A. Thißen

Changes in the crystal and electronic structure of LiCoO2 and LiNiO2 upon Li intercalation and de-intercalation

Li(x)CoO(2) and Li(x)NiO(2) (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of Li(x)CoO(2) and Li(x)NiO(2) as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Li(x)(Co,Ni)O(2) (0 < x < or = 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.

Band energy diagram of CdTe thin film solar cells

Abstract The knowledge of band energy diagrams of solar cells is essential for a fundamental understanding of their function. We have used photoelectron spectroscopy (PES) as a powerful tool for a systematic study of the formation of interfaces of CdTe solar cells in which the different layers CdS/SnO2, CdTe/CdS and Te/CdTe are deposited step by step by thermal evaporation in model experiments. The results of these studies show that in contrast to other investigations the energy converting heterojunction is not a simple n-CdS/p-CdTe contact. Although depth profiling reveals a homogeneously intrinsic CdTe bulk layer, contact formation and CdCl2-activation are assumed to form an n–i–p CdTe absorber. Such non-ideal conditions may strongly affect optimization processes of conversion efficiencies. The main limitations are evidently due to back-contact formation. Our results do not confirm that the electrochemically formed Te layer produces a good ohmic contact between the CdTe layer and the metallic back contact. From model experiments, we assume the formation of a tunneling contact, instead.

Photoelectron spectroscopic study of Li intercalation into V2O5 thin films

In this work the intercalation of lithium into V2O5 thin films prepared by physical vapor deposition (PVD) on highly oriented pyrolytic graphite (HOPG) substrates is studied by X-ray and UV induced photoelectron spectroscopy (XPS and UPS). The vanadium ions in the as-prepared V2O5 are mostly in a pentavalent V5+ state. The intercalated lithium strongly affects the electronic structure and causes a Fermi level shift of about 0.5 eV. The core level spectra show that the vanadium gets partially reduced to V4+ and even V3+ states. About 0.32 electrons per intercalated lithium atom are transferred from the Li2s-orbitals into V3d-like states. For a completely lithium-intercalated V2O5 film with a composition of LizV2O5 (z ≈ 2.4) a further increase in deposition time leads to lithium adsorption on the V2O5 surface and thus to the formation of lithium oxides (LixOy). Partially the vanadium gets reduced even further to V2+ and V1+ states indicating a decomposition reaction. The work function decreases with increasing lithium deposition time due to lithium intercalation into the V2O5, as well as to the lithium adsorption accompanied by the formation of surface dipoles and finally slightly increases again because of depolarization effects.

Surface analysis of CdTe thin film solar cells

The surface properties of CdTe thin film solar cells prepared by ANTEC using the close-space sublimation — (CSS) — technique have been analyzed by X-ray diffraction (XRD), atomic force microscopy (AFM), photoelectron emission microscopy (PEEM), high-resolution scanning electron microscopy (HRSEM) and photoelectron spectroscopy (XPS) after different pretreatment conditions. Exposure of the CdTe films to air leads to surface oxidation with the formation of TeO2 and CdO. The amount of surface oxides depends on the CdCl2 activation process. Activated surfaces are less oxidized than non-activated surfaces. Due to that surface oxidation, the surface is more n-type, indicating the formation of a surface barrier. The surface oxide can be removed by mild sputtering. The results suggest that no extra surface defects are introduced by this procedure. Before sputtering, Cl is found on the surface of the activated material, although no such contamination is found in the stoichiometric bulk material using XPS. A variation in the Fermi level position is observed for the non-activated to the activated CdTe material from weakly to higher p-doped levels. This type of conversion is evidently restricted to the near surface area as further in the bulk, weakly p-doped CdTe is found again. The results indicate that, besides the surface composition, the electronic properties of the film also depend on the different pretreatment steps.

Theoretical and experimental determination of the electronic structure of V2O5, reduced V2O5-x and sodium intercalated NaV2O5

In this work the electronic structure of V(2)O(5), reduced V(2)O(5-x) (V(16)O(39)) and sodium intercalated NaV(2)O(5) has been studied by both theoretical and experimental methods. Theoretical band structure calculations have been performed using density functional methods (DFT). We have investigated the electron density distribution of the valence states, the total density of states (total DOS) and the partial valence band density of states (PVBDOS). Experimentally, amorphous V(2)O(5) thin films have been prepared by physical vapour deposition (PVD) on freshly cleaved highly oriented pyrolytic graphite (HOPG) substrates at room temperature with an initial oxygen understoichiometry of about 4%, resulting in a net stoichiometry of V(2)O(4.8). These films have been intercalated by sodium using vacuum deposition with subsequent spontaneous intercalation (NaV(2)O(5)) at room temperature. Resonant V3p-V3d photoelectron spectroscopy (ResPES) experiments have been performed to determine the PVBDOS focusing on the calculation of occupation numbers and the determination of effective oxidation state, reflecting ionicity and covalency of the V-O bonds. Using X-ray absorption near edge spectra (XANES) an attempt is made to visualize the changes in the unoccupied DOS due to sodium intercalation. For comparison measurements on nearly stoichiometric V(2)O(5) single crystals have been performed. The experimental data for the freshly cleaved and only marginally reduced V(2)O(5) single crystals and the NaV(2)O(5) results are in good agreement with the calculated values. The ResPES results for V(2)O(4.8) agree in principle with the calculations, but the trends in the change of the ionicity differ between experiment and theory. Experimentally we find partly occupied V 3d states above the oxygen 2p-like states and a band gap between these and the unoccupied states. In theory one finds this occupation scheme assuming oxygen vacancies in V(2)O(5) and by performing a spin-polarized calculation of an antiferromagnetic ordered NaV(2)O(5.).

Utilization of sputter depth profiling for the determination of band alignment at polycrystalline CdTe/CdS heterointerfaces

The band alignment at polycrystalline CdS/CdTe heterointerfaces for thin-film solar cells is determined by photoelectron spectroscopy from stepwise CdTe deposition on polycrystalline CdS substrates and from subsequent sputter depth profiling. Identical values of 0.94±0.05 eV for the valence band offset are obtained.

Interface modification of CdTe thin film solar cells by CdCl2-activation

Abstract In thin film solar cell production several materials are subsequently deposited onto a glass substrate. The interface properties between the different layers are important for the opto-electrical performance of the solar cell device. CdTe thin film solar cells are currently produced using a layer sequence of CdTe/CdS/SnO 2 /ITO/glass. In order to reach reasonable conversion efficiencies the device has to be activated in a CdCl 2 atmosphere. Finally, the back contact is prepared. The influence of the activation step on the solar cell is still not understood in detail. Therefore in this study model experiments have been carried out in which CdS and CdTe thin films with a thickness of 100 nm have been deposited by thermal evaporation onto ITO/SnO 2 -coated glass substrates in an UHV system. The influence of the CdCl 2 -activation step on the morphology, chemistry and band alignment of the interfaces has been investigated with atomic force microscopy and sputter depth profiling using X-ray photoelectron spectroscopy. A change in surface morphology due to the CdCl 2 -activation has only been found for the CdTe layer, while the SnO 2 and CdS films are unaffected. It can be shown that the activation step leads to diffusion processes at both interfaces. For the CdS/CdTe interface an interdiffusion of CdS and CdTe takes place. At the SnO 2 /CdS interface Cd diffuses into the SnO 2 layer with a constant amount of approximately 5% Cd in the whole SnO 2 layer. The diffusion and interdiffusion changes the electronic properties of the interfaces. A strong n-type doping for all semiconductor films is observed after the activation.

Effect of in situ UHV CdCl2-activation on the electronic properties of CdTe thin film solar cells

Abstract To reach reasonable conversion efficiencies of approximately 10% and above with CdTe thin film solar cells an activation step involving chlorine at elevated temperatures seems to be necessary before back contact formation. This activation process has been simulated in an ultrahigh-vacuum (UHV) system. Solar cells with a maximum efficiency of 9.1% have been prepared using this process. In addition the effect of the CdCl 2 activation process on the electronic properties of each solar cell layer, SnO 2 , CdS and CdTe has been investigated in situ using photoelectron spectroscopy. The effects of the activation on the Fermi level position of all investigated layers is presented and discussed.

Oriented growth and band alignment at the CdTe/CdS interface

The CdTe/CdS interface has been investigated by photoemission and low energy electron diffraction. The growth of CdS on single crystalline CdTe substrates at elevated temperatures proceeds with the conservation of rotational symmetry. Initial results for the dependence of the band alignment on the crystallographic orientation based on calibrated core level binding energies are presented.

Untersuchung des Sputter‐Ätzens auf die Eigenschaften von PVD‐CrN Hartstoffschichten auf Magnesium AZ91hp

Eine gangige Methode, zu beschichtende Oberflachen von Verunreinigungen zu befreien und die Adhasionseigenschaften von PVD-Schichten zu verbessern, ist das sogenannte Sputter-Atzen vor dem eigentlichen PVD-Beschichtungsprozess. Beim Sputter-Atzen wird der Beschichtungsprozess umgekehrt und die Oberflache des Substrates selbst wird gesputtert. Im Rahmen dieser Veroffentlichung wurden vor der Abscheidung von PVD-CrN Schichten die Substrate der Magnesium Druckgusslegierung AZ91 unterschiedlichen Atzzeiten unterzogen und deren jeweiliger Einfluss auf die Grenzflache Schicht-Grundwerkstoff, auf die Schichtoberflache und auf die mechanischen Eigenschaften des beschichteten Systems hin untersucht. Die Grenzflachenuntersuchungen wurden mit tiefenaufgeloster XPS und mit SIMS durchgefuhrt. Die Oberflachen wurden mit hochauflosender Rasterelektronenmikroskopie und AFM untersucht. Die Charakterisierung der mechanischen Eigenschaften beinhaltet Schichtdicke, Schichtharte, Hartetiefenprofile, Schichthaftung, Schichteigenspannungen und die Schichtstruktur. Es wurden zum Teil sehr starke Eigenschaftsanderungen mit zunehmender Atzzeit festgestellt. Dies betrifft vor allem einige mechanische Eigenschaften sowie die Oberflachenrauheit der beschichteten Systeme. Beim Schichtsystem AZ91-Cr/CrN fuhren langere Atzzeiten zu einer Verbesserung der Schichtqualitaten. Investigating the Influence of the Sputter Etching Process on the Properties of PVD-CrN Coatings on Magnesium Die Cast Alloy AZ91hp A common method prior to the PVD deposition is the sputter etching process of the substrate itself to clean the surface from adhesion products and to improve the coating adhesion. This report deals with the sputter etching of magnesium die cast alloy AZ91hp to investigate the influences on the coating-substrate interface, the surface properties and the mechanical properties of PVD-CrN hard coatings. The coating-substrate interface of the Cr-AZ91 coating systems was investigated with XPS and SIMS. Surface studies were carried out by high resolution electron microscopy and AFM. The characterization of the mechanical properties of the CrN-AZ91 compound systems includes thickness, coating hardness and hardness depth profiles, coating adhesion, structure and residual stresses. Some properties show a strong dependency of the etching time, especially the mechanical properties and the coating roughness. Increasing etching times lead to an improvement of the coating quality.