In situ hydrogen plasma treatment for improved transport of (4 0 0) oriented polycrystalline silicon films

Polycrystalline silicon (poly-Si) films were deposited on glass by very high-frequency (100 MHz) plasma enhanced chemical vapor deposition from a gaseous mixture of SiF₄ and H₂ with small amounts of SiH₄. (2 2 0) oriented films prepared at small SiF₄/H₂ ratios (<30/40 sccm) showed intrinsic transport properties of poly-Si. However, the room temperature dark conductivity (σ_d) of the (4 0 0) oriented film was very high for intrinsic poly-Si, 7.2×10⁻⁴S/cm. This conductivity exhibited a T^−1/4 behavior, suggesting a high defect density at the grain boundaries. It was found that in situ hydrogen plasma treatment successfully produced (4 0 0) oriented poly-Si with a reasonably low σ_d of 4.5×10⁻⁷S/cm and a good photoconductivity of 1.3×10⁻⁴S/cm.     

Low-temperature deposition of polycrystalline silicon thin films by hot-wire CVD

Polycrystalline silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430°C. The material properties have been optimized for photovoltaic applications by varying the hydrogen dilution of the silane feedstock gas, the gas pressure and the wire temperature. The optimized material has 95% crystalline volume fraction and an average grain size of 70 nm. The grains have a preferential orientation along the (2 2 0) direction. The optical band gap calculated from optical absorption by photothermal deflection spectroscopy (PDS) showed a value of 1.1 eV, equal to crystalline silicon. An activation energy of 0.54 eV for the electrical transport confirmed the intrinsic nature of the films. The material has a low dangling bond-defect density of 10¹⁷ cm³. A photo conductivity of 1.9 × 10⁻⁵ Ω⁻¹cm⁻¹ and a photoresponse (σ_ph/σ_d) of 1.4 × 10² were achieved. A high minority-carrier diffusion length of 334 nm as measured by the steady-state photocarrier grating technique (SSPG) and a large majority-carrier mobility-lifetime (μτ) product of 7.1 × 10⁻⁷cm²V⁻¹ from steady-state photoconductivity measurement ensure that the poly-Si : H films possess device quality. A single junction n---i---p cell made in the configuration n⁺-c-Si/i-poly-Si: H/p-μc-Si : H/ITO yielded 3.15% efficiency under 100 mW/cm² AM 1.5 illumination.     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

Effect of proton irradiation on electrical properties of CuInSe2 thin films

High-energy proton irradiation (380 keV and 1 MeV) on the electrical properties of CuInSe₂ (CIS) thin films has been investigated. The samples were epitaxially grown on GaAs (0 0 1) substrates by Radio Frequency sputtering. As the proton fluence exceeded 1×10¹³ cm⁻², the carrier concentration and mobility of the CIS thin films were decreased. The carrier removal rate with proton fluence was estimated to be about 1000 cm⁻¹. The electrical properties of CIS thin films before and after irradiation were studied between 80 and 300 K. From the temperature dependence of the carrier concentration in CIS thin films, we found N_D=9.5×10¹⁶ cm⁻³, N_A=3.7×10¹⁶ cm⁻³ and E_D=21 meV from the fitting to the experimental data on the basis of the charge balance equation. After irradiation, a defect level was created, and N_T=1×10¹⁷ cm⁻³ for a fluence of 3×10¹³ cm⁻², N_T=5.7×10¹⁷ cm⁻³ for a fluence of 1×10¹⁴ cm⁻² and E_T=95 meV were also obtained from the same fitting. The new defect, which acted as an electron trap, was due to proton irradiation, and the defect density was increased with proton fluence.     

Thin film poly-Si formation for solar cells by Flux method and Cat-CVD method

Two methods were examined for the formation of poly-Si films. One is flux method and the other is Cat-CVD method. Flux method was used for forming poly-Si seed films on glass substrates covered with rear electrode. Poly-Si films of a few μm grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 600°C. To make the seed films function as BSF layer for solar cell, boron doping was applied and carrier concentration of 2×10¹⁹/cm³ was obtained which is suitable for highly efficient solar cells. Cat (catalytic)-CVD method was examined for forming poly-Si photo-active layers on the seed films. The films showed deposition gas pressure-dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented films by Cat-CVD method when deposited on (1 1 1) oriented seed films prepared by Flux method. The electrical properties of the film are insufficient at present, may be due to high defect density and the film structure which allows impurity contaminations of oxygen and carbon after film deposition. Although the film quality needs to be improved, poly-Si films whose crystal fraction is more than 85% were obtained at deposition rate of up to around 40 Å/s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin film silicon solar cells.     

Effect of 8 MeV electron irradiation on electrical properties of CuInSe2 thin films

Radiation damages due to 8 MeV electron irradiation in electrical properties of CuInSe₂ thin films have been investigated. The n-type CuInSe₂ films in which the carrier concentration was about 3×10¹⁶ cm⁻³, were epitaxially grown on a GaAs(0 0 1) substrate by RF diode sputtering. No significant change in the electrical properties was observed under the electron fluence <3×10¹⁶ e cm⁻². As the electron fluence exceeded 10¹⁷ e cm⁻², both the carrier concentration and Hall mobility slightly decreased. The carrier removal rate was estimated to be about 0.8 cm⁻¹, which is slightly lower than that of III–V compound materials.     

Polycrystalline silicon film deposited by ICP-CVD

We studied the deposition of polycrystalline silicon (poly-Si) using SiH₄/SiH₂Cl₂/H₂ mixtures by inductively coupled plasma chemical vapor deposition. The deposition rate and crystalline quality were improved by increasing RF power. The poly-Si film deposited with the [SiH₂Cl₂]/[SiH₄] ratio of 2 and the RF power of 1500 W exhibited the deposition rate of 4.2 Å/s, the polycrystalline volume fraction of 88%, the Raman FWHM of 7 cm⁻¹, and the TEM grain size of 1200 Å. The solar cell made of this material exhibited a conversion efficiency of 3.14%.     

Structural, thermal and electrochemical properties of layered perovskite SmBaCo2O5+d, a potential cathode material for intermediate-temperature solid oxide fuel cells

The synthesis, conductivity properties, area specific resistance (ASR) and thermal expansion behaviour of the layered perovskite SmBaCo₂O_5+d (SBCO) are investigated for use as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The SBCO is prepared and shows the expected orthorhombic pattern. The electrical conductivity of SBCO exhibits a metal–insulator transition at about 200 °C. The maximum conductivity is 570 S cm⁻¹ at 200 °C and its value is higher than 170 S cm⁻¹ over the whole temperature range investigated. Under variable oxygen partial pressure SBCO is found to be a p-type conductor. The ASR of a composite cathode (50 wt% SBCO and 50 wt% Ce_0.9Gd_0.1O_2−d, SBCO:50) on a Ce_0.9Gd_0.1O_2−d (CGO91) electrolyte is 0.05 Ω cm² at 700 °C. An abrupt increase in thermal expansion is observed in the vicinity of 320 °C and is ascribed to the generation of oxygen vacancies. The coefficients of thermal expansion (CTE) of SBCO is 19.7 and 20.0 × 10⁻⁶ K⁻¹ at 600 and 700 °C, respectively. By contrast, CTE values for SBCO:50 are 12.3, 12.5 and 12.7 × 10⁻⁶ K⁻¹ at 500, 600 and 700 °C, that is, very similar to the value of the CGO91 electrolyte.     

Preparation of sulfonated poly(ether ether ketone)s containing amino groups/epoxy resin composite membranes and their in situ crosslinking for application in fuel cells

A series of amino-containing sulfonated poly(aryl ether ketone)/4,4′-diglycidyl(biphenyl) epoxy resin (DGBP) composite membranes for proton exchange membranes fuel cells (PEMFCs) are prepared by solution blending and casting. The reaction kinetics and the effects of introduction of DGBP content on the properties of the composite membranes are thoroughly investigated. The crosslinked composite membranes after treatment at either 120 °C or 200 °C have improved oxidative and dimensional stability than those without crosslinking. Despite the fact that crosslinked membranes generally have lower proton conductivity in comparison with the original ones, the proton conductivities of the membranes treated at 120 °C are above 2.22 × 10⁻² S cm⁻¹ at room temperature and 9.42 × 10⁻² S cm⁻¹ at 100 °C. Even for the samples treated at 200 °C, their proton conductivities are still higher than 1.26 × 10⁻² S cm⁻¹ at room temperature and higher than 8.67 × 10⁻² S cm⁻¹ at 100 °C, which are well satisfied with elementary requirement of fuel cells. In addition, all the evaluated membranes have low methanol permeability. For example, the methanol permeability of AP6FSPEEK/DGBP1 cured at 200 °C is 0.33 × 10⁻⁶ cm² s⁻¹, which is an order magnitude lower than Nafion 117. Therefore, these novel crosslinked composite membranes could be potential usage in fuel cells.     

Preparation and characterisation of electrodeposited n-CdS/p-CdTe thin film solar cells

Cadmium sulphide and cadmium telluride films have been electrodeposited for n-CdS/p-CdTe solar cells. Cell efficiency varied considerably from 9.5% to 11.5% for each deposition set. The reverse saturation currents of 9.5% and 11.5% cells at 298 K were 25 and 6.7 nA cm⁻², respectively. The cells with higher efficiency has a lower number of interface states than the less efficient cells. The 11.5% cell had interface states (N_IS) of 3× 10¹⁰ cm⁻² eV⁻¹ at zero volt bias in dark and when it was illuminated with 35 mW cm⁻² light at zero volt bias N_IS increased by two orders to 1.2×10¹² cm⁻² eV⁻¹. At higher frequency the large voltage intercept of the Mott-Schottky plot indicates the existence of the near intrinsic layer of the polycrystalline heterojunction.     

Synthesis and characterization of aluminum-doped CdO thin films by sol–gel process

Aluminum-doped cadmium oxide (CdO:Al) thin films are deposited on glass substrates by the sol–gel dip-coating method, taking cadmium acetate dihydrate as the precursor material. Aluminum nitrate has been taken as a source of Al-dopant. XRD pattern reveals the good crystallinity of CdO thin films. SEM micrograph showed the presence of faceted crystallites. Optical study shows 40–85% transparency with a bandgap value lying in the range 2.76–2.52 eV, depending upon the Al content in the films. Optimum percentage of Al was 5.22 for a maximum room temperature conductivity of 2.81×10³ (Ω cm)⁻¹. Hall measurement confirmed that the material is of n-type, with mobility and carrier concentrations lying in the range 413–14.7 cm²/V s, and 3.4×10¹⁹–8.11×10²⁰ cm⁻³, when percentage of Al varies in the range 1.32–7.24.     

Reduction of defects of polycrystalline silicon thin films by heat treatment with high-pressure H2O vapor

An improvement of electrical properties of pulsed laser crystalllized silicon films was achieved by simple heat treatment with high-pressure H₂O vapor. The electrical conductivity of 7.4×10¹⁷ cm⁻³ phosphorus-doped 50-nm-thick pulsed laser crystallized silicon films was markedly increased from 1.6×10⁻⁵ S/cm (as crystallized) to 2 S/cm by heat treatment at 270°C for 3 h with 1.25×10⁶ Pa H₂O vapor because of reduction of density of defect states localized at grain boundaries. Spin density was reduced from 1.7×10¹⁸ cm⁻³ (as crystallized) to 1.2×10¹⁷ cm⁻³ by heat treatment at 310°C for 3 h with 1.25×10⁶ Pa H₂O vapor.     

Crystallographic analysis of high quality poly-Si thin films deposited by atmospheric pressure chemical vapor deposition

Crystallinity of thin film polycrystalline silicon (poly-Si) grown by atmospheric pressure chemical vapor deposition has been investigated by X-ray diffraction measurement and Raman spectroscopy. Poly-Si films deposited at high temperatures of 850–1050°C preferred to 2 2 0 direction. By Raman spectroscopy, the broad peak of around 480–500 cm⁻¹ belonged to microcrystalline Si (μc-Si) phase was observed even for the poly-Si deposited at 950°C. After high-temperature annealing (1050°C) 3 3 1 direction of poly-Si increased. This result indicates that the μc-Si phase at grain boundary became poly-Si phase preferred to 3 3 1 direction by high-temperature annealing. Effective diffusion length of poly-Si films deposited at 1000°C was estimated to be 11.9–13.5 μm and 10.2–12.9 μm before and after annealing, respectively.     

Effect of film thickness on electrical property of microcrystalline silicon

We report dependences of electrical properties on SiH₄/H₂ dilution rate and film thickness for microcrystalline silicon films formed by a hydrogen radical-induced chemical vapor deposition (HRCVD) method. The electrical conductivity of the films at SiH₄ 18 sccm /H₂ 120 sccm was markedly increased to 10⁻³ S/cm as film thickness increased above 100 nm. Crystalline grains with (2 2 0) orientation were formed. Theoretical analysis revealed that grain boundaries among (2 2 0) grains had a low defect density of 1×10¹² cm⁻² so that the high conductivity was achieved.     

Solar cells using iodine-doped polythiophene–porphyrin polymer films

Wet-type organic solar cells containing 5,10,15,20-3-tetrathienylporphyrin (TThP) and polythiophene (PTh) films were fabricated. The TThP/PTh film was prepared on indium-tin-oxide (ITO) glass using an electrochemical polymerization method in an n-Bu₄NPF₆/CH₂Cl₂ solution. It was found that a small amount of iodine doping of the film improved the incident photon-to-electron conversion efficiency (IPCE) of a solar cell consisting of a TThP/PTh film and an aqueous electrolyte. A HOMO level measurement suggested that a modified HOMO level of the low iodine-doped TThP/PTh film allowed a fast electron transfer from PTh to a porphyrin moiety. To obtain further improvement, a sandwich-type solar cell using a 5% (w/w) aqueous solution of acetonitrile containing 0.05 M iodine and 0.5 M lithium iodide as an electrolyte was then fabricated. The solar cell absorbed light in the 300–800 nm wavelength range, converting this to a cathodic photocurrent with a maximum IPCE of 32% at 430 nm under irradiation of 5.0×10¹⁴ photon cm⁻² s⁻¹. This value is about 10 times higher than that of the solar cells using an aqueous electrolyte. The total energy conversion efficiency (η) of the solar cell under simulated sunlight reached 0.12% for 2.59 mW cm⁻² at AM1.5 and 0.05% for 100 mW cm⁻² at air mass 1.5.     

A novel organic n-type material: fluorinated perylene diimide

In order to increase the dissociation efficiency of electron–hole pairs (excitons) in organic solar cells, and to enhance the solubility of organic materials for easy fabrication, a novel perylene diimide with multiple strongly electron-withdrawing F atoms, N,N′-diperfluorophenyl-3,4,9,10-perylenetetracarboxylic diimide (DFPP), was designed and synthesized. The solubility of DFPP was improved greatly compared to its non-fluorinated parent, N,N′-diphenyl-3,4,9,10-perylenetetracarboxylic diimide (DPP), which might relate to the spatial arrangement of the phenyl versus the pentafluorophenyl groups. The field-effect-transistor (FET) with spin-coated DFPP film as transport layer was prepared, and its electron mobility was measured as 1.29×10⁻⁵ cm² V⁻¹ s⁻¹, 3.4 times higher than that from vacuum-deposited DPP film, demonstrating its potential applications in organic optoelectronic devices.     

Interface characterisation of ZnSe/CuGaSe2 heterojunction

The ZnSe/CuGaSe₂ heterojunctions were fabricated by flash evaporation technique of CuGaSe₂ onto the (110) surface of ZnSe crystals. CuGaSe₂ layers had thickness 2–4 μm and showed a hole concentration up to (1.5–18.0)×10¹⁸ cm⁻³ and mobility μ4–24 cm² V⁻¹ s⁻¹ at 300 K. The charge carrier concentration in ZnSe crystals at 300 K was n=5.6×10¹⁶ cm⁻³ and their mobility μ=300 cm² V⁻¹ s⁻¹. The investigated ZnSe/CuGaSe₂ heterojunctions have at the interface an intermediate layer with a thickness of 450–750 Å and a linear graded band gap as well as an i-ZnSe compensated layer with a thickness of 1–2 μm and resistivity ρ10⁸–10⁹ Ω cm. The i-ZnSe layer is highly compensated due to the presence of Cu acceptor impurities. In this layer the Fermi level position E_c−F₀690 meV and a trap level position E_t−F₀17 meV were determined. The total trap concentration in the i-ZnSe layer is N_t5×10¹⁴ cm⁻³. The mean free path of excited charge carriers in the graded band gap region was calculated as λ55 Å. On the basis of experimental data analysis of electrophysical properties of both ZnSe/CuGaSe₂ heterojunctions and constituent materials the energetic band diagram of the investigated heterostructures is proposed. The current transport mechanism through ZnSe/CuGaSe₂ heterojunctions is consequently elucidated.     

Oxygen- and hydrogen-permeation measurements on-mixed conducting SrFeCo0.5Oy ceramic membrane material

The SrFeCo_0.5O_y system combines high electronic/ionic conductivity with appreciable oxygen permeability at elevated temperatures. This system has potential use in high-temperature electrochemical applications such as solid oxide fuel cells, batteries, sensors, and oxygen separation membranes. Dense ceramic membranes of SrFeCo_0.5O_y are prepared by pressing a ceramic powder prepared by using a sol–gel combustion technique. Oxygen and hydrogen permeation at high temperature on this material are studied. Measurements are conducted using a time-dependent permeation method at the temperature in the range of 1073–1273 K with oxygen- and hydrogen-driving pressures in the range (3×10²)–(1×10⁵) Pa (300–1000 mbar). The maximum oxygen-permeated flux at 1273 K is 6.5×10⁻³ mol m⁻² s⁻¹. The activation energies for the O₂-permeation fluxes and diffusivities are 240 and 194 kJ/mol, respectively. Due to the high fragility, the high temperature for the measurements and the high oxygen permeation through such material, a special membrane holder, and compression sealing system have been designed and realized for the permeation apparatus.     

Porous alumina templates and nanostructured CdS for thin film solar cell applications

Nanostructured CdS was grown by electrodeposition of cadmium sulfide inside a porous alumina template. Uniform pore size and spacing in the template was achieved when the starting material for the template was aluminum foil. Typical pore size was 45 nm. Nanostructured CdS was also deposited by electrodeposition on indium tin oxide (ITO)-coated glass and by solution growth on ITO-coated glass. Schottky diodes were formed on nanocrystalline CdS and the analysis of their current–voltage characteristics yielded a diode ideality factor (n) of 2.6 and a reverse saturation current density (J_S) of 1.00×10⁻⁵ A/cm². Corresponding values for the Schottky diode on polycrystalline CdS were 3.4 and 1.93×10⁻⁶ A/cm².     

A preliminary study of the electro-oxidation of l-ascorbic acid on polycrystalline silver in alkaline solution

Electrochemical oxidation of l-ascorbic acid on polycrystalline silver in alkaline aqueous solutions is studied by cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (IS). The anodic electro-oxidation starts at −500 mV versus SCE and shows continued anodic oxidation in the cathodic half cycle in the CV regime signifying slowly oxidizing adsorbates. Diffusion coefficient of ascorbate ion measured under both voltammetric regimes is around 1.4 × 10⁻⁵ cm² s⁻¹. Impedance spectroscopy measures the capacitances associated with double layer and adsorption around 50 μF cm⁻² and 4 mF cm⁻² as well as the adsorption and decomposition resistances (rates).