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.     

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.     

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.     

Analysis of heterointerface recombination by Zn1−xMgxO for window layer of Cu(In,Ga)Se2 solar cells

An adjustment of a conduction band offset (CBO) of a window/absorber heterointerface is important for high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. In this study, the heterointerface recombination was characterized by the reduction of the thickness of a CdS layer and the adjustment of a CBO value by a Zn_1−xMg_xO (ZMO) layer. In ZnO/CdS/CIGS solar cells, open-circuit voltage (V_oc) and shunt resistance (R_sh) decreased with reducing the CdS thickness. In constant, significant reductions of V_oc and R_sh were not observed in ZMO/CdS/CIGS solar cells. With decreasing the CdS thickness, the CBO of (ZnO or ZMO)/CIGS become dominant for recombination. Also, the dominant mechanisms of recombination of the CIGS solar cells are discussed by the estimation of an activation energy obtained from temperature-dependent current-voltage measurements.     

New trends to grow the n-CdZn (S1−xSex)2/p-CuIn(S1−xSex)2 heterojunction thin films for solar cell applications

The n-CdZn(S_1−xSe_x) and p-CuIn(S_1−xSe_x)₂ thin films have been grown by the solution growth technique (SGT) on glass substrates. Also the heterojunction (p–n) based on n-CdZn (S_1−xSe_x)₂ and p-CuIn (S_1−xSe_x)₂ thin films fabricated by same technique. The n-CdZn(S_1−xSe_x)₂ thin film has been used as a window material which reduced the lattice mismatch problem at the junction with CuIn (S_1−xSe_x)₂ thin film as an absorber layer for stable solar cell preparation. Elemental analysis of the n-CdZn (S_1−xSe_x)₂ and p-CuIn(S_1−xSe_x)₂ thin films was confirmed by energy-dispersive analysis of X-ray (EDAX). The structural and optical properties were changed with respect to composition ‘x’ values. The best results of these parameters were obtained at x=0.5 composition. The uniform morphology of each film as well as the continuous smooth thickness deposition onto the glass substrates was confirmed by SEM study. The optical band gaps were determined from transmittance spectra in the range of 350–1000 nm. These values are 1.22 and 2.39 eV for CuIn(S_0.5Se_0.5)₂ and CdZn(S_0.5Se_0.5)₂ thin films, respectively. J–V characteristic was measured for the n-CdZn(S_1−xSe_x)₂/p-CuIn(S_1−xSe_x)₂ heterojunction thin films under light illumination. The device parameters V_oc=474.4 mV, J_sc=13.21 mA/cm², FF=47.8% and η=3.5% under an illumination of 85 mW/cm² on a cell active area of 1 cm² have been calculated for solar cell fabrication. The J–V characteristic of the device under dark condition was also studied and the ideality factor was calculated which is equal to 1.9 for n-CdZn(S_0.5Se_0.5)₂/p-CuIn(S_0.5Se_0.5)₂ heterojunction thin films.     

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.     

Highly transparent and conductive Zn0.85Mg0.15O:Al thin films prepared by pulsed laser deposition

Zn_1−xMg_xO:Al thin films have been prepared on glass substrates by pulsed laser deposition (PLD). The effect of substrate temperature has been investigated from room temperature to 500 °C by analyzing the structural, optical and electrical properties. The best sample deposited at 250 °C shows the lowest room-temperature resistivity of 5.16×10⁻⁴ Ω cm, and optical transmittance higher than 80% in the visible region. It is observed that the optical band gap decreases from 3.92 to 3.68 eV when the substrate temperature increases from 100 to 500 °C. The probable mechanism is discussed.     

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.     

Annealing effect on electrochromic properties of tungsten oxide nanowires

The effect of thermal annealing on the electrochromic properties of the tungsten oxide (WO_3−x) nanowires deposited on a transparent conducting substrate by vapor evaporation was investigated. The X-ray diffraction (XRD) indicated that the structures of the nanowries annealed below 500 °C had no significant change. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the O/W ratio and the amount of W⁶⁺ ions in the annealed nanowire films could be increased as increasing annealing temperature. Increased annealing temperature could promote the coloration efficiency and contrast of the nanowire films; however, it could also affect the switching speed of the nanowire films.     

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.     

Stable, easily sintered BaCe0.5Zr0.3Y0.16Zn0.04O3−δ electrolyte-based protonic ceramic membrane fuel cells with Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite cathode

A stable, easily sintered perovskite oxide BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3−δ (BCZYZn) as an electrolyte for protonic ceramic membrane fuel cells (PCMFCs) with Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) perovskite cathode was investigated. The BCZYZn perovskite electrolyte synthesized by a modified Pechini method exhibited higher sinterability and reached 97.4% relative density at 1200 °C for 5 h in air, which is about 200 °C lower than that without Zn dopant. By fabricating thin membrane BCZYZn electrolyte (about 30 μm in thickness) on NiO–BCZYZn anode support, PCMFCs were assembled and tested by selecting stable BSZF perovskite cathode. An open-circuit potential of 1.00 V, a maximum power density of 236 mW cm⁻², and a low polarization resistance of the electrodes of 0.17 Ω cm² were achieved at 700 °C. This investigation indicated that proton conducting electrolyte BCZYZn with BSZF perovskite cathode is a promising material system for the next generation solid oxide fuel cells.     

ZnO-doped BaZr0.85Y0.15O3−δ proton-conducting electrolytes: Characterization and fabrication of thin films

ZnO-doped BaZr_0.85Y_0.15O_3−δ perovskite oxide sintered at 1500 °C has bulk conductivity of the order of 10⁻² S cm⁻¹ above 650 °C, which makes it an attractive proton-conducting electrolyte for intermediate-temperature solid oxide fuel cells. The structure, morphology and electrical conductivity of the electrolyte vary with sintering temperature. Optimal electrochemical performance is achieved when the sintering temperature is about 1500 °C. Cathode-supported electrolyte assemblies were prepared using spin coating technique. Thin film electrolytes were shown to be dense using SEM and EDX analyses.     

Structural and optical properties of MgyNi1–yHx gradient thin films in relation to the as-deposited metallic state

Thin Mg_yNi_1−yH_x films with a gradient in chemical composition are investigated by optical spectrophotometry, dc resistivity and X-ray diffraction measurements before and after exposure to hydrogen. In the metallic state crystalline Mg₂Ni is present for 0.6 ≤ y ≤ 0.8 and coexists with amorphous Mg and/or Ni. The hydride state is mainly characterized by the presence of Mg₂NiH₄ around the stoichiometric [Mg]/[Ni] = 2 composition and some MgH₂ on the Mg-rich side. The abrupt microstructural changes found in the as-deposited metallic state around the Mg–Mg₂Ni eutectic point correlate well with the compositional dependence of the optical properties in the hydride state. We conclude that the formation of the hydride depends directly on the detailed nature of the metallic parent phase. Furthermore, we demonstrate that high-throughput compositional screening via fiber-optic spectrophotometry is useful for hydride identification. When no structural long-range order is present, this provides a new tool for the search for hydrogen storage materials.     

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 Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).     

Field effect surface passivation of SiO2/Si interfaces by heat treatment with high-pressure H2O vapor

We investigated a simple field effect passivation of the silicon surfaces using the high-pressure H₂O vapor heating. Heat treatment with 2.1×10⁶ Pa H₂O vapor at 260°C for 3 h reduced the surface recombination velocity from 405 cm/s (before the heat treatment) to 38 cm/s for the thermally evaporated SiO_x film/Si. Additional deposition of 140 nm-SiO_x films (x<2) with a high density of fixed positive charges on the SiO₂/Si samples further decreased the surface recombination velocity to 22 cm/s. We also demonstrated the field effect passivation for n-type silicon wafer coated with thermally grown SiO₂. Additional deposition of 210 nm SiO_x films on both the front and rear surfaces increased the effective lifetime from 1.4 to 4.6 ms. Combination of thermal evaporation of SiO_x film and the heat treatment with high-pressure H₂O vapor is effective for low-temperature passivation of the silicon surface.     

Nearly carbon-free printable CIGS thin films for solar cell applications

In order to fabricate low cost and printable CuIn_xGa_1−xSe_yS_2−y (CIGS) thin film solar cells, a precursor solution based method was developed. Particularly, in this method, nearly carbon-free CIGS film was obtained by applying a three-step heat treatment process: the first for the elimination of carbon residue by air annealing, the second for the formation of CIGS alloy by sulfurization, and the third for grain growth and densification in the CIGS film by selenization. The film also revealed very large grains with a low degree of porosity, similar to those produced by the vacuum based method. A solar cell device with this film showed current-voltage characteristics of J_sc=21.02 mA/cm², V_oc=451 mV, FF=47.3%, and η=4.48% at standard conditions.     

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.     

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.