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 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.     

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 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.     

Optical absorption and photosensitivity of CuInxGa1-x Se2 thin film structures

Thin polycrystalline films of CuIn_xGa_1−x Se₂ (0⩽x⩽1) were fabricated by pulsed laser evaporation. Results of measurements of the optical properties, photocurrent polarization indicatrices, and spectral dependence of the photoconversion quantum yield of In-p-CuIn_xGa_1−x Se₂ structures are discussed. A window effect in the photosensitivity has been observed, and it is concluded that it is possible to use CuIn_xGa_1−x Se₂ thin films as photoconverters of solar radiation. Fiz. Tekh. Poluprovodn. 32, 432–435 (April 1998)     

High-Vacuum Evaporation of <Emphasis Type="Italic">n</Emphasis>-CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> Photoabsorber Films for Hybrid PV Structures

Thin films of Cu-In-Se (CISe) photoabsorber with an overall composition of CuIn₃Se₅ were deposited onto glass/indium tin oxide (ITO) substrates from a polycrystalline bulk CuIn₃Se₅ source using the high-vacuum evaporation technique. Thermal conditions for the substrates during the evaporation process and the subsequent annealing in vacuum were selected to prepare polycrystalline n-CuIn₃Se₅ photoabsorber layers for use in hybrid photovoltaic structures based on an inorganic photoabsorber and conductive polymer functional layers. The CISe layers were deposited at a substrate temperature of 200°C and were annealed at temperatures from 300°C to 500°C in vacuum. Part of the as-deposited CISe was annealed twice, in argon and in vacuum at 500°C. These layers exhibited high photosensitivity and photoconductivity when illuminated with white light at an intensity of 100 mW/cm². The results showed that the chalcopyrite structure of the prepared CISe photoabsorber films adhered well to the glass/ITO substrate. The average value of charge carrier concentration and the profile of charge carrier concentration in the annealed CISe photoabsorber layer were calculated using impedance spectroscopy.     

Photosensitivity of structures based on I-IIIn-VIm ternary compounds containing ordered vacancies

Homogeneous crystals of CuIn₃Se₅, CuGa₃Se₅, and CuGa₅Se₈ ternary compounds were grown, and their physical and chemical properties were investigated. Photosensitive structures were fabricated for the first time on the basis of these compounds, and the spectral dependence of the relative quantum photoconversion efficiency was measured. The bandgap of these compounds was also estimated, and it was shown that direct interband transitions are typical in them. It was found that the content and chemical nature of atoms forming an elementary cell in a I-III_n-VI_m ternary compound control the relevant band gap.     

Optical properties of CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> single crystals

Single crystals of the CuIn₅Se₈ ternary compound are grown by the Bridgman-Stockbarger method (with the vertical layout of the procedure). The composition and structure of the crystals are determined. The spectra of transmittance and photoluminescence are studied in the temperature range from 10 to 300 K. The transmittance spectra and the photoluminescence spectra are used to determine, correspondingly, the band gap and the energy of donor-acceptor transitions in the CuIn₅Se₈ crystals. The temperature dependences of these parameters are obtained.     

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.     

Study of single crystals of the CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> ternary compound

Single crystals of the CuIn₃Se₅ ternary compound are grown from the melt by the Bridgman-Stockbarger method. The composition, structure, and electrical properties of the crystals are studied. From the transmittance spectra in the region of the optical absorption edge, the band gap is determined in the temperature range from 10 to 300 K. Using the dilatometry method, the relative elongation is measured for crystals oriented parallel and orthogonal to their principal axis, and the coefficients of thermal expansion are calculated.     

Anomalies in the thermal and electrical conductivity of CuIn5Se8 crystals

Anisotropy, asymmetry, and other anomalies of electrical and thermal properties are revealed in CuIn₅Se₈ crystals. It is shown that the nature of these anomalies is associated with the natural coexistence of order and disorder in these ordered-defect compounds.     

On the band gap in (In2S3) x (CuIn5S8)1 ? x alloy single crystals

For In₂S₃, CuIn₅S₈ and (In₂S₃) _x (CuIn₅S₈)_{1 ? x} alloy single crystals grown by the Bridgman method (vertical variant), the transmittance spectra in the region of the fundamental absorption-band edge are studied at 80 and 295 K. From the spectra, the band gaps of the In₂S₃ and CuIn₅S₈ compounds and the alloys based on them are determined, and the dependences of the band gap on the composition parameter x of the alloys are established. It is shown that, at 80 and 295 K, the band gap nonlinearly varies with x and the variation is described by a quadratic function.     

Photosensitive structures based on CuIn<Subscript>5</Subscript>Te<Subscript>8</Subscript> single crystals: Development and properties

A new ternary compound is synthesized for the first time, and bulk CuIn₅Te₈ single crystals are grown by directed crystallization of near-stoichiometric melt. It is established from X-ray diffraction patterns of grown crystals that they exhibit the structure of imperfect chalcopyrite with parameters of the unit cell of CuIn₅Te₈, which were close to those known for the CuInTe₂ ternary compound with the composition index n = 0. First, photosensitive structures are fabricated based on CuIn₅Te₈ crystals, and photosensitivity spectra are obtained for them; it is shown that it is possible to achieve broadband photosensitivity under illumination of the barrier side of these crystals. From the analysis of photosensitivity spectra, the character of band-to-band transitions and corresponding energies of these transitions in CuIn₅Te₈ are determined. This opens up prospects to use this new semiconductor in photoconverters of solar radiation.     

Specific features of the low-temperature conductivity and photoconductivity of CuInSe2-ZnIn2Se4 alloys

n-type CuInSe₂-ZnIn₂Se₄ alloy single crystals are grown by the horizontal variant of the Bridgman method. The slight temperature dependence of the conductivity, high electron concentration, and the low photoconductivity of single crystals containing a low (5–10 mol %) fraction of ZnIn₂Se₄ are indicative of the nearly degenerate state of the crystals. It is established that, in the CuInSe₂-ZnIn₂Se₄ single crystals containing 15 and 20 mol % of ZnIn₂Se₄, the hopping mechanism of conductivity is dominant at temperatures of T ～ 27–110 K. At T ≥ 110 K, hopping conductivity gives way to activated conductivity. It is found that the specific feature of the low-temperature (27–77 K) photoconductivity spectrum of single crystals with ～15 and 20 mol % of ZnIn₂Se₄ is a single narrow peak at a wavelength of λ_max = 1190–1160 nm.     

Specific optical and photoelectric properties of thin CuIn3Se5 films synthesized by laser deposition

Optical and photovoltaic properties of films produced by laser deposition of CuIn₃Se₅ and the effect of the temperature conditions of film synthesis on these properties are studied. The composition of the films deposited onto substrates held at room temperature coincides with that of the target, with the films obtained in the vitreous state. Heating of substrates during deposition leads to a larger fraction of the crystalline phase and lower fraction of Se in a film. Annealing of glassy CuIn₃Se₅ films in a vacuum at temperatures of up to 700 K causes their crystallization without changes in composition. The photosensitivity of the films is highest at annealing temperatures of 510–600 K.     

Effect of γ-ray radiation on photosensitivity of ZnO/CuIn3Se5 heterojunctions

The effect of γ-ray radiation (⁶⁰Co) on the photosensitivity of ZnO/CuIn₃Se₅ heterojunctions is studied. It is established that the spectral dependence of the photoconversion efficiency is retained if the heterojunctions are irradiated with γ-ray fluxes no higher than 2 × 10¹⁹ photon/cm². Dependences of the open-circuit photovoltage and the short-circuit current on the γ-photon flux are studied. It is shown that the photoelectric parameters of heterojunctions remain unchanged under irradiation with the fluxes Φ < 10¹⁷ photon/cm²; in contrast, if the flux increases in the region of Φ > 10¹⁷ photon/cm², the photocurrent decreases monotonically, while the photovoltage exhibits a maximum, after which the photovoltage returns to the initial value. The potential for using the ZnO/CuIn₃Se₅ heterojunctions under the conditions of a high background radiation is established.     

Doping of silicon with selenium by diffusion from the gas phase

Selenium-doped silicon single crystals are studied for the case of an impurity introduced by diffusion from the gas phase. Doping is performed in sealed quartz ampules at a temperature of 1240°C over the course of 240 h. The dependence of the concentration of the introduced deep donor centers of various types on the diffusant vapor pressure p _Se is examined. It is found that samples with a concentration of atomic Se centers exceeding 10¹⁵ cm^?3 can be obtained at comparatively low p _Se (0.02–0.06 atm). In this case, the content of diatomic Se₂ complexes is lower by an order of magnitude and more. The results obtained may be of interest for those who study nonlinear optical phenomena involving deep donor centers in silicon.     

Interface engineering and characterization at the atomic‐scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin‐film solar cells

In this work, we investigate the p–n junction region for two different buffer/Cu(In,Ga)(Se,S)₂ (CIGSSe) samples having different conversion efficiencies (the cell with pure In₂S₃ buffer shows a lower efficiency than the nano‐ZnS/In₂S₃ buffered one). To explain the better efficiency of the sample with nano‐ZnS/In₂S₃ buffer layer, combined transmission electron microscopy, atom probe tomography, and X‐ray photoelectron spectroscopy studies were performed. In the pure In₂S₃ buffered sample, a CuIn₃Se₅ ordered‐defect compound is observed at the CIGSSe surface, whereas in the nano‐ZnS/In₂S₃ buffered sample no such compound is detected. The absence of an ordered‐defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn₃Se₅, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered‐defect compound. In the nano‐ZnS/In₂S₃ sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se₂) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X‐ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p–n junction at the nanoscale in CIGSSe solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

The effect of annealing on the physical properties of thermally evaporated CuIn2n+1S3n+2 thin films (n=0, 1, 2 and 3)

In this study, the annealing effect on structural, electrical and optical properties of CuIn_2n+1S_3n+2 thin films (n=0, 1, 2 and 3) are investigated. CuIn_2n+1S_3n+2 films were elaborated by vacuum thermal evaporation and annealed at 150 and 250 °C during 2 h in air atmosphere. XRD data analysis shows that CuInS₂ and CuIn₃S₅ (n=0 and 1) crystallize in the chalcopyrite structure according to a preferential direction (112), CuIn₅S₈ and CuIn₇S₁₁ (n=2 and 3) crystallize in the cubic spinel structure with a preferential direction (311). The optical characterization allowed us to determine the optical constants (refractive indexes 2.2–3.1, optical thicknesses 250–500 nm, coefficients of absorption 10⁵ cm^?1, coefficients of extinction <1, and the values of the optical transitions 1.80–2.22 eV) of the samples of all materials. We exploited the models of Cauchy, Wemple–DiDomenico and Spitzer–Fan for the analysis of the dispersion of the refractive index and the determination of the optical and dielectric constants.     

Surface-barrier In/p-CuGa3Te5 and In/p-CuGa5Te8 structures: Fabrication and properties

Single crystals of the ternary CuGa₃Te₅ and CuGa₅Te₈ ternary compounds are grown and their properties are studied. The photosensitive In/p-CuGa₃Te₅ and In/p-CuGa₅Te₈ structures were formed on homogeneous crystals for the first time. Photoelectric properties of new structures were studied and the parameters of the structures and of new semiconductors were determined; it is concluded that these structures can be used in broadband photoconverters of nonpolarized radiation.