Search

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]

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]

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 variable angle spectroscopic ellipsometry technique is employed to determine the dielectric functions and optical constants of the different [Cu.sub.2]ZnGe[S.sub.4] bulk crystals grown by the Bridgman method. An increase in the Ge-S hybridization in the system is shown to lead to a significant reduction in the first gap.

Spectroscopic ellipsometry is used to characterize the dielectric functions of bulk Cu[([In.sub.1-x][Ga.sub.x]).sub.5][Se.sub.8] crystals. The photon transition energies [E.sub.0] and [E.sub.1A] of the crystals are found to shift linearly to higher energies as the gallium content increases.

The analysis of the optical properties of bulk Cu[([In.sub.1-x][Ga.sub.x]).sub.3][Se.sub.5] mixed crystals prepared from the elements as a function of the Ga content is described. The values of the interband transitions are shown to increase linearly with the increasing Ga content.

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]

The crystal structure identification of the photovoltaic material Cu2ZnSnSe4 (CZTSe) is challenging due to the distinguishing feature between the two polymorphs, kesterite and stannite, being the arrangement of Cu and Zn ions. Here an energy dispersive X-ray (EDX) technique, based on electron beam channeling along specific crystallographic planes in a transmission electron microscope (TEM), is used to identify the structure. Regions a few 100 nm in size can be analysed using this method, unlike neutron or anomalous X-ray scattering. The parent crystal structure of CZTSe, annealed on either side of the order–disorder transition temperature, was correctly identified as being kesterite. The presence of 1/2[110] (001) and 1/4[201] (101) anti-site boundaries (ASBs) has also been investigated. The density of ASBs is higher above the transition temperature, due to a smaller energy penalty for disordering on the 2c and 2d Wyckoff sites. A nearest neighbour cation analysis predicted the 1/2[110] (001) ASB to have the lowest formation energy. From density functional theory (DFT) simulations the 1/2[110] (001) ASB energy is only 43 mJ m−2 and furthermore it is not a recombination site or current blocking layer, so that photovoltaic device performance is not significantly degraded. [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]

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]

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]

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]

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]

The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnGeSe4 (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu-Se and Zn-Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells., (© 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).)

The solid solution series between Cu 2 ZnSnSe 4 , crystallizing in the kesterite type structure, and Cu 2 MnSnSe 4 , adopting the stannite type structure, i.e. Cu 2 (Zn 1- x Mn x )SnSe 4 , was studied by a combination of neutron and X-ray powder diffraction. Powder samples with 0 ≤ x ≤ 1 were synthesized by the solid state reaction of the pure elements and it was confirmed by wavelength-dispersive X-ray spectroscopy that each contained a homogeneous, off-stoichiometric quaternary phase. The lattice parameters and cation site occupancy factors were determined simultaneously by the Rietveld analysis of the neutron and X-ray powder diffraction data. The refined site occupancy factors were used to determine the average neutron scattering length of the cation sites in the crystal structure of the Cu 2 (Zn 1- x Mn x )SnSe 4 mixed crystals, from which a cation distribution model was derived. For the end member Cu 2 ZnSnSe 4 , the disordered kesterite structure was confirmed and for Cu 2 MnSnSe 4 , the stannite structure was confirmed. The cross-over from the kesterite to stannite type structure by Zn 2+ ↔ Mn 2+ substitution in the Cu 2 Zn 1- x Mn x SnSe 4 solid solution can be seen as a cation re-distribution process among the positions (0, 0, 0), (0, ½, ¼) and (0, ¼, ¾), including Cu + , Zn 2+ and Mn 2+ . The Sn 4+ cation does not take part in this process and remains on the 2b site. Moreover, the cross-over is also visible in the ratio of the lattice parameters c /(2 a ), showing a characteristic dependence on the chemical composition. The order parameter Q , the quantitative measure of Cu/B II disorder (B II = Zn and Mn), shows a distinct dependence on the Mn/(Mn + Zn) ratio. In Zn-rich Cu 2 (Zn 1- x Mn x )SnSe 4 mixed crystals, the order parameter Q ∼ 0.7 and drops to Q ∼ 0 (complete Cu/B II disorder) in the compositional region 0.3 ≥ x ≥ 0.7. In Mn-rich Cu 2 (Zn 1- x Mn x )SnSe 4 mixed crystals, adopting the stannite type structure, the order parameter reaches almost Q ∼ 1 (order). Thus, it can be concluded that only Mn-rich Cu 2 (Zn 1- x Mn x )SnSe 4 mixed crystals do not show Cu/B II disorder. A similar trend of the dependence on the chemical composition of both Cu/B II -disorder and the band gap energy E g in Cu 2 (Zn 1- x Mn x )SnSe 4 mixed crystals was observed.

The earth-abundant ternary compound BaZrS 3 , which crystallizes in the perovskite-type structure, has come into view as a promising candidate for photovoltaic applications. We present the synthesis and characterization of polycrystalline perovskite-type BaZrS 3 thin films. BaZrO 3 precursor layers were deposited by pulsed laser deposition and sulfurized at various temperatures in an argon-diluted H 2 S atmosphere. We observe increasing incorporation of sulfur for higher annealing temperatures, accompanied by a red shift of the absorption edge, with a bandgap of E g = 1.99 eV and a large absorption strength >10 5 cm -1 obtained for sulfurization temperatures of 1000 °C. X-ray diffraction analysis and SEM indicate enhanced crystallization at the higher annealing temperatures, but no evidence for a crystalline solid solution between the BaZrO 3 and BaZrS 3 phases is found. The charge carrier sum mobility estimated from optical-pump-terahertz-probe spectroscopy indicates increasing mobilities with increasing sulfurization temperature, reaching maximum values of up to ∼2 cm 2 V -1 s -1 .

Multiple-Edge Anomalous Diffraction (MEAD) has been applied to various quaternary sulfosalts belonging to the adamantine compound family in order to validate the distribution of copper, zinc and iron cations in the structure. Semiconductors from this group of materials are promising candidates for photovoltaic applications. Their properties strongly depend on point defects, in particular related to cation order-disorder. However, Cu + , Zn 2+ and Fe 2+ have very similar scattering factors and are all but indistinguishable in usual X-ray diffraction experiments. Anomalous diffraction utilizes the dependency of the atomic scattering factors f' and f'' of the energy of the radiation, especially close to the element-specific absorption edges. In the MEAD technique, individual Bragg peaks are tracked over an absorption edge. The intensity changes depending on the structure factor can be highly characteristic for Miller indices selected for a specific structural problem, but require very exact measurements. Beamline KMC-2 at synchrotron BESSY II, Berlin, has been recently upgraded for this technique. Anomalous X-ray powder diffraction and XAFS compliment the data. Application of this technique confirmed established cation distribution in Cu 2 ZnSnSe 4 (CZTSe) and Cu 2 FeSnS 4 (CFTS). In contrast to the literature, cation distribution in Cu 2 ZnSiSe 4 (CZSiSe) is shown to adopt a highly ordered wurtz-kesterite structure type., (open access.)

Microstructural properties of Cu2ZnSn(S(x)Se(1-x))4 kesterite solid solutions were investigated using grazing incidence X-ray diffraction for the full interval of anion compositions in order to explore the influence of S and Se atoms on the thin film morphology. Thin films were prepared by sputtering deposition of metallic precursors, which were then submitted to a high temperature sulfo-selenization process. By adjusting process parameters samples from sulfur- to selenium-pure (0 ≤ x ≤ 1) were made. Microstructural analysis shows a strong dependence of domain size and microstrain on composition. Both values increase with higher sulfur content, and depth profile analysis by grazing incidence X-ray diffraction shows selenium-rich films tend to have a more homogeneous depth distribution of domain size. The increasing trend in domain size of S-rich absorbers can be related to lower formation energies of the sulfur binary phases leading to formation of kesterites, while the increase in the microstrain is explained by the substitution of larger Se atoms with smaller S atoms in the host lattice and the presence of secondary phases.