Susan Schorr

A neutron diffraction study of the stannite-kesterite solid solution series

The intermixing of stannite and kesterite, i.e. Cu 2 Fe 1−x Zn x SnS 4 , was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ x ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 750°C and quenched after the final annealing. The lattice parameter, cation site occupancies and the isotropic temperature factors were determined by simultaneously Rietveld analysis of X-ray and neutron powder diffraction data. The refined lattice constants are in agreement with the literature. The refined site occupancy factors were used to determine the average neutron scattering length of the cation sites in stannite, kesterite and Cu 2 Fe 1−x Zn x SnS 4 solid solutions, giving new insight into the cation distribution. For Cu 2 FeSnS 4 the stannite type cation ordering was approved, whereas a new ordering was obtained in Cu 2 ZnSnS 4 . In the latter Cu occupies the 2a site, Zn and the remaining Cu are disordered on 2c (0, ½, ¼) and 2d (0, ¼, ¾). The disorder may be due to sample synthesis, i.e. to the quenching of the samples. The cross-over from stannite to kesterite by Fe 2+ ↔ Zn 2+ substitution in the Cu 2 Fe 1−x Zn x SnS 4 solid solution can be foreseen as a 3-stage process of cation exchange among the positions at (0, 0, 0), (0, ½, ¼) and (0, ¼, ¾), including Cu + , Zn 2+ and Fe 2+ . The Sn 4+ cation does not take part in this process and remains on the 2b site. Moreover the crossover is also visible in the ratio of the lattice parameter c/(2a), showing a characteristic dependence on chemical composition. The refined isotropic temperature factors were also taken into account. They give a consistent picture and to a degree support the developed cation distribution.

Determination of secondary phases in kesterite Cu2ZnSnS4 thin films by x-ray absorption near edge structure analysis

Secondary phases in Cu2ZnSnS4 (CZTS) are investigated by x-ray absorption spectroscopy. Evaluating the x-ray absorption near edge structure at the sulfur K-edge, we show that secondary phases exhibit sufficiently distinct features to allow their quantitative determination with high accuracy. We are able to quantify the ZnS fraction with an absolute accuracy of ±3%, by applying linear combination analysis using reference spectra. We find that even in CZTS thin films with [Sn]/[Zn] ≈ 1, a significant amount of ZnS can be present. A strong correlation of the ZnS-content with the degradation of the electrical performance of solar cells is observed.

Comprehensive comparison of various techniques for the analysis of elemental distributions in thin films

In a recent publication by Abou-Ras et al., various techniques for the analysis of elemental distribution in thin films were compared, using the example of a 2-µm thick Cu(In,Ga)Se2 thin film applied as an absorber material in a solar cell. The authors of this work found that similar relative Ga distributions perpendicular to the substrate across the Cu(In,Ga)Se2 thin film were determined by 18 different techniques, applied on samples from the same identical deposition run. Their spatial and depth resolutions, their measuring speeds, their availabilities, as well as their detection limits were discussed. The present work adds two further techniques to this comparison: laser-induced breakdown spectroscopy and grazing-incidence X-ray fluorescence analysis.The present work shows results on elemental distribution analyses in Cu(In,Ga)Se2 thin films for solar cells performed by use of wavelength-dispersive and energy-dispersive X-ray spectrometry (EDX) in a scanning electron microscope, EDX in a transmission electron microscope, X-ray photoelectron, angle-dependent soft X-ray emission, secondary ion-mass (SIMS), time-of-flight SIMS, sputtered neutral mass, glow-discharge optical emission and glow-discharge mass, Auger electron, and Rutherford backscattering spectrometry, by use of scanning Auger electron microscopy, Raman depth profiling, and Raman mapping, as well as by use of elastic recoil detection analysis, grazing-incidence X-ray and electron backscatter diffraction, and grazing-incidence X-ray fluorescence analysis. The Cu(In,Ga)Se2 thin films used for the present comparison were produced during the same identical deposition run and exhibit thicknesses of about 2 μm. The analysis techniques were compared with respect to their spatial and depth resolutions, measuring speeds, availabilities, and detection limits.

Comprehensive insights into point defect and defect cluster formation in CuInSe2

The concentration of native point defects in CuInSe2 powder material as a function of stoichiometry has been experimentally determined by neutron powder diffraction. A correlation between the Cu/In ratio and the density of VCu as well as InCu has been established and their concentrations are quantified. It is demonstrated, that assuming the spontaneous formation of defect pairs, the density of native point defects is reduced significantly by an order of magnitude. The functionality of a solar device, assuming same conditions like in the analyzed material, may be explained by a neutralization due to the formation of electrically inactive defect complexes.

Optical constants of Cu2ZnGeS4 bulk crystals

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.

Optically Induced Structural Transformation in Disordered Kesterite Cu 2 ZnSnS 4

The kesterite-structured semiconductor Cu2ZnSnS4 is one of the most promising compound for earth-abundant low-cost solar cells. One of the complex problem on this way deals with its stoichiometry. In this work Raman spectra of Cu-rich Cu2ZnSnS4 crystals are discussed in connection with the non-stoichiometric composition and disordering within the cation sublattice of the kesterite. The shift of the main A-peak from 338 to 331 cm−1 and its broadening are attributed here to transition from the kesterite (I

Multiwavelength excitation Raman scattering of Cu2ZnSn(SxSe1−x)4 (0 ≤ x ≤ 1) polycrystalline thin films: Vibrational properties of sulfoselenide solid solutions

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Formation of Cu 2 ZnSnS 4 and Cu 2 ZnSnS 4 -CuInS 2 Thin Films Investigated by In-Situ Energy Dispersive X-Ray Diffraction

symmetry) to the disordered kesterite structure (I

Single Crystal X-ray Structure Investigation of Cu2ZnSnSe4

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The multiwavelength cold neutron time-of-flight spectrometer project IN500 at LANSCE

2m symmetry). It is shown that this transition may also be driven by an intense light, which could stimulate transformation of Cu+-ion to Cu2+-ions and facilitates generation of CuZn-defects on 2d-crystalographic positions.