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An important component of the deep decarbonization of the worldwide energy system is to build up the large-scale utilization of hydrogen to substitute for fossil fuels in all sectors including industry, the electricity sector, transportation and heating. Hence, apart from reducing hydrogen production costs, establishing an efficient and suitable infrastructure for the storage, transportation and distribution of hydrogen becomes essential. This article provides a technically detailed overview of the state-of-the-art technologies for hydrogen infrastructure, including the physical- and material-based hydrogen storage technologies. Physical-based storage means the storage of hydrogen in its compressed gaseous, liquid or supercritical state. Hydrogen storage in the form of liquid-organic hydrogen carriers, metal hydrides or power fuels is denoted as material-based storage. Furthermore, primary ways to transport hydrogen, such as land transportation via trailer and pipeline, overseas shipping and some related commercial data, are reviewed. As the key results of this article, hydrogen storage and transportation technologies are compared with each other. This comparison provides recommendations for building appropriate hydrogen infrastructure systems according to different application scenarios.

Light-induced metastabilities of Cu(In,Ga)Se2 (CIGS)-based thin film solar cells have been extensively researched for many years. It is commonly observed that junction capacitance and p-doping level in CIGS absorbers increase considerably after light soaking (LS). In this work, we focus on the LS behaviors of cells with different minority carrier lifetimes (τn). Experiments show that high efficiency cells with long τn lose open circuit voltage (Voc) and fill factor (FF) upon LS, whereas low efficiency cells with short τn lose less or even gain Voc and FF. The sodium content measured with glow discharge optical emission spectroscopy (GD-OES) increases in the region close to the CdS/CIGS interface with LS and may contribute to the observed LS behaviors. The change in electrical parameters is explained with simulations, which relate the Voc and FF changes to a reduced recombination rate in the space charge region due to the light-induced doping increase. The simulations also suggest that cells with higher n-doping in the CdS are less sensitive to changes in interface recombination rate and doping of CIGS, which agrees with the hypothesis that the CdS buffer deposition is important for the LS behavior.

A one-step electrochemical grafting process using Grignard reagents has been performed to achieve a complete monolayer methyl-terminated Si(111) surfaces. Anodic treatment (0.5 mA/cm2 for 300 s) ha...

Se/Ru nanoparticles – a potent non-platinum catalyst towards oxygen reduction reaction – were modified by hydrated WO3 and investigated using the rotating disk/ring electrode methods and by synchrotron X-ray photoelectron spectroscopy. The modification resulted in an enhanced catalytic activity towards oxygen reduction reaction (ORR). Our data indicate that the oxygen reduction current starts ca. 70 mV more positive and formation of undesirable hydrogen peroxide has significantly decreased following the modification of Se/Ru with WO3. X-ray photoelectron spectroscopy reveals that WO3 interacts electronically with Se/Ru as the W 4f and Se 3d line-shapes change. We therefore conclude that the electronic interactions between Se/Ru and WO3 are primarily responsible for the increase in activity and selectivity of the WO3-modified Se/Ru towards ORR.

Ultra thin organic films (about 5 nm thick) of nitrobenzene and 4-methoxydiphenylamine were deposited electrochemically on p-Si(111) surfaces from benzene diazonium compounds. Studies based on atomic force microscopy, infrared spectroscopic ellipsometry and x-ray photoelectron spectroscopy showed that upon exposure to atmospheric conditions the oxidation of the silicon interface proceed slower on organically modified surfaces than on unmodified hydrogen passivated p-Si(111) surfaces. Effects of HF treatment on the oxidized organic/Si interface and on the organic layer itself are discussed.

Functionalisation of Si surfaces by organic molecules is of high interest for a wide range of possible applications such as Si based sensors [1-3], photovoltaics [4], microelectronics [5, 6] or bio-chips [7]. Hydrogenated Si(111) surfaces have a low rate of surface charge carrier recombination, which means that this sur-face is well-passivated [8]. However, this surface is not stable and oxidizes rapidly in contact with ambient air [9]. For this reason, a way to avoid this degradation is to form covalent Si-C bonds which can be obtained by various methods initiated chemically [10], electrochemically [11, 12], thermally [13] or photochemically [14]. Such modi-fied surfaces are expected to exhibit a better chemical sta-bility and good electronic passivation (low concentration of interface states in the band gap) [15, 16]. Pulsed PL and SPV are two complementary methods to obtain informa-tions about the amount of non-radiative active defect cen-ters at the interface and to measure the minority carrier lifetime and diffusion length [17, 18]. In this work, electrochemical processing has been cho-sen as method to graft ethynyl derivatives onto Si(111) sur-faces by the use of Grignard reagents. Recently, it has been shown that this electrochemical grafting method lead to formation of polymeric layers with thicknesses of about 10-20 nm when a charge flow of 450 mC/cm

Three different experimental routes to in situ characterization of electronic structure and chemical composition of thin film cathode surfaces used in lithium ion batteries are presented. The focus is laid on changes in electronic structure and chemical composition during lithium intercalation and deintercalation studied by photoelectron spectroscopy and related techniques. At first, results are shown obtained from spontaneous intercalation into amorphous or polycrystalline V2O5 thin films after lithium deposition. Although this technique is simple and clean, it is nonreversible and only applicable to the first lithium intercalation cycle into the cathode only to be applied to host materials stable in the delithiated stage. For other cathode materials, as LiCoO2, a real electrochemical setup has to be used. In our second approach, the experiments are performed in a specially designed electrochemical cell directly connected to the vacuum system. First experimental results of RF magnetron sputtered V2O5 and LiCoO2 thin film cathodes are presented. In the third approach, an all solid-state microbattery cell must be prepared inside the vacuum chamber, which allows electrochemical processing and characterization by photoelectron spectroscopy in real time. We will present our status and experimental difficulties in preparing such cells.

This study seeks to explore the effects of the electronic structure of Pt/Ru alloy nanoparticles on reactivity of small organic molecules of relevance to fuel cell applications through the combined use of synchrotron radiation photoelectron spectroscopy and electrochemistry. Platinum core-level binding energies were found to increase linearly with the addition of ruthenium. This effect is a product of lattice strain and charge transfer, and is explained in terms of the d-band center theory proposed by Norskov and co-workers (Hammer, B.; Norskov, J. K. Surf. Sci. 1995, 343, 211). In the course of the study of electrooxidation of methanol we have found that it is very difficult, if not impossible, to separate the effects of the bifunctional mechanism and the electronic structure effects that might play a role in the activity. However, data for electrooxidation of formic acid, when studied at a short reaction time (where the indirect reaction pathways and poisoning intermediates are assumed to play a negligi...

In the Anderson model the alignment of electronic energy levels follows from the assumption that the vacuum-levels of the contacting phases are at equal height. Engineering of the line up may be attempted by changing the ionization energy of the substrate and/or of the adsorbate. We report on the variation of the Si(111) ionization energy by induced surface dipoles of –CH3, –H, and –GaSe terminations and the induced variation of the HOMO line up of PTCDA and ZnPc layers. In addition the variation of the line up by changing the organic molecule ionization energy is exemplified by F substitution in ZnPc. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The relative coverage, thermal stability, and electronic properties of CH_(3-) and C_2H_(5-)functionalized Si(111) surfaces prepared by a two-step chlorination/alkylation procedure have been compared using high-resolution synchrotron photoemission spectroscopy. Whereas the CH_(3-) terminated Si(111) surface showed only one C 2s peak for the occupied σ orbitals, the C 2s spectra of C_2H_(5-)terminated Si(111) surfaces showed a symmetric splitting of the occupied σ orbitals, as expected for an ethyl moiety bonded to the surface. The C_2H_5 termination resulted in an unpinning of the Si surface Fermi level, with a band bending of ∼0.2 eV, and produced a surface dipole potential step of −0.23(15) eV. The observed close-to-flat-band condition is similar to that of CH_3−Si(111) and is consistent with H termination of the non-alkylated Si atop sites in the two-step chlorination/alkylation process. The C_2H_(5-)functionalized Si(111) surfaces decomposed at temperatures >300 °C, whereas CH_3−Si(111) surfaces were stable up to at least 440 °C. The data clearly highlight the similarities and identify some significant differences between the behavior of the CH_3- and C_2H_(5-)functionalized Si(111) surfaces.

Synchrotron measurements of Pt and Ru core-level binding energies, BE’s, in Pt–Ru nanoparticles, as a function of Pt content, quantify earlier indications that the Pt 4f BE shift is much larger than the Ru 3d BE shift. A complementary theoretical analysis relates the BE shifts to changes in the metal–metal distances as the composition of the nanoparticle changes. We establish that the large Pt and small Ru BE shifts arise from the different response of these metals to changes in the bond distances, an unexpected result. Our results give evidence that the magnitudes of the BE shifts depend on whether the d band is open, as for Ru, or essentially filled, as for Pt.

The initial stage of oxidative CuI film formation on Cu(111) has been studied in an electrochemical environment by means of cyclic voltammetry (CV), in situ scanning tunneling microscopy (STM) and ex situ synchrotron X-ray photoemission spectroscopy (SXPS). Cyclic voltammetric studies indicate a significant acceleration of copper oxidation in the presence of iodide. The reason for that is the iodide-mediated stabilization of cuprous species resulting in a downward shift of the onset potential for copper oxidation. Reactive sites for the copper oxidation followed by iodide complexation are exclusively defects such as substrate step edges. It is the surface-confined supersaturation of mobile CuI species that leads to the two-dimensional (2D) CuI film formation via nucleation and growth of a Cu/I bilayer on top of the preadsorbed iodide phase. In an advanced stage of copper oxidation, however, terraces are directly transformed into the 2D CuI film at the reactive boundary between metallic copper and the grow...

The Schottky junction formation by the stepwise evaporation of gold and copper, respectively, onto methyl-terminated silicon, CH 3 –Si(1 1 1), was investigated by synchrotron X-ray photoelectron spectroscopy. During the junction formation process, interface reactions occurred as revealed by the appearance of chemically shifted Si 2p components. Upon deposition of Au, the formation of about one monolayer of gold silicide, SiAu 3 , with a Si 2p chemical shift of +0.75(2) eV, was observed. The SiAu 3 floated on top of the growing gold layer. Similarly, for the deposition of Cu, the methyl termination layer was partially disrupted, as indicated by the appearance of a −0.28(2) eV chemically shifted Si 2p component attributable to an interfacial copper silicide phase, SiCu 3 . Hence, the termination of the Si(1 1 1) surface by methyl groups did not completely prevent interfacial reactions, but did reduce the amount interfacial reaction products as compared to bare Si(1 1 1)-(7 × 7) surfaces. Electron Schottky barrier heights of 0.78(8) eV (Au) and 0.61(8) eV (Cu) were measured. Within the experimental uncertainty the observed Schottky barriers were identical to those ones obtained on non-passivated, (7 × 7)-reconstructed Si(1 1 1) surfaces. Thus, the modification of the electronic properties of the silicon–metal contact requires the complete absence of interfacial reactions.

In this paper we present results of efficiency optimizations in conjunction with improved CIGS interface control. During large area thin film growth, understanding and control of the interfaces is an important key to enable a stable production. Our results indicate that efficiency, and weak light performance of CIGS modules can be increased and stabilized if wait time management and the related surface processes were better understood and controlled. The experiments indicate that oxide formation impacts the Na diffusion and has an impact on the module voltage. On the other hand defect formation before the absorber is observed to generate shunts between back contact and transparent conducting oxide with a significant drawback on the fill factor. Furthermore we show that internal sodium control helps to decouple wait times from module efficiency.

The adsorption of Ru dye N3 [RuII(2,2‘-bipyridyl-4,4‘-dicarboxylate)2(NCS)2] from ethanol solution and gas-phase adsorption and coadsorption of the solvent acetonitrile on liquid nitrogen cooled nanocrystalline (nc) TiO2 films have been investigated using highly surface sensitive synchrotron-induced photoelectron spectroscopy. Resonantly excited Ti3+-related band gap states form a peak 2.2 eV above the leading edge of the valence band, and additional states up to the Fermi level are found. The intensities of the gap states and related Ti3+ 2p emissions of pristine samples depend strongly on sample preparation and history. Gap-state-related emissions are partially quenched by dye as well as acetonitrile adsorption. Quenching of gap states due to acetonitrile is reversible. The maximum of the highest occupied molecular orbital (HOMO) emission of the dye is found 1.6 eV above the leading edge of the nc-TiO2 valence band. The binding energies of dye orbitals are increased upon acetonitrile coadsorption. The H...

The properties of Si(111) surfaces grafted with benzene derivatives were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The investigated materials were nitro-, bromo-, and methoxybenzene layers (-C(6)H(4)-X, with X = NO(2), Br, O-CH(3)) deposited from diazonium salt solutions in a potentiostatic electrochemical process. The UPS spectra of the valence band region are governed by the molecular orbital density of states of the adsorbates, which is modified from the isolated state in the gas phase due to molecule-molecule and molecule-substrate interaction. Depending on the adsorbate, clearly different emission features are observed. The analysis of XPS intensities clearly proves multilayer formation for bromo- and nitrobenzene in agreement with the amount of charge transferred during the grafting process. Methoxybenzene forms only a sub-monolayer coverage. The detailed analysis of binding energy shifts of the XPS emissions for determining the band bending and the secondary electron onset in UPS spectra for determining the work function allow one to discriminate between surface dipole layers--changing the electron affinity--and band bending, affecting only the work function. Thus, complete energy band diagrams of the grafted Si(111) surfaces can be constructed. It was found that silicon surface engineering can be accomplished by the electrochemical grafting process using nitrobenzene and bromobenzene: silicon-derived interface gap states are chemically passivated, and the adsorbate-related surface dipole effects an increase of the electron affinity.

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials.

Although chalcopyrites have been used as absorber in thin film solar cells for nearly 30 years, not much is known about their surface structure, which might be an important information towards growth and interface optimization. Surface reconstructions, a general feature of semiconductor surfaces, have not been reported on Cu-III–VI 2 (001) surfaces so far. We have thus studied the chalcopyrite CuGaSe 2 (001) surface preparation by Ar + ion bombardment and annealing. The samples were grown by MOCVD on a (001) GaAs substrate on axis with a near-stoichiometric ratio Cu/Ga. The preparation was optimized so that we finally obtained a flat, well-ordered surface. The surface composition and structure was analyzed with AES, XPS, and LEED. On uncontaminated surfaces, a (1×4) reconstruction was observed with LEED which we tentatively interpret as a Se-terminated surface. XPS characterization shows a surface core level shift of −0.8 eV for the selenium surface atoms of the (1×4) reconstruction. Further investigations were carried out via AFM to characterize the surface morphology.

The chemical composition of the Cu(In,Ga)Se 2 /CdS interface is studied using photoelectron spectroscopy, with monochromatized Al Kα and synchrotron radiation as excitation source. The samples were prepared by the decapping of Se layers, yielding a Cu-poor surface composition. CdS deposition and photoemission were performed in the same vacuum system. An excess of sulfur is detected at very low CdS thickness. However, reference experiments suggest that the interface is atomically abrupt. In contrast to the interface of CdS with a stoichiometric CuInSe 2 single-crystal surface, the formation of Cu x S at elevated temperatures is suppressed by the Cu-poor surface phase.

The chemical modification of polycrystalline CuInSe 2 absorber surfaces by the so-called Cd partial electrolyte (PE) treatment was studied by synchrotron X-ray photoelectron spectroscopy (SXPS). The Cd PE treatment was found to remove surface indium oxides and hydroxides and segregated sodium compounds. A hydroxide-terminated CdSe surface layer of one monolayer thickness is formed by the partial electrolyte treatment. The reaction mechanism is discussed as substrate site-controlled exchange reaction, where surface indium is removed and replaced by cadmium. Electronically, the Cd PE treated surface is inverted and exhibits a surface barrier which is by 0.2 eV higher than a comparable structure that was prepared by the vacuum deposition of one monolayer of CdS onto clean CuInSe 2 .

A selenium capping and decapping process for the protection of CuInSe 2 surfaces during storage in air was investigated. The quality and cleanness of CuInSe 2 (0 0 1) surfaces restored in UHV by thermal evaporation of the covering protective Se cap was assessed. For the study, MBE-grown heteroepitaxial CuInSe 2 /GaAs (0 0 1) films were employed. The Se capping and decapping process was monitored by reflection high-energy electron diffraction. The Se cap layer is amorphous and, after its reevaporation, the initial diffraction pattern of CuInSe 2 (0 0 1) is restored. The decapping process and the quality of decapped CuInSe 2 (0 0 1) surfaces were investigated by means of photoelectron spectroscopy using synchrotron radiation with excitation energies of 1050 and 95 eV. For a decapping temperature of 390 °C, an O 1s contamination signal as small as the detection limit was found. Photoelectron spectra taken with the highest surface-sensitivity ( hν =95 eV) yield well-resolved valence band emissions confirming the low contamination level and high quality of the decapped surfaces. The investigations show that the Se capping and decapping process is an efficient means for the conservation and restoration of clean and well-defined Cu chalcopyrite surfaces in UHV.

Synchrotron-induced photoelectron spectroscopy was used to investigate the native-oxide-covered GaAs(1 0 0) surface and changes induced by etching with aqueous ammonia solution and by annealing in vacuum. The etching step removes arsenic and gallium oxides from the surface and the surface gets covered by elemental arsenic and tiny amounts of gallium suboxide. The surface oxygen content is reduced by an order of magnitude after etching, whereas the surface carbon content is somewhat increased. Annealing of this surface at 450 °C results in the disappearance of elemental arsenic and a considerable decrease in surface carbon and oxygen contents. The valence band spectra exhibit clear features typical for As-terminated GaAs(1 0 0) surfaces, as also obtained after As decapping.

The absorber/buffer interface is essential for high-performance thin film chalcopyrite solar cells. In this contribution we present studies of interface formation between Cu(In,Ga)Se 2 with 30 and 100% Ga content and the II–VI semiconductor CdS using in situ photoelectron spectroscopy. Clean Cu(In,Ga)Se 2 surfaces with a Cu-deficient surface composition were prepared by heating-off of Se layers, which were deposited onto the absorber layers in the deposition chamber directly after absorber deposition. Interfaces with CdS were prepared by stepwise evaporation. The determined band alignments are compared to theoretical calculations.

Recently, we have developed an improved three-stage growth method by simultaneously using a pyrometer and a thermocouple, to accurately control thickness and composition during growth of CuInGaSe 2 (CIGS). As a result, we have obtained solar cells that show preliminary efficiencies up to 16.4% without anti-reflection coating. Using the technique, we have investigated possible mechanism behind non-optimal cell efficiency, focusing on the effects of the molybdenum contact layer. Some additional advantages of our pyrometer monitoring technique are described. Optimizing the properties of Mo layer leads to suppression of Na segregation and distinctive damage to the CIGS surface, improving both cell performance and reproducibility.

The surfaces of sputtered zinc oxide (ZnO) films and its interfaces with CdS are investigated with X-ray photoelectron spectroscopy and soft X-ray photoelectron spectroscopy measured at the synchrotron radiation facility BESSY II. Cadmium sulfide (CdS) films are prepared by thermal evaporation from the compound and the zinc oxide films by DC magnetron sputtering from an undoped zinc oxide target at low power densities with and without addition of oxygen to the sputtering gas. ZnO films deposited at room temperatures show an additional oxygen component located at the surface that can be mostly removed by heat treatment. The electronic interface properties are studied in dependence to ZnO deposition conditions.

The controllability of the three-stage-growth process of Cu(In,Ga)Se 2 films was investigated, implementing two different substrate temperature monitoring channels simultaneously: (a) a thermocouple at the rear side of the substrate, and (b) a pyrometer, measuring the emission of heat radiation from the front surface of the growing film. By this setup not only the film composition, i.e. the relative incorporation rate of Cu and group III elements, but as well the absolute growth rate can be monitored on-line during growth. The heat radiation intensity during the first growth stage (deposition of (In,Ga) 2 Se 3 precursor films) exhibits characteristic oscillations. The oscillation period is inversely proportional to the growth rate. The oscillations are caused by thickness interference effects of black body radiation within the growing film. By a simulation of the infrared emissivity, the refractive index n igs =2.9 and absorption coefficient k igs =0.2 of the growing film could be extracted. Employing our process, Mo/CIGS/CdS/ZnO/Al solar cells with an efficiency up to 16.4% could be realized.

Preparation steps of Pt/n-GaAs Schottky contacts as applied in the fabrication process of varactor diode arrays for THz applications are analysed by photoelectron spectroscopy. Pulsed cathodic deposition of Pt onto GaAs (1 0 0) wafer surfaces from acidic solution has been studied by core level photoelectron spectroscopy using different excitation energies. A laboratory AlKα source as well as synchrotron radiation of hν =130 and 645 eV at BESSY was used. Chemical analyses and semiquantitative estimates of layer thickness are given for the natural oxide of an untreated wafer surface, a surface conditioning NH 3 etching step, and stepwise pulse plating of Pt. The structural arrangement of the detected species and interface potentials are considered.

It has been theoretically predicted that the codoping of an acceptor (nitrogen) and a donor (aluminium, gallium, indium) in a 2 : 1 ratio will enable the fabrication of low resistivity p-type ZnO due to reduction of the Madelung energy making the nitrogen acceptor energy level shallower. We have been growing N-doped and Ga+N codoped ZnO films by radical-source molecular beam epitaxy. Nitrogen and oxygen radicals were separately supplied via radio-frequency radical cells. Nitrogen did not incorporate in ZnO films at the high substrate temperature of 600°C. Lower substrate temperatures were necessary for nitrogen incorporation. From the fact that the nitrogen concentration also depended on the Zn fluxes, it is concluded that Zn-rich conditions are indispensable for nitrogen doping. The presence of Ga was observed to enhance nitrogen incorporation, one of the predicted effects of codoping. However, high Ga concentrations were found to lead to formation of the additional phase of ZnGa 2 O 4 in ZnO films.

CuInSe 2 (CIS)-based semiconductors which are strong candidates for high-efficiency low-cost thin-film solar cells, have been investigated for device applications. First, high-quality CIS and CuGaSe 2 (CGS) epitaxial films were grown by molecular beam epitaxy. CIS films showing predominantly free exciton emissions in photoluminescence spectra were grown for the first time by optimizing the growth conditions. Next, native defects, control of which is critical for solar cell performance, have been systematically investigated. A common acceptor-type defect has been identified as Cu-vacancies (V Cu ) from positron lifetime measurements and the Cu/In composition ratio of the CIS films. Unlike films grown under Cu-excess conditions, films grown under an In-excess tend to be heavily compensated, and the origin of the donor-type defects present in these films was found to be Cu–Se divacancies (V Cu–Se ). Reduction of Se-vacancies (V Se ) was found to be critical for improving solar cell performance. Air-annealing, the presence of a Cu–Se second phase and the interdiffusion of Na from the sodalime glass substrate were all found to be effective in suppressing V Se formation. Post-growth annealing has been extensively used to understand the oxidation process of CIS and CGS. Annealing of CGS films at temperatures higher than 450°C resulted in a significant improvement in CGS film quality as well as the formation of a Ga–O second phase. The Cu/Ga ratios of oxygen-annealed films after the selective etching of Ga–O by H 2 SO 4 were found to become nearly unity, suggesting that the formation of the Ga–O phase enhanced Cu outdiffusion leaving Cu-deficient CGS regions.

In this paper we present results of efficiency optimizations in conjunction with increased CIGS deposition rates. Furthermore, we describe Solibro's recent technology improvements for higher module efficiencies and our roadmap for further efficiency improvements. Our results indicate that with co-evaporation of CIGS absorbers high module efficiencies can be realized at high dynamic deposition rates. Various improvements have recently been introduced in Solibro's CIGS production process, which led to a champion module efficiency of 14.4% (total area, externally confirmed). Solibro's efficiency roadmap shows that the technology is capable of producing average efficiencies of 17% within the next 3 years.

(1 1 1), (0 0 1), and (1 1 0) oriented CuInS 2 films were grown epitaxially on H-terminated Si substrates by molecular beam epitaxy. The epilayers had surface compositions in the range 0.27 sf 2 (1 1 1)-(1×2), CuInS 2 (0 0 1)-(1×1), and CuInS 2 (1 1 0)-(1×1) were observed for near-stoichiometric films (cubic indices). Based upon simulations of LEED patterns, the origin of the CuInS 2 /Si(1 1 1) cub -(1×2) superstructure is attributed to a cation ordering of CuAu type in the film surface. For (1 1 0) oriented films, a complete faceting of the film surface into CuInS 2 {1 1 1} surface facets was found, independent of the surface composition. A partial {1 1 1}-faceting was obtained for near-stoichiometric CuInS 2 /Si(0 0 1), which, however, could be suppressed under Cu-rich growth conditions. Using valence band spectroscopy, a clear signature of the (0 0 1) surface was detected and is interpreted as due to a surface resonance. Ionization energies and surface Fermi energies of differently oriented CuInS 2 epilayers were found to be non orientation dependent. For stoichiometric and Cu-poor CuInS 2 an electron affinity χ of (4.9±0.15) eV was found, which is distinctly higher than previously reported χ values of CuInS 2 . The electron affinity decreases significantly with increasing Cu/In ratio in the film surface, down to ∼4.1 eV for [In]/([Cu]+[In]) sf ∼0.3. The surface Fermi level of stoichiometric films is energetically located ∼0.8 eV above the valence band edge.

CuInS 2 films grown epitaxially on Si (111) substrates exhibit a number of extended defects such as stacking faults, coherent and incoherent twin boundaries, and micro twins. These defects are partly induced by the interface strain due to the lattice or thermal mismatch between Si and CuInS 2 . The appearance and density of the planar defects depend on film stoichiometry: In Cu-rich films, coherent twin boundaries form on (112) surfaces but are absent on (112). The density of incoherent twin boundaries and dislocation lines is small. Film growth appears to apply a step flow mechanism on 〈110〉 steps with the result of defect free regions laterally extending up to 1.3 μm. Incoherent twin boundaries and stacking faults are concentrated near the interface region. For Cu-poor films, the density of stacking faults and dislocation lines on the (112) plane is much higher. Stacking fault and twin formation is also found on (112) planes. The film volume free of planar defects is smaller. This difference between Cu-rich and Cu-poor CuInS 2 films is discussed in relation to the different point defects induced by the stoichiometry deviation. A model for the formation of stacking faults is proposed.

In this paper we present results of the efficiency optimization study performed in our production line for CIGS solar modules. For three different Ga/(Ga+In) ratios two different grading settings were tested. Results indicate high impact of these parameters on efficiency, in the range of 0.5% (abs.). Increasing the Ga content in the film, results in an increase of module voltage. This is almost completely compensated by the decrease of Isc. The resulting change in efficiency is FF driven. The FF increases slightly with higher overall Ga content, but the grading plays a more important role. For the grading setting 1, which was steeper than the setting 2, the FF and consequently efficiency were significantly improved. Optimum Ga content and grading has been defined accordingly.

The contact formation of thin-film epitaxial CuInSe₂(001) with a physical-vapor-deposited CdS layer is presented in this work. Synchrotron-excited photoelectron spectroscopy was used for this investigation. The epitaxial CuInSe₂ films contain a surface layer of reduced Cu stoichiometry similar to the ordered defect compound CuIn₃Se₅. A valence band offset of 0.79±0.15 eV has been determined for this heterojunction. The comparison to literature data indicates that neither surface orientation nor surface copper content have a major impact on the valence band offset of CuIn₃Se₅, respectively, CuInSe₂ with CdS.

It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4.

The interface construction of tilt growth that is sometimes observed for epilayer growth on a lattice mismatched substrate is modeled on the basis that tilt relieves misfit strain in the epilayer. In this model off-axis misfit accommodation is assumed to be due to only tilt dislocations or tilt dislocations combined with misfit dislocations. The average interval between successive dislocations which are formed along the interface can be estimated using the lattice units of both materials and that of the epilayer in another principal axis direction; the tilt angle can also be calculated geometrically. The tilt angle predicted by this model agrees well with experimental results for several examples of mismatched epilayer growth by molecular-beam epitaxy. The model suggests a method to grow a single domain of stress-free epilayer by using a substrate cut to an angle that allows off-axis fit to the lattice unit of the epilayer.

Covalent grafting of ethynyl derivatives (-C triple bond C-H, -C triple bond C-CH3, -C triple bond C-aryl) onto H-terminated Si(111) surfaces was performed by a one-step anodic treatment in Grignard electrolytes. The electrochemical grafting of such ethynyl derivatives, which tends to form ultrathin polymeric layers, can be controlled by the current and charge flow passing through the Si electrode. The prepared ultrathin layers cover the Si surface and had a thickness up to 20 nm, as investigated by the scanning electron microscopy (SEM) technique. Exchanging Cl for Br in the ethynyl Grignard reagent leads to very thin layers, even under the same electrochemical conditions. However, for all ethynyl derivatives, high-resolution synchrotron X-ray photoelectron spectroscopy (SXPS) investigations reveal the incorporation of halogen atoms in the organic layers obtained. Moreover, it was observed that the larger the end group of the ethynyl derivative, the thinner the thickness of the ultrathin polymeric layers as measured by both SXPS and SEM techniques after low and high current flow respectively. For the first time, these new types of ultrathin organic layers on Si surfaces were investigated using infrared spectroscopic ellipsometry (IRSE). The different possible reaction pathways are discussed.

We report the results of an investigation of the interaction between Se and Ru in Ru−Se electrocatalysts showing high activity and methanol tolerance in the oxygen reduction reaction (ORR). The res...

We report the results of an investigation of the interaction between Se and Ru in Ru−Se electrocatalysts showing high activity and methanol tolerance in the oxygen reduction reaction (ORR). The res...

The irradiation-induced reduction of electrochemically grafted nitrobenzene films on Si(111) was monitored by high-resolution photoelectron spectroscopy. The experiments were performed using synchrotron soft X-ray irradiation at the BESSY II synchrotron facility. The evolution of different chemical species was monitored as a function of time. Careful fitting of the Si2p, C1s, N1s, and O1s core level spectra allowed us to follow this process in detail and to determine the constants of growth and decay of the specific components. The chemical changes were caused by the X-ray irradiation-induced secondary electron current through the aryl layer. A minor fraction (approximately 25%) of the initial nitro groups was split off and desorbed. The bulk of the NO2 groups was reduced to species in an amino-like chemical environment. A desorption of carbon fragments was not observed, and benzene ring specific shakeup satellites indicated that the aromatic ring structure remained intact. Irradiation-induced line-shape changes suggest a polymerization via -NH- bridges, which were formed after the irradiation-induced N-O bond splitting. A significant part of the released oxygen appeared to contribute to an oxidation of the silicon substrate at the Si(111)/benzene interface. The irradiation-induced aryl layer modification can be exploited for chemical lithography (i.e., a lateral structuring of functionalized silicon surfaces).

Ordering of Cu and In atoms in near-stoichiometric CuInS2 epitaxial films grown on Si (111) by molecular beam epitaxy was studied by transmission electron microscopy. Nonchalcopyrite ordering of the metal atoms in CuInS2 is observed, which is identified as CuAu-type ordering. Sharp spots in electron diffraction patterns reveal the ordered Cu and In atom planes alternating along the [001] direction over a long range. High-resolution electron microscopy confirms this ordering. The CuAu-ordered structure coexists with the chalcopyrite ordered structure, in agreement with theoretical prediction.

Perspectives of a new approach for the synchrotron photoemission spectroscopic analysis of chemical processes at solid/liquid interfaces under UHV conditions have been explored. A thin layer of HCl-2-propanol solution was frozen-in on the semiconductor GaAs(100) wafer surface by cooling the substrate to liquid nitrogen temperature after etching off the native oxide layer under N2 atmosphere. Chemical reactions induced in situ by exposure to synchrotron radiation (SR) and by stepwise heating have been monitored. Right after etching and freezing, the surface is covered by gallium chlorides with 1, 2, 3, and 4 Cl ions attached and lattice back-bonded to As atoms, as well as by elemental arsenic As0 and 2-propanol. Exposure to SR at low temperature produces surface As chlorides at the expense of As0. The GaCl3 and GaCl2 emissions diminish while GaCl is enhanced. On the other hand, heating the sample to approximately 130 K just above H2O desorption causes the thermodynamically expected reaction of AsCl3 with the substrate GaAs to form Ga chloride species and As0. Heating the sample to room temperature leaves only As0 on the surface and for gallium the content of all surface chlorides is drastically reduced. By further heating to 400 K elemental arsenic starts to desorb and the Ga chloride surface content is reduced. Using different excitation energies the depth composition of the reaction products has been monitored indicating a tendency of decreasing chlorination numbers and increasing Ga vs As chloride content toward the pristine substrate at each stage of the reaction.

Interfaces are important for the efficiencies of thin film solar cells. In particular, for polycrystalline chalcogenide semiconductors as CdTe and Cu(In,Ga)(S,Se)/sub 2/ (CIGS) the existing physical concepts, which describe the electronic properties of elemental or III-V compound semiconductor interfaces quite well, are not sufficient. The increased complexity is mostly due to the non-abruptness of the interfaces and the strong tendency for the formation of defects in the more polar bonded II-VI compounds. Photoelectron spectroscopy has significantly contributed to the understanding of the mechanisms governing the properties of semiconductor interfaces in thin film solar cells. The experimental approach using integrated surface analysis and thin film deposition systems and selected results will be presented.

The chemical state, electronic properties, and geometric structure of methyl-terminated Si(111) surfaces prepared using a two-step chlorination/alkylation process were investigated using high-resolution synchrotron photoelectron spectroscopy and low-energy electron diffraction methods. The electron diffraction data indicated that the methylated Si surfaces maintained a (1×1) structure, where the dangling bonds of the silicon surface atoms were terminated by methyl groups. The surfaces were stable to annealing at 720 K. The high degree of ordering was reflected in a well-resolved vibrational fine structure of the carbon 1s photoelectron emission, with the fine structure arising from the excitation of C-H stretching vibrations having hnu=0.38±0.01 eV. The carbon-bonded surface Si atoms exhibited a well-defined x-ray photoelectron signal having a core level shift of 0.30±0.01 eV relative to bulk Si. Electronically, the Si surface was close to the flat-band condition. The methyl termination produced a surface dipole of –0.4 eV. Surface states related to piCH3 and sigmaSi-C bonding orbitals were identified at binding energies of 7.7 and 5.4 eV, respectively. Nearly ideal passivation of Si(111) surfaces can thus be achieved by methyl termination using the two-step chlorination/alkylation process.

Clean and ordered chalcopyrite CuInSe2 surfaces are a precondition for the study of the electronic structure by angle-resolved photoelectron spectroscopy. The preparation of welldefined CuInSe2(001) surfaces by the combination of molecular beam epitaxy and a selenium capping and decapping process is described. The surface structure of CuInSe2 epilayers with different bulk composition is compared and analysed by low-energy electron diffraction.Employing near-stoichiometric surfaces, the valence electronic structure of CuInSe2 was investigated by angle-resolved photoelectron spectroscopy at the synchrotron source BESSY 2. This is the first study of the valence band structure of a copper chalcopyrite semiconductor material by photoelectron spectroscopy. The valence band dispersion along τT, i.e. the [001] direction, was investigated by a variation of the excitation energy from 10 to 35 eV under normal emission, and the band dispersion along τT, i.e. the [110] direction, was analysed by angular scans with hv = 13 eV.The valence bands derived from antibonding and bonding Se4p-Cu3d hybrid orbitals, nonbonding Cu3d states and In-Se hybrid states are clearly indentified. The strongest dispersion is found for the topmost valence band with a bandwidth of ∼0.7 eV from τ to T. From τ to N, the observed dispersion was 0.5 eV. The experimental valence bands are discussed in relation to calculated band structures in the literature.

Epitaxial layers of CuInS2 are grown on chemically hydrogen terminated Si(111) surfaces with 4° miscut by molecular beam epitaxy. The morphological and structural properties are determined by scanning electron microscopy, transmission electron microscopy, x‐ray diffraction, and texture analysis. The data show growth in the 〈112〉 direction and substantial twinning of the 75‐nm‐thick films. High‐resolution cross‐sectional micrographs of the interface indicate semicoherent epitaxial growth via an interfacial indium‐rich secondary phase. The pronounced faceting of the film surface is discussed in relation to twin lamellae.

CuInS2 films were grown by molecular beam epitaxy (MBE) on hydrogen terminated Si(111) substrates with 4° miscut. X-ray diffraction (XRD) texture analysis reveals that CuInS2 was grown heteroepitaxially with the epitaxial relationships CuInS2(112) ‖ Si(111) and CuInS2 % MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqr1ngB % PrgifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr0x % c9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8fr % Fve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaadaWada % qaaiaaigdacaaIXaGabGymayaaraaacaGLBbGaayzxaaaaaa!3BE6! $$\left[ {11\bar 1} \right]$$ ‖ Si % MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqr1ngB % PrgifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr0x % c9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8fr % Fve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaadaaada % qaaiaaigdacaaIXaGabGOmayaaraaacaGLPmIaayPkJaaaaa!3BC5! $$\left\langle {11\bar 2} \right\rangle $$ Moreover, a substantial amount of rotational twins is observed. The crystalline order is maintained across the interface as observed by cross-sectional transmission electron microscopy (XTEM). XRD and scanning electron microscopy (SEM) investigations show that non-stoichiometric preparation greatly influences the growth morphology and leads to the formation of secondary phases.

The adsorption of viologens (1,1'-disubstituted-4,4'-bipyridinium molecules) on a chloride-modified copper electrode has been studied using a combination of cyclic voltammetry (CV), in-situ scanning tunneling microscopy (STM) and ex-situ photoemission spectroscopy (XPS). Two prototypes of viologens could be identified with respect to their redox behavior upon adsorption, namely those which retain (non-reactive adsorption) and those which change their redox state (reactive adsorption) upon interaction with the chloride-modified copper surface at given potential. The first class of viologens represented by 1,1'-dibenzyl-4,4'-bipyridinium molecules (dibenzyl-viologens, abbreviated as DBV) can be adsorbed and stabilized on this electrode surface in their di-cationic state at potentials more positive than the reduction potential of the solution species. XPS N1s core level shifts verify that the adsorbed DBV molecules on the electrode are in their oxidized di-cationic state. Electrostatic attraction between the partially solvated viologen di-cations and the anionic chloride layer is discussed as the main driving force for the DBV stabilization on the electrode surface. Analysis of the N1s and O1s core level shifts points to a non-reactive DBV adsorption leaving the DBVads2+ solvation shell partly intact. The laterally ordered DBVads2+ monolayer turns out to be hydrophilic with at least four water molecules per viologen present within this cationic organic film. The analysis of the Cl2p core level reveals that no further chloride species are present at the surface besides those which are specifically adsorbed, i.e. which are in direct contact with the metallic copper surface underneath the organic layer. The reduction of these adsorbed DBVads2+ surface species takes place only in the same potential regime where the solvated DBVaq2+ bulk solution species react and is accompanied by a pronounced structural change from the di-cationic 'cavitand'-structure to a 'stripe'-structure of chains of π-stacked DBV•+ mono-cation radicals as verified by in-situ STM. The second class of viologens represented by 1,1'-diphenyl-4,4'-bipyridinium molecules (diphenyl-viologens, abbreviated as DPV) is much more reactive upon adsorption and cannot be stabilized on the electrode surface in a di-cationic state, at least within the narrow potential window of copper. The N1s core level binding energy indicates only the presence of the corresponding mono-reduced DPVads•+ species on the surface even at potentials more positive than the redox potential of the bulk solution species. This process leads to the formation of a hydrophobic viologen monolayer with stacked polymeric chains as the characteristic structural motif. The wet electrochemical reduction of viologens is further compared with a dry reduction under UHV conditions. The latter reaction inevitably affects the di-cationic viologen species in the course of the photoemission experiment. Slow photoelectrons and secondary electrons are assumed to transform the di-cationic viologens into the corresponding radical mono-cations upon irradiation.