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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.
Na has deliberately been incorporated into Cu(In,Ga)Se2 (“CIGSe”) chalcopyrite thin-film solar cell absorbers deposited on Mo-coated polyimide flexible substrates by adding differently thick layers of NaF in-between CIGSe absorber and Mo back contact. The impact of Na on the chemical and electronic surface structure of CIGSe absorbers with various Cu-contents deposited at comparatively low temperature (420 °C) has been studied using x-ray photoelectron and x-ray excited Auger electron spectroscopy. We observe a higher Nasurface content for the Cu-richer CIGSe samples and can distinguish between two different chemical Na environments, best described as selenide-like and oxidized Na species, respectively. Furthermore, we find a Cu-poor surface composition of the CIGSe samples independent of Na content and — for very high Na contents — indications for the formation of a (Cu,Na)–(In,Ga)–Se like compound. With increasing Nasurface content, also a shift of the photoemission lines to lower binding energies could be identified, which we interpret as a reduction of the downward band bending toward the CIGSe surface explained by the Na-induced elimination of InCu defects.
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.
The μm-scale spatial distribution of the elements of polycrystalline Cu(In1− x Ga x )Se2 absorber surfaces is examined using x-ray photoelectron emission microscopy. The chemical composition varies from grain to grain, and a direct, linear anticorrelation between the In 3d and Ga 2p photoemission line intensities is observed. The line intensities are interpreted in terms of a varying value of x= Ga/(In+ Ga); the band gaps calculated from the inferred compositions of the grains are shown to be normally distributed with a standard deviation of 40 meV.
The chemical and electronic properties of a-
Si:H(B)/ZnO:Al and μc-Si:H(B)/ZnO:Al thin-film solar cell structures
are studied by hard X-ray photoelectron spectroscopy (HAXPES).
Using a combination of different X-ray excitation energies
and deliberate sample design, we were able to select the probed
volume, i.e., the silicon capping layer only or the silicon and zinc
oxide layer (including the buried interface). For the a-Si:H(B) material,
we find a higher deposition rate and a smaller value for the
modified Auger parameter than for μc-Si:H(B). In addition, we
find indications of a pronounced band bending limited to the very
surface of the a-Si:H(B) and the μc-Si:H(B) layers, which is more
distinct in the latter case.
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.