Stabilization of hexagonal phase in spray-deposited CdTe films by the presence of electric field

The metastable hexagonal phase of cadmium telluride is stabilized in polycrystalline thin films by spray deposition in presence of a high DC voltage. The X-ray diffraction pattern shows peaks closely corresponding to the hexagonal phase. The CdTe films are grown in the stable cubic phase if the electric field is removed. A higher value of the band gap (1.47 eV) of the films deposited in presence of electric field compared to that (1.43 eV) for the films deposited in the absence of the electric field supports the formation of hexagonal regions. The AFM image reveals the formation of rod-shaped particles for the films deposited in the presence of electric field that contribute to the stabilization of hexagonal phase and randomly distributed spherical particles in the absence of electric field.     

Enhancement of photoelectrochemical response by aligned nanorods in ZnO thin films

ZnO thin films are deposited in pure Ar and mixed Ar and N₂ gas ambient at various substrate temperatures by rf sputtering ZnO targets. We find that the deposition in pure Ar ambient leads to polycrystalline ZnO thin films. However, the presence of N₂ in the deposition ambient promotes the formation of aligned nanorods at temperatures above 300 °C. ZnO films with aligned nanorods deposited at 500 °C exhibit significantly enhanced photoelectrochemical response, compared to polycrystalline ZnO thin films grown at the same temperature. Our results suggest that aligned nanostructures may offer potential advantages for improving the efficiency of photoelectrochemical water-splitting for H₂ production.     

Enhanced oxidation resistance of magnesium nanorods grown by glancing angle deposition

Oxidation behavior of magnesium thin films and nanorods were investigated in the temperature range of 25-550 °C by using thermal gravimetric analysis. Arrays of vertical magnesium nanorods were deposited by the DC magnetron sputtering glancing angle deposition technique, while the magnesium thin films were deposited using the same system but at normal incidence. The morphologies and corresponding crystal structure of the samples were analyzed by scanning electron microscopy, transmission electron microscopy and X-ray diffraction methods, respectively. We report that the Mg thin films showed oxidation induced weight gain starting from room temperature. On the other hand, Mg nanorods did not show any indication of significant oxidation at temperatures below 350 °C. Enhanced oxidation resistance of Mg nanorods was also confirmed by quartz crystal microbalance measurements. At temperatures higher than 350 °C, Mg nanorods started to get oxidized and their weight increased at a similar rate to that of Mg thin films. We argue that reduced oxidation of Mg nanorods is mainly attributed to their single crystal nature. Magnesium nanorods’ reduced oxidation can potentially play a key role in hydrogen storage and gas sensing applications.     

Some physical properties of Cd1−xSnxS films used as window layer in heterojunction solar cells

CdS has been proved to be an ideal material for use as the window layer for heterojunction solar cells especially with n-CdS/p-CdTe. CdS, Cd_0.9Sn_0.1S and Cd_0.8Sn_0.2S films were deposited onto glass substrates at 300 °C substrate temperature by using ultrasonic spray pyrolysis technique (USP). The effect of Sn concentration on some structural, optical and electrical properties of the films was presented. The crystal structure and orientation of the films were investigated by X-ray diffraction (XRD) patterns. XRD patterns showed that films have polycrystalline nature with a hexagonal structure. The grain size of the films decreased with increasing x values. The optical band gap values were obtained from optical absorption spectra of the films. The optical band gap values of the films were found to be between 2.44 and 2.45 eV. The variations of conductivity of Cd_1−xSn_xS (0 ≤ x ≤ 0.2) films have been investigated depending on applied voltage in dark and under illumination. The resistivity significantly decreased with increasing tin concentration and under illumination.     

Seed-layer-free deposition of well-oriented ZnO nanorods thin films by SILAR and their photoelectrochemical studies

Morphological forms of ZnO nanostructures play a vital role in deciding properties such as high internal surface area, efficient light scattering and harvesting, lowest charge transfer resistance, etc. which are important for photoelectrochemical (PEC) performance. Herein successful deposition of well oriented ZnO nanorods thin films over fluorine doped tin oxide (FTO) coated glass substrate is achieved by using simple, soft solution and scalable method known as successive ionic layer adsorption and reaction (SILAR). For the first time a compact ZnO layer over large area is deposited in one step synthesis approach, without any assistance of seed layer, by using hydrazine hydrate as a source of hydroxyl ions. The plausible growth mechanism of the morphological variation (alignment and orientation) happening with increasing SILAR cycles and its consequence on PEC performance are discussed in detail. All ZnO thin films show wurtzite crystal structure, however variations in their texture coefficients were found with SILAR cycles, which turns out to be a major aspect for PEC application. Anodic shift was observed in flat band potential values with increment in number of SILAR cycles. The ZnO thin films deposited for 120 cycles showed preferential orientation along (0002) plane and showed better PEC performance with photocurrent of 0.19 mA/cm² (1 V) and maximum photo conversion efficiency of 0.084% at 0.45 V. On the other hand, film deposited for 60 (photocurrent of 0.11 mA/cm² (1 V); efficiency of 0.055%) and 180 cycles (photocurrent of 0.15 mA/cm² (1 V); efficiency of 0.063%) demonstrated inferior PEC performance.     

High near-infrared transparent molybdenum-doped indium oxide thin films for nanocrystalline silicon solar cell applications

Molybdenum-doped indium oxide (IMO) thin films were deposited at 450 °C for varying molybdenum concentrations in the range of 0.5-2 at% by the spray pyrolysis technique. These films confirmed the cubic bixbyite structure of polycrystalline In₂O_3. The preferred growth orientation along the (2 2 2) plane shifts to (4 0 0) on higher Mo doping levels. The films doped with 0.5 at% Mo showed high mobility of 76.9 cm²/(V s). The high visible transmittance extends well into the near-infrared region. A possibility of using the produced IMO films in nanocrystalline (nc) silicon solar cell applications is discussed in this article. The morphological studies showed a change in the microstructure, which is consistent with the change in crystallographic orientation.     

Pulsed electrodeposition of CdTe thin films: effect of pulse parameters over structure, stoichiometry and optical absorption

CdTe thin films were electrochemically deposited using unipolar current pulses of high magnitude between 2.5 and 30 mA/cm² in an aqueous solution. Parametric study of the effect of periodic current pulse magnitude, average current and ON and OFF duration was undertaken to understand the effect of pulse variables on CdTe film properties. Increasing pulse deposition current modifies crystalline growth phase from single cubic to mixed cubic and hexagonal growth phases. In addition to the modification in CdTe growth phases, there is an increasing tendency of the oxide formation particularly CdTeO₃. Increase in pulse current density or average current yields Cd rich CdTe films. The optical absorption coefficient decreases with the decrease in pulse current density, whereas an increase is observed as the OFF time decreases. The optical energy gap is found to increase with OFF time. A systematic study on the effect of pulse variables over the structure, compositional and optical properties of CdTe film is described.     

Synthesis and characterization of nanocrystalline Zn1−xMxO (M = Ni, Cr) thin films for efficient photoelectrochemical splitting of water under UV irradiation

Nanocrystalline thin films of Zn_1−xM_xO (M = Ni, Cr) were deposited on glass substrate by sol-gel method. To a solution of zinc acetate 2-hydrate in dimethyl formamide, calculated quantities of nickel nitrate or chromium acetate were added. The clear solution, obtained after 2 h of continuous stirring, was coated on conducting glass (ITO plates). After preannealing at 250 °C to remove organic impurities, films were sintered at 400, 500 and 600 °C. XRD analysis reveals dominant evolution of hexagonal ZnO with a possible simultaneous growth of meta-stable cubic ZnO. AFM analysis indicated preferential growth of nanocrystallites along c-axis, while SEM analysis confirmed films having uniform morphology. Optical characterization led to two band gap values; one matching with the band gap of bulk ZnO and the second slightly higher, which suggest quantum confinement effect in nanocrystallites. Ni and Cr incorporation influenced the two band gap energies differently. Photoelectrochemical (PEC) splitting of water was attempted, using prepared thin films as working electrode, in conjunction with Pt counter electrode and saturated calomel reference electrode along with 150 W Xenon Arc light source and aqueous solution of NaOH (0.01 M). Results indicate Ni:ZnO films yielding improved photoresponse compared to Cr:ZnO films. Ni:ZnO (5 % at.) films sintered at 600 °C resulted in significantly enhanced photocurrent due to improved optical absorption and decrease in resistivity.     

Shape effects of CdS photocatalysts on hydrogen production

Granular and lamellar structured cadmium sulfide (CdS) with the shapes of branches, cauliflower and nanorods, were synthesized via a hydrothermal process. During the synthesis, ethylenediamine (EN) was used as a template and deionized water (DIW) as a coordination agent. Experimental results show that the morphology of CdS nanoparticles was controlled by EN concentration, CdS precursor concentration and molar ratio of Cd(NO₃)₂·4H₂O to thiourea (NH₂CSNH₂). It was found that the key shape-controlling step is the formation of CdS nuclei via the decomposition of the cadmium-ethylenediamine complex. Various shapes of CdS nanoparticles were obtained based on the concentrations of EN in water. CdS particles synthesized in pure water show granular and lamellar shapes with a mixture of hexagonal and cubic crystal structures. When EN concentration was increased to 30%, branched morphology of CdS particles was observed. Further increasing EN concentration to 70% CdS catalyst particles resulted in a cauliflower-like shape. Finally, CdS nanorod particles with a hexagonal structure were developed when synthesized in a pure EN solution. Although EN concentration plays an important role in the shapes of CdS particles, experimental observation showed that the diameter and aspect ratio of as prepared CdS nanorods were determined by concentrations of CdS precursors. In the course of photocatalytic hydrogen production, nearly 2577 μmol H₂ was produced over 0.05 g CdS nanorods in 4.0 h. The rate of hydrogen evolution over the CdS nanorods based photocatalyst was approximately 42.6 times higher than that over granular and agglomerated lamellar CdS.     

Pulse plated CdSxTe1−x films and their properties

CdS_xTe_1−x films were deposited on titanium and conducting glass substrates at room temperature using 0.25 M cadmium sulphate, the concentration of sodium thiosulphate and TeO₂ dissolved in sodium hydroxide was varied in the range of 0.01-0.05 M. The as deposited films exhibited hexagonal structure irrespective of the composition. The FWHM maximum of the x-ray diffraction peaks were found to decrease with increase of duty cycle. The optical energy gap values are in the range of 1.54-2.32 eV for films of different composition, it is observed that the band gap shifts towards CdS side as the concentration of CdS in the films increase. XPS studies indicated the formation of CdSTe solid solution. The grain size increases from 11.54 to 99.40 nm as the value of x increases from 0.2 to 0.8. The surface roughness is found to increase from 0.22 to 2.50 nm as the value of ‘x’ increases from 0.2 to 0.8. The resistivity is found to vary from 53 to 8 ohm cm as the ‘x’ value decreases from 1 to 0.     

Electrochromic behavior of WO3 nanotree films prepared by hydrothermal oxidation

Hexagonal structured WO₃ films with tree-like morphology were synthesized on tungsten foils by a hydrothermal method. Each nanotree was composed of several (typically six) nanosheet-shaped “branches”. TEM examination revealed that the nanosheet was a single crystal and its long axis was oriented toward 〈0 0 1〉 direction. The WO₃ nanotree films retain the hexagonal structure after being annealed up to 400 °C for 2 h, while they have a phase transition to monoclinic structure after being annealed at 500 °C for 2 h. Electrochromic (EC) performance of the films was examined in a propylene carbonate solution of 1 M LiClO₄ using an electrochemical workstation and an UV-Vis spectrometer. Due to the large tunnels of hexagonal structure and highly porous surface morphology, a large modulation of optical reflectance of WO₃ nanotree films up to 30% and coloration efficiency of 43.6 cm² C⁻¹ at 500 nm were achieved by annealing the WO₃ nanotree films at 400 °C for 2 h.     

Optical, structural and electrochromic properties of nickel oxide films produced by sol-gel technique

Nickel oxide films have been deposited from nickel acetate precursor using a sol-gel dip coating method, onto glass and conducting fluorine doped tin oxide (FTO) glass substrate. The direct energy gap (E_gd) values for the 2-10 layered films are in the range of 3.62 eV-3.72 eV. X-ray diffraction (XRD) analysis reveals that films consisting of 2-6 layers are amorphous, while films consisting of 8-10 layers are poly-crystalline with cubic grains of around 12 nm-20 nm and preferential growth along the (1 1 1) and (2 0 0) planes. Fourier Transform Infrared (FTIR) spectrum confirms the formation of Ni-O. Electrochromic properties of the nickel oxide coatings were studied using cyclic voltammetric (CV) technique. The 8 layered NiO films exhibit the anodic/cathodic diffusion coefficient of 16.7/5.73 × 10⁻¹³ cm²/s and the change in optical transmission is ΔT_630nm = 53% with a photopic contrast ratio of 2.87.     

Photoelectrochemical cells based on CdSe films brush plated on high-temperature substrates

CdSe thin films were brush plated on substrates maintained at temperatures in the range 30–90 °C from the precursors. The films exhibited hexagonal crystal structure. Optical band gap of 1.65 eV was obtained. XPS measurements indicated the formation of CdSe. Capacitance–voltage measurements indicated the films to exhibit n-type behaviour. A carrier density of 10¹⁷ cm⁻³ was obtained. Photoelectrochemical cells have exhibited higher efficiency compared to earlier reports on brush-plated films. Spectral response measurements indicated a peak quantum efficiency of 75% at 1.65 eV.     

Effect of Mg content on structural, electrical and optical properties of Zn1−xMgxO nanocomposite thin films

We report on the growth of Zn_1−xMg_xO (ZMO) thin films on quartz substrate using pulsed laser deposition (PLD) technique. The influence of varying Mg composition on structural, electrical and optical properties of ZMO films has been systematically investigated. Increase in Mg content (in the range 0.0?x?1.0), reflects the structural phase transition from wurtzite via mixed phase region to cubic one. X-ray diffraction (XRD) studies indicate the hexagonal wurtzite phase at Mg composition ranging from 0% to 30%; mixture of wurtzite and cubic phases for 40% and single cubic phase at Mg content greater than 50%. The variation of the cation-anion bond length to Mg content shows that the lattice constant of the hexagonal ZMO decreases with corresponding increase in Mg content, which result in the structure gradually deviating from the wurtzite structure. The optical measurements reveal a blue shift in absorption edge and increase in transmittance from 75% to 96% with increase in Mg content. Tuning of the band gap has been obtained from 3.41 to 6.58 eV with corresponding increase in Mg content from x=0.0 to 1.0, which demonstrates that the films are useful for window layer of solar cells that improve the overall efficiency by decreasing the absorption loss.     

Growth and characterization of ZnO thin films prepared by electrodeposition technique

ZnO thin films were deposited on either indium tin oxide-coated glass or copper substrate by the electrodeposition process, using zinc chloride and flowing air as precursors. The effect of pH on the structural and morphological ZnO films was studied and the optimum deposition conditions have been outlined. The kinetics of the growth of the films have been investigated. We note that the rate of deposition of ZnO in an acidic solution was larger than in a basic solution. The structure of the films was studied using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The surface morphology and thickness of the films were determined using scanning electron microscopy. The X-ray diffraction analysis shows that the films are polycrystalline with hexagonal crystal structure (zincite) at pH 4. The optical transmittance of ZnO decreases with varying film thickness. The optical energy bandgap was found to be 3.26 eV.     

Effect of wire temperature on the microstructural evolution of Si films in hot-wire chemical vapor deposition for digital display and photovoltaic devices

The formation of crystalline phase in Si by hot-wire chemical vapor deposition (HWCVD) was investigated, focusing on the microstructural evolution as a function of hot-wire temperature. The microstructure of films deposited on a Si wafer was compared between hot-wire temperatures of 1590, 1670, and 1800 °C. A heavily twinned structure was observed at the wire temperature of 1670 °C, which resulted in the apparent intensity peak of (1 1 1) hexagonal-closed packed (HCP) crystalline Si from a typical face-centered cubic (FCC) crystalline Si structure in the X-ray diffraction analysis. The twin-related HCP crystalline phase was markedly diminished at 1800 °C and hardly observed at 1590 °C. The observed deposition behavior was approached by the effect of the wire temperature on the size of charged nanoparticles formed in the gas phase in the HWCVD process.     

Structural and optical investigations of cadmium sulfide nanostructures for optoelectronic applications

Cadmium sulfide (CdS) nanostructures were deposited on glass substrates by sol–gel spin coating technique. X-Ray Diffraction (XRD) results have indicated that the formation of CdS has hexagonal structure. The lattice constants are investigated for CdS nanostructures deposited on glass substrates with different spin coating speeds. The thickness of CdS nanostructures as measured by the Atmoic Force Microscopy (AFM) and found to be in the range of 150 nm and 10 nm at 1000 rpm and 5000 rpm spin coating speed, respectively. The transmissions of CdS nanostructures are investigated systematically by Ultra Violet (UV–vis) spectrophotometer. The measured values using XRD and Photoluminescence (PL) spectra and the calculated results by specific models of refractive index are in agreement with experimental data.     

A study on the improved growth rate and morphology of chemically deposited ZnS thin film buffer layer for thin film solar cells in acidic medium

Zinc sulfide (ZnS) thin films have been prepared by chemical bath deposition method with improving growth rate and morphology using the mixed complexing agents of ethylenediamine tetra-acetate disodium salt (Na₂EDTA) and hexamethylenetetramine (HMTA). The effects of HMTA quantity on the morphological, compositional, optical, structural and electrical properties of ZnS thin films with fixed Na₂EDTA concentration have been investigated. ZnS thin films were deposited on glass substrates using aqueous solutions containing zinc acetate dehydrate and thioacetamide in acidic medium (pH 4). Field emission scanning electron microscopy results show that the morphology of a deposited ZnS thin film using HMTA as a complexing agent is rough. However, very uniform and smooth ZnS thin films are obtained using mixed complexing agents of Na₂EDTA and HMTA. The growth rate and root mean square of ZnS thin films are improved with increasing HMTA quantities. X-ray diffraction patterns show that all the ZnS thin films are grown as a hexagonal structure without secondary phase (ZnO) regardless of HMTA quantity. Optical band gap energy of ZnS thin films deposited using mixed complexing agents increase from 3.75 to 3.87 eV with increasing quantity of HMTA.     

Highly stable transparent and conducting gallium-doped zinc oxide thin films for photovoltaic applications

Transparent and highly conducting gallium zinc oxide (GZO) films were successfully deposited by RF sputtering at room temperature. A lowest resistivity of ∼2.8×10⁻⁴ Ω cm was achieved for a film thickness of 1100 nm (sheet resistance ∼2.5 Ω/□), with a Hall mobility of 18 cm²/V s and a carrier concentration of 1.3×10²¹ cm⁻³. The films are polycrystalline with a hexagonal structure having a strong crystallographic c-axis orientation. A linear dependence between the mobility and the crystallite size was obtained. The films are highly transparent (between 80% and 90% including the glass substrate) in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk material. These films were applied to single glass/TCO/pin hydrogenated amorphous silicon solar cells as front layer contact, leading to solar cells with efficiencies of about 9.52%. With the optimized deposition conditions, GZO films were also deposited on polymer (PEN) substrates and the obtained results are discussed.     

Direct synthesis of nanostructured V2O5 films using solution plasma spray approach for lithium battery applications

We demonstrate for the first time, the synthesis of vanadium pentoxide (V₂O₅) nanoparticles and nanorods in the films using a high throughput solution plasma spray deposition approach. The scalable plasma spray method enables the direct deposition of large area nanostructured films of V₂O₅ with controllable particle size and morphology. In this approach, the solution precursors (vanadium oxychloride and ammonium metavanadate) were injected externally into the plasma jet, which atomizes and pyrolyzes the precursors in-flight, resulting in the desired films on the current collectors. The microstructure analysis of the as synthesized films revealed pure nanocrystalline phase for V₂O₅ with particles in the size range of 20-50 nm. The V₂O₅ film based electrodes showed stable reversible discharge capacity in the range of 200-250 mAh g⁻¹ when cycled in the voltage window 2-4 V. We further discuss the mechanism for controlling the particle growth and morphology, and also the optimization of reversible lithium storage capacity. The nanorods of V₂O₅ formed after the anneal treatment also show reversible storage capacity indicative of the potential use of such film based electrodes for energy storage.