Többens, Daniel M.; Valle-Rios, Laura E.; Neldner, Kai; Gurieva, Galina; Zander, Stefan; Gunder, Rene; Schorr, Susan: Quantitative cation occupancies in CZTS materials by anomalous powder diffraction. 2015 MRS Spring Meeting & Exhibit San Francisco, California, USA, 06.04.2015 - 10.04.2015 (2015)
Poster
Abstract:
The compound semiconductor Cu2ZnSnS4 (CZTS), is a promising alternative for absorber layers in thin film solar cells. Replacement of some elements with chemically similar ones, in particular (S,Se), allows to optimize desired characteristics; Cu2ZnSn(S,Se)4 have already achieved efficiencies above 12% [1]. Moreover, off-stoichiometric composition allows introducing vacancies in the structure. Concentration and distribution of point defects depend on thermal history, in particular low-temperature thermal history, of the compound [2]. Besides chemical composition and phase purity, the efficiency of CZTSSe thin film solar cells depends strongly on the concentration of these vacancies and Cu- and Zn-antisites. However, Cu(I) and Zn(II) are isoelectric and thus cannot be distinguished by conventional X-ray diffraction. In prior work [3] we determined Cu-Zn-distribution successfully using the different neutron scattering lengths of Cu and Zn. Anomalous X-ray diffraction can also be used to this effect. Anomalous scattering coefficients are highly wavelength-dependent close to the absorption edges of the respective element. This can be utilized for contrast enhancement. The Diffraction station at KMC-2 beamline at BESSY is well suited for these experiments. The Cu-K edge (8979 eV) and Zn-K edge (9659 eV) are in the center of the available energy range. The narrow energy width of the monochromator allows to use energies very close to the absorption edge, where f' contrast enhancement is at a maximum. Changes in the diffraction pattern at different wavelengths are highly significant at the attainable counting statistics. However, even in effective total absence of uncertainty from counting and particle statistics an unusual high degree of variance was found in the values for cation site occupation factors resulting from straightforward Rietveld refinement of data sets taken at multiple wavelengths. A detailed analysis reveals this to be the result of extremely high correlation between various structural parameters. These in turn result from the extreme crystallographic similarity of the different sites, which in the Kesterite structure are non-equivalent only due to the very cation ordering analyzed. While this can be handled comparatively easy in single-crystal experiments, it is amplified by the loss of information in powder diffraction. We present a recipe to circumvent this problem by use of selected derived parameters. This allows quantification of Cu and Zn site occupancies, going beyond currently published papers, which either restrict themselves to qualitative changes [4,5] or use single crystal data [6]. [1] W. Wang, et al., Advanced Energy Materials, (2014) 4(7) [2] J.J.S. Scragg, et al., Appl. Phys. Let. (2014) 104, 041911 [3] S. Schorr, Sol. Energy Mat. and Sol. Cells. (2011) 95, 1482 [4] H. Nozakia et al., J. Alloys Comp. (2012) 524, 22 [5] T. Washio et al., J. Appl. Phys. (2011) 110, 074511 [6] A.0. Lafond et al., Acta Cryst. B (2014) 70, 390