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.     

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.     

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.     

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.     

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.     

Preparation and characterization of chemically deposited CuInS2 thin films

The structural and electrical properties of thin films of CuInS₂ prepared by the chemical deposition technique are described. The composition of the polycrystalline films produced deviate from the ideal composition as shown by energy dispersive X-ray spectrometer (EDS) analysis. Transmission electron diffraction (TED) and X-ray powder analyses showed that the films were single phase with chalcopyrite structure. The films were p-type and the resistivities were in the range of 0.1 to 10 μ cm. Influence of deposition parameters on the surface morphology was observed by SEM.     

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.     

Influence of CdCl2 treatment on structural and optical properties of vacuum evaporated CdTe thin films

In this article we have discussed the structural, optical properties of vacuum evaporated CdTe thin films before and after CdCl₂ treatment. The CdTe thin films were prepared by vacuum evaporation. Films were prepared under the vacuum of 10⁻⁶ Torr. The structural studies have been performed by the X-ray diffraction (XRD) technique. The XRD analysis of vacuum evaporated CdTe films reveals that the structure of films is polycrystalline in nature. However, the crystallinity has been improved after the CdCl₂ treatment as shown by an increase of the diffraction peak intensities. This is due to the enhancement in the atomic mobility of CdTe. The optical properties of the CdTe thin films have been studied by the spectrophotometer in the 300–800 nm wavelength range. It is observed that the optical band gap energy is highly dependent on CdCl₂ treatments. The optical transitions in these films are found to be direct and allowed.     

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.     

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.     

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.     

Development of copperindiumdisulfide into a solar material

The historic development of the ternary chalcopyrite CuInS₂ into an efficient thin film solar cell absorber material is reviewed beginning with the preparation and analysis of crystalline and large grain polycrystalline material. The room temperature phase relations obtained by a combination of the nuclear probe method perturbed angular correlations and X-ray diffraction are presented. It is shown that despite the efforts in analysis and crystal preparation, the solar-to-electrical conversion efficiency remained generally restricted to values around 4%. Whereas a singular growth result yielding 9.7% efficiency in a photoelectro-chemical solar cell was obtained on n-type material, the subsequent successful development of thin film absorbers prepared by physical vapor deposition used p-type samples. The compositional and optoelectronic properties of these samples are described and a selection of recently achieved improvements, based on electrochemical conditioning, is presented. In the effort of elucidating the interface behavior of the {1 1 2} growth face which dominates at the polycrystalline absorber surface the preparation of epitaxial thin films have been prepared and their properties are shortly described. Aspects for implementation of this chalcopyrite in a solar technology are discussed.     

Growth, structural and optical properties of copper indium diselenide thin films deposited by thermal evaporation method

Copper indium diselenide (CuInSe₂) compound was synthesized by reacting its constituent’s elements copper, indium and selenium in near stoichiometric proportions (i.e. 1:1:2 with 5% excess selenium) in an evacuated quartz ampoule. Synthesized pulverized compound material was used as an evaporant material to deposit thin films of CuInSe₂ onto organically cleaned sodalime glass substrates, held at different temperatures (300-573 K), by means of single source thermal evaporation method. The phase structure and the composition of chemical constituents present in the synthesized compound and thin films have been investigated using X-ray diffraction and energy dispersive X-ray analysis, respectively. The investigations show that CuInSe₂ thin films grown above 423 K are single phase, having preferred orientation of grains along the (112) direction, and having near stoichiometric composition of elements. The surface morphology of CuInSe₂ films, deposited at different substrate temperatures, has been studied using the atomic force microscopy to estimate its surface roughness. An analysis of the transmission spectra of CuInSe₂ films, recorded in the wavelength range of 500-1500 nm, revealed that the optical absorption coefficient and the energy band gap for CuInSe₂ films, deposited at different substrate temperatures, are ∼10⁴ cm⁻¹ and 1.01-1.06 eV, respectively. The transmission spectrum was analyzed using iterative method to calculate the refractive index and the extinction coefficient of CuInSe₂ thin film deposited at 523 K. The Hall effect measurements and the temperature dependence of the electrical conductivity of CuInSe₂ thin films, deposited at different substrate temperatures, revealed that the films had electrical resistivity in the range of 0.15-20 ohm cm, and the activation energy 82-42 meV, both being influenced by the substrate temperature.     

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.     

CuGaxSey chalcopyrite-related thin films grown by chemical close-spaced vapor transport (CCSVT) for photovoltaic application: Surface- and bulk material properties, oxidation and surface Ge-doping

Device-grade ternary Cu-Ga-Se chalcopyrite thin films used for photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapor transport (CCSVT) technique developed for a deposition on areas of up to 10×10 cm². A two-step process has been developed which allows the fine tuning of the film composition and the electronic properties. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the structural phase transitions. In this paper the main focus of interest is related to the material properties of the device-grade thin films prepared by CCSVT technique. We present our recent studies on (i) the growth, compositional, structural and electronic structural properties, (ii) the degradation under ambient conditions and (iii) the feasibility of n-type doping this p-type semiconducting material by germanium. Thin films were grown with chalcopyrite (1:1:2) and CuGaSe₂-related defect compound structures (DC) with stoichiometries of CuGa₃Se₅ and CuGa₅Se₈. In order to derive the DC structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe₂-related compounds grown by elemental synthesis (powder) and CCSVT (thin films), respectively. We found no hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures. From our results a growth model is presented for CuGa₃Se₅ formation in gallium-rich CCSVT-grown CuGa_xSe_y films. The chemical and electronic surface and interface structure of CuGaSe₂ thin films with bulk [Ga]/[Cu] ratios between 0.94 and 1.39 is investigated by X-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). A transition of the Cu:Ga:Se surface composition from 1:1:2 for the Cu-rich bulk sample to 1:3:5 for the sample with the highest bulk [Ga]/[Cu] ratio is observed. Simultaneously, a downward shift of the valence band maximum position with respect to the Fermi energy is found. The comparison of the estimated conduction band minimum with that of CdS reveals the formation of a pronounced “cliff-like” conduction band offset at the respective interface.Furthermore, the CuGaSe₂ thin film degradation under ambient as well as under thermal conditions of CuGaSe₂ thin films has been studied by XPS. During thermal oxidation, the formation of predominantly Ga₂O₃ and some amount of SeO₂ were observed, but no copper oxides could be detected in the near-surface region of the thin films. The same oxides are found after native oxidation in air under ambient conditions. An additional sodium oxide compound formed at the thin film surface, Na_xO and Na₂CO₃ after thermal and native oxidation, respectively.Germanium ion implantation technique of the near-surface region of CuGaSe₂ thin films has been used in order to prove the feasibility of n-type doping. In photoluminescence (PL) studies, the occurrence of a new emission line is identified as Ge related and explained as a donor-acceptor-pair (DAP) recombination. The precise role the Ge is playing in this doping of CuGaSe₂ is revealed by X-ray absorption spectroscopy (XANES and EXAFS) and ab initio calculations based on the density functional theory. The studies indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe₂ originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted anti-bonding Ge-Se4sp³ defect band appearing well above the Fermi level for the Ge¹⁺_Ga point defect system can be directly linked to a Ge-dopant-related donor-acceptor-pair transition as observed in our photoluminescence spectra.     

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.     

Structure changes and optical properties of Mg2Ni switchable mirrors

A multilayer film of Mg and Ni was prepared by dc/ac magnetron sputtering and annealed below 623 K in vacuum to obtain polycrystalline Mg₂Ni thin films. The phase transformation during heating process and optical switching properties of the films were investigated. The influence of the original crystalline state of Mg₂Ni films on optical switching properties such as transmission, optical band gaps and the cycle times was discussed. The indirect optical band gaps of the fully hydrogenated amorphous Mg₂Ni films were estimated by linear extrapolation.     

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.     

Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process

Cu₂ZnSnS₄ thin films were deposited on corning 7059 glass substrates without substrates heating by rf magnetron sputtering. The Cu/(Zn+Sn) ratio of the thin film sputtered at 75 W was close to the stoichiometry of Cu₂ZnSnS₄. However, the S/(Cu+Zn+Sn) ratio was less than the stoichiometry. The as-deposited films were amorphous and annealed in the atmosphere of Ar+S₂ (g). The annealed (1 1 2), (2 0 0), (2 2 0), (3 1 2) planes were conformed to all the reflection of a kesterite structure. A preferred (1 1 2) orientation was observed with the increase of the annealing temperature. The optical absorption coefficient of the thin film was about 1.0×10⁴ cm⁻¹. The optical band energy was derived to be 1.51 eV. The optical absorption coefficient of the sputtered Cu₂ZnSnS₄ thin films was less than that of CuInS₂ thin film, however, the band gap energy was more appropriate for photovoltaic materials.     

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.