Effect of CdCl2 treatment on physical properties of CdTe films with different compositions fabricated by chemical molecular beam deposition

CdTe films with different compositions (Cd-rich, Te-rich and stoichiometric) were fabricated by revolutionary novel and low cost chemical molecular beam deposition (CMBD) method in the atmospheric pressure hydrogen flow. Cd and Te granules were used as precursors. The films were deposited on ceramic (SiO₂: Al₂O₃) substrates at 600°C. The growth rate was varied in the range of 20–30 Å/s. The composition of the samples was changed by controlling the molecular beam intensity (MBI) ratio Cd/Te. Effect of CdCl₂ treatment on morphology, photoluminescence and electrical properties of CdTe films was investigated by AFM, Raman, photoluminescence (PL) and Hall methods.     

Fabrication of nanostructured titania on flexible substrate by electrochemical anodization

Titanium films were deposited on ITO (indium tin oxide)-coated PEN (polyethylene naphthalate) and flexible clay substrates by ion beam sputter deposition method. The surface morphology of the deposited films was smooth on PEN and rough on clay substrates. The titanium film deposited on Clay-mo (98% montmorillonite) substrate was anodized in ethylene glycol + 2 vol% H₂O + 0.3 wt% NH₄F solution, and the titanium films deposited on Clay-st (99% stevensite) substrate was anodized in 2-propanol + 16 vol% H₂O + 0.14 M NH₄F solution. Then nanohole-structured titania (TiO₂) films were firstly and successfully fabricated on the flexible transparent clay substrates. The nanohole structures of TiO₂ on both clay substrates were similar to those on PEN and glass substrates. The TiO₂ nanohole structure was almost maintained after annealing at 450 °C for 4 h in air. The optical transmittance of the nanohole-structured TiO₂ films on Clay-st increased from 26% to 54% at 800 nm in wavelength after annealing at 450 °C for 1 h in air.     

Fabrication of β-FeSi2/Si composite films for photovoltaic applications by using scanning annealing

For use as superior photovoltaic materials, thin β-iron silicide (β-FeSi₂) nanoparticles/Si composite films were fabricated by using rf-magnetron sputtering and post scanning annealing. After the scanning annealing at a preheating temperature of 800 °C, transmission electron microscopy (TEM) observation was then used to confirm 10-50-nm nanoparticles in FeSi₂=8.4 at% composite films. Amorphous and single-phase FeSi₂ films were also crystallized by the scanning annealing to determine the type of crystal phase of the nanoparticles. Lower preheating temperature (<800 °C) yielded β-FeSi₂ films, whereas higher preheating temperature (>900 °C) yielded α-FeSi₂ films. TEM observation and X-ray diffraction (XRD) analysis confirmed the successful fabrication of β-FeSi₂ nanoparticles/Si composite films.In conclusion, the photoabsorbance of thin Si films at wavelengths between 500 and 800 nm was significantly enhanced by fabrication with β-FeSi₂. The photoabsorption spectra could be controlled by changing the diameter of the nanoparticles. The nanoparticle diameter depended on the deposition conditions and the preheating temperature.     

Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering

Polycrystalline WO₃ thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O₂ gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO₃, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm⁻² at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO₃ films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm.     

Behavior of 3 mol% yttria-stabilized tetragonal zirconia polycrystal film prepared by slurry spin coating

A dense and uniform 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3YSZ) electrolyte film of 6 μm in thickness was fabricated by slurry spin coating on a porous NiO/3YSZ anode substrate. Composite cathodes of La_0.7Sr_0.3MnO₃ impregnated with Sm_0.2Ce_0.8O_1.9 were fabricated on the 3YSZ films. A single cell produced in this way was tested at 700, 750 and 800 °C with hydrogen as fuel and stationary air as oxidant. Test results revealed an open-circuit voltage of 1.04 V at 800 °C, and maximum power density of 551, 895 and 1143 mW cm⁻² at 700, 750 and 800 °C, respectively. Impedance spectra results demonstrated that the cell performance was determined by the polarization resistance of the cathode.     

Fabrication of Cu2ZnSnS4 films by sulfurization of Cu/ZnSn/Cu precursor layers in sulfur atmosphere for solar cells

Cu₂ZnSnS₄ (CZTS) absorbers were grown by sulfurization of Cu/ZnSn/Cu precursors in sulfur atmosphere. The reaction mechanism of CZTS formation from the precursor was analyzed using XRD and Raman spectroscopy. The films with a single phase CZTS were formed at 560 and 580 °C by sulfurization for 30 min. The film grown at 560 °C showed bi-layer morphology with grooved large grains on the top and dense small grains near the bottom of the film. On the other hand, the film grown at 580 °C showed large grains with grooves that are extended from surface top to bottom of the film. The solar cell fabricated with the CZTS film grown at 560 °C showed the best conversion efficiency of 4.59% for 0.44 cm² with V_oc=0.545 V, J_sc=15.44 mA/cm², and FF=54.6. We found that further improvement of the microstructure of CZTS films can increase the efficiency of CZTS solar cells.     

Chemically-deposited Te layers improving the parameters of back contacts for CdTe solar cells

A chemical bath process was carried out for the deposition of Te layers on CdTe films grown by the close-space vapor transport technique (CSVT) on conducting SnO₂:F substrates. The Te layers were chemically-deposited on as-grown CdTe films and on previously CdCl₂-treated CdTe ones. After Te deposition, the CdTe films were annealed at temperatures from 200 to 400 °C. The Te layer on top of the CdTe films was studied by Raman Spectroscopy and by Scanning Electron Microscopy. The electrical resistivity of the annealed CdTe films was determined from current versus voltage measurements in a sandwich configuration, employing gold contacts on top of the CdTe modified surface. The results show that the combined effect of the Te layer on CdTe together with previous CdCl₂ treatment improves the electrical properties of CSVT-CdTe films. These results are quite promising for increasing performance of CdS/CdTe solar cells.     

Electrochemical evaluation of nanocrystalline Zn-doped tin oxides as anodes for lithium ion microbatteries

Nanocrystalline tin oxides were synthesized using electron beam evaporation (e-beam) and further heat treatment. X-ray diffraction (XRD) revealed that as-deposited samples were amorphous SnO. Heat treatment of the as-deposited thin films at 250 °C for 2 h and 500 °C for 10 h led to the formation of romarchite SnO and tetragonal SnO₂, respectively. Scanning electron microscopy (SEM) showed a compact morphology of the coatings. Elemental mapping of the films also represented homogeneous distribution of the zinc atoms in the SnO₂ structure. Atomic force microscopy (AFM) images demonstrated a fine and smooth surface of the e-beam evaporated films for the SnO samples, and rough topography for the SnO₂. Doping led to the formation of finer and more uniform surface morphology. Anodic behavior of the thin film during charge/discharge process showed that specific capacity of the pure SnO₂ increased from 502 to 903 μAh cm⁻² μm⁻¹ for nanocrystalline Zn-doped SnO₂. Moreover, specific capacity of the doped film enhanced to 137.6 that is, higher than 69.5 μAh cm⁻² μm⁻¹ for the pure SnO₂. XRD results also show that Zn doping decreased Sn clustering during cycling.     

Performance of electron beam deposited tungsten doped indium oxide films as anodes in organic solar cells

Tungsten doped indium oxide (IWO) thin films have been investigated as an alternative to indium tin oxide (ITO) anodes in organic solar cells (OSCs). The surface morphology, electrical, and optical properties of the IWO films grown by electron beam deposition were studied as a function of oxygen flow rate. For 120 nm thick IWO films deposited on float glass substrates at 350 °C and oxygen flow rate of 35 sccm, an electrical resistivity of 4.78×10⁻⁴ Ω cm and average transmittance of over 78% between 400 and 2000 nm were obtained. OSCs based on poly(3-hexylthiophene) and [6,6]-phenyl C₆₁-butlyric acid methyl ester were prepared on glass/IWO electrodes and the device performance was investigated as a function of IWO films with different oxygen flow rates. OSCs fabricated on the optimum IWO anode (oxygen flow rate of 30-35 sccm) exhibited a power conversion efficiency of ∼3.5%, which is comparable with the same device made on commercial glass/ITO electrodes (3.75%).     

Flame spray deposition of La0.6Sr0.4CoO3−δ thin films: Microstructural characterization, electrochemical performance and degradation

Thin films of La_0.6Sr_0.4CoO_3−δ (LSC) were deposited by flame spray deposition at a deposition temperature of 200 °C. The as-deposited LSC films were dense, particle-free and amorphous. Upon annealing above 600 °C, the films crystallized into the rhombohedral perovskite LSC phase. For isothermal annealing at higher temperatures parabolic grain growth kinetics occur in parallel to densification of the thin films. Electrochemical measurements on symmetrical cells with LSC films on CGO pellets showed lowest area specific resistance (ASR) of 0.96 Ω cm² at 600 °C for films of 38 nm in grain size annealed at 700 °C. Degradation of the ASR of the LSC films of 3.9% was found after 5 days operation at 550 °C in ambient air.     

Size effects of WO3 nanocrystals for photooxidation of water in particulate suspension and photoelectrochemical film systems

Monoclinic WO₃ nanocrystals were synthesized by a hydrothermal reaction and post calcination. Their particle sizes were varied from 30 nm to 500 nm by changing calcination temperature from 500 °C to 800 °C. Photooxidation of water was studied in particulate suspension (PS) system and photoelectrochemical (PEC) film system. For PS system, WO₃ nanocrystals were suspended in 50 mM AgNO₃ solution to measure O₂ evolution rate. For PEC system, WO₃ films were fabricated by doctor blade method using synthesized nanocrystals. Photocurrent density was measured at AM 1.5 G (1 sun) solar condition in 0.5 M H₂SO₄. In PS system, the sample calcined at the highest temperature generated the largest amount of oxygen, whereas in PEC system the sample calcined at 600 °C showed the maximum photocurrent. The two systems also showed opposite response to deposition of the Pt co-catalyst. These different behaviors were attributed to different mechanisms of charge separation in the two systems.     

Impact of substrate material and annealing conditions on the microstructure and chemistry of yttria-stabilized-zirconia thin films

Si-diffusion from Si-based substrates into yttria-stabilized-zirconia (YSZ) thin films and its impact on their microstructure and chemistry is investigated. YSZ thin films used in electrochemical applications based on micro-electrochemical systems (MEMS) are deposited via spray pyrolysis onto silicon-based and silicon-free substrates, i.e. Si_xN_y-coated Si wafer, SiO₂ single crystals and Al₂O₃, sapphire. The samples are annealed at 600 °C and 1000 °C for 20 h in air. Transmission electron microscopy (TEM) showed that the Si_xN_y-coated Si wafer is oxidized to SiO_z at the interface to the YSZ thin film at temperatures as low as 600 °C. On all YSZ thin films, silica is detected by X-ray photoelectron spectroscopy (XPS). A particular large Si concentration of up to 11 at% is detected at the surface of the YSZ thin films when deposited on silicon-based substrates after annealing at 1000 °C. Their grain boundary mobility is reduced 2.5 times due to the incorporation of SiO₂. YSZ films on Si-based substrates annealed at 600 °C show a grain size gradient from the interface to the surface of 3 nm to 10 nm. For these films, the silicon content is about 1.5 at% at the thin film's surface.     

Optical, electrical and structural properties of indium-doped cadmium oxide films obtained by the sol-gel technique

Indium-doped cadmium oxide films were obtained by mixing cadmium oxide and indium oxide precursor solutions by the sol-gel technique. The indium atomic concentrations in solution (x) studied were 0, 2, 5 and 10 at%. The films were sintered at two different sintering temperatures (T_s) 350 and 450 °C, and after that annealed in a 96:4 N₂/H₂ gas mixture atmosphere at 350 °C. X-ray diffraction patterns showed that all films sintered at T_s=350 °C only consisted of cadmium oxide crystals. The films sintered at T_s=450 °C consisted of cadmium oxide crystals also; however, for the highest indium atomic concentration (10 at%) the formation of cadmium indate oxide crystals was evident. All films show high optical transmission (>85%) and an increase of the direct band gap value from 2.4 to 3.1 eV, as the indium atomic concentration in solution increases. The minimum resistivity value obtained was 6.3×10⁻⁴ Ω cm for the films with x=5 at%, T_s=450 °C and annealed at 350 °C.     

Fabrication of bilayered YSZ/SDC electrolyte film by electrophoretic deposition for reduced-temperature operating anode-supported SOFC

Bilayered Y₂O₃-stabilized ZrO₂ (YSZ)/Sm₂O₃-doped CeO₂ (SDC) electrolyte films were successfully fabricated on porous NiO–YSZ composite substrates by electrophoretic deposition (EPD) based on electrophoretic filtration followed by co-firing with the substrates. In EPD, positively charged YSZ and SDC powders were deposited directly on the substrates, layer by layer from ethanol-based suspensions. Delamination between YSZ and SDC films was avoided by reducing the SDC films’ thickness to ca. 1 μm. A single cell was constructed on the bilayered electrolyte films composed of ca. 4 μm-thick YSZ and ca. 1 μm-thick SDC films. As a cathode in the cell, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−x (LSCF) was used. Maximum output power densities greater than 0.6 W cm⁻² were obtained at 700 °C for the bilayered YSZ/SDC electrolyte cells thus constructed.     

Ga homogenization by simultaneous H2Se/H2S reaction of Cu-Ga-In precursor

The compositional distribution of Ga and S in Cu(InGa)(SeS)₂ films fabricated by a simultaneous selenization and sulfization process was systematically investigated. At low H₂Se/H₂S reaction temperature (490 °C), most Ga remains at the back of the film adjacent to the Mo back contact. However, the Ga/III ratios at the top and bottom of the Cu(InGa)(SeS)₂ layer monotonically increase and decrease with reaction temperatures, respectively. At T>550 °C, homogeneous distribution of elemental Ga and In through film is achieved. Further increase of the reaction temperature (e.g., T>550 °C) causes phase segregation on the surface of the Cu(InGa)(SeS)₂ film confirmed by XRD, GIXRD and EDS analysis.     

Effects of substrate temperature on the properties of facing-target sputtered Al-doped ZnO films

ZnO:Al films (Al 2.5 wt%) were deposited using a DC facing targets magnetron sputtering via two ZnO targets mixed with Al₂O₃. The structural, electrical and optical properties of the deposited films were strongly influenced by substrate temperature. Films with better texture, higher transmission, lower resistivity and larger carrier concentration were obtained for the samples fabricated at higher substrate temperature. The optimal condition for deposition of ZnO:Al film with the lowest resistivity of 3.18×10⁻⁴ Ω cm, the highest carrier concentration of 4.58×10²⁰ cm⁻³, and a transmission toward 85% in the visible range was obtained at 200 °C. This film proposes a promising future for the application of the practical window and contact layers for solar cells.     

Optical and structural properties of the sol-gel-prepared ZnO thin films and their effect on the photocatalytic activity

ZnO thin films were obtained by the sol-gel method, using the dip-coating procedure. Glass slides were used as substrates. The sintering temperature (T_s) was varied in the range of 200-600 °C in intervals of 50 °C, in an open atmosphere. Films with 1 and 5 coatings were prepared for each T_s. An increase of the grain size from 10 to 34 nm as the T_s increased was observed from X-ray diffraction measurements. The thickness of the films prepared starting from five coatings, decreased by 36% when T_s increased, and denser films were obtained. This result was corroborated with the refractive index values, calculated from the UV-Vis transmission spectra. The films were tested as a photocatalyst by the photobleaching of methylene blue in an aqueous solution under UV light exposure during 5 h. The photocatalytic activity (PA) increased with T_s, around 72% for the films with one coating and 66% for those with five coatings. The samples with one coating and a T_s=500 °C showed the best PA. However, the glass substrate had a negative effect on the PA for T_s>500 °C, even when the surface morphology of the samples showed an increase in roughness when T_s increased. The observed negative effect can be due to the presence of an amorphous compound formed by Si, Zn and O at the glass-ZnO interface.     

Plasma-induced TCO texture of ZnO:Ga back contacts on silicon thin film solar cells

This paper considers texturing of ZnO:Ga (GZO) films used as back contacts in amorphous silicon (a-Si) thin film solar cells. GZO thin films are first prepared by conventional methods. The as-deposited GZO surface properties are modified so that their use as back contacts on a-Si solar cells is enhanced. Texturing is performed by simple dry plasma etching in a CVD process chamber,at power=100 W, substrate temperature=190 °C (temperature is held at 190 °C because thin film solar cells are damaged above 200 °C), pressure=400 Pa and process gas H₂ flow=700 sccm. Conventional a-Si solar cells are fabricated with and without GZO back contact surface treatment. Comparison of the with/without texturing GZO films shows that plasma etching increases optical scattering reflectance and reflection haze. SEM and TEM are used to evaluate the morphological treatment-induced changes in the films. Comparison of the a-Si solar cells with/without texturing shows that the plasma treatment increases both the short-circuit current density and fill factor. Consequently, a-Si solar cell efficiency is relatively improved by 4.6%.     

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.     

Development of LSM-based cathodes for solid oxide fuel cells based on YSZ films

In an attempt to achieve desirable cell performance, the effects of La_0.7Sr_0.3MnO₃ (LSM)-based cathodes on the anode-supported solid oxide fuel cells (SOFCs) were investigated in the present study. Three types of cathodes were fabricated on the anode-supported yttria-stabilized zirconia (YSZ) thin films to constitute several single cells, i.e., pure LSM cathode, LSM/YSZ composite by solid mixing, LSM/Sm_0.2Ce_0.8O_1.9 (SDC) composite by the ion-impregnation process. Among the three single cells, the highest cell output performance 1.25 W cm⁻² at 800 °C, was achieved by the cell using LSM/SDC cathode when the cathode was exposed to the stationary air. Whereas, the most considerable cell performance of 2.32 W cm⁻² was derived from the cell with LSM/YSZ cathode, using 100 ml min⁻¹ oxygen flow as the oxidant. At reduced temperatures down to 700 °C, the LSM/SDC cathode was the most suitable cathode for zirconia-based electrolyte SOFC in the present study. The variation in the cell performances was attributed to the mutual effects between the gas diffusing rate and three-phase boundary length of the cathode.