Structural and photoluminescence study of the quaternary alloys system CuIn(SxSe1−x)2

The full composition range CuIn(S_xSe_1−x)₂ alloy system has been studied using 40 mm length crystal cuts from 10 mm diameter ingots grown by the classical Bridgman method. X-ray diffraction diffractographs show that the CuIn(S_xSe_1−x)₂ compounds have a chalcopyrite structure for each composition x, they exhibit an expansion on the unit cell characteristics by the tetragonal distortion which depends linearly on the electronegativity of the atoms. The photoluminescence spectra is investigated as a function of various compositions, temperature and excitation intensities. Photoluminescence spectra shows a wide variation in the dominant peak location and an overall blue shift with the increase of sulphur content. Photoluminescence CuInS₂ and CuIn(S_0.72Se_0.28)₂ have been studied in detail.     

Synthesis and transport properties of prospective photovoltaic systems related to CuInSe2 and CuGaSe2

Solid solutions in CuGaSe₂–ZnSe and CuInSe₂–ZnSe systems have been obtained by radio frequency heating. In order to prepare n-type phases based on CuGaSe₂, p-type (CuGa)_1−xZn_2xSe₄ and (CuIn)_1−xZn_2xSe₄ (0.05x0.1) single crystals were doped by Ag, Hg, Cd, Zn implantation. The crystal structure of the solid solutions was studied by X-ray diffraction; the substitutors as well as the implantant valence states were analyzed using X-ray photoelectron spectroscopy. Hall effect, electrical conductivity, and the charge carrier mobility of an n-type zinc-implantated solid solution (CuGa)_1−xZn_2xSe₄ and (CuIn)_1−xZn_2xSe₄ (0.05x0.1) were studied.     

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.     

WO3−x nanowires based electrochromic devices

A simple method was developed to fabricate tungsten oxide (WO_3−x) nanowires based electrochromic devices. The WO_3−x nanowires are grown directly from tungsten oxide powders in a tube furnace. The WO_3−x nanowires have diameters ranging from 30 to 70 nm and lengths up to several micrometers. The WO_3−x nanowires based device has short bleach-coloration transition time and can be grown on a large scale directly onto an ITO-coated glass that makes it potential in many electrochromic applications. The structure, morphology, and composition of the WO_3−x nanowires were characterized using the scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive spectrometer. The optical and electrochromic performance of the nanowires layer under lithium intercalation was studied in detail by UV–VIS–NIR spectroscope and cyclic voltameter.     

Pyrite (FeS2) thin films prepared by spray method using FeSO4 and (NH4)2Sx

A simple spray method for the preparation of pyrite (FeS₂) thin films has been studied using FeSO₄ and (NH₄)₂S_x as precursors for Fe and S, respectively. Aqueous solutions of these precursors are sprayed alternately onto a substrate heated up to 120°C. Although Fe–S compounds including pyrite are formed on the substrate by the spraying, sulfurization of deposited films is needed to convert other phases such as FeS or marcasite into pyrite. A single-phase pyrite film is obtained after the sulfurization in a H₂S atmosphere at around 500°C for 30 min. All pyrite films prepared show p-type conduction. They have a carrier concentration (p) in the range 10¹⁶–10²⁰ cm⁻³ and a Hall mobility (μ_H) in the range 200–1 cm²/V s. The best electrical properties (p=7×10¹⁶ cm⁻³, μ_H=210 cm²/V s) for a pyrite film prepared here show the excellence of this method. The use of a lower concentration FeSO₄ solution is found to enhance grain growth of pyrite crystals and also to improve electrical properties of pyrite films.     

Optical and electrical studies on molybdenum sulphoselenide [Mo(S1−xSex)2] thin films prepared by arrested precipitation technique (APT)

Nanocrystalline stoichiometric [Mo(S_1−xSe_x)₂] thin films were deposited by using arrested precipitation technique (APT) developed in our laboratory. The precursors used for this are namely, molybdenum triethanolamine complex, thioacetamide and sodium selenosulphite; and various preparative conditions are finalised at the initial stages of deposition. Formation of [Mo(S_1−xSe_x)₂] semiconducting thin films are confirmed by studying growth mechanism, optical and electrical properties. X-ray diffraction analysis showed that the composites are nanocrystalline being mixed ternary chalcogenides of the general formula [Mo(S_1−xSe_x)₂]. The optical studies revealed that the films are highly absorptive (α×10⁴ cm⁻¹) with a band-to-band direct type of transitions and the energy gap decreased typically from 1.86 eV for pure MoS₂ down to 1.42 eV for MoSe₂. The thermoelectrical power measurement shows negative polarity for the generated voltage across the two ends of semiconductor thin films. This indicate that the [Mo(S_1−xSe_x)₂] thin film samples show n-type conduction.     

Sputtered Cd1−xZnxTe films for top junctions in tandem solar cells

Cd_1−xZn_xTe alloy films with 1.6 and 1.7 eV band gaps were deposited by RF magnetron sputtering from targets made either of mixed powders or alloys of CdTe and ZnTe (25% and 40%). High-quality polycrystalline films with the (1 1 1) preferred orientation were obtained. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy, resistivity, optical absorption, Raman, and photoluminescence. The EDS, XRD, and optical absorption analysis indicated that the x-value of the as-grown films were typically 0.20 and 0.30 for films sputtered from 25% and 40% ZnTe containing targets, respectively. The as-deposited alloy films exhibit quite low photovoltaic performance when used to make cells with CdS as the hetero-junction partner. Therefore, we have studied various post-deposition treatments with vapors of chlorine-containing materials, CdCl₂ and ZnCl₂, in dry air or H₂/Ar ambient at 390 °C. The best performance of a Cd_1−xZn_xTe cell (, ) was found for treatment with vapors of the mixed CdCl₂+0.5%ZnCl₂ in an H₂/Ar ambient after pre-annealing at 520 °C in pure H₂/Ar.     

Growth of Zn1−xMgxO films with single wurtzite structure by MOCVD process and their application to Cu(InGa)(SSe)2 solar cells

The effect of the growth temperature and Mg/(Mg+Zn) molar flow rate ratio of metal organic sources on the crystalline structure of Zn_1−xMg_xO (ZMO) films is investigated in thin films prepared by metal organic chemical vapor deposition (MOCVD) process on fused silica in order to obtain the wide-bandgap ZMO films with single wurtzite structure, which is very important to achieve high-efficiency chalcopyrite solar cells. Based on the measurements and analysis of the fabricated samples, the ZMO films with the controllable bandgap from 3.3 to 3.72 eV can exhibit a single wurtzite phase depending on the growth temperature and Mg content. Furthermore, the resistivity of ZMO films is comparable to that of ZnO film. It is a good indication that ZMO film is superior to CdS or ZnO films as buffer and window layers mainly due to its controllable bandgap energy and safety. As a result, the solar cells with ZMO buffer were fabricated without any surface treatment of Cu(InGa)(SSe)₂ (CIGSSe) absorber or antireflection coating, and the efficiency of 10.24% was obtained.     

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.     

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.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

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.     

Cu(In,Ga)Se2 based photovoltaic structure by electrodeposition and processing

CuIn_1−xGa_xSe₂ (CIGS) thin films were formed from an electrodeposited CuInSe₂ (CIS) precursor by thermal processing in vacuum in which the film stoichiometry was adjusted by adding In, Ga and Se. The structure, composition, morphology and opto-electronic properties of the as-deposited and selenized CIS precursors were characterized by various techniques. A 9.8% CIGS based thin film solar cell was developed using the electrodeposited and processed film. The cell structure consisted of Mo/CIGS/CdS/ZnO/MgF₂. The cell parameters such as J_sc, V_oc, FF and η were determined from I–V characterization of the cell.     

CuInxGa1−xSe2 thin films prepared by electron beam evaporation

CuIn_xGa_1−xSe₂ bulk compound of three different compositions x=0.75, 0.80 and 0.85 have been prepared using individual elements of copper, indium, gallium and selenium. Thin films of CuIn_xGa_1−xSe₂ have been deposited using the prepared bulk by electron beam evaporation method. The structural studies carried on the deposited films revealed that films annealed at 400 °C are crystalline in nature exhibiting chalcopyrite phase. The position of the (1 1 2) peak in the X-ray diffractogram corresponding to the chalcopyrite phase has been found to be dependent on the percentage of gallium in the films. The composition of the prepared bulk and thin films has been identified using energy dispersive X-ray analysis. The photoluminescence spectra of the CuIn_xGa_1−xSe₂ films exhibited sharp luminescence peaks corresponding to the band gap of the material.     

Temperature dependence of Cu2ZnSn(SexS1−x)4 monograin solar cells

The temperature dependence of open-circuit voltage (V_oc), short-circuit current (I_sc), fill factor (FF), and relative efficiency of monograin Cu₂ZnSn(Se_xS_1−x)₄ solar cell was measured. The light intensity was varied from 2.2 to 100 mW/cm² and temperatures were in the range of T = 175-300 K. With a light intensity of 100 mW/cm²dV_oc/dT was determined to be −1.91 mV/K and the dominating recombination process at temperatures close to room temperature was found to be related to the recombination in the space-charge region. The solar cell relative efficiency decreases with temperature by 0.013%/K. Our results show that the diode ideality factor n does not show remarkable temperature dependence and slightly increases from n = 1.85 to n = 2.05 in the temperature range between 175 and 300 K.     

Fabrication of CuInSe2 films and solar cells by the sequential evaporation of In2Se3 and Cu2Se binary compounds

Cu₂Se/In_xSe(x≈1) double layers were prepared by sequentially evaporating In₂Se₃ and Cu₂Se binary compounds at room temperature on glass or Mo-coated glass substrates and CuInSe₂ films were formed by annealing them in a Se atmosphere at 550°C in the same vacuum chamber. The In_xSe thickness was fixed at 1 μm and the Cu₂Se thickness was varied from 0.2 to 0.5 μm. The CuInSe₂ films were single phase and the compositions were Cu-rich when the Cu₂Se thickness was above 0.35 μm. And then, a thin CuIn₃Se₅ layer was formed on the top of the CuInSe₂ film by co-evaporating In₂Se₃ and Se at 550°C. When the thickness of CuIn₃Se₅ layer was about 150 nm, the CuInSe₂ cell showed the active area efficiency of 5.4% with V_oc=286 mV, J_sc=36 mA/cm² and FF=0.52. As the CuIn₃Se₅ thickness increased further, the efficiency decreased.     

Photoconductivity of Cu(In, Ga) Se2 films

Polycrystalline CuIn_{1 − x}Ga_xSe₂ (0 ≤ x < 0.3) films (CIGS) were deposited by coevaporating the elements from appropriate sources onto glass substrates (substrate temperature 720 to 820 K). Photoconductivity of the polycrystalline CIGS films with partially depleted grains were studied in the temperature range 130–285 K at various illumination levels (0–100 mW/cm²). The data at low temperature (T < 170 K) were analyzed by the grain boundary trapping model with monovalent trapping states. The grain boundary barrier height in the dark and under illumination were obtained for different x-values of CuIn_1−xGa_xSe₂ films. Addition of Ga in the polycrystalline films resulted in a significant decrease in the barrier height. Variation of the barrier height with incident intensity indicated a complex recombination mechanism to be effective in the CIGS films.     

Electrical conduction studies of hot wall deposited CdSexTe1−x thin films

CdSe_xTe_1−x thin films of different compositions have been deposited on cleaned glass substrates using the hot wall deposition technique under conditions very close to thermodynamical equilibrium with minimum loss of material. The electrical conductivity of the deposited films has been studied as a function of temperature. All the films showed a transition from phonon-assisted hopping conduction through the impurity band to grain-boundary-limited conduction in the conduction/valence band at temperature around 325 K. The conductivity has been found to vary with composition; it varied from 0.0027 to 0.0198 Ω⁻¹ cm⁻¹ when x changed from 0 to 1. The activation energies of the films of different compositions determined at 225 and 400 K have been observed to lie in the range 0.0031–0.0098 and 0.0285–0.0750 eV, respectively. The Hall-effect studies carried out on the deposited films revealed that the nature of conductivity (p or n-type) was dependent on film composition; films with composition x=0 and 0.15 have been found to be p-type and the ones with composition x=0.4, 0.6, 0.7, 0.85 and 1 have been observed to exhibit n-type conductivity. The carrier concentration has been determined and is of the order of 10¹⁷ cm⁻³. The majority of carrier mobilities of the films have been observed to vary from 0.032 to 0.183 cm² V⁻¹ s⁻¹ depending on film composition. The study of the mobility of the charge carriers with temperature in the range of 300–450 K showed that the mobility increased with power of temperature indicating that the type of scattering mechanism in the studied temperature range is the ionized impurity scattering mechanism.     

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.     

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se₂ (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn_1−xMg_xO film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn_1−xMg_xO/CdS/CIGS/Mo/glass structure with various CIGS band gaps (E_g≈0.97–1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.