CISSY: A station for preparation and surface/interface analysis of thin film materials and devices

The CISSY end station combines thin film deposition (sputtering, molecular beam epitaxy ambient-pressure methods) with surface and bulk-sensitive analysis (photo emission, x-ray emission, x-ray absorption) in the same UHV system, allowing fast and contamination–free transfer between deposition and analysis. It is mainly used for the fabrication and characterization of thin film devices and their components like thin film photovoltaic cells, water-splitting devices and other functional thin film materials

CTZS Thin Films Grown by a Sequential Deposition of Precursors

A comparative study of the structural, optical and morphological properties of Cu2ZnSnS4 (CZTS) thin films prepared by two different techniques was performed. One consists of sequential evaporation of the elemental metallic precursors under a flux of sulphur supplied by evaporation from an effusion cell (physical vapor deposition-PVD) and the second one is a solution-based chemical route where thin layers of CuS, SnS and ZnS are deposited sequentially by diffusion membrane- assisted chemical bath deposition techniques; the membranes are used to optimize the kinetic growth through a moderate control of the release of the metal into CBD solution by osmosis. The present comparative study is helpful to the synthesis of kesterite nanostructured thin films.Comment: Published 201

Theory and Experiment

Selective modification of the morphology and intrinsic electrocatalytic activity of porous electrodes is urgently required to improve the performance of vanadium redox flow batteries (VRFBs). For this purpose, electrospinning was exploited to prepare high‐performance nanofiber‐based composites. Blends of polyacrylonitrile, polyacrylic acid, and polyaniline with carbon black were electrospun into a 3D free‐standing nanofibrous web, which was utilized as a novel electrode. By extending the recent theory of cyclic voltammetry at porous electrodes to account for interfacial double‐layer capacities, nonlinear effects of ohmic resistances, and parasitic reactions, we could quantitatively investigate non‐faradaic as well as desired and undesired faradaic current contributions. Combination of experimental and theoretical studies allowed a unique quantitative assessment of the intrinsic catalytic activity of selected electrode materials concerning the VO2+/VO2+ redox reaction

Assessment of Chemical and Electronic Surface Properties of the Cu2ZnSn(SSe)4 after Different Etching Procedures by Synchrotron-based Spectroscopies

Kesterite Cu2ZnSn(S,Se)4 absorber layers with different [S]/([S]+[Se]) ratios were studied using XPS, UPS, Hard X-ray (HIKE) photoemission and the Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS). The samples were prepared by IREC using sequentially sputtered metallic precursor stacks with metal ratios of [Cu]/([Zn]+[Sn])=0.80, [Zn]/[Sn]=1.20 followed by annealing under S+Se+Sn atmosphere. Different etching procedures were used depending on the sample's composition. It is shown that the surface composition varies from that of the bulk, especially for the Se-rich samples. Contamination with sulfur is detected after using a Na2S etching solution for the pure Se kesterite. A Cu-depleted surface was found for all samples before and after etching. HIKE measurements show a higher [Zn]/[Sn] ratio in the near surface region than on the very surface. This is explained by the fact, the etching procedure removes secondary phases from the very few surface layers, while some of ZnS(e) is still buried underneath. In order to investigate the band gap transition from the pure sulfide (1.5 eV) to the pure selenide (1eV), the valence and conduction band of the respective absorbers were probed. According to UPS and HIKE measurements, the relative distance between Fermi level (Ef) and valance band maximum (VBM) for sulfide sample was 130 meV larger than for selenide. Using NEXAFS on the copper, zinc and tin edges, the development of the conduction band with increasing [S]/([S]+[Se]) ratios was studied. Stoichiometric powder samples were used as reference materials. © 2015 Published by Elsevier Ltd.Peer ReviewedPostprint (published version

Reliable wet-chemical cleaning of natively oxidized high-efficiency Cu(In,Ga)Se2 thin-film solar cell absorbers

Currently, Cu-containing chalcopyrite-based solar cells provide the highest conversion efficiencies among all thin-film photovoltaic (PV) technologies. They have reached efficiency values above 20%, the same performance level as multi-crystalline silicon-wafer technology that dominates the commercial PV market. Chalcopyrite thin-film heterostructures consist of a layer stack with a variety of interfaces between different materials. It is the chalcopyrite/buffer region (forming the p-n junction), which is of crucial importance and therefore frequently investigated using surface and interface science tools, such as photoelectron spectroscopy and scanning probe microscopy. To ensure comparability and validity of the results, a general preparation guide for “realistic” surfaces of polycrystalline chalcopyrite thin films is highly desirable. We present results on wet-chemical cleaning procedures of polycrystalline Cu(In1-xGax)Se2 thin films with an average x = [Ga]/([In] + [Ga]) = 0.29, which were exposed to ambient conditions for different times. The hence natively oxidized sample surfaces were etched in KCN- or NH3-based aqueous solutions. By x-ray photoelectron spectroscopy, we find that the KCN treatment results in a chemical surface structure which is – apart from a slight change in surface composition – identical to a pristine as-received sample surface. Additionally, we discover a different oxidation behavior of In and Ga, in agreement with thermodynamic reference data, and we find indications for the segregation and removal of copper selenide surface phases from the polycrystalline material

Comportamento alimentar do peixe recifal Sargentinho (Abudefduf saxatilis) em Santa Catarina

TCC(graduação) - Universidade Federal de Santa Catarina. Centro de Ciências Biológicas. Biologia.Estudos da ecologia alimentar de peixes recifais são essenciais para compreender o fluxo energético nestes ecossistemas. Diversos estudos abordam o comportamento alimentar das espécies, porém, são poucos os estudos que avaliam esse comportamento ao longo do dia. Visto isso, este estudo busca observar e quantificar o comportamento alimentar do peixe recifal Sargentinho (Abudefduf saxatilis) no litoral de Santa Catarina, Brasil. Foram realizados mergulhos livres e autônomos, nos quais empregou-se busca ativa por indivíduos de A. saxatilis. Utilizando o método de animal focal, um indivíduo era seguido de cada vez por um minuto ou até sua primeira mordida, quando era anotado em qual substrato o mesmo estava mordendo, seu comprimento total e o horário. Para verificar se [1] há diferenças no substrato de forrageio ao longo do dia; e se [2] há diferença no substrato de forrageio entre classes de tamanho, foram utilizados teste-G. Sete substratos foram utilizados pela espécie para forrageio, sendo a coluna d’água o predominante (79,7% dos indivíduos observados), seguido pelos substratos bentônicos: matriz algal epilítica (16,6%), Sargassum sp. (3,3%), alga calcária (0,22%), Palythoa sp. (0,12%) e desova de A. saxatilis (0,06%). As análises sugerem A. saxatilis prefere forragear na coluna d’água durante toda a manhã e no final da tarde, com maior intensidade antes do meio dia, enquanto no começo da tarde preferem o componente bentônico. Estas diferenças podem ser explicadas pelo comportamento oportunista da espécie, que o permite explorar diferentes substratos para forrageio. Também foi demonstrado que as classes de tamanho utilizam diferentes substratos de forrageio, indicando diferenças nas demandas energéticas ao longo de sua ontogenia. Por fim este trabalho revela a grande importância do comportamento alimentar na compreensão geral da ecologia alimentar para a espécie.Reef fish feeding ecology is essential for understating the energy flow of these ecosystems. Many studies approach feeding behaviour, although those approaching the variation of feeding behaviours during the day are scarce. This study aims to observe the feeding behaviour of the reef fish Sargeant major (Abudefduf saxatilis) on the coast of Santa Catarina state, Brazil. For this, free and SCUBA dives were conducted, in which active search for A. saxatilis was performed. Using the focal animal method, an individual was followed each time for one minute or until it’s first bite, when the bitten substrate, total length and time was recorded. To verify if [1] there are differences among the foraging substratum during the day; and if [2] there are differences among foraging substratum between different size classes, a G-test was used. Seven substrata were utilized, being the water column predominant (79,7% of observed individuals), followed by the benthic substrata: epilitic algal matrix (16,6%), Sargassum sp. (3,3%), calcareous algae (0,22%), Palythoa sp. (0,12%) and A. saxatilis eggs (0,06%). The analysis suggests that A. saxatilis prefer to forage on the water column during the morning and end of the evening, with a higher intensity of bites before midday, while at the beginning of the evening benthic substrata are preferred. These differences can be explained by the opportunistic behaviour of the species, that allows the exploration of different substrata. It was also demonstrated that individual size classes use different foraging substrata, indicating different energetic demands according to the species different ontogeny. Lastly, this study verified the important role of feeding behaviour in the general understanding of the species feeding ecology

Gallium gradients in chalcopyrite thin films: Depth profile analyses of films grown at different temperatures

The following article appeared in Journal of Applied Physics 110.9 (2011): 093509 and may be found at http://scitation.aip.org/content/aip/journal/jap/110/9/10.1063/1.3656986Cu(In,Ga)Se2 films are used as absorber layers in chalcopyrite thin film solar cells. As the gallium concentration in the absorber can be used to control the band gap, there have been many efforts to vary the gallium concentration in depth to gain an optimum balance of light absorption, carrier collection, and recombination at different depths of the absorber film, leading to improved quantum efficiency. In this study, we investigate the effect of the maximum substrate temperature during film growth on the depth dependent gallium concentration. For the in-depth gallium concentration analyses, we use two techniques, covering complementary depth ranges. Angle dependent soft x-ray emission spectroscopy provides access to information depths between 20 and 470 nm, which covers the depth range of the space charge region, where most of the photoexcited carriers are generated. Therefore, this depth range is of particular interest. To complement this investigation we use secondary neutral mass spectrometry, which destructively probes the whole thickness of the absorber (≈2 µm). The two methods show increasingly pronounced gallium and indium gradients with decreasing maximum substrate temperature. The probing of the complementary depth ranges of the absorbers gives a consistent picture of the in-depth gallium distribution, which provides a solid basis for a comprehensive discussion about the effect of a reduced substrate temperature on the formation of gallium gradients in Cu(In,Ga)Se2 and the device performance of the corresponding reference solar cells.The authors acknowledge the support of the European Commission in the framework of the ATHLET-project (Project No. 019670)

Crystallisation Phenomena of In2O3:H Films

The crystallisation of sputter-deposited, amorphous In2O3:H films was investigated. The influence of deposition and crystallisation parameters onto crystallinity and electron hall mobility was explored. Significant precipitation of metallic indium was discovered in the crystallised films by electron energy loss spectroscopy. Melting of metallic indium at ~160 °C was suggested to promote primary crystallisation of the amorphous In2O3:H films. The presence of hydroxyl was ascribed to be responsible for the recrystallization and grain growth accompanying the inter-grain In-O-In bounding. Metallic indium was suggested to provide an excess of free electrons in as-deposited In2O3 and In2O3:H films. According to the ultraviolet photoelectron spectroscopy, the work function of In2O3:H increased during crystallisation from 4 eV to 4.4 eV, which corresponds to the oxidation process. Furthermore, transparency simultaneously increased in the infraredspectral region. Water was queried to oxidise metallic indium in UHV at higher temperature as compared to oxygen in ambient air. Secondary ion mass-spectroscopy results revealed that the former process takes place mostly within the top ~50 nm. The optical band gap of In2O3:H increased by about 0.2 eV during annealing, indicating a doping effect. This was considered as a likely intra-grain phenomenon caused by both (In0)O•• and (OH−)O• point defects. The inconsistencies in understanding of In2O3:H crystallisation, which existed in the literature so far, were considered and explained by the multiplicity and disequilibrium of the processes running simultaneously.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

Point contacts at the copper indium gallium selenide interface a theoretical outlook

For a long time, it has been assumed that recombination in the space-charge region of copper-indium-gallium-selenide (CIGS) is dominant, at least in high efficiency solar cells with low band gap. The recent developments like potassium fluoride post deposition treatment and point-contact junction may call this into question. In this work, a theoretical outlook is made using three-dimensional simulations to investigate the effect of point-contact openings through a passivation layer on CIGS solar cell performance. A large set of solar cells is modeled under different scenarios for the charged defect levels and density, radius of the openings, interface quality, and conduction band offset. The positive surface charge created by the passivation layer induces band bending and this influences the contact (CdS) properties, making it beneficial for the open circuit voltage and efficiency, and the effect is even more pronounced when coverage area is more than 95%, and also makes a positive impact on the device performance, even in the presence of a spike at CIGS/CdS heterojunction

Dye Sensitization of Nanocrystalline Titanium Dioxide with Osmium and Ruthenium Polypyridyl Complexes

A series of osmium polypyridyl complexes having various ground-state reduction potentials has been synthesized and used to sensitize nanoporous titanium dioxide electrodes to solar illumination. The spectral response and current vs potential properties of electrodes modified with these dyes have been compared with the behavior of their ruthenium analogues. The trends can be explained by the differences in absorption spectra and ground-state redox potentials. The osmium complexes appear to be promising candidates for further optimization in operating photoelectrochemical cells for solar energy conversion applications. Of the materials studied, all complexes having ground-state redox potentials in methanol more positive than ∼0.4 V vs aqueous SCE were able to sustain oxidation of I-/I_3- with a high steady-state quantum yield. For electrodes with very low dye coverages, the open-circuit voltage was mainly determined by the rate of reduction of I_2, whereas for high dye coverages, the open-circuit voltage depended on the nature of the complex and on the dye loading level