Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells

Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu₁₁In₉ precursors to be employed as light absorbing CuInS₂ (CIS) layers in CIS-CdS heterojunction thin-film solar cells has been investigated. The Cu₁₁In₉ precursor films were produced by DC-sputtering from a single-phase Cu₁₁In₉ target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In₂S compound, the formation of which is favoured by the phase transition of Cu₁₁In₉ to Cu₁₆In₉ at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuS_x surface phase.     

Structure and phase relations in the 2(CuInS2)-Cu2ZnSnS4 solid solution system

The intermixing of roquesite (CuInS₂) and kesterite (Cu₂ZnSnS₄), i. e. Cu(In_x(ZnSn)_1−xS₂ was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ × ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 800 °C and cooled with a 10 K/h rate after the final annealing. The structural parameters of CuIn_x(ZnSn)_1−xS₂ were determined by simultaneous Rietveld refinement of neutron and X-ray diffraction data. The microstructure and chemical composition of the samples were investigated by electron microprobe analysis. A broad miscibility gap exists in the region 0.4 ≤ × < 0.8 indicated by the coexistence of two phases, an In-rich (x ~ 0.77) and a Zn-Sn-rich (x ~ 0.33) phase. Cu(In_x(ZnSn)_1−xS₂ mixed crystals with 0 ≤ x < 0.4 crystallize in the kesterite type structure, and with 0.8 ≤ × ≤ 1.0 in the chalcopyrite type structure. In the latter In, Zn and Sn are disordered on the In site. In the mixed crystals the lattice constant a and c show a linear dependence on chemical composition, whereas the tetragonal deformation Δ = 1−c/2a varies nonlinearly. Moreover in the mixed crystal with x ~ 0.15 the tetragonal deformation is equal zero and thus its structure is characterized by a pseudo-cubic unit cell.     

Epitaxial and polycrystalline CuInS2 layers: Structural metastability and its influence on the photoluminescence

Thin epitaxial and polycrystalline CuInS₂ (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.     

Epitaxial and polycrystalline CuInS2 thin films: A comparison of opto-electronic properties

Epitaxial and polycrystalline thin CuInS₂ (CIS) layers were grown by means of molecular beam epitaxy (MBE) on single crystalline silicon substrates of 4 inch diameter. Photoluminescence (PL) studies were performed to investigate the opto-electronic properties of these layers. For the epitaxial CIS, low-energy-hydrogen implantation leads to the passivation of deep defects and several donor-acceptor (DA) pair recombinations (from 1.034 eV to 1.439 eV) and two free-to-bound (FB) transitions (at 1.436 eV and 1.485 eV) become observable at low temperatures (5 to 100 K). Excitonic luminescence is completely absent for all investigated epitaxial CIS layers. This contrasts sharply with the PL of the polycrystalline films which is dominated by excitonic luminescence (1.527 eV). Also a donor-to-valence band transition at 1.465 eV (BF-1) and one donor-acceptor recombination at 1.435 eV (DA-1) were observed, while luminescence from deep levels is not present at all. Based on these data, a refined defect model for CuInS₂ with two donor and two acceptor states is presented. Under comparable growth conditions, the electronic quality of polycrystalline CIS is superior to epitaxially grown material.     

Progress in the development of CuInS2 based mini-modules

A sequential process is used to synthesise CuInS₂ absorber layers for photovoltaic application. In this process CuIn precursor layers sputtered on molybdenum coated float glass are converted to CuInS₂ via sulphurisation in an elemental sulphur vapour ambient. A re-evaluation of process parameters has been performed including fine tuning of numerous minor aspects. Using optimised process conditions has led to improved device performance, especially a narrowed distribution at higher module efficiencies is achieved. At the same time the process yield is improved resulting in fewer devices with poor electrical quality.     

Characterization of sprayed CuInS2 films by XRD and Raman spectroscopy measurements

We studied CuInS₂ (CIS) film growth using two deposition methods, which were high electrostatic field assisted ultrasonic spray (HEFAUS) deposition and sulfurization of Cu-In metallic film. The sprayed-films were grown with chalcopyrite ordering and Cu-Au ordering mixed. In order to obtain higher quality CIS films, post-sulfurization was carried out for sprayed-films. The post-sulfurization induced improvement of crystallinity and enhancement of chalcopyrite ordering. However, it was observed that Cu-Au ordering still coexisted in the CIS film after post-sulfurization. With the same sulfurization condition, sulfurization was done to transform Cu-In metallic film into CIS film. The sulfurized metallic film was turned out to be formed as CIS film with higher crystallinity and better chalcopyrite ordering than sulfurized sprayed-films. All fabricated films were characterized by X-ray diffraction, Raman scattering, scanning electron microscope and energy dispersive X-ray analysis measurements.     

The formation of the thin-film solar cell absorber CuInS2 by annealing of Cu-In-S stacked elemental layer precursors — A comparison of selenisation and sulfurisation

We present results of in-situ X-ray diffraction experiments on the formation of CuInS₂ thin film solar cell absorbers. The experiments have been performed while annealing Cu-In-S stacked elemental layer precursors produced by sputtering and thermal evaporation to investigate the crystallisation process of the chalcopyrite CuInS₂. Rietveld refinement has been performed to obtain the quantitative phase evolution of crystalline phases while annealing. The annealing process is characterised by a rapid sulfurisation of the initially present intermetallic alloy Cu₁₁In₉ forming the sulfide phases CuS, Dg-Cu_2 − xS, InS and/or CuIn₅S₈. The chalcopyrite CuInS₂ crystallises at elevated sample temperatures by the consumption of these sulfide phases as educts. Three different chalcopyrite formation reactions have been identified by an analysis of the quantitative phase evolution. A comparison to earlier investigations on the formation of CuInSe₂ from Cu-In-Se precursors is presented to show similarities and differences of sulfurisation and selenisation processes. The chalcopyrite forms from chalcogenide educts in both cases. However, distinct differences concerning the chalcogenisation kinetics of sulfur and selenium containing Cu-In precursors have been revealed. The chalcogenisation of the intermetallic alloy phase Cu₁₁In₉ proceeds extremely fast for Cu-In-S precursors as compared to Cu-In-Se samples.     

Optical constants of Zn-doped CuInS2 thin films

Optical properties of Zn-doped CuInS₂ thin films grown by double source thermal evaporation method have been studied. The amount of the Zn source was determined to be 0%-4% molecular weight compared with CuInS₂ source. After that, samples were annealed in vacuum at the temperature of 450 °C in quartz tube. The optical constants of the deposited films were obtained from the analysis of the experimental recorded transmission and reflexion spectral data over the wavelength range 300-1800 nm. It is observed that there is an increase in optical band gap with increasing Zn % molecular weight. It has been found that the refractive index and extinction coefficient are dependent on Zn incorporation. The complex dielectric constants of Zn-doped CuInS₂ films have been calculated in the investigated wavelength range. It was found that the refractive index dispersion data obeyed the single oscillator of the Wemple-DiDomenico model, from which the dispersion parameters and the high-frequency dielectric constant were determined. The electric free carrier susceptibility and the carrier concentration on the effective mass ratio were estimated according to the model of Spitzer and Fan.     

One-pot route for preparation of monodisperse CuInS2 nanocrystals

Chalcopyrite copper indium sulfide (CuInS₂) nanocrystals (NCs) were synthesized by a one-pot route and characterized by XRD, TEM, HRTEM, EDS, UV-vis and SPS. The experimental results demonstrated that the CuInS₂ NCs synthesized by metal compound [Sn(acac)₂Cl₂] had good crystallinity, monodispersity and stoichiometric composition. The UV-vis absorption spectra showed the as-synthesized CuInS₂ NCs had fine absorption in the visible light region and the energy band gap was estimated to be 1.58 eV. Moreover, the metal compound could improve the photoinduced charge separation and transfer effect of NCs based on the SPS study. The synthesis strategy developed in this work may be used as a general process for the synthesis of pure or doped chalcogenide NCs, and may have great potential to be applied in high efficiency, yet low cost photovoltaic areas.     

Addition of Na into CuInS2 thin film via co-evaporation

While most of studies focus on the addition of Na into CuInGaSe₂ as well as CuInGaS₂ thin films, this study examines the addition of Na into CuInS₂ (CIS) thin films. Moreover, an alternative approach has been used to incorporate Na into CIS thin films. Two source evaporation (Cu and In) was first performed to obtain Cu-In layers with desired thicknesses. Three source evaporation (Cu, In, and NaF) then followed subsequently to produce Na-doped Cu-In precursor films having different Na concentrations. The precursor films were immediately sulfurized in the same evaporation chamber to form CIS thin films. The addition of Na was found to enhance (112) preferred orientation and reduce the grain size. Raman spectra show that the addition of Na does not alter the needed phase transformation from CuInS₂-CuAu structure to CuInS₂-chalcopyrite structure during the sulfurization. Blue shift of the CIS Raman CH mode occurs as a result. The doping of Na was also found to decrease the film resistivity or increase the hole concentration in the films.     

Dependence of optical and structural properties of ZnS and MgF2 multilayers as a function of the number of layers

In this paper, optical and structural properties of ZnS and MgF₂ multilayers grown by thermal evaporation are studied. Effects of annealing at different temperatures on samples with different number of layers are investigated. The maximum of reflection is shifted to different wavelengths, depending on the number of layers of the annealed samples. Using X-ray diffraction analysis, structural properties have been studied, and grain size and microstrain have been obtained by the Scherrer-Wilson formula, with grain sizes ranging from 10 nm to 22 nm for MgF₂ and from 0.9 nm to 210 nm for ZnS, and microstrain values from 2.5 × 10^− 3 to 3 × 10^− 3 for MgF₂, and from 1.2 × 10^− 3 to 2.6 × 10^− 3 for ZnS. Competition between crystallite size and microstrain is observed.     

Solvothermal synthesis, nanocrystal print and photoelectrochemical properties of CuInS2 thin film

CuInS₂ nanoflakes were synthesized successfully by a convenient solvothermal route in ethylene glycol under the open-air condition using copper chloride, indium chloride and thiourea as starting materials. The factors which might affect the purity of the product during the synthesis were discussed. It's found that the products were significantly affected by the reaction time, temperature and the diffusion of the reactors. CuInS₂ nanoflakes ink was prepared and coated onto Mo substrate using blade technology, and after sintering, a dense and compact CuInS₂ film was produced. The morphological of the precursor films and CuInS₂ films were done by scanning electron microscope (SEM), and the photoelectrochemical properties and morphology of the CuInS₂ thin film were characterized.     

Simplified modulated evaporation process for the production of CuInS2 films with reduced substrate temperatures

CuInS₂ films with sub-micrometer thickness have been grown onto soda-lime glass substrates from the elemental constituents by a modulated flux deposition procedure. A reduced substrate temperature of about 350 °C was used during the process. Morphological characterization of the films suggests the formation of an In-rich layer in a first step of the deposition process. Adequate modulation of the In and Cu evaporation fluxes during a second stage makes the film evolving to the ternary CuInS₂ compound. The absence of any copper sulfur phases on the film surface would make unnecessary the use of any etching treatment after deposition of the film.     

Preparation of CuInS2 microspheres via a facile solution-chemical method

With a mixed solvent of triethylenetetramine-ethylene glycol (1:1, v/v), CuInS₂ microspheres were synthesized by a facile solution-chemical method under the open-air condition. Morphology, structure, phase constituents and optical properties of the as-prepared CuInS₂ powders were characterized by X-ray diffraction (XRD), Energy Dispersive Spectrometer (EDS), scanning electron microscope (SEM) and ultraviolet-visible (UV-vis) spectrophotometry. The characterizations showed that the synthesized CuInS₂ powders had single phase, good crystallinity and stoichiometric composition. Moreover, the prepared CuInS₂ powders showed microspheres with the size from 200 to 400 nm, and their energy band gaps were 1.52 eV, which made them promising candidates as absorber materials for photovoltaic applications.     

Reactive magnetron sputtering of CuInS2 absorbers for thin film solar cells: Problems and prospects

We have investigated the reactive magnetron sputtering process from copper and indium targets in Ar/H₂S sputtering atmospheres for the deposition of CuInS₂ absorber films. This large area deposition method is in principle suited for the preparation of absorbers of good electronic quality. One of our best CuInS₂ cells exhibits an efficiency of 11.4% and an open circuit voltage of 745 mV, the highest reported value for CuInS₂ cells.However, it was found that the reproducibility of the photovoltaic layer properties is low. By analyzing the different defects in the films, it was found that the main defects are microscopic pin holes and crevices, which lead to short circuits. The intrinsic electronic quality of the magnetron sputtered CuInS₂ films is excellent, proven by room temperature photoluminescence spectroscopy.The occurrence of short circuits depends decisively on the film morphology, which is determined by the copper-to-indium ratio during the deposition. Even for constant external deposition parameters this ratio changes over the life time of the targets, mainly caused by a significant decrease of the indium deposition rate. For a stable CuInS₂ film deposition process an in situ control of the Cu/In ratio and the film morphology is advisable.     

Fabrication of photocatalytic heat-mirror with TiO2/TiN/TiO2 stacked layers

The photocatalytic heat-mirror based on TiO₂/TiN/TiO₂ stacked layers is prepared by reactive magnetron sputtering on quartz substrates under substrate-heating condition. We find that the addition of a thin Ti interlayer between the TiN and the outer TiO₂ layers drastically improves the heat-insulating performance. This type of stacked layer also exhibits higher photocatalytic activity for decomposition of acetaldehyde gas, compared with a TiO₂ single layer. The optical property of the TiN in TiO₂/TiN/TiO₂ stacked layers is the key not only revealing excellent heat-insulating effect but also improving the photocatalytic performance of the outer TiO₂ layers in the stacked layers.     

Role of oxygen in enhancing N-type conductivity of CuInS2 thin films

Post-growth treatments in air atmosphere were performed on CuInS₂ films prepared by the single-source thermal evaporation method. Their effect on the structural, optical and electrical properties of the films was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical reflection and transmission and resistance measurements. The films were annealed from 100 to 350 °C in air. The stability of the observed N-type conductivity after annealing depends strongly on the annealing temperature. Indeed it is shown that for annealing temperatures above 200 °C the N-type conductivity is stable. The resistance of the N-CuInS₂ thin films correlates well with the corresponding annealing temperature. The samples after annealing have direct bandgap energies of 1.45-1.50 eV.     

Transfer of CuInS2 thin film by lift-off process and application to superstrate-type thin-film solar cells

Transfer of a CuInS₂ thin film grown on a Mo/soda-lime glass substrate was investigated using a lift-off process. The CuInS₂ thin film was flatly exfoliated, with preferential peeling occurring in the CuInS₂/MoS₂ interface vicinity. This suggests that the interfacial MoS₂ layer behaves as a sacrificial layer. The lift-off process was also applied to solar cell fabrication. A superstrate-type CuInS₂ thin-film solar cell was fabricated and exhibited no significant degradation of conversion efficiency compared with a substrate-type CuInS₂ thin-film solar cell. The lift-off process could therefore also be applied to fabricate the upper part of a tandem solar cell structure.     

Preparation of Cu2ZnSnS4 thin films by sulfurization of stacked metallic layers

Stacked precursors of Cu, Sn, and Zn were fabricated on glass/Mo substrates by electron beam evaporation. Six kinds of precursors with different stacking sequences were prepared by sequential evaporation of Cu, Sn, and Zn with substrate heating. The precursors were sulfurized at temperatures of 560 °C for 2 h in an atmosphere of N₂ + sulfur vapor to fabricate Cu₂ZnSnS₄ (CZTS) thin films for solar cells. The sulfurized films exhibited X-ray diffraction peaks attributable to CZTS. Solar cells using CZTS thin films prepared from six kinds of precursors were fabricated. As a result, the solar cell using a CZTS thin film produced by sulfurization of the Mo/Zn/Cu/Sn precursor exhibited an open-circuit voltage of 478 mV, a short-circuit current of 9.78 mA/cm², a fill factor of 0.38, and a conversion efficiency of 1.79%.     

A mild solvothermal route for preparation of cubic-like CuInS2 crystals

A mild solvothermal route for preparation of cubic-like CuInS₂ crystals was developed by using benzyl alcohol as the reaction medium and In(NO₃)₃·9H₂O, CuCl₂·2H₂O and thiourea as the precursors. The as-synthesized CuInS₂ crystals were characterized by XRD, XPS, SEM, and TEM. The characterizations showed that the synthesized CuInS₂ crystals had stoichiometric composition and good crystallinity. Moreover, the prepared CuInS₂ crystals show good cubic-like morphology with the size from 100 to 200 nm.