Properties of CuIn1−xGaxSe2

Polycrystalline bulk samples of CuIn_1−xGa_xSe₂ weregrown with nominal x = 0.15, 0.25 and 0.5. Mobility, conductivity and band gap were measured at room and low temperatures. Mobilities for x = 0.21 were several hundred cm² V⁻¹s⁻¹ at room temperature and for x = 0.15 were 10³ cm² V⁻¹ s⁻¹, all n type. The band gaps were estimated from the spectra of photoelectrochemical cells at room temperature (with 8.5 K photoluminescence estimates shown in brackets), as 1.10 eV (1.14) for x = 0.21, and 1.07 eV (1.093) for x = 0.15. Crystal mechanical properties as regards cracks were not as good as for CuInSe₂, using similar growth techniques.     

Photoconductivity and defect levels in ZnxCd1−xSe with (x=0.5, 0.55) crystals

Gold surface barriers on Zn_xCd_1−xSe alloys have been investigated for composition with x=(0.5, 0.55). The electrical characteristics were studied as a function of air annealing. The common feature of all the Schottky devices was the reduction of reverse bias leakage current after heating in air. Typical measurements of capacitance as a function of bias voltage can provide information on the charge density and diffusion potential. The barrier height was found to increase after air annealing at 200°C for 2 min. The spectral response of the photocurrent and photocapacitance associated with these device layers enable a donor level at 0.13 eV and acceptor levels at 1.08, 1.3 and 1.45 eV below the bottom of the conduction band to be identified. The results are discussed in terms of the effects of oxygen absorption.     

Electrical and photoluminescence properties of evaporated CuIn1−xGaxTe2 thin films

Polycrystalline thin films of CuIn_1−xGa_xTe₂ have been deposited by flash evaporation on Corning glass 7059 substrates at T_s=200°C. Hall and resistivity measurements have been carried out down to 77 K. These films are p-type and the variation of the resistivity may be linked to defects, disorder of the material or grain boundaries. The PL spectra of these films after annealing in argon atmosphere at T_a=450°C have showed a broad band emission between 0.98 and 1.12 eV in which the main peak appears at 1.05 eV (at 4.2 K).     

High-temperature optical properties and stability of AlxOy–AlNx–Al solar selective absorbing surface prepared by DC magnetron reactive sputtering

Al_xO_y–AlN_x–Al selective absorbing surface was prepared by DC magnetron reactive sputtering with aluminum alloy (LY13)¹ in air and argon. The studies were carried out to access the high-temperature (400°C–600°C) optical properties and stability of the coatings. The coatings were found to withstand heating at 600°C for 30 min in 4.5×10⁻³ Pa vacuum with absorptance 0.94 and emittance 0.07 after annealing. After heating at 450°C for 10 h, the specimen still had good performance whose absorptance and emittance was 0.93 and 0.07, respectively. Auger electron spectroscopy was used to analyse the structure of the solar selective surface before and after annealing.     

Compositional dependence of the optical properties and the H local bonding in a-Si1−xGex: H (x<0.4) thin films

A study on hydrogenated amorphous silicon-germanium alloys is presented. Amorphous thin films are prepared by RF magnetron co-sputtering. The dependence of the optical properties and parameters and the local hydrogen bonding on the composition of a-Si_1−xGe_x: H films (x < 0.4) has been investigated. It is very important to take into account the variation of hydrogen concentration when the influence of Ge content on optical properties and parameters is considered.     

PtxNi1−x nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

We report on Pt_xNi_1−x (x = 0, 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane (NH₃BH₃). The Pt_xNi_1−x catalysts were prepared through a redox replacement reaction with a reverse microemulsion technique. The structure, morphology, and chemical composition of the obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive X-ray (EDX), and inductively coupled plasma emission spectroscopy (ICP). The results show that the diameters of the Pt_xNi_1−x nanoparticles are about 2–4 nm, and the Pt atomic contents in the catalysts were 35%, 44%, 65%, 75%, and 93%, respectively. It is found that the catalytic activity toward the hydrolysis of NH₃BH₃ is correlated with the composition of the Pt_xNi_1−x catalysts. The annealing of Pt_0.65Ni_0.35 at 300 °C for 1 h increases the crystallinity of the nanoparticles, but shows almost the same activity as that without annealing. Among the as-prepared Pt_xNi_1−x nanoparticles, Pt_0.65Ni_0.35 displays the highest catalytic performance, delivering a high hydrogen-release rate of 4784.7 mL min⁻¹ g⁻¹ and a low activation energy of 39.0 kJ mol⁻¹.     

Preparation of CuIn1−xGaxSe2 thin films by sputtering and selenization process

CuIn_1−xGa_xSe₂ polycrystalline thin films were prepared by a two-step method. The metal precursors were deposited either sequentially or simultaneously using Cu–Ga (23 at%) alloy and In targets by DC magnetron sputtering. The Cu–In–Ga alloy precursor was deposited on glass or on Mo/glass substrates at either room temperature or 150°C. These metallic precursors were then selenized with Se pellets in a vacuum furnace. The CuIn_1−xGa_xSe₂ films had a smooth surface morphology and a single chalcopyrite phase.     

Structure and hydrogen storage properties of Mg2Cu1−xNix (x = 0–1) alloys

The effect of Ni-substitution on the structure and hydrogen storage properties of Mg₂Cu_1−xNi_x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys prepared by a method combining electric resistance melting with isothermal evaporation casting process (IECP) has been studied. The X-ray single-crystal diffraction analysis results showed that the cell volume decreases with increasing Ni concentration, and crystal structure transforms Mg₂Cu with face-centered orthorhombic into Ni-containing alloys with hexagonal structure. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg₂Cu and Mg₂Ni alloys absorbed 2.54 and 3.58 wt% H, respectively, at 573 K under 50 bar H₂. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH₂, MgCu₂ and Mg₂NiH₄ while the discharged samples contained ternary alloys of Mg–Cu–Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg–Cu and Mg–Ni alloys. With increasing nickel content, the effect of Ni is actually effective in MgH₂ and Mg₂NiH₄ destabilization, leading to a decrease of the desorption temperature of these two phases.     

Application of scanning mid-IR-laser microscopy for characterization of semiconductor materials for photovoltaics

Scanning mid-IR-laser microscopy was previously demonstrated as an effective tool for the characterization of different semiconductor crystals. Now the technique has been successfully applied for the investigation of CZ Si_xGe_1−x – a promising material for photovoltaics – and multicrystalline silicon for solar cells. In addition, this technique was shown to be appropriate for the imaging of polishing-induced defects as well as huge defects such as “pin holes”. Besides, previously unexplained “anomalous” (cubic power) dependence of signal of the scanning mid-IR-laser microscope in the optical-beam-induced light scattering mode on the photoexcitation power obtained for mechanically polished samples has now been attributed to the excess carrier scattering on charged linear defects, likely dislocation lines. The conclusion is made in the article that scanning mid-IR-laser microscopy may serve as a very effective tool for defect investigations in materials for modern photovoltaics.     

Annealing effects on Zn1−xMgxO/CIGS interfaces characterized by ultraviolet light excited time-resolved photoluminescence

Annealed Zn_1−xMg_xO/Cu(In,Ga)Se₂ (CIGS) interfaces have been characterized by ultraviolet light excited time-resolved photoluminescence (TRPL). The TRPL lifetime of the Zn_1−xMg_xO/CIGS film increased on increasing the annealing temperature to 250 °C, whereas the TRPL lifetime of the CdS/CIGS film had little change by annealing at temperatures lower than 200 °C. This is attributed to the recovery of physical damages by annealing, induced by sputtering of the Zn_1−xMg_xO film. The TRPL lifetime abruptly decreased with annealing at 300 °C. The diffusion of excess Zn from the Zn_1−xMg_xO film into the CIGS interface is clearly observed in secondary ion mass spectroscopy (SIMS) depth profiles. These results indicate that excess Zn at the vicinity of the CIGS surface acts as non-radiative centers at the interface. The TRPL lifetime of the Zn_1−xMg_xO/CIGS film annealed at 250 °C reached values to be comparable to that of the as-deposited CdS/CIGS film. Performance of the Zn_1−xMg_xO/CIGS cells varied with the annealing temperature in the same manner as the TRPL lifetime. The highest efficiency of the Zn_1−xMg_xO/CIGS solar cells was achieved for annealing at 250 °C. The results of the TRPL lifetime on annealing show that the cell efficiency is strongly influenced by the Zn_1−xMg_xO/CIGS interface states related to the damages and diffusion of Zn.     

Electrochromic properties of NiOxHy thin films

NiO_xH_y films were prepared by DC magnetron sputtering in H₂/O₂ atmosphere. NiO_xH_y coatings with transparency and high electrochromic efficiency were obtained by changing H₂ content. A 60 nm thick NiO_xH_y film with transmittance of 0.57 (as-deposited state), 0.78 (bleached state) and 0.24 (coloured state) at wavelength of 550 nm was deposited in an atmosphere of H₂(60%)+O₂(40%). Analysis of infrared spectra (60002400 cm⁻¹) showed that the absorption peaks for bleached and colored states are associated with free ‘OH’ and OH stretching vibrations, respectively. XPS Ni2p core level spectra of colored NiO_xH_y film exhibited a peak at 856.2±0.2 eV which is attributed to Ni³⁺. Ni2p core level spectra of the bleached and as-deposited films exhibited two peaks at 856.4±0.2 and 854.6±0.2 eV which are attributed to Ni³⁺ and Ni²⁺.     

Characterization and electrochromic properties of CuxNi1−xO films prepared by sol–gel dip-coating

Cu_xNi_1−xO electrochromic thin films were prepared by sol–gel dip coating and characterized by XRD, UV–vis absorption and electrochromic test. XRD results show that the structure of the Cu_x Ni_1−xO thin films is still in cubic NiO structure. UV–vis absorption spectra show that the absorption edges of the Cu_xNi_1−xO films can be tuned from 335 nm (x = 0) to 550 nm (x = 0.3), and the transmittance of the colored films decrease as the content of Cu increases. Cu_xNi_1−xO films show good electrochromic behavior, both the coloring and bleaching time for a Cu_0.2Ni_0.8O film were less than 1 s, with a variation of transmittance up to 75% at the wavelength of 632.8 nm.     

Composition, surface topography, structure, Raman, and electrochemical/photoelectrochemical characterisation of CdxHg1−xTe films

Cd-rich Cd_xHg_{1 − x}Te films have been electrodeposited under potentiostatic conditions on conducting glass and Ti substrates from an acidic solution containing the respective ions as Cd²⁺:Hg²⁺:HTeO₂⁺ = 100:1:2. Six films one after another have been prepared from a single electrochemical cell. EDAX analysis of the air annealed films show decreasing Hg content in the deposit as the number of film preparation increases. SEM analysis indicate undulatory surface with Hg-rich clusters at the top surface. XRD analysis indicate the presence of Cd_xHg_{1 − x}Te along with . The Cd_xHg_{1 − x}Te alloy formation have been confirmed from Raman shift measurements which change with composition, x. The as-deposited films are n-type but converts to p-type after air annealing. Spectral response measurements gave band gap values that change with Hg content in the deposit. Band gap values ranging from 1.1 eV to 1.45 eV have been estimated. Photoelectrochemical solar cells using polysulphide electrolyte have been fabricated which gave an open-circuit photovoltage and short-circuit photocurrent, respectively, as 325 mV and 5.5 mA/cm² under 60 mW/cm² intensity of illumination.     

Optical, electrical and structural investigations on Cd1−xZnxSe sintered films for photovoltaic applications

II–VI polycrystalline semiconducting materials have come under increased scrutiny because of their wide use in the cost reduction of devices for photovoltaic applications. Cd_1−xZn_xSe is an important semiconducting alloy because of the tunability of its physical parameters such as band gap and lattice parameters by controlling its stoichiometry. Many more material characteristics of it would be altered and excellently controlled by controlling system composition x.Polycrystalline thin films of Cd_1−xZn_xSe with variable composition (0x1) have been deposited onto ultra-clean glass substrates by sintering process. The optical, structural and electrical transport properties of Cd_1−xZn_xSe thin films have been examined. The optical band gap and optical constants of these films were determined by using double beam spectrophotometer. The DC conductivity and activation energy of the films were measured in vacuum by two-probe technique. The Schottky junction of Cd_1−xZn_xSe with indium was made and the barrier height and ideality factor were determined using current–voltage characteristics. The nature of sample, crystal structure and lattice parameters were determined from X-ray diffraction patterns. The films were polycrystalline in nature having cubic zinc-blende structure over the whole range studied.Sintering is very simple and viable compared to other cost intensive methods. The results of the present investigation will be useful in characterizing the material, Cd_1−xZn_xSe, for its applications in photovoltaics.     

Novel cobalt-free cathode materials BaCexFe1−xO3−δ for proton-conducting solid oxide fuel cells

A series of cobalt-free and low cost BaCe_xFe_1−xO_3−δ (x = 0.15, 0.50, 0.85) materials are successful synthesized and used as the cathode materials for proton-conducting solid oxide fuel cells (SOFCs). The single cell, consisting of a BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7)-NiO anode substrate, a BZCY7 anode functional layer, a BZCY7 electrolyte membrane and a BaCe_xFe_1−xO_3−δ cathode layer, is assembled and tested from 600 to 700 °C with humidified hydrogen (3% H₂O) as the fuel and the static air as the oxidant. Within all the cathode materials above, the cathode BaCe_0.5Fe_0.5O_3−δ shows the highest cell performance which could obtain an open-circuit potential of 0.99 V and a maximum power density of 395 mW cm⁻² at 700 °C. The results indicate that the Fe-doped barium cerates can be promising cathodes for proton-conducting SOFCs.     

High efficiency AlxGa1−xAs/GaAs solar cells: Fabrication, irradiation and annealing effect

High efficiency Al_xGa_1−xAs/GaAs heteroface solar cells have been fabricated by an improved multi-wafer squeezing graphite boat liquid phase epitaxy (LPE) technique, which enables simultaneous growth of twenty 2.3 × 2.3cm² epilayers in one run. A total area conversion efficiency of 17.33% is exhibited (1sun, AMO, 2.0 × 2.0cm²). The shallow junction cell shows more resistance to 1 MeV electron radiation than the deep one. After isochronal or isothermal annealing the density and the number of deep level traps induced by irradiation are reduced effectively for the solar cells with deep junction and bombardment under high electron fluences.     

Solar absorber coatings based on CoCuMnOx spinels prepared via the sol–gel process: structural and optical properties

Titanium-doped and undoped CuCoMnO_x spinel films were deposited on Al substrates from sols which were made from the following: Co-acetate, Cu-chloride and Mn-acetate (Ti:CoCuMnO_x-I); and Co-acetate, Cu-nitrate and Mn-acetate (CoCuMnO_x-II). The precursors’ ratio Co:Cu:Mn was equal to 1:3:3. The solar absorptance (α_s) and the thermal emittance (_T) of the films, which were annealed at 450°C for 15 or 30 min, were determined from the corresponding diffuse reflectance spectra in the 0.32–20 μm range. The results show that the CoCuMnO_x-II films with SiO_x protective over-coatings exhibited values of α_s=0.85–0.91 and _T below 0.036 after just a single dipping/annealing cycle.The structure of the films was studied with X-ray diffraction (XRD), infrared (IR) absorbance and near-grazing incidence angle (NGIA) reflection-absorption spectroscopy. Our results suggested that the films have a spinel structure with the composition CoCuMnO_x. The stability of the films was tested by soaking them in boiling water for 2 h. NGIA IR spectra of the treated films confirmed the formation of the hydrated mixed oxide (Mn-, Co-, Cu-) phases. To improve the stability of the films two kinds of protective over-coatings were tested: one over-coating was based on polysiloxane resin and the other on high-density silica (T-resin). Films that were resistant to boiling water were obtained by applying the high-density silica protective over-coating, which was cured at 140°C for 30 min.     

Synthesis of LiFe1−xNixPO4/C composites and their electrochemical performance

LiFePO₄/C and LiFe_1−xNi_xPO₄/C (x = 0, 0.02, 0.04, and 0.06) composites were prepared using solid-state reaction. The as-prepared composites were characterized by using X-ray diffraction, charge–discharge cycling, cyclic voltammograms, Raman spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and other techniques. The experimental results show that all as-prepared composites have a single phase of orthorhombic olivine-type structure with the Pnmb space group. The as-prepared LiFe_1−xNi_xPO₄/C composites exhibit high capacities and good cycling performance; e.g., the LiFe_0.98Ni_0.02PO₄/C composite delivers 142 and 138 mAh g⁻¹ at the 0.1 C rate for the first and fifth cycles, respectively. Such composites also show good rate capabilities; e.g., when discharged at the 2 C rate the LiFe_0.98Ni_0.02PO₄/C composite delivers an initial capacity of 121 mAh g⁻¹, 85% of the discharge capacity at the 0.1 C rate. The reason why the LiFe_1−xNi_xPO₄/C composites have better electrochemical performance compared to the LiFePO₄/C composites is because nickel doping enhances the PO bond, stabilizes the structure, and thus the charge-transfer resistance and cathode particle resistance of the composites are decreased.     

Investigation of non-aqueous electrodeposited CdS/Cd1−xZnxTe heterojunction solar cells

Polycrystalline Cd_1−xZn_xTe solar cells with efficiency of 8.3% were grown by cathodic electrodeposition on glass/ITO/CdS substrates using non-aqueous ethylene glycol bath. The deposit is characterised versus the process conditions by XRD and found to possess a preferred (1 1 1) orientation on Sb doping in the electroplating bath. The surface morphology of the deposit is studied using atomic force microscope. The average RMS roughness for the ternary film was higher than that for the binary CdTe. Optical properties of the films were carried out to study the band gap and calculation of molar concentration ‘x’. The effects of Sb doping in CdS/Cd_1−xZn_xTe heterojunctions have been studied. The short circuit current density (c) was found to improve and series resistance (R_s) reduced drastically upon Sb doping. This improvement in J_sc is attributed to an increase in quantum efficiency. The evaluation of solar cell parameters was also carried out using the current–voltage characteristics in dark and illumination. The best results were obtained when 2×10⁻³ M ZnCl₂ along with antimony were present in the deposition bath. Under AM 1.5 conditions the open circuit voltage, short circuit current density, and fill factor of our best cell were V_oc=600 mV, J_sc=26.66 mA/cm², FF=0.42 and efficiency, η=8.3%. The carrier concentration and built-in potential of Cd_1−xZn_xTe calculated from Mott–Schottky plot was 2.72×10¹⁷ cm⁻³ and 1.02 eV.     

Metastable volume changes of hydrogenated amorphous silicon and silicon–germanium alloys produced by exposure to light

Exposure of hydrogenated amorphous silicon, a-Si:H, to light produces large-scale structural changes and increases the density of dangling Si bond defects acting as efficient carrier recombination centers. The latter is the well-studied Staebler–Wronski effect (SWE). All light-induced changes are metastable and disappear after annealing to approximately 200°C. This review focuses on one of the large-scale changes, namely that of the macroscopic density of the material. In all device quality materials, the initial stress is compressive with values typically in the range of 10⁸–10⁹ Pa. Exposure to light produces additional compressive stress, which can exceed 2×10⁷ Pa. The observed change of stress is due to a change of the volume of the unsupported material and not of its elastic modulus. The relative volume change, ΔV/V, at 300 K becomes detectable at values in excess of about 10⁻⁶ after only a few photons per Si atom have been absorbed. ΔV/V saturates above 10⁻³, under high-intensity light after an average of more than 10⁶ photons per Si atom have been absorbed. ΔV/V initially grows with t^0.50±0.04 under CW illumination producing carrier generation rate G in the range of 10²¹ to a few 10²³ cm⁻³ s⁻¹. The approach to saturation is well fitted by a stretched exponential function with stretch exponent close to 0.5. ΔV/V is approximately proportional to G. The fastest and largest photo-expansion has been observed in the so-called “edge material” between the amorphous and microcrystalline state, produced by plasma enhanced CVD from increasingly diluted silane/hydrogen gas mixtures. The quantum efficiency of volume expansion has been observed to increase with the photon energy of the light in contrast to the SWE. No volume increase is observed in Ge rich a-Si_1−xGe_x:H alloys and in hydrogenated microcrystalline material. Photo-expansion and the SWE show marked difference in spatial extend in the network, different evolution in time and different wavelength dependence. Hence, the two effects appear to be independent even though both involve hydrogen.