Daniel Lincot

Mechanistic Study of Cathodic Electrodeposition of Zinc Oxide and Zinc Hydroxychloride Films from Oxygenated Aqueous Zinc Chloride Solutions

Films of zinc oxide and related compounds [ZnO, ZnO x (OH) y , Zn(OH) x Cl y ] are electrodeposited cathodically in aqueous zinc chloride solutions using dissolved oxygen as a precursor. The influence of the precursor concentrations, pH, and deposition temperature on the growth, composition, and properties of the films are investigated by means of in situ techniques : voltammetry, electrochemical quartz-crystal microgravimetry, surface pH, and ex situ techniques: X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The deposition mechanism is analyzed in terms of electrochemically induced surface precipitation due to an increase of local pH resulting from the oxygen reduction reaction. This approach allows us to explain the formation of either zinc hydroxychloride compounds or zinc oxide from their calculated solubility diagrams. In conditions of the formation of ZnO, a dramatic effect of temperature is observed, with a transition between amorphous insulating zinc oxyhydroxide to well-crystallized and conducting zinc oxide when the temperature increases (T transition 50 °C).

Mechanism of Chemical Bath Deposition of Cadmium Sulfide Thin Films in the Ammonia‐Thiourea System In Situ Kinetic Study and Modelization

The mechanism of chemical bath deposition of cadmium sulfide thin films from the ammonia-thiourea system is studied in situ by means of quartz crystal microbalance technique (QCM). The influence of reaction parameters (concentration of reactans, pH, anions, temperature, stirring rate) is determined. The growth is thermally activated with an activation energy of about 85 kJmol, which probably corresponds to a chemical step related to the decomposition of thiourea

Electrodeposition of semiconductors for optoelectronic devices: results on zinc oxide

Electrodeposition of polycrystalline semiconductor thin films is mainly used for photovoltaic applications. Most of the work concerns chalcogenide compounds like CdTe, CuInSe2, CdS… This route is also emerging for semiconductor oxide preparation. Results concerning the electrodeposition of zinc oxide layers by reduction of dissolved oxygen in presence of Zn(II) ions are presented. Epitaxial growth can be achieved in zinc chloride solutions on single crystal GaN layers. Bandgap (Eg) variations and effects on photoluminescence are observed by changing the deposition conditions and post-deposition treatments. For instance, changing the potential from −1.1 to −1.5 V versus MSE in chloride solutions increases Eg from 3.4 to 3.55 eV. Changing chloride to perchlorate anions shifts Eg from 3.52 to 3.38 eV at −1.4 V. A thermal annealing in air further decreases the Eg down to the classical value (3.27 eV). This is correlated to a decrease in ZnO lattice parameters. A concomitant variation of the photoluminescence emission in the blue is observed. The highest yield is achieved on the epitaxial layers after annealing (500°C, 1 h). Specific aspects about the deposition mechanism are also discussed.

Electrochemical deposition of zinc oxide films from non-aqueous solution : a new buffer/window process for thin film solar cells

Abstract Zinc oxide is a wide band gap semiconductor with wide application in thin film devices such as n-type window layers for thin film solar cells, piezoelectric and luminescent devices, and for catalytic applications. We have cathodically electrodeposited films of ZnO by reduction of dissolved oxygen in a non-aqueous solution (dimethylsulfoxide) containing a Zn salt. This method allows a large deposition potential window and gives films with high transparency, good crystallinity and adherence. The ZnO was electrodeposited on Cu(In,Ga)Se2 as a buffer layer. The resulting solar cells gave higher light-to-electricity conversion efficiencies (>11%) than those made with conventional r.f. sputtered insulating ZnO.

Study of atomic layer epitaxy of zinc oxide by in-situ quartz crystal microgravimetry

The deposition of ZnO thin films by atomic layer epitaxy (ALE) from diethylzinc and water precursors is studied for the first time by an in-situ quartz crystal microbalance technique. Quantitative measurement of the growth with a resolution down to the monolayer level is demonstrated. Influence of temperature, pulse lengths and substrate characteristics has been studied. The site saturation regime (ALE window) is between 100°C and 160°C and corresponds to the growth of one (100) monolayer with possible effects of roughness and surface reconstruction. Effect of nucleation and coalescence has been clearly evidenced both on foreign substrates and more surprisingly on ZnO substrates depending of the duration of the rest period. The analysis of the mass variation during individual cycles raises some questions about the growth mechanism.

Hydrogen Peroxide Oxygen Precursor for Zinc Oxide Electrodeposition I. Deposition in Perchlorate Medium

Cathodic electrodeposition of zinc oxide is demonstrated using dissolved hydrogen peroxide as the oxygen precursor. Unlike molecular oxygen, H 2 O 2 is a very soluble compound in water and can consequently be used at high concentrations allowing high deposition rates. Unlike nitrate ions, the reduction reaction does not give rise to by-products, which may progressively contaminate the deposition bath. ZnO films have been deposited at 70°C and - 1.4 V vs. a saturated sulfate mercurous electrode onto tin oxide over a large range of peroxide concentration with perchlorate as the supporting electrolyte. The films are well covered and crystallized with the wurtzite structure and textured with the c axis perpendicular to the electrode surface. A transition between dense films and porous films is observed at the higher H 2 O 2 concentration investigated. At 40 mM H 2 O 2 , the porosity of the films deposited in the presence of 80 mM zinc perchlorate is estimated at 33% with deposition rate of ca. 16 μm h -1 .

Hydrogen peroxide oxygen precursor for zinc oxide electrodeposition II—Mechanistic aspects

Zinc oxide thin films have been deposited cathodically at 70 °C from a chloride aqueous solution containing dissolved hydrogen peroxide and a zinc(II) chloride. The electrodeposition process has been studied by cyclic voltammetry (CV), chronoamperometry, and quartz crystal microbalance techniques. A parametric study has been carried out by varying the solution composition and the substrate. It is shown that the film growth is under kinetic control since, the catalytic activity of ZnO surfaces for the reduction of hydrogen peroxide species is low under these conditions. The nature of the substrates (tin oxide, gold) and their treatments prior to the deposition experiments are shown to have a marked influence on the electrochemical behavior of the system. Besides, we show that it is possible to normalize the film growth behavior by taking the mean current density as the key parameter. For values below the limiting diffusion current of zinc ions, the faradaic efficiency is close to 100%, whereas, above this value the faradaic efficiency decreases markedly. The results have been modeled with the help of a simple competition mechanism in which we assume that the hydroxide ions produced in excess at the surface diffuse towards the solution and react partly with zinc ions diffusing towards the surface. That reaction in solution reduces the availability of zinc ions for the heterogeneous deposition reaction at the surface. We have proposed an empirical law to describe this phenomenon.

Chemical Bath Deposition of Cadmium Sulfide Thin Films. In Situ Growth and Structural Studies by Combined Quartz Crystal Microbalance and Electrochemical Impedance Techniques

Chemical bath deposition of CdS layers, using the ammonia process, have been studied for the first time by combined in situ quartz crystal microbalance and electrochemical impedance techniques. Both allow monitoring of the growth kinetics, but an important result is that information about the covering properties of the film, its internal structure, and the evolution of the structure during the growth, have been obtained from combined experiments. The film is shown to have in general a duplex structure with an inner compact layer (only measured by capacitance) and an outer porous layer, growing at longer reaction times. In this paper the influence of the thiourea concentration on this structure is studies and discusses. Thiourea in excess is found to be very favorable for obtaining total coverage of the substrate with a minimum thickness of the CdS film ({approx}30 nm on unactivated gold substrate), which is important for applications. A simple columnar growth model has been derived which accounts well for the experimental results.

Toward laser emission of epitaxial nanorod arrays of ZnO grown by electrodeposition

Arrays of epitaxial, vertically oriented nanorods of ZnO are grown at low temperature (80°C) on the (0002) plane of GaN single crystals by electrodeposition in an aqueous solution. The freestanding nanocolumns are prepared by a template-free method based on simple solution chemistry. At room temperature, the nanorods present an amplified ultraviolet emission centered at 381nm with an excitation threshold at 4.4MWcm−2. The amplified emission of the ZnO columns is dominated by the radiative recombination of excitons.

One step electrodeposition of CuInSe2: Improved structural, electronic, and photovoltaic properties by annealing under high selenium pressure

Films of Cu–In–Se alloys can be electrodeposited in a wide range of controlled composition. Annealing treatments under Se pressure transform these precursor films in large grain CuInSe2 films with improved electronic properties. These modifications are shown to depend on the Se pressure imposed during the treatment allowing a certain tailoring of the electronic properties of the films. The properties of electrodeposited/selenized films are presented as obtained from luminescence measurements, Hall effect, and photoelectrochemical characterization. An efficiency of 6.5% (total area, without antireflecting coating) is reported for the best CuInSe2/CdS/ZnO solar cell. An analysis of the device is also presented where limitations by interface recombination are shown to be the dominant loss mechanism.