Metallic Precursor Layers Research Articles

Thermochemical and kinetic aspects of the sulfurization of Cu–Sb and Cu–Bi thin films

CuSbS2 and Cu3BiS3 are being investigated as part of a search for new absorber materials for photovoltaic devices. Thin films of these chalcogenides were produced by conversion of stacked and co-electroplated metal precursor layers in the presence of elemental sulfur vapour. Ex-situ XRD and SEM/EDS analyses of the processed samples were employed to study the reaction sequence with the aim of achieving compact layer morphologies. A new “Time-Temperature-Reaction” (TTR) diagram and modified Pilling–Bedworth coefficients have been introduced for the description and interpretation of the reaction kinetics. For equal processing times, the minimum temperature required for CuSbS2 to appear is substantially lower than for Cu3BiS3, suggesting that interdiffusion across the interfaces between the binary sulfides is a key step in the formation of the ternary compounds. The effects of the heating rate and sulfur partial pressure on the phase evolution as well as the potential losses of Sb and Bi during the processes have been investigated experimentally and the results related to the equilibrium pressure diagrams obtained via thermochemical computation.

Formation of semitransparent CuAlSe2 thin films grown on transparent conducting oxide substrates by selenization

Wide band-gap semiconductors have been studied for applications as buffer layers in thin film solar cells and as top cell in tandem devices. CuAlSe2 (CAS) thin films were deposited onto bare and two different transparent conducting oxide (TCO)-coated glass substrates, In2O3:Sn (ITO) and ZnO:Al (AZO), by a two stage process consisting on the selenization of metallic precursor layers. Homogeneous and crystalline formation of CAS thin films is not trivial and it is strongly influenced by selenization conditions, type of substrate and the film thicknesses. Under certain conditions, polycrystalline CuAlSe2 thin films with chalcopyrite structure and preferential orientation along the (112) plane were obtained onto bare glass susbtrates. However, formation and crystallization of homogeneous CAS thin films was promoted by transparent conducting oxides (ITO and AZO)-coated glass substrates and take place in a wide range of thicknesses and Se amounts with high degree of reproducibility. TCO-coated substrates promoted larger grains when the CAS compound was formed. The band-gap energy, preferential orientation, crystallite size and the average surface roughness varied depending on the film thickness and type of substrate.

Electrochemical study of one-step electrodeposition of copper–indium–gallium alloys in acidic conditions as precursor layers for Cu(In,Ga)Se 2 thin film solar cells

This paper investigates one step electrodeposition of copper indium gallium metallic precursor layers for preparing CuIn 1− x Ga x Se 2 (CIGS) absorber layers in thin film solar cells (0 ≤ x ≤ 1). Electrodeposition was carried out in acidic aqueous solutions at about pH 2. At first partial single or binary electrodeposition systems Cu, In, Ga, Cu–Ga, Cu–In were investigated by cyclic voltammetry. Then ternary Cu–In–Ga electrodeposition system was studied. The nature of the supporting electrolyte (sodium sulfate vs. sodium chloride) and the influence of sodium citrate were more specifically investigated. The applied potential, the pH and the nature of the electrolyte were optimized to obtain x values around 0.3 needed for high efficiency devices. Depositions were carried out under potentiostatic conditions in a paddle cell configuration. The electrodeposited Cu–In–Ga alloys were annealed under Se atmosphere at temperatures between 400 and 600 °C to produce CIGS absorbers. Films were characterized by XRF, SEM and XRD analysis. Device efficiencies up to 9.3% are achieved for optimal gallium content.

Preparation and properties of SnS film grown by two-stage process

SnS films have been prepared by a novel two-stage process. It involved sputtering of Sn film on glass substrate and sulfurization of the thin metallic tin precursor layers in a vacuum furnace. The X-ray diffraction results showed that the SnS layers had orthorhombic structure and (0 4 0) preferential growth is more and more obvious with the increase of sulfurization time. The SnS film obtained by this work shows high optical absorption efficiency, and the film has a direct optical band gap of about 1.3 eV. The films show p-type conductivity and the resistivity of SnS film decreased obviously under illumination.

Wide-bandgap CuIn 1− xAl xSe 2 thin films deposited on transparent conducting oxides

The development of wide-bandgap materials deposited on transparent conducting oxides for high efficiency and low cost semitransparent photovoltaic devices or tandem cells is becoming important in the last few years. CuIn 1− x Al x Se 2 (CIAS) thin films with bandgap above 1.95 eV for 0.7≤ x≤0.9 were deposited onto bare and two different (tin-doped indium oxide, ITO and aluminium-doped zinc oxide, AZO) coated glass substrates by a two stage process consisting of the selenization of metallic precursor layers. Polycrystalline CIAS thin films orientated preferentially along the (1 1 2) plane with chalcopyrite structure were obtained. The bandgap energy increased no linearly with the Al addition. X-ray diffractograms showed the coexistence of several CIAS phases. Optical, structural and composition analysis revealed that the ITO and AZO substrates promote not only the incorporation of Se, but also a more homogenous distribution in depth regarding the CIAS samples on bare glass, and acted like a barrier to the oxidation from the glass substrate.

Optimal deposition conditions of TiN barrier layers for the growth of vertically aligned carbon nanotubes onto metallic substrates

Plasma enhanced chemical deposition (PECVD) has proven over the years to be the preferred method for the growth of vertically aligned carbon nanotubes and nanofibres (VACNTs and VACNFs, respectively). In particular, carbon nanotubes (CNTs) grown on metallic surfaces present a great potential for high power applications, including low resistance electrical contacts, high power switches, electron guns or supercapacitors. Nevertheless, the deposition of CNTs onto metallic substrates is challenging, due to the intrinsic incompatibility between such substrates and the metallic precursor layers required to promote the growth of CNTs. In particular, the formation of CNT films is assisted by the presence of a nanometric (10–100 nm) monolayer of catalyst clusters, which act as nucleation sites for CNTs. The nanometric character of the precursor layer, together with the high growth temperature involved during the PECVD process (∼700 °C), strongly favours the in-diffusion of the catalyst nanoclusters into the bulk of the metallic substrate, which results in a dramatic reduction in the nucleation of CNTs. In order to overcome this problem, it is necessary to coat the metallic substrate with a diffusion barrier layer, prior to the growth of the catalyst precursor. Unlike other conventional ceramic barrier layers, TiN provides high electrical conductivity, thus being a promising candidate for use as barrier material in applications involving low resistance contacts. In this work we investigate the anti-diffusion properties of TiN sputtered coatings and its potential applicability to the growth of CNTs onto copper substrates, using Fe as catalyst material. The barrier and catalyst layers were deposited by magnetron sputtering. Auger electron spectroscopy was used to determine the diffusivity of Fe into TiN. Morphological characterization of the CNTs coatings was performed on scanning and transmission electron microscopes. Raman spectroscopy and x-ray diffraction were employed to establish a correlation between TiN characteristics and deposition parameters.

Cu 2ZnSnSe 4 films by selenization of Sn–Zn–Cu sequential films

This paper deals with the formation of Cu 2ZnSnSe 4 (CZTS) in the process of selenization of metal precursor layers in elemental selenium vapour. Metallic precursors were sequentially evaported from Sn, Zn and Cu sources. Precursor Sn–Zn–Cu films have a “mesa-like” structure and consist mainly of Cu 5Zn 8 and Cu 6Sn 5 phases. It was confirmed that the formation of different binary copper selenides is the dominating process of selenization in elemental Se vapour at temperatures up to 300 °C. The formation of kesterite CZTS films begins at 300 °C and dominates at higher temperatures, always resulting in multiphase films that consist of high-quality Cu 2ZnSnSe 4 crystals and of a separate phase of ZnSe.

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.

Homogeneity of single phase Cu(In,Ga)Se 2 produced by selenisation of metal precursors: An optical investigation

Two-stage processes involving the selenisation of metallic precursor layers are among the most promising techniques for the formation of chalcopyrite-based solar cell absorber layers on a commercial scale. In this paper, the homogeneity of Cu(In 0.75Ga 0.25)Se 2 prepared by a new two-stage technique [V. Alberts, Semicond. Sci. Technol., 19 (2004) 65.], which involves the selenisation of sputtered CuIn 0.75Ga 0.25 precursor films in steps designed to control the reaction rates of the binary selenide phases and to prevent the formation of the more stable CuGaSe 2 phase, is studied. Photoluminescence spectroscopy, optical absorption measurements and X-ray diffraction measurements confirm that layers grown by a traditional process, which involves a single selenisation step, contain separate quaternary phases: gallium-rich phases are found closer to the substrate, while gallium-poor phases reside near the front surface. Layers produced by the novel process do not show this grading. A line appearing at ∼ 0.8 eV is ascribed to Na III, which results from the out-diffusion of Na from the glass substrate.

CuIn1−xGaxSe2‐based thin‐film solar cells by the selenization of sequentially evaporated metallic layers

AbstractPolycrystalline CuIn1−xGaxSe2 (CIGS) thin films were deposited by the non‐vacuum, near‐atmospheric‐pressure selenization of stacked metallic precursor layers. A study was carried out to investigate the influence of significant factors of the absorber on the solar cells performance. An efficiency enhancement was obtained for Cu/(In+Ga) atomic ratios between 0·93 and 0·95. The slope of the observed energy bandgap grading showed a strong influence on the VOC and the short circuit current density JSC. An increase of the Ga content in the active region of the absorber was achieved by the introduction of a thin Ga layer on the Mo back contact. This led to an improvement of efficiency and VOC. Furthermore, an enhanced carrier collection was detected by quantum efficiency measurements when the absorber layer thickness was slightly decreased. Conversion efficiencies close to 10% have been obtained for these devices. Copyright © 2005 John Wiley & Sons, Ltd.

Real-time study of phase transformations in Cu–In chalcogenide thin films using in situ Raman spectroscopy and XRD

The growth of polycrystalline CuInSe 2 and CuInS 2 thin films from metallic precursor layers is investigated using two complementary in situ methods which give bulk sensitive (XRD) and surface sensitive (Raman) information. From the time evolution of the XRD and Raman peak intensities the phase transformation sequences and the reaction kinetics can be derived. In both cases the chalcogenization of the Cu–In precursors proceeds at the top surface. Thus, the process is at least limited by cation diffusion through the metallic precursor. However, the growth kinetics of CuInSe 2 and CuInS 2 films differ. While the CuInSe 2 growth is limited by the reaction of binary phases, CuInS 2 growth is controlled by fast diffusion which is solely restricted to the period of raising temperature during the process.

Electrosprayed and selenized Cu/In metal particle films

Nano-particulate copper and indium metal layers of 1–2 μm have been deposited by non-vacuum techniques such as doctor blade, screen printing and electrospray using alcoholic suspension pastes. Electrospray showed a high efficiency of material usage and yielded the most uniform morphology. The metal precursor layers were subjected to a thermal treatment (500–600 °C) in selenium vapor to convert the porous metal layers into CuInSe 2 compound layers. The chemical conversion, investigated by X-ray diffraction, showed the presence of the In 2O 3 impurity phase in the precursor as well as in the selenized layers.

Steps toward industrialization of Cu-III-VI 2 thin-film solar cells: a novel full in-line concept

In order to manufacture Cu-III-VI 2 thin-film solar cells in much larger area and full speed as compared to the conventional evaporation method with the CdS layer deposited by CBD, a novel full in-line fabrication concept was developed, which includes in-line cleaning of substrates, sequential in-line sputtering/RTP steps for the fabrication of absorber, buffer layer and window-layer. A novel method named ‘MO sputter’ was developed in which the metal–organic (MO) TMGa used as a reactant gas, was introduced into the sputter for very fine-tuning the sputter-deposited metal precursor layers compositions, which could fulfill the requirement of Cu(In x Ga 1− x )(S,Se) 2 with the gradient bandgap, which help to obtain the precisely controlled, designated electrical characteristics for the device making. Then rapid thermal process (RTP) was utilized for sulfurization or selenization of the metal precursor layers. It has been found that Ga/(Ga+In) ratio incorporated in the films was increased linearly from 0.47 to 0.60 with increasing the TMGa gas flow rate at 450 °C substrate temperature. Single phase Cu(In x Ga 1− x )S 2 films have been obtained by optimizing the deposition parameters. Using the in-line sputtering/RTP sulfurization techniques, the ZnS buffer layer can also be prepared which can ensure not to use the hazardous CdS layer, and together with the sputter-deposited ZnO electrode, this could promote genuine realization of the large-scale production and lay down foundation for the full in-line manufacture of the ultra-high efficiency Cu-III-VI 2 thin film solar cells.

Development of sputtering systems for large-area deposition of CuIn1−xGaxSe1−ySy thin-film solar cells

CuIn 1−x Ga x Se 1−y S y (CIGS) thin-film modules are expected to become cheaper than crystalline silicon modules within 5 yr. At present, commissioning and reaching full production of thin film modules is delayed because of nonavailability of turnkey manufacturing plants. Very few universities are conducting research on development of PV plants. CIGS thin-film solar cells are being prepared routinely at Florida Solar Energy Center (FSEC) on glass and metallic foil substrates for terrestrial and space applications. Earlier, the substrate size was limited to 3×3 cm2. This article presents results of development of large-area sputtering systems for preparation of large (15.2×15.2 cm2) CIGS thin-film solar cells. The facilities have the potential of serving as a nucleus of a pilot plant for fabrication of CIGS minimodules. Initial problems of bowing of the brass diaphragm, restriction of effective water flow and consequent heating of the target material were resolved by increasing the thickness of the backing plate and redesigning the structural members. Thickness uniformity was improved by modifying the magnetic field distribution in the middle 15 cm portion of the 10.2×30.5 cm2 magnetron sputtering sources by selectively removing nickel-coated soft-iron pieces at the rear. This resulted in Mo layer thickness uniformity of ±3% over 10.2×10.2 cm2. The magnetic field was boosted at extremities to avoid precipitous ∼15% drop beyond 10.2 cm. With this, thickness uniformities of ±2.5% for Mo and ±4.5% for ZnO over 12.7×10.2 cm2 have been achieved however with a continuing drop beyond 12.7 cm width. Modifying the magnetic field to achieve better distribution by preferentially removing soft irons pieces and also boosting of the magnetic field at the ends are two new concepts introduced and successfully utilized in this study. Scaling up of the large-area uniform deposition of metallic precursor layers was a challenging task. The efforts were directed towards obtaining similar thickness and uniformity that have provided very good photovoltaic efficiencies of 10.4% (Air mass AM 1.5) in small area CIGS thin film solar cells on stainless steel foils in earlier research. Preliminary results on large area CIGS solar cells are encouraging.

Sulfurization of Cu/In Precursors for CuInS2-Based Solar Cells

The details of the sulfurization reaction of Cu/In metallic precursor layers for the preparation of photovoltaic absorbers have been investigated. For this, an experiment has been designed in order to establish the main mechanisms that govern the sulfurization reaction. A metallic layer containing Cu and In has been processed using a combined sequence of sulfurization and selenization steps. Afterwards, this sample was investigated by Auger electron spectroscopy, Raman spectroscopy, and transmission electron microscopy in order to study its microstructure and chemical composition. The results show that the sulfurization reaction of these films is dominated by the out-diffusion of the metallic atoms through the chalcogen layer. Moreover, taking into account our previous investigations we have developed a consistent model for the formation of these absorber layers. © 2003 The Electrochemical Society. All rights reserved.

Structural studies on SnS films grown by a two-stage process

Thin films of SnS have been grown by a novel two-stage process. It involves sulphurisation of thin metallic tin precursor layers in a vacuum furnace using a graphite box. The precursor layers were annealed at different temperatures in the range of 100–400 °C in excess ambient sulphur. The layers, annealed at temperatures 300–350 °C, were polycrystalline with a strong {111} preferred orientation. These layers had orthorhombic crystal structure with a grain size of 100 nm and exhibited stoichiometric composition. The precursor layers, reactively annealed at temperatures lower than 300 °C, had secondary phases such as SnS 2 and Sn 2S 3 with a grain size of 50 nm. However, the Sn layers, annealed at higher temperature >350 °C, had an additional phase of SnS 2 along with the SnS phase and showed deviation from stoichiometry with a deficiency of sulphur. The results were analysed systematically to develop the growth process.

Growth and characterization of ultrathin zirconia dielectrics grown by ultraviolet ozone oxidation

In this letter, we report on the growth of ultrathin films of zirconia on silicon oxide and silicon oxynitride passivated (001) Si by ultraviolet ozone oxidation of Zr metal precursor layers. The oxidation kinetics has been measured using an accelerator-based nuclear reaction analysis. It was found that oxide films up to 55 Å could be grown at room temperature by oxidation at 600 Torr while oxidation at 80 mTorr is self-limiting at 20 Å. The interfaces between the dielectric and the substrate have been characterized by scanning transmission electron microscopy. The ZrO2 films were found to be crystalline as grown. Electrical measurements on capacitor structures with 30-Å-thick ZrO2 films grown on native oxide on silicon show a capacitance–voltage hysteresis of 15 mV and a capacitance-based equivalent oxide thickness of 17 Å at 100 kHz.

Synthesis of Bi-Pb-Sr-Ca-Cu oxide/Ag superconductors by melt dipping and oxidation of metallic precursor alloys

Bi-Pb-Sr-Ca-Cu-Ag alloy coatings were made by vacuum induction melting followed by melt dipping on substrates of MgO, SrTiO3, and Al2O3. The metallic precursor layers were then subjected to a controlled atmosphere oxidation and annealing to produce oxide superconductors. Superconducting coatings with onset temperatures of 112 K and zero resistance temperatures of 80 K were obtained in this way. The addition of Ag aided in the alloy melting and dipping processes, and produced a superconducting oxide/noble metal composite after oxidation. The resulting superconducting phases were analyzed by x-ray powder diffraction. The influence of the substrate, dipping processes, oxidation, and annealing conditions on the superconducting properties are discussed.