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.     

Sulfurization of Cu–In electrodeposited precursors for CuInS2-based solar cells

Cu–In electrodeposited layers were annealed using rapid thermal processing (RTP) in a reactive atmosphere containing sulfur vapors. The CuInS₂ formation mechanism during sulfurization of electrodeposited precursors proceeds mainly through direct sulfurization of the metallic Cu–In alloy, forming spinel CuIn₅S₈ and chalcopyrite CuInS₂ ternary phases. During the heating step, the Cu–In metallic alloy gets richer in copper as the temperature increases and transforms from CuIn₂ to Cu₁₁In₉, then Cu₁₆In₉ and finally to Cu₇In₃. The use of rapidly cooled samples stopped after different durations of the process along with ex-situ XRD analysis enabled us to differentiate the Cu₁₆In₉ and Cu₇In₃ phases. Finally, the efficiency of the solar cells made with the two-step electrodeposition and RTP low-cost process reaches 11% (active area 0.421 cm²), which is close to the results obtained for cells made with PVD precursors.     

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.     

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.     

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 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.     

Electrodeposition of CuInSe2 and CuInS2 films

Various methods to electrodeposit films of CuInSe(S)₂ are discussed. Two basically different methods are treated separately: electrodeposition of the ternary CuInSe(S) systems and deposition of a CuIn alloy followed by annealing in an Se(S)-containing atmosphere.Electrodeposition of the ternary CuInSe(S) systems includes CuInS₂ (and CuIn₅S₈) plated from a thiourea bath. Morphological, compositional and photoelectrochemical behaviour of these layers is discussed. Attempts to plate CuInSe₂ from an SeO₂-containing bath and CuInS₂ from a non-aqueous sulphur-containing bath are also treated.Discussion of the second method, deposition of a CuIn alloy, concentrates on the alloy deposition step. Both co-deposition of copper and indium and sequential deposition of indium on copper are treated, with emphasis on the morphology of the layers.     

Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor

Surface sulfurization of Cu(In,Ga)Se₂ (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H₂S gas; a sequential evaporation of In₂S₃ after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga) (S,Se)₂ thin layer was grown on the surface of the CIGS thin film after sulfurization using In₂S₃, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques.     

Thermal diffusion of Cu into In2Se3: A better approach to SEL technique in the deposition of CuInSe2 thin films

This paper reports the modifications made in the preparation techniques of getting CuInSe₂ thin films starting with chemical bath deposited (CBD) selenium films. In the present study, CBD Se film was converted into CuInSe₂ by stacked elemental layer (SEL) technique and also by thermal diffusion of Cu into In₂Se₃. In both the cases CBD Se films were used to avoid toxic Se vapor and H₂Se gas. Improvements were made in these techniques through a detailed study, varying the composition of the films over a wide range by changing the Cu/In ratio. Structural, optical and electrical characterizations were performed. On comparing the material properties of CuInSe₂ deposited by these two techniques, it was found that photosensitivity was better for samples prepared by thermal diffusion of Cu into In₂Se₃. So the technique of thermal diffusion of Cu into In₂Se₃ was found to be better than SEL technique in the preparation of CuInSe₂ using CBD Se. Cu-rich, In-rich and nearly stoichiometric samples could be prepared by thermal diffusion of Cu into In₂Se₃. These samples were analyzed using energy dispersive spectroscopy, Raman spectroscopy and atomic force microscopy also.     

Influence of deposition parameters on surface morphology and application of CuInS2 thin films in solar cell and photocatalysis

Copper indium disulfide (CuInS₂) thin films as an absorption layer for solar cell and photocatalytic degradation of organic pollutants, were successfully electrodeposited on the FTO coated glass substrate using the simple and inexpensive electrodeposition method and a sulfurization process. The effects of the Cu/In molar ratio, annealing temperature and kind of Cu²⁺ precursor (Cu(salen) and Cu(acac)₂ as novel Cu²⁺ precursors) on the structural and morphological properties of samples were examined. The XRD diffraction patterns and energy dispersive spectroscopy measurements exhibit that high-quality film with superior crystalline structure was formed in the presence of Cu(salen) as Cu²⁺ precursor. Also, we found that a suitable heat treatment temperature could suppress the CuS phases and form well-crystallized CIS. As we know, this is the first reported efficiency for any CuInS₂ superstrate solar cell to date that fabricated using Cu(salen) as Cu²⁺ precursor. In addition, the photocatalytic degradation of organic dye in the presence of as-synthesized CuInS₂ thin films was studied. The as-prepared semiconductor photocatalysts have a good reusability; it can be successfully reused for 5 times recycling photoactivity tests.     

Hydrothermal synthesis of pyramid-like In2S3 film for efficient photoelectrochemical hydrogen generation

Herein, the pyramid-like In₂S₃ film was synthesized for photoelectrochemical (PEC) hydrogen generation using a facile hydrothermal route for the first time. The hydrothermal time was crucial to determine the morphology and composition of the as-prepared films. When the hydrothermal time lasted for 24 h, the pyramid-like In₂S₃ film could be achieved. With the increased hydrothermal time, the photocurrent first increased and then decreased, and the pyramid-like In₂S₃ film prepared for 24 h showed the highest photocurrent. After an annealing treatment, the photocurrent could be improved to 2.7 mA cm^?2 at 0.78 V vs. RHE, and a substantial hydrogen generation was observed. Compared to previously reported In₂S₃ films with different morphologies, the pyramid-like In₂S₃ film exhibited better PEC property. It was revealed that the remaining In₂O₃ after the synthesis of In₂S₃ had a great influence on the PEC performance. The pyramid-like In₂S₃ film with a moderate amount of In₂O₃ induced an efficient charge separation and transfer, as well as good light-absorption ability, which led to the superior PEC performance. This work has demonstrated great potentials of the pyramid-like In₂S₃ film for PEC hydrogen generation and opened a promising avenue towards the design and fabrication of novel pyramid-like nanostructures.     

Ex-situ and in-situ analyses on reaction mechanism of CuInSe2 (CIS) formed by selenization of sputter deposited CuIn precursor with Se vapor

Formation mechanism of CIS thin films by selenization of sputter deposited CuIn precursor with Se vapor was investigated by ex-situ and in-situ phase analysis tools. Precursor films were composed of In, CuIn and Cu₂In compounds, and their relative fractions were systematically changed with Cu/In ratios. Those films were found to have a double layered structure with nearly pure In particles (top layer) placed on the flat Cu-rich bottom layer, and the morphologies were also significantly affected by Cu/In ratio. At the initial stage of selenization, the outer In-rich layer reacted with Se vapor to form In-Se binary, which is the first selenide phase appeared, and inner Cu-rich phases acted as a Cu source to supply Cu to outer In-Se phase to form ordered vacancy compounds (OVC). As these reactions continues, in conjunction with Se incorporation into inner Cu-rich region, the films gradually changes from OVC to α-CIS, reflecting that the formation route of CIS is closely related to the elemental and phase distribution in precursor films. Selenized CIS films were further processed to fabricate CIS thin film solar cells, resulting in the best cell efficiency of 10.44%.     

Optical characterization of In2S3 solar cell buffer layers grown by chemical bath and physical vapor deposition

In this paper, we study the optical properties of indium sulfide thin films to establish the best conditions to obtain a good solar cell buffer layer. The In₂S₃ buffer layers have been prepared by chemical bath deposition (CBD) and thermal evaporation (PVD). Optical behavior differences have been found between CBD and PVD In₂S₃ thin films that have been explained as due to structural, morphological and compositional differences observed in the films prepared by both methods. The resultant refractive index difference has to be attributed to the lower density of the CBD films, which can be related to the presence of oxygen. Its higher refractive index makes PVD film better suited to reduce overall reflectance in a typical CIGS solar cell.     

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.     

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.     

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.     

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.     

Fabrication of CuInSe2 films and solar cells by the sequential evaporation of In2Se3 and Cu2Se binary compounds

Cu₂Se/In_xSe(x≈1) double layers were prepared by sequentially evaporating In₂Se₃ and Cu₂Se binary compounds at room temperature on glass or Mo-coated glass substrates and CuInSe₂ films were formed by annealing them in a Se atmosphere at 550°C in the same vacuum chamber. The In_xSe thickness was fixed at 1 μm and the Cu₂Se thickness was varied from 0.2 to 0.5 μm. The CuInSe₂ films were single phase and the compositions were Cu-rich when the Cu₂Se thickness was above 0.35 μm. And then, a thin CuIn₃Se₅ layer was formed on the top of the CuInSe₂ film by co-evaporating In₂Se₃ and Se at 550°C. When the thickness of CuIn₃Se₅ layer was about 150 nm, the CuInSe₂ cell showed the active area efficiency of 5.4% with V_oc=286 mV, J_sc=36 mA/cm² and FF=0.52. As the CuIn₃Se₅ thickness increased further, the efficiency decreased.     

Colloidal synthesis of CuInS2 nanoparticles: Crystal phase design and thin film fabrication for photoelectrochemical solar cells

This study reports the colloidal synthesis of copper indium disulfide (CuInS₂) nanoparticles in different crystal phases to be employed as thin film photoanodes in photoelectrochemical water splitting process. First, CuInS₂ nanoparticles with chalcopyrite-, zincblende-, wurtzite-as well as polytypic-phases have been synthesized using hot injection method. The effects of solvent, temperature and type of precursors on the phase design have been thoroughly investigated via various spectroscopic techniques such as XRD, SEM, HRTEM, UV-Vis and PL spectroscopy and Zeta particle size analysis. The XRD spectra have been revealed that the all the targeted nanoparticles had good crystallinity and free from undesired binary sulfides. The synthesized nanoparticles have been re-dispersed in N, N-dimethylformamide (DMF) to form nanoink paste and applied on fluorine doped tin oxide coated glass substrate by doctor blade technique. DMF has been found to be an enviable solvent for thin film fabrication since it could lead to the crack free and uniform surface formation. The chalcopyrite thin film has shown the best photoelectrochemical performance with the photocurrent density of ∼15 mA cm⁻² and conversion efficiency of 6.7%. Howbeit, thin films photoanodes bearing wurtzite, zincblende and polytypic CuInS₂ nanoparticles have been investigated to compare the performance of different crystal phases for photoelectrochemical solar cell applications. Moreover, it should be emphasized that all thin film electrodes have been investigated under 1-sun condition without any surface modification, chemical treatment and etching. Additionally, the thin films except wurtzite structure exhibited good stability along 2 h under dark and illuminated conditions.     

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%.