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The crystal structure of the quaternary compound semiconductor Cu 2 ZnGeSe 4 (CZGSe) was investigated by the complementary use of neutron diffraction, and Raman spectroscopy. The powder sample, which resulting from wavelength dispersive X-ray spectroscopy (WDX) turned out to be single phase Cu-rich CZGSe, was synthesized by solid state reaction of the pure elements in an evacuated silica tube at 700 °C. Raman spectroscopy confirmed the homogeneity and phase purity of the sample, in addition, the kesterite type structure was suggested. Rietveld analysis of the neutron diffraction data confirmed that the compound crystallizes in the tetragonal kesterite type structure. The refined site occupancy factors were used to determine the average neutron scattering lengths of the cation sites, giving insights into cation distribution and finally point defect types. Thus it has been shown, that additional to the Cu Zn -Zn Cu anti-site defects in the lattice planes at z =¼ and ¾ (Cu-Zn disorder) also the off-stoichiometry type related point defects like Cu i and Cu Zn occur in Cu-rich CZGSe. [ABSTRACT FROM AUTHOR]

Kesterite-type based thin film solar cell technologies are mainly based on polycrystalline absorber layers. A promising low cost alternative technology uses Cu 2 ZnSn(S x Se 1-x) 4 (CZTSSe) monograins (single crystals of 20–100 μm size) which are fixed in a polymer matrix to form a flexible solar cell. The Cu/Zn disorder is discussed as a possible reason for band tailing causing voltage losses limiting the efficiency of CZTSSe-based devices. The experimental determination of the order parameter Q which is a quantitative measure of Cu/Zn disorder, requires a differentiation between the isoelectronic cations Cu+ and Zn2+. An in-depth analysis of neutron diffraction data allows the determination of type and concentration of intrinsic point defects including a distinction between Cu and Zn. Neutron diffraction requires large sample volumes, thus monograins offer the unique possibility to correlate structural disorder in CZTSSe with device performance parameters. In this study we tackle the influence of grinding the monograins on stoichiometry deviations, the Cu/Zn disorder as well as intrinsic point defects and optoelectronic properties of CZTSSe monograins. Moreover, an easy methodology based on Raman scattering spectroscopy is proposed for the assessment of Cu/Zn disorder in the CZTSSe compounds. • a significant impact on the order parameter Q in CZTSSe monograins from the "ordering" and "disordering" procedures. • The possibility to develop a fast and easy methodology to estimate the level of Cu/Zn disorder from Raman measurements. • Clear independence of the main structural, compositional and optoelectronic properties from the mechanical treatment. [ABSTRACT FROM AUTHOR]

Quaternary Cu2ZnSn(S1-xSex)4 compounds are promising semiconductor materials for absorber layer in thin film solar cells. Record efficiency devices (12.6%) exhibit an off-stoichiometric composition of the absorber layer. These deviations from stoichiometry cause intrinsic point defects, which determine the electronic properties of the semiconductor significantly. A structural analysis of a nearly C-type off-stoichiometric Cu and Sn rich and Zn poor Cu2ZnSn(S1-xSex)4 mixed crystal with x=0.48 by a neutron diffraction investigation was performed. The average neutron scattering length method was applied, showing that the mixed crystal adopts the kesterite type structure. A detailed cation distribution analysis revealed the occurrence of ZnCu(2a) and SnZn anti-site point defects additional to the Cu-Zn disorder in the planes at z=1/4 and 3/4. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Cu2ZnSnS4 (CZTS) thin films were deposited on glass substrates by spray pyrolysis a fast, cost effective and vacuum-free method. X-ray fluorescence, Raman spectroscopy and grazing incidence X-ray diffraction, were used to characterize the obtained thin films. The analysis of these data showed a close to stoichiometry composition of the films and CZTS adopts the kesterite type structure but with poor crystalline quality and possible existence of secondary phases, such as ZnS and Cu2SnS3. An annealing procedure in presence of elemental Sn and S was applied in order to improve the quality of the films. The annealing leads to an improvement of the crystalline quality of the thin films. The lattice parameters a and c of the annealed CZTS thin films were obtained as result of the Rietveld analysis of the gracing incidence X-ray diffraction data. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: Variable angle spectroscopic ellipsometry has been applied to characterize the optical constants of bulk Cu(In0.7Ga0.3)5Se8 and Cu(In0.4Ga0.6)5Se8 crystals grown by the Bridgman method. The spectra were measured at room temperature over the energy range 0.8–4.4eV. Adachi’s model was used to calculate the dielectric functions as well as the spectral dependence of complex refractive index, absorption coefficient, and normal-incidence reflectivity. The calculated data are in good agreement with the experimental ones over the entire range of photon energies. The parameters such as strength, threshold energy, and broadening, corresponding to the E 0, E 1A, and E 1B interband transitions, have been determined using the simulated annealing algorithm. [ABSTRACT FROM AUTHOR]

Abstract: Spectroscopic ellipsometry has been used to characterize the dielectric functions of bulk Cu2SnSe3 crystals, grown by modified Bridgman technique. Spectral measurements were performed at room temperature over the energy range 1.0 to 4.7eV. The spectral dependence of the complex dielectric functions, complex refractive index, the absorption coefficient and the normal-incidence reflectivity of Cu2SnSe3 was derived. The structures observed in the measured spectra have been associated with critical points higher than band gap interband transitions. [Copyright &y& Elsevier]

Spectroscopic ellipsometry has been used to characterize the dielectric functions of bulk Cu(In1-xGax)5Se8 crystals. Spectra were measured at room temperature over the energy range 0.74–5.2 eV. The dielectric functions as well as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity have been modeled using a modification of the Adachi model. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters (strength, threshold energy, and broadening) have been determined using the simulated annealing algorithm. The transition energies E0 and E1A are found to shift linearly to higher energies as the gallium content increases. [ABSTRACT FROM AUTHOR]

The substitution of Ge4+ for Sn4+ in Cu2ZnSn(S,Se)4 (CZTSSe) kesterite-type absorber layers for thin film solar cells has been proven to enhance the opto-electronic properties of the material. By cationic substitution, in general, the optical bandgap can be more readily designed for the purpose of bandgap engineering, and the substitution of Sn4+ by Ge4+, in particular, widens the optical bandgap such that it can be employed for both photovoltaics and solar fuel quarrying by photocatalytic water splitting. This work is an experimental study of intrinsic point defects in off-stoichiometric kesterite-type Cu2ZnGeSe4 (CZGSe) by means of neutron powder diffraction. We revealed the existence of copper vacancies (VCu), various cation antisite defects (CuZn, ZnCu, ZnGe, CuGe) as well as interstitials (Cui, Zni) in a wide range of off-stoichiometric polycrystalline materials synthesized by solid state reaction. In addition to the off-stoichiometry-type specific defects, Cu/Zn disorder is always present in the kesterite-type CZGSe phase. While compositional changes are clearly reflected by the tetragonal deformation c/2a, the lattice parameters a and c seem differently responding to point defect types and concentration variations, respectively. The CuGe antisite defect which is known to greatly deteriorate the opto-electronic properties exists only in Cu-rich CZGSe, but appears already in CZGSe with Cu/(Zn + Ge) ≈ 1. Furthermore, we showed by diffuse reflectance hyperspectral imaging a widening of the energy bandgap in off-stoichiometric kesterite-type CZGSe with decreasing Cu/(Zn + Ge). [ABSTRACT FROM AUTHOR]

Abstract: A comparative study of chalcopyrite-related Cu2In7Se11.5 and CuIn5Se8 crystals by spectroscopic ellipsometry is presented. Their complex dielectric function ɛ(ω)= ɛ 1(ω)+ iɛ 2(ω) has been determined in the 0.8–4.7eV photon energy range. The spectral dependence of ɛ 1(ω) and ɛ 2(ω) as well as the complex refractive index, the absorption coefficient and the normal-incidence reflectivity for tetragonal Cu2In7Se11.5 and trigonal CuIn5Se8 crystals have been modelled using the Adachi''s model for interband transitions. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap (E 0) and higher critical points (E 1A , E 1B ), have been determined using the simulated annealing algorithm. The results show that the tetragonal CuIn3Se5, the tetragonal Cu2In7Se11.5 and the trigonal CuIn5Se8 crystals belong to different phases of the Cu4In2n Se2+3n system, n =6, n =7 and n =10, respectively. [Copyright &y& Elsevier]

The dielectric functions of Cu2ZnGeS4 bulk crystals grown by the Bridgman method were measured over the energy range 1.4 to 4.7 eV at room temperature using variable angle spectroscopic ellipsometry. The observed structures in the dielectric functions were adjusted using the Adachi's model and attributed to interband transitions E0, E1A, and E1B at Γ:(000), N(A):2π/a(0.5 0.5 0.5), and T(Z):2π/a(0 0 0.5) points of the first Brillouin zone, respectively. The model parameters (threshold energy, strength, and broadening) have been determined using the simulated annealing algorithm. The decrease in the first gap, E0, has been attributed to a higher Ge-S hybridization. The spectral dependence of the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived. [ABSTRACT FROM AUTHOR]

Analysis of the optical properties of bulk Cu(In1-xGax)3Se5 mixed crystals synthesized from the elements as a function of the Ga content is presented. Measurements of the complex dielectric function [variant_greek_epsilon](ω)=[variant_greek_epsilon]1(ω)+i[variant_greek_epsilon]2(ω) were performed at room temperature in the photon energy range of 0.8–4.7 eV using a variable angle of incidence ellipsometer. The spectral dependence of the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived. The structure observed in the dielectric functions attributed to the interband transitions E0, E1A, and E1B has been modeled using a modification of the Adachi’s model. The results are in excellent agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap and higher critical points, have been determined using the simulated annealing algorithm. The values of E0 and E1A are found to increase linearly with the increasing Ga content. [ABSTRACT FROM AUTHOR]

The recent investigation of kesterite type quaternary compounds showed that one of the main detrimental problems, which limits the efficiency of the solar cells based on these materials, is related to the high defect concentrations, mainly Cu Zn disorder. To overcome this problem partial replacement of Cu or Zn cations in the classical Cu 2 ZnSn(S,Se) 4 compounds was proposed. One of the promising cations was found to be Ag, which partially replaces Cu and the solid solution of (Ag x Cu 1-x) 2 ZnSn(S,Se) 4 (ACZTSSe) is formed. In the present study, ACZTSSe thin films with different Ag concentrations were deposited by the spray pyrolysis method, with subsequent annealing in S + Se atmosphere. From the analysis of the chemical composition of the thin films a good correlation of measured silver concentration with initial concentration (dissolved in the main solution for deposition) for the samples with up to 15% Ag were found. These results were also confirmed by the structural studies and analysis of the Raman scattering spectra. On the other hand, thin film with higher Ag concentration (20%) has shown strong compositional inhomogeneity and presence of secondary phases was detected. Analysis of the modulated surface photo-voltage yielded the optical transition at ~ 1.5 eV, correlated with the band gap energy, and a transition at lower energy. From the temperature dependence of resistivity investigation no significant change in the resistivity for the samples with low silver concentration and strong increase for the samples with 20% of Ag were found. • Thin films of kesterite type compounds were deposited by a spray pyrolysis. • Thin films with partial replacement of Cu by Ag were deposited. • A good quality thin films were obtained with Ag concentration up to 15%. • Electrical properties with Ag concentration increase didn't changed significantly. [ABSTRACT FROM AUTHOR]

The resistivity, ρ( T ), of Cu 2 ZnSnS 4 powder samples, obtained by a solid-state reaction method from pure elements, exhibits an activated character. The Mott variable-range hopping charge transfer is observed between T ∼100–230 K, followed by the conductivity due to activation of holes into the states above the mobility threshold when T is increased. Such behavior, accompanied by semi-width W of the acceptor band, exceeding the mean acceptor energy, indicates overlap of the acceptor band states with those of the valence band, forming a joint near-edge hole spectrum in conditions of the strong degree of the compensation. The details of this spectrum have been determined, including positions of the Fermi level, μ, and the mobility threshold, as well as the density of the localized states at the Fermi level, g (μ). The acceptor concentration and the hole localization radius have been obtained, too. These parameters indicate the material to lie relatively far from the metal-insulator transition. [ABSTRACT FROM AUTHOR]

The influence of thickness on the structural parameters, off-stoichiometry and cation disorder in kesterite Cu 2 ZnSnSe 4 films grown by flash deposition for solar cell absorbers is investigated employing X-ray diffraction, energy-dispersive X-ray and Raman spectroscopies. It is shown that the lattice parameters of Cu 2 ZnSnSe 4 changed depending on film thickness: the 100 nm film turned out to be weakly stretched on the molybdenum-coated glass substrates, while, in the thicker films, the compressive deformation is defined. The causes of the changes in film structure are outlined. Raman spectra revealed secondary phases like Cu 2 SnSe 3 detecting reduction of its fraction with an increment in thickness; also, the Sn Zn antisite defect fraction decreases. Simultaneously, the share of disordered kesterite phase associated with Cu Zn antisite defects rises with thickness. The obtained results can be useful for optimization of technological growth process. [ABSTRACT FROM AUTHOR]

Using spectroscopic ellipsometry we investigated and analyzed the pseudo-optical constants of Cu2ZnSnSe4 bulk crystals, grown by the Bridgman method, over 0.8-4.5 eV photon energy range. The structures found in the spectra of the complex pseudodielectric functions were associated to E0, E1A, and E1B interband transitions and were analyzed in frame of the Adachi's model. The interband transition parameters such as strength, threshold energy, and broadening were evaluated by using the simulated annealing algorithm. In addition, the pseudo-complex refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity were derived over 0.8-4.5 eV photon energy range. [ABSTRACT FROM AUTHOR]

Abstract: Exciton spectra of Cu2ZnSiS4 single crystals are investigated by reflection spectroscopy at 10 and 300K for light polarized perpendicular (E⊥c) and parallel (E∥c) to the optical axis. The parameters of the excitons and dielectric constant are determined. The free carriers effective masses have been estimated. The room temperature reflectivity spectra at photon energies higher than the fundamental band gap in the polarization Е⊥с and E∥с were measured and related to the electronic band structure of Cu2ZnSiS4. [Copyright &y& Elsevier]

Highlights: [•] Reflection spectra of Cu2ZnSiSe4 were studied for E ⊥ c and E || c light polarizations. [•] Four excitonic series are revealed in the reflection spectra at 10K. [•] Model of exciton dispersion and the presence of a dead-layer. [•] Exciton Rydberg energies and free carriers effective masses were calculated. [•] Reflectivity for E ⊥ c and E || c were analyzed in the region 3–6eV at 300K. [ABSTRACT FROM AUTHOR]

Highlights: [•] Resistivity of single crystalline p-Cu2ZnSiSe4 is investigated between 10 and 300K. [•] Variable-range hopping conduction of the Mott type is established below ∼200K. [•] Conductivity connected to the activation of holes is observed between ∼200 and 300K. [•] Width and density of the localized states of the acceptor band are determined. [•] Localization radii and Bohr radius of the localized carriers are obtained. [ABSTRACT FROM AUTHOR]

Abstract: We report the room temperature spectroscopic ellipsometry study of Cu2ZnGeSe4 and Cu2ZnSiSe4 crystals, grown by modified Bridgman technique. Optical measurements were performed in the range 1.2–4.6 eV. The spectral dependence of the complex pseudodielectric functions as well as pseudo- complex refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity of Cu2ZnGeSe4 and Cu2ZnSiSe4 crystals were derived. The observed structures in the optical spectra were analyzed by Adachi's model and attributed to the band edge transitions and higher lying interband transitions. The parameters such as strength, threshold energy, and broadening, corresponding to the E 0, E 1A and E 1B interband transitions, have been determined using the simulated annealing algorithm. [Copyright &y& Elsevier]

Optical properties of the Cu2ZnGeS4 bulk single crystals were characterized by using Raman spectroscopy and photoluminescence spectroscopy technique. The strongest lattice vibration mode of this compound observed at 359 cm-1 and less intensive modes at 273, 291, 332, 370, 383, 405 and 414 cm-1. PL spectra of the Cu2ZnGeS4 consist of dominant broad emission band located at about 1.40 eV and a less intensive near 2.00 eV at 4.5 K. The observed PL bands were attributed to the donor acceptor pair transitions, with activation energy of 37 (140) meV for dominant and less intensive band, respectively. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]