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.     

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.     

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.     

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.     

Structural and optical properties of sulfur-annealed CuInS2 thin films

CuInS₂ thin films were deposited by single source vacuum thermal evaporation method on substrates submitted to longitudinal thermal gradient. Some of these films were annealed in sulfur atmosphere and converted into CuInS₂ homogenous layers. Both of the as-deposited and sulfurated films were characterized by X-ray diffraction, optical transmission and reflection measurements. The optical band gap of films after sulfurization was 1.50 eV which is near the optimum value for photovoltaic energy conversion.     

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%.     

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.     

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.     

Solar cells with Cu(In1−xGax)S2 thin films prepared by sulfurization

By rapid thermal processing of Cu/In/GaS precursors, good-quality CuIn_1–xGa_xS₂ films are synthesized. By suppressing the formation of In-rich hillocks, we could obtain homogeneous CuIn_1–xGa_xS₂ surfaces. A conversion efficiency of 12% has been achieved using a relatively low (1.2) Cu/In ratio.     

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.     

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.     

Photoelectrochemical properties of AgInS2 thin films prepared using electrodeposition

Ternary silver-indium-sulfide samples were deposited on fluorine-doped tin oxide (FTO) coated glass substrates using a one-step electrodeposition method. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in solution bath on the structural, morphological, and photoelectrochemical properties of samples was examined. X-ray diffraction patterns of samples show that the films are the AgInS₂ phase. The thickness, direct band gap, and indirect band gap of the films were in the ranges 209-1021 nm, 1.82-1.85 eV, and 1.44-1.51 eV, respectively. The carrier densities and flat-band potentials of films obtained from Mott-Schottky and open-circuit potential measurements were in the ranges of 4.2×10¹⁹-9.5×10¹⁹ cm⁻³ and −0.736 to −0.946 V vs. the normal hydrogen electrode (NHE), respectively. It was found that the samples with molar ratio [Ag]/[In]=0.8 in solution bath had a maximum photocurrent density of 9.28 mA/cm² with an applied bias of +1.0 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.25 M K₂SO₃ and 0.35 M Na₂S. The results show that high-quality AgInS₂ films can be deposited on FTO-coated glass substrates for photoelectrochemical (PEC) applications.     

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.     

Influence of H2S concentration on the properties of Cu2ZnSnS4 thin films and solar cells prepared by sol-gel sulfurization

Cu₂ZnSnS₄ (CZTS) thin films prepared by a non-vacuum process based on the sulfurization of precursor coatings, consisting of a sol-gel solution of Cu, Zn, and Sn, under H₂S+N₂ atmosphere were investigated. The structure, microstructure, and electronic properties of the CZTS thin films as well as solar cell parameters were studied in dependence on the H₂S concentration. The sulfurization process was carried out at 500 °C for 1 h in an H₂S+N₂ mixed-gas atmosphere with H₂S concentrations of 3%, 5%, 10%, and 20%. As the H₂S concentration decreased from 20% to 5%, the S content of the CZTS thin films decreased. However, when the H₂S concentration was decreased below 3%, the S content of the films began to increase. A CZTS thin film prepared with an H₂S concentration of 3% had grains in the order of 1 μm in size, which were larger than those of films prepared at other H₂S concentrations. Furthermore, the most efficient solar cell, with a conversion efficiency of 2.23%, was obtained from a sample sulfurized at an H₂S concentration of 3%.     

Control of valence states by a codoping method in CuInS2

We deviate the valence and conduction band energies of stoichiometric CuInS₂ crystals based on ab initio electronic band structure calculations using the augmented spherical wave (ASW) method and discuss that at low doping levels, the Madelung energy is a good intrinsic parameter for stabilization of p- or n-type doped CuInS₂ crystals. We find that P and Sb atoms are eminently suitable dopants substituted for S atoms for p-type doped CuInS₂ crystals with lower resistivity from both the character of electronic states around E_F and the Madelung energy. A closer study of the nature of chemical bonds of CuInS₂ crystals using first-principles band structure calculation method reveals that In with polyvalence codoping for p-type CuInS₂ doped with P results in a decrease of the Madelung energy compared with CuInS₂: P, to be an effective method for stabilizing of its ionic charge distributions.     

Deposition of CuInS2 films by electrostatic field assisted ultrasonic spray pyrolysis

We studied CuInS₂ (CIS) film growth using high electrostatic field assisted ultrasonic spray (HEFAUS) deposition. CIS films were fabricated with various precursors and substrate temperatures. All the as-sprayed CIS films were observed to be grown with mixed ordering mode, where chalcopyrite (CH) and CuAu (CA) orderings coexisted. It was found that application of additional sulfurization to sprayed CIS films induced re-crystallization of films accompanied by enhancement of CH ordering. After the post-sulfurization, the most improved film showed nearly the same CH-fraction as that for a CIS film which was made by sulfurization of sputtered Cu-In alloy film. These results indicate that our modified spray deposition could be used for fabrication of CIS photoabsorbing layer instead of high-cost vacuum-based process. All fabricated films were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscope and energy dispersive X-ray analysis measurements.     

Investigation of capacitance transients in CuInS2 due to ionic migration

Transport of mobile ions in n-TiO₂/n-CuInS₂/p-CuInS₂ thin-film devices is studied with the transient ion-drift (TID) method. In contrast to the normal TID method, a mobile ion profile is created in the CuInS₂ layer, which can be described by the Gouy-Chapman theory. Activation energies for diffusion of 0.5 and 1.0 eV are found. We postulate that these activation energies are related to the associated defect, ( In_Cu)^x, which introduces a deep electronic state inside the bandgap of CuInS₂. This defect can accept or release an electron and drift out of the depletion region. This will lower the concentration of recombination centers in the depletion region, which explains the self-healing property of CuInS₂.     

Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E---B evaporated precursors

By sulfurization of E---B evaporated precursors, CZTS(Cu₂ZnSnS₄) films could be prepared successfully. This semiconductor does not consist of any rare-metal such as In. The X-ray diffraction pattern of CZTS thin films showed that these films had a stannite structure. This study estimated the optical band gap energy as 1.45 eV. The optical absorption coefficient was in the order of 10⁴cm⁻¹. The resistivity was in the the order of 10⁴ Ω cm and the conduction type was p-type. Fabricated solar cells, Al/ZnO/CdS/CZTS/Mo/Soda Lime Glass, showed an open-circuit voltage up to 400 mV.     

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.