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.     

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.     

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.     

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.     

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.     

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.     

Photo-fixation of SO2 in nanocrystalline TiO2 films prepared by reactive DC magnetron sputtering

We report on photo-fixation of SO₂ onto nanostructured TiO₂ thin films prepared by reactive DC magnetron sputtering. The films were exposed to 50 ppm SO₂ gas mixed in synthetic air and illuminated with UV light at 298 and 473 K. The evolution of the adsorbed SO_x species was monitored by in situ Fourier transform infrared specular reflection spectroscopy. Significant photo-fixation occurred only in the presence of UV illumination. The SO₂ uptake was dramatically enhanced at elevated temperatures and then produced strongly bonded surface-coordinated SO_x complexes. The total SO_x uptake is consistent with Langmuir adsorption kinetics. The sulfur doping at saturation was estimated from X-ray photoelectron spectroscopy to be ~ 2.2 at.% at 473 K. These films were pale yellowish and had an optical absorption coefficient being ~ 3 times higher than in undoped film. The S-doped films exhibit interesting oleophobic properties, exemplified by the poor adherence of stearic acid. Our results suggest a new method for sulfur doping of TiO₂ to achieve combined anti-grease and photocatalytic properties.     

Numerical modeling of SiH4 discharge for Si thin film deposition for thin film transistor and solar cells

Amorphous and microcrystalline silicon thin films are used in solar cells as a multi-junction photovoltaic device. Plasma enhanced chemical vapor deposition is used and high deposition rate of a few nm/s is required while keeping film quality. SiH₄ is used as a precursor diluted with H₂. Electron impact processes give complex interdependent plasma chemical reactions. Many researchers suggest keeping high H/SiH_x ratio is important. Numerical modeling of this process for capacitively coupled plasma and inductively coupled plasma is done to investigate which process parameters are playing key roles in determining it. A full set of 67 volume reactions and reduced set are used. Under most of conditions, CCP shows 100 times higher H/SiH₃ ratio over ICP case due to its spatially localized two electron temperature distribution. Multi hollow cathode type CCP is also modeled as a 2 × 2 hole array. For Ar, the discharge is well localized at the neck of the hole at a few Torr of gas pressure. H₂ and SiH₄ + H₂ needed higher gas pressure and power density to get a multi hole localized density profile. H/SiH₃ was calculated to be about 1/10.     

Reactive sputtering of precursors for Cu2ZnSnS4 thin film solar cells

The quaternary semiconductor Cu₂ZnSnS₄ (CZTS) is a possible In-free replacement for Cu(In,Ga)Se₂. Here we present reactive sputtering with the possibility to obtain homogeneous CZTS-precursors with tunable composition and a stoichiometric quantity of sulfur. The precursors can be rapidly annealed to create large grained films to be used in solar cells. The reactive sputtering process is flexible, and morphology, stress and metal and sulfur contents were varied by changing the H₂S/Ar-flow ratio, pressure and substrate temperature. A process curve for the reactive sputtering from CuSn and Zn targets is presented. The Zn-target is shown to switch to compound mode earlier and faster compared to the CuSn-target. The precursors containing a stoichiometric amount of sulfur exhibit columnar grains, have a crystal structure best matching ZnS and give a broad peak, best matching CZTS, in Raman scattering. In comparing process gas flows it is shown that the sulfur content is strongly dependent on the H₂S partial pressure but the total pressures compared in this study have little effect on the precursor properties. Increasing the substrate temperature changes the film composition due to the high vapor pressures of Zn, SnS and S. High substrate temperatures also give slightly denser and increasingly oriented films. The precursors are under compressive stress, which is reduced with higher deposition temperatures.     

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.     

Fabrication of YbAl3 film via annealing amorphous Yb-Al film deposited by RF magnetron sputtering

YbAl₃ single-phase bulk alloy was synthesized by melting at 1258 K using raw Yb and Al metals in the ratio of 1:2.2. A sputtering YbAl₃ target was prepared using the precursor YbAl₃ bulk alloy with the spark plasma sintering process. An amorphous Yb-Al film was fabricated by RF magnetron sputtering using the YbAl₃ target, and a single-phase crystalline YbAl₃ film was fabricated by annealing the amorphous Yb-Al film at temperatures higher than 923 K in Ar atmosphere.     

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.     

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.     

Development of CZTS-based thin film solar cells

The low cost, environmental harmless Cu₂ZnSnS₄ (CZTS)-based thin film solar cells are fabricated by using abundant materials. The CZTS film possesses promising characteristic optical properties; band-gap energy of about 1.5 eV and large absorption coefficient in the order of 10⁴ cm^− 1. All constituents of this CZTS film, which are abundant in the crust of the earth, are non-toxic. Therefore, if we can use CZTS film practically as the absorber of solar cells, we will be free from both of the resource saving problem and the environmental pollution.In our CZTS project, CZTS absorber films were prepared by two independent techniques. One is three rf sources co-sputtering followed by annealing in sulfurized atmosphere. The latest conversion efficiency of over 6.7% was achieved by this technique. The other is co-evaporation technique. CZTS films were grown on Si (100) by vacuum co-evaporation using elemental Cu, Sn, S and binary ZnS as sources. XRD patterns indicated that the polycrystalline growth was suppressed and the orientational growth was relatively induced in a film grown at higher temperatures.In this presentation, the development of CZTS-based thin film solar cells will be surveyed.     

Surface sulfurization studies of thin film Cu(InGa)Se2 solar cells

Sulfurization of copper indium gallium diselenide (CIGS) thin films solar cell absorber has been attempted to enhance the open circuit voltage of the device by increasing the band gap of the absorber near the interface. The homogeneous co-evaporated Cu(InGa)Se₂ thin film was used for sulfurization. The sulfurization was studied in hydrogen sulfide and mixture of hydrogen sulfide and hydrogen selenide gases with the inclusion of oxygen. The structural and compositional properties of the absorber layers were investigated by XRD, EDS and AES analysis. The device results were characterized using current-voltage (I-V) and quantum efficiency. Sulfurization in hydrogen sulfide gas forms a fully converted sulfide layer at the top of the absorber layer, which in turn forms a barrier for the current collection; the device results also support this observation. Sulfurization in mixture of hydrogen sulfide and hydrogen selenide gases forms a wide band gap Cu(InGa)(SeS)₂ layer at the surface by partial replacement of Se by S. The device does not show an increase in open circuit voltage. This may be attributed to the diffusion of Ga away from the surface with the inclusion of sulfur at the surface, which counters the beneficial effect of sulfur at the surface.     

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.     

The crystallisation of Cu2ZnSnS4 thin film solar cell absorbers from co-electroplated Cu-Zn-Sn precursors

The best CZTS solar cell so far was produced by co-sputtering continued with vapour phase sulfurization method. Efficiencies of up to 5.74% were reached by Katagiri et al. The one step electrochemical deposition of copper, zinc, tin and subsequent sulfurization is an alternative fabrication technique for the production of Cu₂ZnSnS₄ based thin film solar cells. A kesterite based solar cell (size 0.5 cm²) with a conversion efficiency of 3.4% (AM1.5) was produced by vapour phase sulfurization of co-electroplated Cu-Zn-Sn films. We report on results of in-situ X-ray diffraction (XRD) experiments during crystallisation of kesterite thin films from electrochemically co-deposited metal films. The kesterite crystallisation is completed by the solid state reaction of Cu₂SnS₃ and ZnS. The measurements show two different reaction paths depending on the metal ratios in the as deposited films. In copper-rich metal films Cu₃Sn and CuZn were found after electrodeposition. In copper-poor or near stoichiometric precursors additional Cu₆Sn₅ and Sn phases were detected. The formation mechanism of Cu₂SnS₃ involves the binary sulphides Cu_2 − xS and SnS₂ in the absence of the binary precursor phase Cu₆Sn₅. The presence of Cu₆Sn₅ leads to a preferred formation of Cu₂SnS₃ via the reaction educts Cu_2 − xS and SnS₂ in the presence of a SnS₂(Cu₄SnS₆) melt. The melt phase may be advantageous in crystallising the kesterite, leading to enhanced grain growth in the presence of a liquid phase.     

Growth and Raman scattering characterization of Cu2ZnSnS4 thin films

In the present work we report the results of the growth, morphological and structural characterization of Cu₂ZnSnS₄ (CZTS) thin films prepared by sulfurization of DC magnetron sputtered Cu/Zn/Sn precursor layers. The adjustment of the thicknesses and the properties of the precursors were used to control the final composition of the films. Its properties were studied by SEM/EDS, XRD and Raman scattering. The influence of the sulfurization temperature on the morphology, composition and structure of the films has been studied. With the presented method we have been able to prepare CZTS thin films with the kesterite structure.     

Characteristics of stacked CuInS2 and CuGaS2 layers as determined by the growth sequence

CuInS₂ and CuGaS₂ thin films have been prepared sequentially from elemental evaporation sources onto conventional soda lime glass substrates heated at 350 °C during the deposition process. The gradient in the structure and composition of the stacked layers has been investigated for the two possible growth sequences. Structural depth profiling and crystallographic phase analysis were performed by grazing incidence X-ray diffraction. The atomic distribution in the films depth was analyzed by X-ray photoelectron spectroscopy combined with sputter etching. Formation of the quaternary compound CuIn_1 − xGa_xS₂, with a high Ga content x > 0.80, has been detected with different distribution depending on the growth sequence.     

Microstructure and electrochemical properties of magnetron-sputtered LiCoO2/LiNiO2 multi-layer thin film electrode

LiCoO₂ single-layer and LiCoO₂/LiNiO₂ multi-layer thin film electrodes were successfully fabricated by magnetron sputtering. Their microstructure and electrochemical properties were investigated. Once annealed, both films had the (0 0 3) preferred orientation to minimize the surface energy. The initial discharge capacity of the multi-layer thin film was approximately 53.1 μAh/cm² μm, which was higher than that of the LiCoO₂ single-layer thin film having similar thickness. The capacity retention of the multi-layer thin film was superior to that of the single-layer thin film. These findings indicate that the multi-layer thin film is a promising cathode material for the fabrication of high-performance thin film batteries.