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In view of the complexity of thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the
chemical and electronic structure of these components. Because of the high complexity of the Cu2ZnSn(S,Se)4 compound semiconductor absorber material alone, this is particularly true for kesterite-based devices. Hence, this paper reviews our recent progress in the characterization of Cu2ZnSnS4 (CZTS) thin films. It is demonstrated that a combination of different soft x-ray spectroscopies is an extraordinarily powerful method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. The focus of the article will be on secondary impurity phases, electronic structure, native oxidation, and the CZTS surface composition.
The electronic structure of gas-phase H2O and D2O molecules has been investigated using resonant
inelastic soft x-ray scattering (RIXS). We observe spectator shifts for all valence orbitals when
exciting into the lowest three absorption resonances. Strong changes of the relative valence orbital
emission intensities are found when exciting into the different absorption resonances, which can be
related to the angular anisotropy of the RIXS process. Furthermore, excitation into the 4a1 resonance
leads to nuclear dynamics on the time scale of the RIXS process; we find evidence for vibrational
coupling and molecular dissociation in both, the spectator and the participant emission.
Resonant inelastic soft X-ray scattering (RIXS) has been used to study the
electronic structure of glycine and lysine in aqueous solution. Upon variation of the pH
value of the solution from acidic to basic, major changes of the nitrogen K edge RIXS data
are observed for both amino acids, which are associated with the protonation and
deprotonation of the amino groups. The experimental results are compared with
simulations based on density functional theory, yielding a detailed understanding of the
spectral changes, as well as insights into the ultrafast proton dynamics in the intermediate
core-excited/ionized state of the RIXS process.
The valence band structures (VBS) of eight
transition metals (Fe, Co, Ni, Cu, Pd, Ag, Pt, Au) were
investigated by photoelectron spectroscopy (PES) using He I,
He II, and monochromatized Al Kα excitation. The influence
of final states, photoionization cross-section, and adsorption of
residual gas molecules in an ultrahigh vacuum environment are
discussed in terms of their impact on the VBS. We find that
VBSs recorded with monochromatized Al Kα radiation are
most closely comparable to the ground state density of states
(DOS) derived from quantum mechanics calculations. We use
the Al Kα-excited PES measurements to correct the energy scale of the calculated ground-state DOS to approximate the “true”
ground-state d-band structure. Finally, we use this data to test the d-band center model commonly used to predict the electronicproperty/
catalytic-activity relationship of metals. We find that a simple continuous dependence of activity on d-band center
position is not supported by our results (both experimentally and computationally).