Polycrystalline CuInS Research Articles

The effect of citric acid and selenization onto electrochemically deposited Copper-Indium thin films for solar cell applications

Copper Indium diselenide (CISe) layers have been synthesized via two stage processes; electrodeposition of Copper-Indium (Cu-In) and selenization. The effect of complexing agent, citric acid (CA) was studied on the growth of Cu, In and Cu-In layers. Polycrystalline Cu-In layers were electrodeposited from aqueous bath at −0.85V versus Ag/AgCl reference electrode. The structural, optical, morphological, compositional, photoelctrochemical and optoelectronic properties for Cu-In layer and selenized Cu-In layers are studied. Three prominent reflections of tetragonal crystal structure of CISe, (112), (204/220) and (312/116) were exhibited in selenized layers. Nearly uniform, void-free surface morphology was observed in selenized samples. Upon selenization of Cu-In layers prepared with 0.1M CA produced stoichiometric CISe thin films. The energy band-gap values are estimated between 1.05eV to 1.18eV. Photoelectrochemical cell measurement revealed the growth of p-type and n-type conductivity of CISe thin films. The peaks related to chalcopyrite CISe and ordered defect compound are observed in Raman analyses. The superstrate solar cell structure prepared with selenized Cu-In layer deposited in presence of 0.1M CA measured 4.05% efficiency. The values of Rs, 7.42Ω-cm2, 0.94Ω-cm2 and G, 1.01mS/cm2 10.25mS/cm2 were calculated under dark and illuminated condition, respectively. We believe that the measured efficiency is low; however the further improvement is possible by optimizing the surface treatment conditions.

Time Effect of Sulfurization on the Characterization of Copper Indium Diselenide

Polycrystalline CuIn(Se,S)2 (CISS) thin films with chalcopyrite structure were synthesized by sol-gel spin-coating method. Sequential reduction, selenization and sulfurization treatments were performed with hydrogen, Se and H2S vapor, respectively. The effects of different sulfurization times on the films were also studyed. X-ray diffraction (XRD), and SEM analysis have been employed to examine the structural properties and surface morphology of polycrystalline CISS thin films. The experimental results demonstrated that the obtained CISS had good crystallinity and uniformity.

Effect of Soft-annealing on the Properties of CIGSe Thin Films Prepared from Solution Precursors

(CIGSe) thin films is well known non-vacuum process for thefabrication of CIGSe solar cells. However, due to the usage of organic chemicals in the preparation of CIGprecursor solutions, the crystallization of the polycrystalline CIGSe and the performance of CIGSe thin filmsolar cells were significantly affected by the carbon residues from the organic chemicals. In this work, we havetried to eliminate the carbon residues in the CIG precursor thin films efficiently by using soft-annealing process.By adjusting soft-annealing temperature, it is possible to control the amount of carbon residues in CIGprecursor thin films. The reduction of the carbon residues in CIG precursors by high temperature soft-annealingimproves the grain size and morphology of polycrystalline CIGSe thin films, which are also closely related withthe electrical properties of CIGSe thin film solar cells.Key Words : CIGSe, Thin film, Solar cell, Solution deposition, Carbon residueIntroductionDue to high absorption coefficient and optimum band gapenergy, solar cells based on polycrystalline CuIn

Sulfurization of Copper Indium Diselenide Synthetized by Sol-Gel Spin-Coating Method

A non-vacuum process for fabrication of polycrystalline CuIn(Se,S)2 (CISS) thin films with chalcopyrite structure were synthesized by sol-gel spin-coating method using nitrates as precursor for low-cost photovoltaic applications. Sequential reduction, selenization and sulfurization treatments were performed with hydrogen, Se and H2S vapor, respectively. X-ray diffraction (XRD), SEM and EDS analysis have been employed to examine the structural properties, surface morphology and atomic concentrations of polycrystalline CISS thin films. The experimental results demonstrated that the obtained CISS had good crystallinity, uniformity and near stoichiometric composition.

First-principles density functional calculation of mechanical, thermodynamic and electronic properties of CuIn and Cu2In crystals

The study aims at assessing the mechanical, thermodynamic and electronic properties of single-crystalline and polycrystalline CuIn and Cu2In intermetallic compound (IMC) crystals using first-principles calculation based on the density functional theory within the generalized gradient approximation. The lattice constants and the five independent elastic constants of the two hexagonal single crystal structures are first calculated as a function of hydrostatic pressure, and their elastic anisotropy is examined through the computation of the crystal direction-dependent elastic modulus and the Zener anisotropy factor. Subsequently, their associated pressure-dependent polycrystalline elastic properties are also predicted, by which the ductility or brittleness of the IMC materials is characterized. Moreover, the temperature-dependent Debye temperature and heat capacity of these two IMC nanocrystals are determined using a quasi-harmonic Debye model, and their electronic band structures and density of states profiles are examined through analysis of electronic characteristics.The calculation results show that these two IMC crystals are not only an elastically anisotropic, low stiff and very ductile material but also a conductor. The elastic anisotropy, Debye temperature and heat capacity of Cu2In single crystal all surpass those of CuIn, and also, Cu2In crystal tends to be much stiffer than CuIn. Besides, the heat capacity of these two nanocrystals strictly follows with the well-known T3-law at temperature below the Debye temperature and would reach the Dulong–Petit limit at temperature above the Debye temperature.

Optical Properties of \(\text{CuInS}_{2}\) Thin Films Prepared by Spray Pyrolysis

Polycrystalline CuInS\(_{2}\) (CIS) absorber films for solar cells were prepared by spray pyrolysis of aqueuos solution of copper chloride, indium chloride and thiourea onto heated glass substrates. By optimizing the spray parameters, such as reducing/increasing the temperature of the substrate and molar ratio of Cu/In in the spraying solution, the optical characteristics of films, which are well matched to the solar spectrum, were identified. In all cases, those CIS thin films were of p-type conductivity. Transmission measurements were performed to examine the optical properties of the films; the absorption coefficient and the optical band gap of the films were calculated by transmission spectra. The absorption spectra of the films showed that this compound is a direct band gap one and its gap varied between 1.30 - 1.78 eV. Those thin films were analyzed by X-ray diffraction in order to understand the effect of layers structure on their optical properties.

Nanometer-scale electronic and microstructural properties of grain boundaries in Cu(In,Ga)Se 2

Despite many recent research efforts, the influence of grain boundaries (GBs) on device properties of CuIn 1−xGa xSe 2 solar cells is still not fully understood Here, we present a microscopic approach to characterizing GBs in polycrystalline CuIn 1−xGa xSe 2 films with x = 0.33. On samples from the same deposition process we applied methods giving complementary information, i.e., electron backscatter diffraction (EBSD), electron-beam induced current measurements (EBIC), conductive atomic force microscopy (c-AFM), variable-temperature Kelvin probe force microscopy (KPFM), and scanning capacitance microscopy (SCM). By combining EBIC with EBSD, we find a decrease in charge-carrier collection for non-∑3 GBs, while ∑ 3 GBs exhibit no variation with respect to grain interiors. In contrast, a higher conductance of GBs compared to grain interiors was found by c-AFM at low bias and under illumination. By KPFM, we directly measured the band bending at GBs, finding a variation from − 80 up to + 115 mV. Depletion and even inversion at GBs was confirmed by SCM. We comparatively discuss the apparent differences between the results obtained by various microscopic techniques.

Identification of the Molecular Precursors for Hydrazine Solution Processed CuIn(Se,S)2 Films and Their Interactions

We investigate the molecular species present in hydrazine CuIn(Se,S)2 precursor solutions, their interactions, and phase formation in solution processed CuIn(Se,S)2 thin films through Raman spectroscopy. The reaction between Cu2S and sulfur yields [Cu6S4]2− ions, while [In2Se4]2− ions are formed by the reaction of In2Se3 with selenium within their respective solutions. Once combined to prepare a CuIn(Se,S)2 precursor solution, these two species appear to be bridged via newly formed In−S bonds. The creation of the In−S bonds in the CuIn(Se,S)2 precursor solution provides strong evidence for the mixing of copper, indium, sulfur, and selenium at a molecular level even prior to deposition. From this configuration, relatively little atomic diffusion is required to reach the chalcopyrite structure, which enables the formation of highly uniform polycrystalline CuIn(Se,S)2 films at relatively low temperatures.

Preparation of CuIn(SxSe1–x)2 thin films with tunable band gap by controlling sulfurization temperature of CuInSe2

In this paper, polycrystalline CuIn(SxSe1–x)2 thin films with tunable x and Eg (band gap) values were prepared by controlling the sulfurization temperature (T) of CuInSe2 thin films. X-ray diffraction indicated the CuIn(SxSe1–x)2 films exhibited a homogeneous chalcopyrite structure. When T increases from 150 to 500 °C, x increases from 0 to 1, and Eg increases from 0.96 to 1.43 eV. The relations between x and Eg and the sulfurization process of CuIn(SxSe1–x)2 thin films have been discussed. This work provides an easy and low-cost technique for preparing large area absorber layers of solar cell with tunable Eg.

Photoreduction of Water by using Modified CuInS2 Electrodes

Polycrystalline CuInS(2) films were fabricated by sulfurization of electrodeposited Cu and In metallic precursor films. Structural analyses revealed that the CuInS(2) film formed compact agglomerates of crystallites with grain sizes of ca. 0.5-1.5 μm. Photoelectrochemical characterization revealed that the film was p-type with a flat band potential of 0.3-0.4 V (vs Ag/AgCl at pH 4), which is suitable for water reduction but cannot be for water oxidation. Upon loading Pt deposits, the film worked as a hydrogen (H(2)) liberation electrode under cathodic polarization. Moreover, by introduction of n-type thin layers such as CdS and ZnS on the CuInS(2) surface before the Pt loading, appreciable improvements of H(2) liberation efficiency were achieved: for the CdS modified sample, spectral response data showed incident photon to current efficiency as high as 20 % at wavelengths ranging from ca. 500 to 750 nm. Appreciable H(2) evolution on this sample under potentials of power-producing regions was also confirmed.

Enhanced electrical properties of pulsed laser-deposited CuIn 0.7Ga 0.3Se 2 thin films via processing control

Polycrystalline CuIn 0.7Ga 0.3Se 2 thin films were prepared on soda-lime glass substrates using pulsed laser deposition (PLD) with various process parameters such as laser energy, repetition rate and substrate temperature. It was confirmed that there existed a limited laser energy, i.e. less than 300 mJ, to get phase pure CIGS thin films at room temperature. Particularly, even at room temperature, distinct crystalline CIGS phase was observed in the films. Crystallinity of the films improved with increasing substrate temperature as evidenced by the decrease of FWHM from 0.65° to 0.54°. Slightly Cu-rich surface with Cu 2− x Se phase was confirmed to exist by Raman spectra, depending on substrate temperature. Improved electrical properties, i.e., carrier concentration of ∼10 18 cm −3 and resistivity of 10 −1 Ω cm at higher substrate temperature for the optimal CIGS films are assumed to be induced by the potential contributions from highly crystallized thin films, existence of Cu 2− x Se phase and diffusion of Na from substrates to films.

Synthesis and characterization of polycrystalline CuInS 2 thin films for solar cell devices at low temperature processing conditions

Polycrystalline CuInS 2 thin films were deposited on glass substrates using a novel solution-based continuous flow microreactor process for the first time. A series of analysis was performed to characterize the CuInS 2 thin films using UV–visible spectrophotometer, scanning electron microscope, X-ray diffraction spectrometer, and X-ray photoelectron spectroscopy. The estimated optical band gaps of CuInS 2 thin films were in the range of 1.52–1.60 eV. The structural and chemical binding information indicated that CuInS 2 thin films with a tetragonal chalcopyrite structure were successfully deposited. Dense film with a thickness around 1 μm could be obtained with a 5 min deposition time. This study demonstrates the solution-based continuous flow microreactor process is a promising low cost alternative for thin film PV manufacturing.

Properties of CuIn xGa 1−xSe 2 thin films grown from electrodeposited precursors with different levels of selenium content

In this paper, polycrystalline CuIn x Ga 1− x Se 2 (CIGS) thin film absorbers were prepared by selenizing electrodeposited (ED) precursors with two different levels of selenium content: rich in selenium and poor in selenium. Comparing the results obtained by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and illuminated current–voltage ( J– V), it was found that absorber layers processed from Se-poor ED precursors shows better crystalline quality and increased gallium incorporation, which thus improved cell performance, compared to the layers grown using Se-rich ED precursors. The best cell fabricated from Se-poor ED precursor shows a conversion efficiency of 1.63% at AM1.5 global light.

Locally resolved surface photo voltage spectroscopy on Zn-doped CuInS 2 polycrystalline thin films

Incorporation of small amounts of Zinc (< 1 at.%) in polycrystalline CuInS 2 thin films for solar cells leads to an increased open circuit voltage. Here we investigate the optoelectronic effect of Zn doping by local surface photovoltage spectroscopy (SPS). SPS is measured using Kelvin probe force microscopy (KPFM) to obtain the surface photovoltage (SPV) and SPS with high lateral resolution, and thereby study the homogeneity of the doping. In our KPFM experimental setup, illumination is realized by a Xe arc lamp and monochromator in the visible spectrum range by means of an optical fiber into the UHV system of the KPFM. We compare CuInS 2 thin film samples with and without Zn doping. The pure CuInS 2 samples show a sharp onset of SPV at the band gap of 1.48 eV, whereas for Zn-doped CuInS 2 we observe a two step onset, with a steep increase of SPV at 1.48 eV. However, already below this band gap, we observe a slight SPV response, even down to about 1.40 eV. This indicates the presence of states in the band gap, likely resulting from disorder induced by the Zn-doping. The absence of lateral differences in the observed SPV spectra favors an explanation by Urbach-tails over the possible existence of a Zn foreign phase. These results are in agreement with transmission/absorption measurements.

Epitaxial and polycrystalline CuInS 2 layers: Structural metastability and its influence on the photoluminescence

Thin epitaxial and polycrystalline CuInS 2 (CIS) films were grown on single crystalline Si(111) and Mo-coated Si substrates, respectively, by means of molecular beam epitaxy from elemental sources. Photoluminescence (PL) measurements were performed to investigate the optical properties of both, epitaxial and polycrystalline CIS films. Epitaxial CIS samples show defect related transitions only and the PL spectra are dominated by broad luminescence peaks of deep levels, while excitonic transitions are completely absent. This contrasts sharply with the PL of the polycrystalline films, which is dominated by excitonic luminescence. Contributions due to shallow defects are observed with a small intensity only. However, luminescence peaks of defects with electronic levels deep in the band gap are not present at all. This includes the broad PL lines around 1.2 or 1.3 eV which are typical for polycrystalline CIS solar-cell material. X-ray diffraction and selected area electron diffraction measurements were employed in order to study the crystal structure. The epitaxial CIS films show a coexistence of the metastable CuAu-type (CA) ordering with the ground-state chalcopyrite (CH) structure, while the polycrystalline layers crystallize exclusively in the ground-state CH ordering. Hence, the coexistence of the metastable CA ordering and the ground-state CH structure in the epitaxial films is accompanied by a high density of electrically active intrinsic defects with levels deep in the band gap.

CuIn 1 − xAl xSe 2 thin films obtained by selenization of evaporated metallic precursor layers

CuIn 1 − x Al x Se 2 (CIAS) thin films were grown by a two stage process. Cu, In and Al layers were sequentially evaporated and subsequently heated with elemental selenium in a quasi-closed graphite box. Different x values (0 ≤ x ≤ 0.6) were obtained by varying the Al and In precursor layers thicknesses. Selenization conditions such as Se amount provided during the selenization process were adjusted in order to optimize the film properties. Polycrystalline CuIn 1 − x Al x Se 2 thin films with chalcopyrite structure were obtained. Referred to CuInSe 2 thin films the lattice parameters, the (112) orientation and the average crystallite size decreased and the band gap energy increased with increasing Al content. To optimize structural properties of the CIAS films a higher Se amount was required as the x value increased. The incorporation of Al changed the thin film morphology towards smaller grain sizes and less compact structures.

Growth of single-phase Cu(In,Al)Se 2 photoabsorbing films by selenization using diethylselenide

Selenization growth of purely single-phase, polycrystalline CuIn 1 − x Al x Se 2 (0 ≤ x ≤ 0.26) alloy films was demonstrated using a less-hazardous metal-organic selenide, diethylselenide [(C 2H 5) 2Se: DESe], by simple thermal annealing of metal precursor. Approximately 2.0 µm thick alloy films exhibited X-ray diffraction peaks originating exclusively from the chalcopyrite structure. Transitions seen in the low temperature photoluminescence spectra were attributed to characteristic donor–acceptor pair emissions of the state-of-the-art CuIn 1 − x Al x Se 2 photoabsorbing layers.

Influence of post-grown treatments on CuInS 2 thin films prepared by sulphurization of Cu-In films

Polycrystalline CuInS 2 (CIS) films were prepared by sulphurization of Cu-In films. The surface morphology and phase composition of the as-grown film, the KCN-etched film, and the annealed KCN-etched film were investigated. During the sulphurization, the secondary Cu x S phase segregated on the surface of the as-grown films. To improve the crystalline quality of CuInS 2 films, a series of post-grown treatments, such as KCN-etching and vacuum annealing KCN-etched films, were performed on the as-grown films. Both as-grown and post-treated films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results indicated that a Cu x S secondary phase segregated on the surface of the as-grown film, which could be removed effectively by KCN etching. After the vacuum annealing treatment, the KCN-etched film had a sphalerite structure with (112) preferred orientation. Meanwhile, the crystalline quality of the CIS film was significantly improved, which provided a novel method to improve the performance of thin film solar cells.

Electrical and optical characterisation of CuInS 2 crystals and polycrystalline coevaporated thin films

Single crystals CuInS 2 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 CuInS 2 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 CuInS 2 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.

Growth of single-phase CuInGaSe 2 photo-absorbing alloy films by the selenization method using diethylselenide as a less-hazardous Se source

Selenization growth of purely single-phase, polycrystalline CuIn 1 − x Ga x Se 2 (0 ≤ x ≤ 1) alloy films was demonstrated using a less-hazardous metalorganic selenide, diethylselenide [(C 2H 5) 2Se: DESe], without additional thermal annealing. Approximately 2.0-μm-thick films of the alloys exhibited X-ray diffraction peaks originating exclusively from the chalcopyrite structure. Low temperature photoluminescence spectra of the alloy films were dominated by a couple of characteristic donor-acceptor pair emissions that are particular to the state-of-the-art CuInGaSe 2 photo-absorbing layers.