Effect of substrate temperature on properties of Cu(In,Ga, Al)Se<Subscript>2</Subscript> films grown by magnetron sputtering

Cu(In, Ga, Al)Se₂ (CIGAS) thin films were deposited by magnetron sputtering on Si(100) and soda-lime glass substrates at different substrate temperatures, followed by post-deposition annealing at 350 or 520 °C for 5 h in vacuum. Electron probe micro-analysis and secondary ion mass spectroscopy were used to determine the composition of the films and the distribution of Al across the film thickness, respectively. X-ray diffraction analysis showed that the (112) peak of CIGAS films shifts to higher 2θ values with increasing substrate temperature but remains unchanged when the films were annealed at 520 °C for 5 h. Scanning electron microscopy and atomic force microscopy images revealed dense and well-defined grains for both as-deposited and annealed films. However, notable increase in grain size and roughness was observed for films deposited at 500 °C. The bandgap of CIGAS films was determined from the optical transmittance and reflectance spectra and was found to increase as the substrate temperature was increased.     

One-step RF magnetron sputtering method for preparing Cu(In,Ga)Se<Subscript>2</Subscript> solar cells

Cu(In, Ga)Se₂ (CIGS) solar cell is one of the most promising thin film solar cells. However the marketization of the CIGS solar cells is hindered by the uncertainty of the element ratios. Traditional sputtering with post selenization is one of the most widespread methods to produce the CIGS solar cells. Nevertheless, the post selenization process is the most difficult part of this technique, which could lead to element mismatch and heterogeneous. To simplify the preparing process, Cu(In, Ga)Se₂ (CIGS) solar cells were prepared without post-selenization process by RF sputtering CIGS target with abundant Se element. We focus on the effect of working pressure, substrate temperature and sputtering power on the properties of CIGS solar cells. When CIGS thin film was deposited at 580 °C, 0.8 Pa working pressure and 160 W sputtering power, the solar cell showed the highest power conversion efficiency (PCE) of 5.77%, which is only 0.64% lower than that of the solar cell prepared by traditional sputtering with post selenization method, and the two kinds of solar cells have same structure without MgF₂ antireflection layer, but the one-step sputtering method could greatly simplify the manufacture process of the CIGS solar cells. Our work makes clear that element Se would run off almost half during the sputtering process. And the element atomic ratios and the photovoltaic properties could be controlled by changing the sputtering parameters.     

Components distribution in Cu(In,Ga)Se<Subscript>2</Subscript> films prepared by selenization of evaporated metallic precursors on bare and ITO-coated glass substrates

Cu(In,Ga)Se₂ thin films have been prepared by selenization of the evaporated metallic precursors on transparent and conductive ITO-coated as well as uncoated glass substrates, by starting from slightly Cu-rich or Cu-poor overall metallic proportions. The objective is to determine the influence of the Cu availability on the constituents distribution achieved after selenization, by means of data obtained at several film depths by grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy that have been related to the overall optical and structural characteristics of the material. This study points out the possibility of achieving homogeneous or graded absorber layers, showing for homogeneous samples an ITO/Cu(In,Ga)Se₂ interface with ohmic electrical characteristics suitable to act as back contact for semitransparent photovoltaic devices.     

Photosensitivity of thin-film structures based on CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> and CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> ternary semiconductor compounds

Polycrystalline n-type CuIn₃Se₅ and CuIn₅Se₈ films with thicknesses from 0.4 to 1 μm have been grown by pulsed laser ablation of bulk p-CuIn₃Se₅ and n-CuIn₅Se₈ crystals in vacuum. The temperature dependences of the resistivities of these crystals are determined by deep donor levels with energies E _D ? 0.2–0.3 eV. Photosensitive thin-film structures based on these films have been created for the first time and their photosensitivity spectra have been measured. The possibility of using thin CuIn₃Se₅ and CuIn₅Se₈ films in broadband photoconverters is demonstrated.     

Optimization of Zn(O,S)/(Zn,Mg)O buffer layer in Cu(In,Ga)Se<Subscript>2</Subscript> based photovoltaic cells

Device modeling of CIGS based thin film solar cell with Zn(O,S)/(Zn,Mg)O buffer layer was simulated in order to find the optimum ratios of magnesium in (Zn_1?x,Mg_x)O and oxygen in Zn(O_y,S_1?y) which led to the optimized values of x = 0.1?0.25 and y = 0.5?0.6. When the oxygen content of Zn(O,S) was lower than 30 %, the recombination at Zn(O,S)/CIGS interface became prominent and J_SC was severely limited. It was found that the V_OC is approximately independent of magnesium content in (Zn,Mg)O and oxygen content in Zn(O,S) layers, and the efficiency is highly affected by the fill factor. Also studied were the effect of thicknesses of (Zn,Mg)O and Zn(O,S) layers while the x and y were set at x = 0.2 and y = 0.6. Our simulations show that the optimum range for thickness of the (Zn,Mg)O layer is from 70 to 100 nm, while it is 20–30 nm for the Zn(O,S) layer.     

Effect of electron irradiation on the photopleochroism of ZnO/CdS/Cu(In,Ga)Se<Subscript>2</Subscript> solar cells

The effect of irradiation with 1-MeV electrons to various doses on the photosensitivity of ZnO/CdS/Cu(In, Ga)Se₂ solar cells and related CdS/Cu(In,Ga)Se₂ and ZnO/Cu(In,Ga)Se₂ heterostructures has been studied. Both the photoconversion efficiency and the coefficient of induced photopleochroism of ZnO/CdS/CIGS solar cells remained practically unchanged upon irradiation up to a total dose of 10⁻¹⁷ cm⁻². It is suggested that the method of polarization photoelectric spectroscopy can be used for evaluating the effect of electron irradiation on the photosensitivity of semiconductor photoconverters.     

X-ray diffraction study of the Cu<Subscript>2</Subscript>Se-In<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Cr<Subscript>2</Subscript>Se<Subscript>3</Subscript> system near CuInCr<Subscript>2</Subscript>Se<Subscript>5</Subscript>

The structure of three compounds in the Cu₂Se-In₂Se₃-Cr₂Se₃ system near CuInCr₂Se₅ is determined by single-crystal x-ray diffraction: CuInCr₄Se₈ (I), Cu₂In₂Se₄ (II), and Cu_0.5In_0.5Se (III). I has a cubic (spinel type) structure: a = 10.606(4) Å, Z = 4, sp. gr. F43m. II has a pseudotetragonal (sphalerite type) structure: a = 5.774(2) Å, c = 11.617(6) Å. The structure of II was solved in a reduced unit cell with a = 5.774(2) Å, b = 5.774(2) Å, c = 7.095(6) Å, = 113.95(5)°, = 113.95(5)°, = 90.00(4)°, Z = 1, sp. gr. P1. III has a triclinic cell (disordered structure of II): a = 4.088(1) Å, b = 4.091(2) Å, c = 4.101(1) Å, = 60.05(1)°, = 60.08(1)°, = 89.98(4)°, Z = 1, sp. gr. P1. The Cu and In atoms in I sit in inequivalent tetrahedral sites, and the Cr atoms reside in octahedral interstices of the close packing of Se atoms. The bond lengths are In–Se = 2.538(6), Cr(1)–Se(1) = 2.514(7), Cr(1)–Se(2) = 2.576(8), and Cu–Se = 2.437(5) Å. In II, all of the atoms sit in tetrahedral sites; the mean bond lengths are In–Se = 2.578(6) and Cu–Se = 2.44(1) Å. In III, the Cu and In atoms are fully disordered in the same tetrahedral site; the mean Cu(In)–Se bond length is 2.508(6) Å.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1435–1439.Original Russian Text Copyright © 2004 by Antsyshkina, Sadikov, Koneshova, Sergienko.     

Photoelectrochemical applications of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films by chemical deposition

Indium selenide films have been synthesized by chemical bath deposition method onto stainless steel plate. The configuration of fabricated cell is n-In₂Se₃| NaOH(1 M) + S(1 M) + Na₂S(1 M) |C_(graphite). Characterization of the photoelectrochemical cell was carried out by studying X-ray diffraction, current–voltage and capacitance–voltage characteristics in the dark, barrier height measurements, power output, photoresponse and spectral response. The study shows that the In₂Se₃ thin films are n-type semiconductor. The junction ideality factor was found to be 3.24. The flat band potential and the barrier height were found to be 0.720 V and 0.196 eV, respectively. From the study of power output characteristics, open circuit voltage, short circuit current, fill factor and efficiency were found to be 310 mV, 20 μA, 37.64 and 0.61%, respectively. Photoresponse studies show that the lighted ideality factor is 2.78. Maximum current was observed at 575 nm.     

Phase equilibria in the Ag<Subscript>2</Subscript>Se-As<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

We have studied phase equilibria in the pseudoternary system Ag₂Se-As₂Se₃-Bi₂Se₃ and constructed the 300-, 600-, and 800-K isothermal sections, a number of partial phase diagrams, and the liquidus projection of this system. The AgAsSe₂-AgBiSe₂ and As₂Se₃-AgBiSe₂ joins are shown to be pseudobinary, and the Ag₃AsSe₃-AgBiSe₂ and AgAs₃Se₅-AgAsSe₂ joins are pseudobinary below the liquidus. Several in- and univariant peritectic, eutectic, and eutectoid equilibria and a broad region of AgBiSe₂-based solid solutions are identified. The homogeneity region of the AgBiSe₂-based phase has the largest extent along the AgAsSe₂-AgBiSe₂ join: 40 mol % (650 K) for the high-temperature form of AgBiSe₂ and 20 mol % (300 K) for its low-temperature form.     

Properties of AgInSe<Subscript>2</Subscript> films grown by magnetron sputtering

n-Type AgInSe₂ films 0.5 to 0.9 μm thick were grown by dc magnetron sputtering. As targets, we used AgInSe₂ crystals grown by a modified Bridgman process using high-purity precursors. The crystal structure, morphology, electrical conductivity, and Hall coefficient of the films were studied at various temperatures. We determined the optimal growth and annealing temperatures of the films (500 and 250°C, respectively). Using structures based on the films, we obtained the spectral dependences of their photoresponse, established the nature of interband transitions in the films, and evaluated their band gap. The ability to vary electrical and optical properties with no changes in stoichiometry is of interest for concentrated solar power applications.     

Extruded thermoelectric materials based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> solid solutions

Extruded n-type materials based on Bi₂Te₃-Bi₂Se₃ alloys containing 6 to 40 mol % Bi₂Se₃ have been investigated using microstructural analysis and thermoelectric measurements at room temperature and in the range 100–400 K. Their electrical properties have been compared to those of single-crystal analogs. Compositions have been found at which the extruded materials offer the highest thermoelectric performance in different temperature ranges.     

Ginzburg-Landau Analysis on the Superconductivity in TlNi<Subscript>2</Subscript>Se<Subscript>2</Subscript>

Based on a two-band isotropic Ginzburg-Landau theory, we study the magnetic properties of the recently observed superconducting crystal TlNi₂Se₂. Our exact solution of upper critical field reproduces the experimental data in a broad temperature range. It directly underlies the multi-gap superconductivity in this crystal. We also show that the effective mass of the electron for one band is about 20m _e while that of the other band is only 0.6m _e. This semi-heavy-fermion feature can qualitatively explain the experimental value of the Kadowaki-Woods ratio in TlNi₂Se₂.     

Structural,optical and microscopic properties of chemically deposited In<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films

Solar cell technologically important binary indium selenide thin film has been developed by relatively simple chemical method. The reaction between indium chloride, tartaric acid, hydrazine hydrate and sodium selenosulphate in an aqueous alkaline medium at room temperature gives deposits In₂Se₃ thin film. Various preparative parameters are discussed. The as grown films were found to be transparent, uniform, well adherent, red in color. The prepared films were studied using X-ray diffraction, scanning electron microscopy, atomic absorption spectroscopy, Energy dispersive atomic X-ray diffraction, optical absorption and electrical conductivity properties. The direct optical band gap value E_g for the films was found to be as the order of 2.35 eV at room temperature and having specific electrical conductivity of the order of 10⁻² (Ω cm)⁻¹ showing n-type conduction mechanism. The utility of the adapted technique is discussed from the point of view of applications considering the optoelectric and structural data obtained.     

New forms of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> and Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> prepared by reacting InAs and GaAs substrates with selenium

Ga₂Se₃ and In₂Se₃ prepared through heterovalent substitution during thermal annealing of single-crystal gallium arsenide and indium arsenide substrates in selenium vapor in a quasi-closed system have been characterized by electron diffraction, scanning electron microscopy, and X-ray microanalysis. Cubic phases of In₂Se₃ (a ₀ = 1.1243 nm and a ₀ = 1.6890 nm) and Ga₂Se₃ (a ₀ = 1.0893 nm and a ₀ = 1.6293 nm) have been identified for the first time.     

Structural,optical and microscopic properties of chemically deposited Mo<Subscript>0.5</Subscript>W<Subscript>0.5</Subscript>Se<Subscript>2</Subscript> thin films

Mo_0.5W_0.5Se₂ thin films were obtained by using relative simple chemical route at room temperature. Various preparative conditions of the thin films are outlined. The films were characterized by X-ray diffraction, scanning electron microscope, optical and electrical properties. The grown films were found to be uniform, well adherent to substrate and brown in color. The X-ray diffraction pattern shows that thin films have a hexagonal phase. Optical properties show a direct band gap nature with band gap energy 1.44 eV and having specific electrical conductivity in the order of 10⁻⁵ (Ωcm)⁻¹.     

Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films

Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.     

Solid solutions in the Ag<Subscript>2</Subscript>Se-PbSe-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

The phase equilibria in the pseudoternary system Ag₂Se-PbSe-Bi₂Se₃ have been studied using differential thermal analysis, x-ray diffraction, and microhardness measurements. The results have been used to construct the T-x phase diagram along the AgBiSe₂-PbSe join and the 900-K section of the ternary phase diagram. The AgBiSe₂-PbSe system contains a continuous series of cubic solid solutions with the NaCl structure. Their lattice parameter is an almost linear function of composition (a = 5.822–6.125 Å). The formation of the solid solutions stabilizes the high-temperature phase of AgBiSe₂: PbSe dissolution in this compound markedly reduces its polymorphic transformation temperature (590 K), down to room temperature at ? 10 mol % PbSe. In the Ag₂Se-PbSe-Bi₂Se₃ system, the γ-phase exists in a broad region around the AgBiSe₂-PbSe pseudobinary join.     

Phase equilibria in the systems AgInSe<Subscript>2</Subscript>-HgIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> and AgInSe<Subscript>2</Subscript>-HgSe

The equilibrium phase diagrams along the AgInSe₂-HgIn₂Se₄ and AgInSe₂-HgSe joins of the ternary system Ag₂Se-HgSe-In₂Se₃ have been constructed using X-ray diffraction and differential thermal analysis. Both joins are pseudobinary, with eutectic phase diagrams (type V in Roseboom’s classification). The eutectics are located at ≃30 mol % HgIn₂Se₄ (melting point of 1000 K) and ≃54 mol % HgSe (993 K), respectively. Both systems have considerable terminal solid-solution ranges.     

Energy-driven surface evolution in beta-MnO<Subscript>2</Subscript> structures

Exposed crystal facets directly affect the electrochemical/catalytic performance of MnO₂ materials during their applications in supercapacitors, rechargeable batteries, and fuel cells. Currently, the facet-controlled synthesis of MnO₂ is facing serious challenges due to the lack of an in-depth understanding of their surface evolution mechanisms. Here, combining aberration-corrected scanning transmission electron microscopy (STEM) and high-resolution TEM, we revealed a mutual energy-driven mechanism between beta-MnO₂ nanowires and microstructures that dominated the evolution of the lateral facets in both structures. The evolution of the lateral surfaces followed the elimination of the {100} facets and increased the occupancy of {110} facets with the increase in hydrothermal retention time. Both self-growth and oriented attachment along their {100} facets were observed as two different ways to reduce the surface energies of the beta-MnO₂ structures. High-density screw dislocations with the 1/2<100> Burgers vector were generated consequently. The observed surface evolution phenomenon offers guidance for the facet-controlled growth of beta-MnO₂ materials with high performances for its application in metal-air batteries, fuel cells, supercapacitors, etc.

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Production technology and optical absorption characteristics of CuIn<Subscript>0.95</Subscript>Ga<Subscript>0.05</Subscript>Se<Subscript>2</Subscript> semiconductor solid solution films

A production technology of thin CuIn_0.95Ga_0.05Se₂ films has been developed based on the method of two-stage selenization of CuIn_0.95Ga_0.05 precursor by a reactive component (selenium) in a carrier gas (nitrogen) flow. The morphology and structure of obtained films were studied by electron microscopy and X-ray diffraction techniques. The spectral dependence of the optical absorption coefficient was measured.