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The electronic structure of the CdS/Cu2ZnSnS4 (CZTS) heterojunction was investigated by direct and inverse photoemission. The effects of a KCN etch of the CZTS absorber prior to CdS deposition on the band alignment at the respective interface were studied. We find a “cliff”-like conduction band offset at the CdS/CZTS interface independent of absorber pretreatment and a significant etch-induced enhancement of the energetic barrier for charge carrier recombination across the CdS/CZTS interface.
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.
Resonant inelastic soft x-ray scattering (RIXS) was used to study the electronic structure of solid
cysteine films. A RIXS map approach, i.e., plotting the x-ray emission intensity as a function of
excitation and emission energy, allows us to separate the contributions of the three chemically nonequivalent
carbon atoms in cysteine. In particular, we can identify orbitals localized near the photoexcited
atoms, as well as orbitals that are delocalized over the entire molecule.
In this paper, X-ray photoelectron and X-ray-excited
Auger electron spectroscopy was used to investigate the chemical
surface structure of CuInS2 thin-film solar cell absorbers.We find
that the [In]/[Cu] surface composition can vary between 1.6 (±0.4)
and 3.7 (±0.7), depending on relatively minor variations in the absorber
formation process and/or whether additional wet-chemical
treatments are performed. These variations are primarily due to
differences in the Cu surface concentration. The corresponding
change of the modified In Auger parameter is interpreted as being
indicative of a change in the chemical environment of In as a
function of Cu off-stoichiometry.
Spatial variations in the chemical composition of the (Zn,Mg)O/CuIn(S,Se)2 thin-film solar cell interface were studied by photoemission electron microscopy (PEEM). Energy filtered PEEM
images indicate significant differences in the magnesium and zinc distribution. Local photoemission measurements reveal a relative difference in the derived Mg/(ZnþMg) composition of the (Zn,Mg)O material of up to (11.460.7)%, which can be expected to induce band gap
fluctuations of (60630) meV. Furthermore, local areas with significant accumulations of sodium could be observed.
The surface properties of CuInS 2 (CIS) thin-fi lm solar cell absorbers are
investigated by a combination of electron and soft X-ray spectroscopies.
Spatially separated regions of varying colors are observed and identifi ed to
be dominated by either CuS or Cu 2 S surface phases. After their removal by
KCN etching, the samples cannot be distinguished by eye and the CIS surface
is found to be Cu-defi cient in both regions. However, a signifi cantly more
pronounced off-stoichiometry in the region initially covered by Cu 2 S can be
identifi ed. In this region, the resulting surface band gap is also signifi cantly
larger than the E g Surf of the initially CuS-terminated region. Such variations
may represent a hidden parameter which, if overlooked, induces irreproducibility
and thus prevents systematic optimization efforts.
Investigation of the ionic hydration in aqueous salt solutions by soft x‑ray emission spectroscopy
(2016)
Characterization of sulfur bonding in CdS:O buffer layers for CdTe-based thin-film solar cells
(2015)
We apply soft x-ray emission spectroscopy (XES) to measure the electronic structure of crystalline silicon supersaturated with sulfur (up to 0.7 at. %), a candidate intermediate-band solar cell material. Si L2,3 emission features are observed above the conventional Si valence band maximum, with intensity scaling linearly with S concentration. The lineshape of the S-induced features change across the insulator-to-metal transition, indicating a significant modification of the local electronic structure concurrent with the change in macroscopic electronic behavior. The relationship between the Si L2,3 XES spectral features and the anomalously high sub-band gap infrared absorption is discussed.
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).
The chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers has been investigated by direct and inverse photoemission. Particular emphasis was placed on the impact of KCN etching, which significantly alters the surface composition and is best explained by a preferred etching of Cu and, to a lesser degree, Sn. As a consequence the surface band gap increased from (1.53 +/- 0.15) eV, which agrees with optically derived bulk band gap values, to (1.91 +/- 0.15) ev.