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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]

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]

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 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]

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]