining high fill factors of 71%.     

A novel one-step electrodeposition to prepare single-phase CuInS2 thin films for solar cells

Copper indium disulfide (CuInS₂) thin films have been successfully prepared on Ni substrates using a novel one-step potentiostatic electrodeposition combined with a potassium hydrogen phthalate (C₈H₅KO₄) complexing agent, accompanied by annealing at 350 °C. Electrodeposition in the solution of Cu and In salts and sodium thiosulfate (Na₂S₂O₃) containing an adequate concentration of C₈H₅KO₄ (e.g., [C₈H₅KO₄]=23 mM) provides thin films comprised of a CuInS₂ single phase as the bulk composition, without forming Cu_xS secondary phases. In addition to the effect on bulk-phase compositions, the adjustment of [C₈H₅KO₄] causes variation in morphology and atomic composition of the film surface. The surface states of the films change from the Cu-rich rough surface at low [C₈H₅KO₄] (15 mM) to the In-rich smooth surface at high [C₈H₅KO₄] (23 mM). The higher [C₈H₅KO₄] induces the grains constructing the film to interconnect and form a densely packed CuInS₂ film without voids and pinholes. The single-phase and void-free CuInS₂ film shows a band gap of 1.54 eV, satisfying the requirement of the absorber layers in solar cells. The electrical properties tests denote its n-type conductivity with a resistivity of 9.6×10⁻⁵ Ω cm, a carrier concentration of 2.9×10²¹ cm⁻³ and a carrier mobility of 22.2 cm² V⁻¹ s⁻¹.     

Preparation of CuInS2 films with sufficient sulfur content and excellent morphology by one-step electrodeposition

One-step electrodeposition using sodium thiosulfate (Na₂S₂O₃) as a sulfur source has been studied for the preparation of Cu---In---S thin films. A deposited film is found to have a sufficiently high sulfur content compared with films deposited using thiourea as a sulfur source. The film deposited using Na₂S₂O₃ is also found to have an excellent morphology compared with electrodeposited Cu---In precursors. Predominant factors to govern film composition, In/Cu and S/(Cu + In) ratios, are also investigated in this study. An HC1 content added in order to decompose S₂O₃²⁻ ions in the solution is found to be one of the important factors to control composition of deposited films. A sulfur cocentration in the solution influences not only S/(Cu + In) ratio but also In/Cu ratio in the film. Reproducibility of film composition is deteriorated as the solution temperature increases.     

Influence of Na on the properties of Cu-rich prepared CuInS2 thin films and the performance of corresponding CuInS2/CdS/ZnO solar cells

Using different glass substrate types the Na content in sequentially and Cu-rich prepared CuInS₂ films and corresponding CuInS₂/CdS/ZnO thin-film solar cells is varied. The purpose was to investigate the influence of different Na concentrations on absorbers and devices. While the morphology of the absorbers seems not to be affected by this variation, corresponding PL spectra differ significantly. The properties of the solar cells, however, show no dependence on the Na concentration. This implies that even though the defect chemistry of CuInS₂, sequentially prepared under Cu excess, is changed by the presence of Na this influence has no impact on properties of corresponding solar cells.     

Study of the Mo/CuInS2/ZnS system by TEM

This work presents results from a study carried out on the Mo/CuInS₂/ZnS stacked layers, using high-resolution transmission electron microscopy (HRTEM). This system will be used later for the fabrication of solar cells with Mo/CuInS₂/ZnS/TCO structure, where the layers conforming it will perform as an electrical contact, absorber layer and buffer layer, respectively. The layers of the Mo/CuInS₂/ZnS system were deposited sequentially on soda lime glass substrates. The Mo film was deposited by DC magnetron sputtering, the CuInS₂ (CIS) layer was grown by co-evaporation of precursors in a two-stage process and the ZnS was deposited by co-evaporation and by CBD (chemical bath deposition) using a solution containing zinc acetate, sodium citrate, ammonia and thiourea.The performed study provided significant information regarding crystalline structure, grain boundaries and defects visualization of each one of the layers as well as of the Mo/CuInS₂ and CuInS₂/ZnS interfaces.     

Preparation and properties of CuInS2 thin film prepared from electroplated precursor

Thin CuInS₂ films were prepared by sulfurization of Cu/In bi-layers. First, the precursor layer was electroplated onto the treated surface of Mo-coated glass. Observation of the cross-section prepared by focused ion beam (FIB) etching revealed that the void-free film was initially formed on the top surface of the precursor layer and continued to grow until the advancing front of the film reached the Mo layer. The nucleation of voids near the bottom of the CuInS₂ film followed. To determine whether the condition of the Cu/In alloy influences the CuInS₂ quality we investigated the Cu/In alloy state using FIB. We found that the annealed precursor of low Cu/In ratio (1.2) has several voids in the mid position in the layer compared with Cu-rich precursor (1.6). The cross-sectional view of the Cu-rich absorber layer is uniform compared with the low copper absorber layer. Thin film solar cells were fabricated using the CuInS₂ film (Cu/In ratio: 1.2) as an optical absorber layer. It was found that the optimization of a sulfurization period is important in order to improve the cell efficiency. We have not yet obtained good results with high Cu-rich absorber because of a blister problem. This blister was found before sulfurization. So, we are going to solve this blister problem before sulfurization.     

Preparation and characterization of CuInS2 thin films solar cells with large grain

The CuInS₂ films with a maximum thickness of about 9 μm and a maximum atomic Cu/In ratio (as-deposited precursor) of 3.0 were prepared, and, to prevent peeling from substrate, were heat treated during Cu/In evaporation and/or intercalated with very thin Pt or Pd (between Mo and CuInS₂ layers). Thus, we could prepare the films with very large grain. It is also worth noting that the large grain films were easily optimized by chemical etching of the films using a thick film and Cu-rich composition. Therefore, the absorber for high-efficiency solar cells can be prepared by varying over a wide range of composition and thickness of precursor. The characterization of CuInS₂ absorbers with various film thickness and compositions were investigated and related with the performance of the photovoltaic device.     

CuInS2/In2S3 thin film solar cell using spray pyrolysis technique having 9.5% efficiency

Copper indium sulfide (CuInS₂)/In₂S₃ solar cells were fabricated using spray pyrolysis method and high short circuit current density and moderate open circuit voltage were obtained by adjusting the condition of deposition and thickness of both the layers. Consequently, a relatively high efficiency of 9.5% (active area) was obtained without any anti-reflection coating. The cell structure was ITO/CuInS₂/In₂S₃/Ag. We avoided the usual cyanide etching and CdS buffer layer, both toxic, for the fabrication of the cell.     

Chemical-bath ZnO buffer layer for CuInS2 thin-film solar cells

ZnO buffer layers were grown by a chemical-bath deposition (CBD) in order to improve the interface quality in p-CuInS₂ based solar cells, to improve the light transmission in the blue wavelength region, but also as an alternative to eliminate the toxic cadmium. The process consists of immersion of different substrates (glass, CIS) in a dilute solution of tetraamminezinc II, [Zn(NH₂)₄]²⁺, complex at 60–95°C. During the growth process, a homogeneous growth mechanism which proceeds by the sedimentation of a mixture of ZnO and Zn(OH)₂ clusters formed in solution, competes with the heterogeneous growth mechanism. The mechanism consists of specific adsorption of a complex Zn(II) followed by a chemical reaction. The last process of growth results in thin, hard, adherent and specularly reflecting films. The characterization of the deposited CBD-ZnO layers was performed by X-ray diffraction (XRD), optical transmittance, scanning electron microscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The as-deposited films on glass show hexagonal zincite structure with two preferred orientations (1 0 0) and (1 0 1). High optical transmittance up to 80% in the near-infrared and part of the visible region was observed. The low growth rate of the films on CIS suggests an atomic layer-by-layer growth process.The device parameters and performance are compared to heterojunction with a standard CdS buffer layer.     

Synthesis of CuInS2 by chemical route: optical characterization

CuInS₂ powder was prepared by wet chemical route. The chalcopyrite structure of the powder was revealed by XRD studies. Raman measurements of the powder sample indicated four prominent peaks at 292, 305, 340 and 472 cm⁻¹. The possible origin of the 305 cm⁻¹ peak was investigated and was found to be some local vibration in the structure. The peaks at 292 and 340 cm⁻¹ were ascribed to A₁ and B₂ modes, respectively. The peak at 472 cm⁻¹ which was due to the formation of SO₄⁻² ion at lower pH value of the precursor solution could be eliminated by using pH>11.0. Photoluminescence (PL) studies of the CuInS₂ powder indicated two distinct peaks at 1.49 and 1.42 eV. Post deposition annealing treatment in H₂ atmosphere revealed the formation of excess sulphur vacancy leading to the peak at 1.42 eV in the PL spectra while O₂ annealing of the powder created a deep defect level at 1.10 eV. Thick CuInS₂ films were prepared by Doctor's blade technique. Optical transmittance studies of these films indicated direct allowed transition at 1.5 eV.     

CuInS2 thin-films solar cells fabricated by sulfurization of oxide precursors

CuInS₂ thin-films were prepared by sulfurization of Cu---In---O precursors in H₂S gas. X-ray diffraction patterns showed that In₂O₃ phases did not remain in the CuInS₂ films sulfurized in a H₂S and H₂ atmosphere, whereas In₂O₃ phase remained in the films sulfurized in a H₂S and Ar atmosphere. The performance of CuInS₂ solar cells were studied as a function of the H₂ gas pressure during sulfurization. The open-circuit voltage, short-circuit current and fill factor increased with increasing the H₂ gas pressure. The conversion efficiency of the CuInS₂ solar cells is strongly affected by the reduction of the Cu---In---O precursors.     

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.     

Electrical and luminescent properties of CuGaSe2 crystals and thin films

CuGaSe₂ thin films with thicknesses of about 2 μm were prepared by flash and single source evaporation onto mica and (1 1 0)-oriented ZnSe substrates in the substrate temperature range 150–450°C. The obtained polycrystalline CuGaSe₂ films had the chalcopyrite structure with the predominant growth direction 2 2 1. Hall effect, conductivity and luminescence measurements have been carried out on CuGaSe₂ thin films and source materials: CuGaSe₂ single crystals grown by Bridgman technique and by chemical vapour transport using I₂ as transport agent. All films and crystals are p-type. Two acceptor levels with ionization energies E_A150–56 meV and E_A2130–150 meV have been identified as due to Ga vacancy and presence of Se atoms on interstitial sites respectively.     

Role of substrate temperature in controlling properties of sprayed CuInS2 absorbers

Optimization of substrate temperature of spray pyrolysed CuInS₂ absorber is discussed along with its effect on the photoactivity of junction fabricated. For CuInS₂ thin films, properties like crystallinity, thickness and composition showed progressive behavior with substrate temperature. X-ray photoelectron spectroscopic depth profile of all the samples showed that the concentration of copper on the surface of the films is significantly lesser than that in the bulk thus avoiding need for toxic cyanide etching. Interestingly, samples prepared at 623 K had higher conductivity compared to those prepared above and below this temperature. Also, the low energy transition, in addition to the direct band gap which was observed in other samples were absent in films prepared at 623 K. From thermally stimulated conductivity studies it was seen that shallow levels present in this sample contribute to its improved conductivity. Also, CuInS₂/In₂S₃ bilayer prepared at this substrate temperature showed higher photoactivity than those prepared at other temperatures.     

Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells

Solar cells of CuInS₂/In₂S₃/ZnO type are studied as a function of the In₂S₃ buffer deposition conditions. In₂S₃ is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In³⁺. In parallel, variable amounts of In₂O₃ are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 °C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In₂O₃ content in the buffer. On the other hand, the film deposited at lower temperature has higher In₂O₃ content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film.