Fabrication and photoelectric properties of the ZnO-Cu(In,Ga)Se<Subscript>2</Subscript> heterojunctions

Thin-film n-ZnO:Al/p-Cu(In,Ga)Se₂ heterojunctions are fabricated by magnetron sputtering of an ZnO target, leading to a deposition of Cu(In,Ga)Se₂ films on the surface. The photoelectric properties of the fabricated heterojunctions are studied under exposure to natural and linearly polarized light. It is concluded that the resulting cadmium-free environmentally safe heterostructures can be used as high-efficiency broad-band photoconverters of natural and linearly polarized light.     

Cadmium-free thin-film Cu(In,Ga)Se<Subscript>2</Subscript>/(In<Subscript>2</Subscript>S<Subscript>3</Subscript>) heterophotoelements: Fabrication and properties

The method of heat treatment of metallic Cu-In-Ga layers in the N₂ inert atmosphere in the presence of selenium and sulfur vapors was used to grow homogeneous films of Cu(In,Ga)(S,Se)₂ alloys onto which the CdS or In₂S₃ films were deposited and, on the basis of these structures, the thin-film glass/Mo/p-Cu(In,Ga) (S,Se)₂/n-(In₂S₃,CdS)/n-ZnO/Ni-Al photoelements were fabricated. The mechanisms of charge transport and the processes of photosensitivity in the obtained structures subjected to irradiation with natural and linearly polarized light are discussed. The broadband photosensitivity of thin-film heterophotoelements and the induced photopleochroism were detected; these findings indicate that there is an interference-related blooming of the structures obtained. It is concluded that it is possible to use ecologically safe cadmium-free thin-film heterostructures as high-efficiency photoconverters of solar radiation.     

Photosensitivity of ZnO/CdS/Cu(In,Ga)Se<Subscript>2</Subscript>/Mo thin-film solar cells fabricated on various substrates

The results of measuring the first spectra of relative quantum efficiency for photoconversion in thin-film ZnO/CdS/Cu(In,Ga)Se₂/Mo solar cells fabricated on rigid (glass) and flexible (polyimide) substrates are reported. The character of interband transitions has been studied and the values of the band gap for direct and indirect transitions in thin Cu(In,Ga)Se₂ films are determined. It is found that a shift of the maximal photosensitivity for the obtained solar cells to shorter wavelengths is observed as rigid substrates are replaced by flexible ones. It is concluded that thin-film Cu(In,Ga)Se₂ structures can be used as broad-band photoconverters of solar radiation.     

Properties of CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> crystals and In/CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> structures

Using the method of planar crystallization from the melt with deviations from the stoichiometric composition, p-CuIn₃Se₅ single crystals are grown. The electrical properties of the homogeneous crystals are studied. It is found that the resistivity of the p-CuIn₃Se₅ crystals depends on the excess Se content in the melt. It is established that the voltaic photosensitivity of the In/CuIn₃Se₅ structures is enhanced with an increasing excess of Se content in the melt. The energy spectrum and the character of interband transitions in the CuIn₃Se₅ crystals are discussed. It is concluded that the CuIn₃Se₅ ternary compound can be used in high efficiency photoelectric converters of solar radiation.     

Fabrication and photoelectric properties of oxide/CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> heterojunctions

Single crystals of the n-CuIn₅Se₈ compound of hexagonal modification have been grown by direct crystallization from melt. On the basis of the experimental study of its thermal interaction with air oxygen, a method for fabricating new oxide/n-CuIn₅Se₈ heterojunctions is proposed. Electrical and photoelectric properties of the structures obtained have been investigated. It is shown that the interaction of n-CuIn₅Se₈ of hexagonal modification with air oxygen makes it possible to obtain heterojunctions with high photosensitivity. The new technology can be used in the design of broadband optical radiation converters based on n-CuIn₅Se₈ crystals.     

The charge-transport mechanisms and photosensitivity of <Emphasis Type="Italic">n</Emphasis>-ZnO:Al/Cu<Emphasis Type="Italic">Pc/p</Emphasis>-Cu(In,Ga)Se<Subscript>2</Subscript> structures

The first photosensitive n-ZnO:Al/CuPc/p-Cu(In,Ga)Se₂ structures are produced by a vacuum sublimation of copper phthalocyanine onto the surface of thin p-Cu(In,Ga)Se₂ films and a subsequent magnetron deposition of n-ZnO:Al films. The steady-state current-voltage characteristics of the resulting structures are studied. The charge-transport and photosensitivity mechanisms of the thin-film structures are discussed. The structures appear promising for the fabrication of wide-range (1.2–3.3 eV) thin-film photoelectric converters.     

A comparative study on the electronic and optical properties of Sb<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin film

The thin film of Sb₂Se₃ was deposited by thermal evaporation method and the film was annealed in N₂ flow in a three zone furnace at a temperature of 290°С for 30 min. The structural properties were characterized by scanning electron microscopy (SEM), transmission electron microscopy (ТЕМ), X-ray diffraction (XRD) and Raman spectroscopy, respectively. It is seen that the as-deposited film is amorphous and the annealed film is polycrystalline in nature. The surface of Sb₂Se₃ film is oxidized with a thickness of 1.15 nm investigated by X-ray photolecetron spectroscopy (XPS) measurement. Spectroscopic ellipsometry (SE) and UV–vis spectroscopy measurements were carried out to study the optical properties of Sb₂Se₃ film. In addition, the first principles calculations were applied to study the electronic and optical properties of Sb₂Se₃. From the theoretical calculation it is seen that Sb₂Se₃ is intrinsically an indirect band gap semiconductor. Importantly, the experimental band gap is in good agreement with the theoretical band gap. Furthermore, the experimental values of n, k, ε₁, and ε₂ are much closer to the theoretical results. However, the obtained large dielectric constants and refractive index values suggest that exciton binding energy in Sb₂Se₃ should be relatively small and an antireflective coating is recommended to enhance the light absorption of Sb₂Se₃ for thin film solar cells application.     

Effect of thallium doping on the mobility of electrons in Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> and holes in Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>

The Shubnikov–de Haas effect and the Hall effect in n-Bi_2–xTl_xSe₃ (x = 0, 0.01, 0.02, 0.04) and p-Sb_2–xTl_xTe₃ (x = 0, 0.005, 0.015, 0.05) single crystals are studied. The carrier mobilities and their changes upon Tl doping are calculated by the Fourier spectra of oscillations. It is found shown that Tl doping decreases the electron concentration in n-Bi_2–xTl_xSe₃ and increases the electron mobility. In p-Sb_2–xTl_xTe₃, both the hole concentration and mobility decrease upon Tl doping. The change in the crystal defect concentration, which leads to these effects, is discussed.     

Optical and photoelectrical properties of AgCd<Subscript>2</Subscript>GaS<Subscript>4</Subscript> single-crystal compounds

The poorly known AgCd₂GaS₄ single-crystal compounds that crystallize in a rhombic structure (space group Pmn2₁) are studied. Deviations from the stoichiometric composition of the samples and random occupation of the cation sublattice sites by Ag and Ga ions result in violation of long-range order in the atomic arrangement and make the AgCd₂GaS₄ compounds structurally closer to disordered systems. In this case, it is found that the fundamental optical-absorption edge is smeared and shifted to longer wavelengths, and is adequately described by the Urbach rule. In addition, a broadening of the spectral peaks of photoconductivity and luminescence is observed. The concentration of charged point defects responsible for the smearing of the absorption edge is calculated. It is found to be 1.2×10²⁰ cm^?3. The AgCd₂GaS₄ single crystals are photosensitive semiconductors. From the position of the absorption edge, the optical band gap of the compound is estimated (E _g0 = 2.28 eV at T = 297 K). The photoluminescence spectra of the AgCd₂GaS₄ single crystals are similar to the spectra of defect-containing CdS single crystals; for the AgCd₂GaS₄ crystals, the emission peaks are shifted to longer wavelengths with respect to the peaks for CdS crystals by Δλ = 0.06?0.1 μm. From the analysis of the experimental data, some conclusions on the nature of photoactive centers in AgCd₂GaS₄ compounds are drawn.     

Photoconversion in <Emphasis Type="Italic">n</Emphasis>-ZnO:Al/Pd<Emphasis Type="Italic">Pc/p</Emphasis>-CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> structures

n-ZnO:Al/PdPc/p-CuIn₃Se₅ photosensitive structures have been proposed and fabricated for the first time by vacuum sublimation of palladium phthalocyanine on the surface of wafers of the ternary semiconductor compound CuIn₃Se₅ and by magnetron sputtering of n-ZnO:Al films on the surface of palladium phthalocyanine films. The current-voltage characteristics and spectra of the photoconversion quantum efficiency of the obtained structures are investigated. It is shown that these structures can be used as multiband white-light converters.     

Fabrication and photosensitivity of heterojunctions based on CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> crystals

Heterojunctions based on p-CuIn₃Se₅ crystals are fabricated by magnetron sputtering of an n-ZnO:Al target and by putting naturally cleaved n-GaSe thin wafers onto polished surfaces of p-CuIn₃Se₅ wafers. The current-voltage characteristics and mechanisms of current flow in the diodes under study are analyzed. The photovoltaic effect revealed in the fabricated structures is discussed. It is shown that the fabricated photosensitive heterojunctions are promising for the development of selective analyzers of linearly polarized radiation.     

Two-Terminal Tandem Solar Cells DSC/<Emphasis Type="Italic">c</Emphasis>-Si: Optimization of TiO<Subscript>2</Subscript>-based Photoelectrode Parameters

New types of two-terminal tandem solar cells DSC/c-Si in which mesoscopic dye-sensitized solar cell (DSC) was connected in parallel with a crystalline silicon (c-Si) solar cell, were developed and investigated. We have measured the optical and photovoltaic parameters for both the individual and the fabricated tandem DSC/c-Si solar cells. It was shown that the highest efficiency of 14.7% for the tandem DSC/c-Si solar cell under standard AM1.5G (100 mW/cm²) illumination conditions was achieved for DSC based on 3.5 μm thick titanium dioxide photoelectrode.     

Role of the CdS buffer layer as an active optical element in Cu(In,Ga)Se2 thin‐film solar cells

ZnO/CdS/Cu(In,Ga)Se₂ (CIGS) thin‐film heterojunction solar cells with CdS buffer layers of thicknesses between 0 and 85 nm are characterized by current–voltage, quantum efficiency, and optical reflection measurements. We investigate the correlation between the short‐circuit current density and the CdS layer thickness, focusing on the counteracting effects of light absorption and reduced optical reflection induced by the CdS layer. Both effects almost compensate each other for CdS layer thicknesses between 0 and 40 nm. Thus, an optimization of the short‐circuit current density is not achieved by omitting the CdS layer, but rather by replacing the CdS buffer with an alternative buffer material with higher bandgap energy and optical constants similar to those of CdS. Copyright © 2002 John Wiley & Sons, Ltd.     

Specific thermoelectric properties of lightly doped Bi<Subscript>2</Subscript>(TeSe)<Subscript>3</Subscript> solid solutions

Electrical and thermoelectric properties of a lightly doped n-Bi₂Te_2.7Se_0.3 solid solution have been studied in the temperature range 77–300 K. The results are compared with data for the compound PbTe_0.9Se_0.1 with a similar magnitude of the Seebeck coefficient S at 84 K. Along with lower thermal conductivity, Bi₂Te_2.7Se_0.3 has a higher electrical conductivity σ and a much weaker temperature dependence. As a result, the power coefficient S ²σ in optimal samples begins to decrease only when the density of minority carriers becomes significant. In this case, |S| considerably exceeds the standard value of 200 μV/K. The reduction of the electron density reduces the thermoelectric figure of merit Z at its maximum and slightly lowers the temperature of the maximum; therefore, the expected effect on the average value of Z in the range 77–300 K is absent. Similar behavior is observed in Bi₂Te_2.88Se_0.12, although the effect is less pronounced. The experimental results are discussed taking into account possible changes in the dominant scattering mechanisms, carrier density, and electron energy spectrum. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 7, 2004, pp. 811–815. Original Russian Text Copyright ? 2004 by Konstantinov, Prokof’eva, Ravich, Fedorov, Kompaniets.     

Optical Properties of K<Subscript>2</Subscript>O-Li<Subscript>2</Subscript>O-WO<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Glasses: Evidence of Mixed Alkali Effect

Glass with compositions xK₂O-(30 ? x)Li₂O-10WO₃-60B₂O₃ for 0 ≤ x ≤ 30 mol.% have been prepared using the normal melt quenching technique. The optical reflection and absorption spectra were recorded at room temperature in the wavelength range 300–800 nm. From the absorption edge studies, the values of the optical band gap (E _opt) and Urbach energy (ΔE) have been evaluated. The values of E _opt and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple Di-Domenico model.     

Anisotropy of the thermal expansion of CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> single crystals in two structural modifications

Thermal expansion in the temperature range 80–700 K is studied for two (trigonal and hexagonal) structural modifications of CuIn₅Se₈ single crystals grown by planar crystallization of the melt. From the data, the thermal-expansion coefficients are calculated for both modifications. It is established that, in the temperature range under study, the thermal expansion of both modifications is anisotropic. For the trigonal modification, the thermal-expansion coefficient in the direction of the c axis (α_c) is larger than that in the direction of the orthogonal a axis (α_a). For the hexagonal modification of the CuIn₅Se₈ crystal, the thermal-expansion coefficient in the direction of the c axis exhibits anomalous behavior: as the temperature is increased, the coefficient α_c increases, after which it decreases to negative values, reaches a minimum, and then increases further. Such behavior of the coefficient α_c is associated with the phase transformation of the hexagonal modification of the CuIn₅Se₈ compound into the trigonal modification.     

Special features of charge transport in PbGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals

The current-voltage (I-V) characteristics of PbGa₂Se₄ single crystals grown by the Bridgman-Stockbarger method with a resistivity of 10¹⁰–10¹² Ω cm were measured. The value of the majority carrier mobility μ=14 cm² V^?1 s^?1, calculated by the differential method of analysis of I-V characteristics, makes it possible to evaluate a number of parameters: the carrier concentration at the cathode (n_c0=2.48 cm^?3), the width of the contact barrier d_c=5.4×10^?8 cm, the cathode transparency D _c ^* =10^?5–10^?4 eV, and the quasi-Fermi level E_F=0.38 eV. It is found that a high electric field provides the charge transport through PbGa₂Se₄ crystals in accordance with the Pool-Frenkel effect. The value of the dielectric constant calculated from the Frenkel factor is found to be equal to 8.4.     

Effect of ZnO Nanoparticles on the Sintering Behavior and Physical Properties of Bi<Subscript>0.5</Subscript>(Na<Subscript>0.8</Subscript>K<Subscript>0.2</Subscript>)<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> Lead-Free Ceramics

Sintered Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ + x wt.% ZnO nanoparticle (BNKT–xZnO_n) ceramics have been fabricated by conventional annealing with the aid of ultrasound waves for preliminary milling. Because of the presence of the liquid Bi₂O₃–ZnO phase at the eutectic point of 738°C, the sintering temperature decreased from 1150°C to 1000°C, and the morphology phase boundary of BNKT–xZnO_n ceramics can be clarified by two separated peaks at (002)_T and (200)_T of 2θ in the x-ray diffraction (XRD) patterns. The improvement of ferroelectric properties has been obtained for BNZT–0.2 wt.% ZnO_n ceramics by the increase of remanent polarization up to 20.4 μC/cm² and a decrease of electric coercive field down to 14.2 kV/cm. The piezoelectric parameters of the ceramic included a piezoelectric charge constant of d ₃₁ = 78 pC/N; electromechanical coupling factors k _p = 0.31 and k _t = 0.34, larger than the values of 42 pC/N, 0.12 and 0.13, respectively, were obtained for the BNKT ceramics.     

AC Electrical Conductivity of FeIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> Single Crystals

The results obtained in a study of the frequency and temperature dependences of the ac electrical conductivity of FeIn₂Se₄ single crystals are presented. It is found that the law σ ~ f ^S (0.1 ≤ S ≤ 1.0) is obeyed for electrical conductivity in the 295–375 K temperature range at frequencies of 2 × 10⁴–10⁶ Hz. It shown that the frequency dependence of the conductivity in an FeIn₂Se₄ single crystal can be accounted for in terms of the multiplet model, and, consequently, the conductivity in these single crystals is characterized by the band-hopping mechanism.     

Potential of using the Cd<Subscript>0.8</Subscript>Hg<Subscript>0.2</Subscript>Te alloy in solar cells

Surface-barrier diodes based on the Cd_xHg_1?x Te alloy (x ～ 0.8) sensitive in the wavelength range 0.3–1.1 μm, which were obtained by etching (bombardment) of the surface of the p-type crystal with argon ions, are studied. Using the measured spectral absorption and reflection curves, as well as the parameters of the diode structure, which were found from electrical characteristics, the spectra of photoelectric quantum efficiency of diodes are calculated. The results of the calculation of photoelectric parameters of the Gd_0.8Hg_0.2Te-based diodes are given in comparison with the CdTe-based and Si-based solar cells. For the AM1.5 solar irradiation conditions, the open-circuit voltage and short-circuit current, as well as values of limiting efficiency, are determined.