Intermixing at the InxSy/Cu2ZnSn(S,Se)4 Heterojunction and Its Impact on the Chemical and Electronic Interface Structure

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)

Local electronic structure of the peptide bond probed by resonant inelastic soft X-ray scattering.

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

Molybdenum Disulfide Catalytic Coatings via Atomic Layer Deposition for Solar Hydrogen Production from Copper Gallium Diselenide Photocathodes

We demonstrate that applying atomic layer deposition-derived molybdenum disulfide (MoS2) catalytic coatings on copper gallium diselenide (CGSe) thin film absorbers can lead to efficient wide band gap photocathodes for photoelectrochemical hydrogen production. We have prepared a device that is free of precious metals, employing a CGSe absorber and a cadmium sulfide (CdS) buffer layer, a titanium dioxide (TiO2) interfacial layer, and a MoS2 catalytic layer. The resulting MoS2/TiO2/CdS/CGSe photocathode exhibits a photocurrent onset of +0.53 V vs RHE and a saturation photocurrent density of -10 mA cm-2, with stable operation for >5 h in acidic electrolyte. Spectroscopic investigations of this device architecture indicate that overlayer degradation occurs inhomogeneously, ultimately exposing the underlying CGSe absorber

Coupling methylammonium and formamidinium cations with halide anions: Hybrid orbitals, hydrogen bonding, and the role of dynamics

The electronic structures of four precursors for organic–inorganic hybrid perovskites, namely, methylammonium chloride and iodide, as well as formamidinium bromide and iodide, are investigated by X-ray emission (XE) spectroscopy at the carbon and nitrogen K-edges. The XE spectra are analyzed based on density functional theory calculations. We simulate the XE spectra at the Kohn–Sham level for ground-state geometries and carry out detailed analyses of the molecular orbitals and the electronic density of states to give a thorough understanding of the spectra. Major parts of the spectra can be described by the model of the corresponding isolated organic cation, whereas high-emission energy peaks in the nitrogen K-edge XE spectra arise from electronic transitions involving hybrids of the molecular and atomic orbitals of the cations and halides, respectively. We find that the interaction of the methylammonium cation is stronger with the chlorine than with the iodine anion. Furthermore, our detailed theoretical analysis highlights the strong influence of ultrafast proton dynamics in the core-excited states, which is an intrinsic effect of the XE process. The inclusion of this effect is necessary for an accurate description of the experimental nitrogen K-edge X-ray emission spectra and gives information on the hydrogen-bonding strengths in the different precursor materials

Spectroscopic investigation of the deeply buried Cu In,Ga S,Se 2 Mo interface in thin film solar cells

The Cu In,Ga S,Se 2 Mo interface in thin film solar cells has been investigated by surface sensitive photoelectron spectroscopy, bulk sensitive X ray emission spectroscopy, and atomic force microscopy. It is possible to access this deeply buried interface by using a suitable lift off technique, which allows to investigate the back side of the absorber layer as well as the front side of the Mo back contact. We find a layer of Mo S,Se 2 on the surface of the Mo back contact and a copper poor stoichiometry at the back side of the Cu In,Ga S,Se 2 absorber. Furthermore, we observe that the Na content at the Cu In,Ga S,Se 2 Mo interface as well as at the inner grain boundaries in the back contact region is significantly lower than at the absorber front surfac