The carbon-rich layer at the back-contact interface of a solution-processed Cu2ZnSn(S,Se)4 (CZTSSe) absorber is investigated with a combination of surface-sensitive X-ray photoelectron and bulk-sensitive X-ray emission spectroscopy. For absorber deposition, an aqueous ammonium-thioglycolate (ATGL) solution was used, and the “buried” back-contact interface was accessed by cleaving in a liquid nitrogen environment. In the pertinent literature, it is reported that such a carbon layer at the absorber/back-contact interface could have beneficial effects, e.g., to reduce series resistance or increase the short circuit current. Here, a detailed picture of the chemical structure of this carbon-rich layer at the back contact is derived, which consists of carbon (74 ± 7%), selenium (19 ± 4%), and sulfur (7 ± 3%). The selenium in this layer is found as elemental inclusions, possibly from not fully reacted selenium during the absorber production. The sulfur content in this carbon-rich layer is twice that of sulfur in the absorber. A detailed analysis of the chemical environment suggests that residuals from the aqueous ATGL solution are the origin of sulfur in this carbon-rich layer. Furthermore, underneath the carbon-rich layer, S-Mo bonds are found at the Mo back contact.

Author(s): Hauschild, D; Mezher, M; Schnabel, T; Spiering, S; Kogler, W; Carter, J; Blum, M; Yang, W; Ahlswede, E; Heske, C; Weinhardt, L | Abstract: © Copyright 2019 American Chemical Society. We report on the chemical and electronic structure of the interface between a thermally co-evaporated InxSy buffer and a Cu2ZnSn(S,Se)4 (CZTSSe) absorber for thin-film solar cells. To date, such cells have achieved energy conversion efficiencies up to 8.6%. Using surface-sensitive X-ray and UV photoelectron spectroscopy, combined with inverse photoemission and bulk-sensitive soft X-ray emission spectroscopy, we find a complex character of the buffer layer. It includes oxygen, as well as selenium and copper that diffused from the absorber into the InxSy buffer, exhibits an electronic band gap of 2.50 ± 0.18 eV at the surface, and leads to a small cliff in the conduction band alignment at the InxSy/CZTSSe interface. After an efficiency-increasing annealing step at 180 °C in nitrogen atmosphere, additional selenium diffusion leads to a reduced band gap at the buffer layer surface (2.28 ± 0.18 eV).

The local valence orbital structure of solid glycine, diglycine, and triglycine is studied using soft X-ray emission spectroscopy (XES), resonant inelastic soft X-ray scattering (RIXS) maps, and spectra calculations based on density-functional theory. Using a building block approach, the contributions of the different functional groups of the peptides are separated. Cuts through the RIXS maps furthermore allow monitoring selective excitations of the amino and peptide functional units, leading to a modification of the currently established assignment of spectral contributions. The results thus paint a new-and-improved picture of the peptide bond, enhance the understanding of larger molecules with peptide bonds, and simplify the investigation of such molecules in aqueous environment.

The occupied and unoccupied electronic structure of imidazole (C3N2H4) and imidazolium (C3N2H5+) in aqueous solutions is studied by X-ray emission spectroscopy (XES) and resonant inelastic soft X-ray scattering (RIXS). Both systems show distinct RIXS fingerprints with strong resonant effects. A comparison with calculated X-ray emission spectra of isolated imidazole and imidazolium suggests only a small influence of hydrogen bonding in the aqueous solution on the electronic structure of imidazole and imidazolium, and allows the attribution of specific spectral features to the non-equivalent nitrogen and carbon atoms in the molecules. In the case of nitrogen, this can also be achieved by site-selective resonant excitation. Furthermore, we find spectator shifts and symmetry selectivity in the RIXS spectra, as well as indications for rapid proton dynamics on the femtosecond timescale of the RIXS process, and derive the HOMO-LUMO gaps for the two molecules in aqueous solution.

Nonresonant N K, O K, C K, and S L2,3 X-ray emission spectra of the 20 most common proteinogenic amino acids in their solid zwitterionic form are reported. They represent a comprehensive database that can serve as a reliable basis for the X-ray absorption spectroscopy (XES) studies of peptides and proteins. At the most important N and O K edges, clear similarities and differences between the spectra of certain amino acids are observed and associated with the specific chemical structure of these molecules and their functional groups. Analysis of these spectra allows the generation of spectral fingerprints of the protonated amino group, the deprotonated carboxylic group, and, using a building block approach, the specific nitrogen- and oxygen-containing functional groups in the side chains of the amino acids. Some of these fingerprints are compared to the spectra of reference compounds with the respective functional groups; they exhibit reasonable similarity, underlining the validity of the spectral fingerprint approach. The C K and S L2,3 XES spectra are found to be specific for each amino acid, in accordance with the different local environments of the involved C and S atoms, respectively.

Understanding the molecular structure of the hydration shells and their impact on the hydrogen bond (HB) network of water in aqueous salt solutions is a fundamentally important and technically relevant question. In the present work, such hydration effects were studied for a series of representative salt solutions (NaCl, KCl, CaCl2, MgCl2, and KBr) by soft X-ray emission spectroscopy (XES) and resonant inelastic soft X-ray scattering (RIXS). The oxygen K-edge XES spectra could be described with three components, attributed to initial state HB configurations in pure water, water molecules that have undergone an ultrafast dissociation initiated by the X-ray excitation, and water molecules in contact with salt ions. The behavior of the individual components, as well as the spectral shape of the latter component, has been analyzed in detail. In view of the role of ions in such effects as protein denaturation (i.e., the Hofmeister series), we discuss the ion-specific nature of the hydration shells and find that the results point to a predominant role of anions as compared to cations. Furthermore, we observe a concentration-dependent suppression of ultrafast dissociation in all salt solutions, associated with a significant distortion of intact HB configurations of water molecules facilitating such a dissociation.

The electronic structure of gas-phase methanol molecules (H3COH, H3COD, and D3COD) at atmospheric pressure was investigated using resonant inelastic soft X-ray scattering (RIXS) at the O K and C K edges. We observe strong changes of the relative emission intensities of all valence orbitals as a function of excitation energy, which can be related to the symmetries of the involved orbitals causing an angularly anisotropic RIXS intensity. Furthermore, all observed emission lines are subject to strong spectator shifts of up to -0.9 eV at the O K edge and up to -0.3 eV at the C K edge. At the lowest O K resonance, we find clear evidence for dissociation of the methanol molecule on the time scale of the RIXS process, which is illustrated by comparing X-ray emission spectra of regular and deuterated methanol.

We have investigated the impact of heat treatments on the chemical structure of the In2S3/CuIn(S,Se)2 thin-film solar cell interface using X-ray photoelectron and soft X-ray emission spectroscopy. As-grown, we find the formation of a sulfur-poor (indium-rich) In2S3 surface, an abrupt interface, and sulfur atoms in both In2S3 and CuIn(S,Se)2 chemical environments (as expected for an abrupt interface and a thin overlayer). After a heat treatment at 200 °C to simulate subsequent process steps, a strong copper and sodium diffusion into the In2S3 layer is observed. This diffusion extends throughout the layer, indicating the formation of a copper–indium–sulfide phase.

To probe the influence of hydrogen bonding on the electronic structure of ammonia, gas phase and aqueous NH3 have been investigated using soft X-ray absorption (XAS), resonant inelastic soft X-ray scattering (RIXS), and electronic structure calculations including dynamical effects. Strong spectral differences in the XAS scans as well as in the RIXS spectra between gas phase and aqueous NH3 are attributed to orbital mixing with the water orbitals, dipole-dipole interactions, differences in vibronic coupling, and nuclear dynamics on the time-scale of the RIXS process. All of these effects are consequences of hydrogen bonding and the impact of the associated orbitals, demonstrating the power of XAS and RIXS as unique tools to study hydrogen bonding in liquids.

Hydrogenated microcrystalline silicon oxide (mu c-SiOx:H) layers are one alternative approach to ensure sufficient interlayer charge transport while maintaining high transparency and good passivation in Si-based solar cells. We have used a combination of complementary x-ray and electron spectroscopies to study the chemical and electronic structure of the. c-SiOx:H material system. With these techniques, we monitor the transition from a purely Si-based crystalline bonding network to a silicon oxide dominated environment, coinciding with a significant decrease of the material's conductivity.Most Si-based solar cell structures contain emitter/contact/passivation layers. Ideally, these layers fulfill their desired task (i.e., induce a sufficiently high internal electric field, ensure a good electric contact, and passivate the interfaces of the absorber) without absorbing light. Usually this leads to a trade-off in which a higher transparency can only be realized at the expense of the layer's ability to properly fulfill its task. One alternative approach is to use hydrogenated microcrystalline silicon oxide (mu c-SiOx:H), a mixture of microcrystalline silicon and amorphous silicon (sub) oxide. The crystalline Si regions allow charge transport, while the oxide matrix maintains a high transparency. To date, it is still unclear how in detail the oxygen content influences the electronic structure of the. c-SiOx:H mixed phase material.To address this question, we have studied the chemical and electronic structure of the. c-SiOx:H (0 <= x = O/Si <= 1) system with a combination of complementary x-ray and electron spectroscopies. The different surface sensitivities of the employed techniques help to reduce the impact of surface oxides on the spectral interpretation. For all samples, we find the valence band maximum to be located at a similar energy with respect to the Fermi energy. However, for x > 0.5, we observe a pronounced decrease of Si 3s - Si 3p hybridization in favor of Si 3p - O 2p hybridization in the upper valence band. This coincides with a significant increase of the material's resistivity, possibly indicating the breakdown of the conducting crystalline Si network.Silicon oxide layers with a thickness of several hundred nanometres were deposited in a PECVD (plasma-enhanced chemical vapor deposition) multi chamber system using an excitation frequency of 13.56 MHz with a plasma power

We present a novel gas cell designed to study the electronic structure of gases and gas/solid interfaces using soft x-ray emission and absorption spectroscopies. In this cell, the sample gas is separated from the vacuum of the analysis chamber by a thin window membrane, allowing in situ measurements under atmospheric pressure. The temperature of the gas can be regulated from room temperature up to approximately 600 °C. To avoid beam damage, a constant mass flow can be maintained to continuously refresh the gaseous sample. Furthermore, the gas cell provides space for solid-state samples, allowing to study the gas/solid interface for surface catalytic reactions at elevated temperatures. To demonstrate the capabilities of the cell, we have investigated a TiO2 sample behind a mixture of N2 and He gas at atmospheric pressure.

The electronic structure of the amino acid L-cysteine in an aqueous environment was studied using resonant inelastic soft X-ray scattering (RIXS) in a 2D map representation and analyzed in the framework of a "building block" approach. The element selectivity of RIXS allows a local investigation of the electronic structure of the three functional groups of cysteine, namely, the carboxyl, amino, and thiol groups, by measuring at the O K, N K, and S L2,3 edges, respectively. Variation of the pH value allows an investigation of molecules with protonated and deprotonated functional groups, which can then be compared with simple reference molecules that represent the isolated functional groups. We find that such building blocks can provide an excellent description of X-ray emission spectroscopy (XES) and RIXS spectra, but only if all nearest-neighbor atoms are included. This finding is analogous to the building block principle commonly used in X-ray absorption spectroscopy. The building blocks show a distinct spectral character (fingerprint) and allow a comprehensive interpretation of the cysteine spectra. This simple approach opens the path to investigate the electronic structure of more complex biological molecules in aqueous solutions using XES and RIXS.

To enable an understanding and optimization of the optoelectronic behavior of CdTe-ZnO nanocomposites, the morphological and chemical properties of annealed CdTe/ZnO interface structures were studied. For that purpose, CdTe layers of varying thickness (4–24 nm) were sputter-deposited on 100 nm-thick ZnO films on surface-oxidized Si(100) substrates. The morphological and chemical effects of annealing at 525 °C were investigated using X-ray Photoelectron Spectroscopy (XPS), X-ray-excited Auger electron spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, and atomic force microscopy. We find a decrease of the Cd and Te surface concentration after annealing, parallel to an increase in Zn and O signals. While the as-deposited film surfaces show small grains (100 nm diameter) of CdTe on the ZnO surface, annealing induces a significant growth of these grains and separation into islands (with diameters as large as 1 μm). The compositional change at the surface is more pronounced for Cd than for Te, as evidenced using component peak fitting of the Cd and Te 3d XPS peaks. The modified Auger parameters of Cd and Te are also calculated to further elucidate the local chemical environment before and after annealing. Together, these results suggest the formation of tellurium and cadmium oxide species at the CdTe/ZnO interface upon annealing, which can create a barrier for charge carrier transport, and might allow for a deliberate modification of interface properties with suitably chosen thermal treatment parameters.

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 non-equivalent 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.

We report on the characterization of the chemical and electronic interface structure of the heavily intermixed In 2 S 3 /Cu(In, Ga)Se 2 (CIGSe) interface. Discussing our findings inferred from direct and inverse photoemission as well as soft x-ray emission spectroscopy, we particularly focus on the impact of the interfacial intermixing/interdiffusion processes on the electronic band alignment at the interface. Furthermore, we present deposition-temperature-dependent data on the (In, Al) 2 S 3 /CIGSe interface. We find that the chemical processes at the buffer/absorber interface are thermally activated with a threshold temperature of approximately 200°C.

Thin-film solar cells based on compound semiconductors consist of a multilayer structure with various interfaces and contain a multitude of elements and impurities, etc. A rapid progress of these photovoltaic technologies can only be achieved by an insight-driven optimization/development. Hence it is crucial to characterize and understand the relationship between the chemical and electronic properties of these components. This paper reviews some examples of our recent work characterizing compound semiconductor thin films using laboratory- and synchrotron-based electron and soft X-ray spectroscopic characterization methods. It is demonstrated how these different analytical techniques are extraordinarily powerful to reveal the material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of the related electronic and chemical properties. As examples, the paper will discuss the electronic surface structure of chalcopyrite thin-film solar cell absorbers, the chemical structure of the CdS/chalcopyrite interface, present the band alignment at the CdS/kesterite interface, and report on how post-deposition treatments cause chemical interaction/interdiffusion processes in CdTe/CdS thin-film solar cell structures. (C) 2012 Elsevier B.V. All rights reserved.

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.

We have investigated the surface and near-surface bulk chemical properties of differently stressed (Au/Cu/)CdTe/CdS thin-film solar cells. The employed Au/Cu contact design created unique samples in which both the back contact surface/interface and the (back) surface of the CdTe absorber were exposed. X-ray photoelectron spectroscopy and soft x-ray emission spectroscopy were employed to determine the chemical composition of the surface and near-surface bulk of both the exposed CdTe surface and the Au/Cu back contact surface of each sample. Sulfur is observed to have migrated (from the CdS) into the CdTe bulk, forming Au2S and Cu2-xS compounds under the back contact. Chlorine is found at the surface and in the near-surface bulk of both the back contact and the exposed CdTe absorber. Under the back contact, Cl is observed to form Cu-Cl bonds only in the samples that were heat stressed under AM 1.5 illumination.

To optimize materials and devices for solar photoelectrochemical hydrogen production, a detailed understanding of the chemical and electronic properties, in particular at the reactive surfaces and interfaces, is needed. In this review article we will show how electron and soft X-ray spectroscopies can provide such information. We will present exemplary studies using X-ray photoelectron spectroscopy, soft X-ray emission spectroscopy, UV photoelectron spectroscopy, and inverse photoemission. While the first two techniques mainly give insight into the chemical properties at and near the surface, the latter two methods allow us to derive the electronic levels relevant for photoelectrochemical water splitting at the surface of the investigated material. Ultimately, the ideal experiment would be performed in situ, in which the device is studied under working conditions, i.e., in a liquid environment and under illumination. We will give a short outlook on how this can be achieved experimentally under the strict requirements of the measurement environment. (C) 2012 Elsevier B.V. All rights reserved.