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.     

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.     

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.     

ZnO/CuInS2 core/shell heterojunction nanoarray for photoelectrochemical water splitting

ZnO/CuInS₂ core/shell nanorods array thin film was synthesized on conducting glass substrates for photoelectrochemical water splitting via a simple hydrothermal and cation exchange reaction, using ZnO nanorods array as reactive template. Uniform CuInS₂ films were obtained on the surface of ZnO nanorods, based on the ion-by-ion growth mechanism. The optical property of core/shell nanoarray was characterized, and enhanced absorption spectrum was observed. Hydrogen generation efficiency of 3.2% at 0.29 V versus saturated calomel electrode was achieved with synthesized ZnO/CuInS₂ core/shell nanoarray electrode due to the improved absorption and appropriate energy gap structure. The synthesized core/shell nanoarray has potential application in photoelectrochemical water splitting.     

Stoichiometry controlled conversion efficiency in nanostructured heterojunction solar cell of CdS/CuInSXSe2−X grown by chemical ion exchange method at room temperature

Here in the present paper, we report on growth of stoichiometric and nonstoichiometric nanostructured heterojunction solar cell of CdS/CuInS_XSe_2-X varying X from 0 to 2 in the interval of 0.5 using cost effective, simple, chemical ion exchange method at room temperature on ITO glass substrate. The as-grown varying composition solar cells annealed at 200 °C in air and characterized for structural, compositional, optical and illumination studies. The X-ray diffraction pattern obtained from CdS/CuInS_XSe_2-X solar cell confirms the formation of CuInSe₂, CuInS_0.5Se_1.5, CuInS₁Se₁, CuInS_1.5Se_0.5 and CuInS₂ phases having tetragonal structure with varying crystallite size from 19, 19.37, 28, 33 and 20 nm respectively. The energy dispersive X-ray analysis (EDAX) confirms the expected elemental composition in the heterojunction solar cell. Optical absorbance analysis confirms composition controlled electronic transitions in the thin films while energy band gap observed to be red shifted with increase the value of X. The solar energy conversion efficiency achieved upon illuminating to 100 mW/cm² observed to be 0.27%, 0.06%, 0.17%, 0.02% and 0.23% for CuInSe₂, CuInS_0.5Se_1.5, CuInS₁Se₁, CuInS_1.5Se_0.5 and CuInS₂ respectively, which correspond for stoichiometric dependent electron-hole pair generation and separation phenomenon.     

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 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.     

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.     

Sprayed CuInS2 films grown under Cu-rich conditions as absorbers for solar cells

CuInS₂ films were prepared by the spray pyrolysis method using either copper-rich solutions or the recrystallization of low-crystallinity film in the presence of an intentionally deposited Cu_xS layer. KCN-etched films were characterized by XRD, SEM and EDX. The Cu/In molar ratio of 1.5–4.0 in the solution resulted in well-crystallized CuInS₂ films with the mean crystallite size of 120 nm. SEM study showed nonuniform surface with irregularly placed large grain domains in the flat film. The two-step process resulted in a uniform film with the crystallite size of 50 nm. Films exhibited an In-rich composition. Solar cells based on a recrystallized absorber showed an improved quantum efficiency spectrum.     

Photoelectric characterization of Cu(In,Ga)S2 solar cells obtained from rapid thermal processing at different temperatures

CuInS₂-based solar cells have a strong potential of achieving high efficiencies due to their ideal band gap of 1.5 eV. A further increase in the efficiency is expected from doping the absorber film with gallium due to enlargement of the band gap (E_g) and correspondingly the open-circuit voltage (V_OC). We investigated Cu(In,Ga)S₂ solar cells obtained from stacked metal layers sputtered from In and (Cu,Ga) targets followed by rapid thermal processing (RTP) in sulfur vapor. Depending on the actual RTP temperature profile, the films might exhibit CuInS₂/CuGaS₂ (top/bottom) segregation, which is rather detrimental for a large V_OC. We found that only precursors sulfurized at sufficiently high temperatures exhibit the desired interdiffusion of the segregated CuInS₂/CuGaS₂ layers resulting in an increased V_OC. Moreover, temperature dependent current-voltage profiling (suns-V_OC-analysis) yielded strong indications for improved current collection and reduced losses for devices with proper interdiffusion of the CuInS₂/CuGaS₂ layers. A more fundamental question is related to the variation and formation of defect states in differently processed absorber films. The studied samples were thus further investigated by means of admittance spectroscopy, which allowed us to confirm the presence of three individual defect states in both absorber configurations. Two defects exhibit activation energies, which remain unchanged upon varying the RTP temperature whereas a third state exhibits significantly increased activation energy in devices showing interdiffusion of CuInS₂/CuGaS₂ layers. According to the characteristic shift of the conduction band edge upon Ga-doping we conclude that the latter defect level corresponds with the minority carriers in the p-type absorbers.     

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 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.     

Sprayed CuInS2 thin films for solar cells: The effect of solution composition and post-deposition treatments

CuInS₂ thin films were prepared by spray pyrolysis from solutions with different compositions. Etching in KCN solution and thermal treatments in vacuum and hydrogen were applied to as-deposited films. KCN etching removes conductive copper sulfide from the surface of Cu-rich films but has no effect on matrix composition. Vacuum annealing at 500°C and hydrogen treatment at 400–500°C purifies the films, prepared from the solutions with the Cu/In=1, from secondary phases, reduces chlorine content and improves crystallinity. Vacuum annealing results in n-type films due to the formation of In₂O₃ phase. Treatment in hydrogen reduces oxygen-containing residues and results in p-type CuInS₂ films with resistivity close to 10 Ω cm.     

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.     

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.     

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.     

Double layer CuInS2 absorber using spray pyrolysis: A better candidate for CuInS2/In2S3 thin film solar cells

In the fabrication of CuInS₂/In₂S₃ solar cell using chemical spray pyrolysis (CSP) deposition technique, one of the major problems is the diffusion of Cu towards the In₂S₃ layer affecting stability and repeatability of the CuInS₂/In₂S₃ cells. In order to ensure a Cu-free In₂S₃ layer, a ‘double layer structure’ of CuInS₂ film, having a Cu-rich first layer and In-rich second layer was deposited using manual CSP technique. In this paper, we present the difference in material properties of single and double layered CuInS₂ films and the results of characterisation of the junctions prepared using such films with β-In₂S₃ films. Better crystallinity as well as larger and densely packed grains were observed for the CuInS₂ films having ‘double layer structure’. Such samples also possessed two band gaps, which was not due to the presence of different phases, but due to the Cu-rich and Cu-poor layers. In addition, their low resistivity makes the double layered CuInS₂ film more beneficial for photovoltaic applications.     

Electrodeposited CuInS2-based thin-film solar cells

CuInS₂ thin films were prepared by sulfurization of electrodeposited Cu–In precursors. Morphological improvement enabled us to fabricate the solar cells using electrodeposited Cu–In precursors. The photovoltaic property of a conversion efficiency of 1.3% was obtained.     

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.