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2013 Spring

Materials for energy and environment


2013 Spring : Symposium A

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Authors : Julien Bachmann
Affiliations : University of Erlangen-Nürnberg, Inorganic Chemistry, Erlangen, Germany

Resume : tba

Authors : Sanghoon Ji, Ikwhang Chang, Suk Won Cha, Min Hwan Lee
Affiliations : Seoul National University; University of California, Merced

Resume : Anodic aluminum oxide (AAO) templates which have well-arrayed nanopores are considered as one of the promising substrate to achieve high-performance large active area thin-film fuel cells (TF-SOFCs). However, the highly rough surface of AAO template can lower micro-structural integrity of a thin-film electrolyte and consequently electrochemical performance of TF-SOFCs significantly degrades. In this study, localized physical properties of AAO template-supported thin-film fuel cells are micro-structurally and electrically characterized with the aid of atomic force microscopy (AFM). The characterization results include the qualitative analysis of AAO pore regularity and the quantitative analysis in terms of electrical short between anode and cathode. A sub-100 nm thick yttria-stabilized zirconia (YSZ) layer is used as an electrolyte and fabricated with atomic layer deposition (ALD). The AAO template with 80 nm pores and 100 μm heights is used as a substrate, and an approximately 250 nm thick dense platinum layer is inserted between an ALD YSZ layer and an AAO template.

Authors : Kristin Bergum, Helmer Fjellvåg, Ola Nilsen
Affiliations : Centre for Materials Science and Nanotechnology, University of Oslo

Resume : With an increasing need for cleaner production and conversion of hydrogen, proton conductors are becoming evermore relevant. La28-xW4+xO54+d, (LWO) is a material with useful properties for the hydrogen fuel economy, both as a proton-conducting electrolyte in an intermediate-temperature solid oxide fuel cell (IT-SOFC) and as a hydrogen gas separating membrane. One of the main areas for improvement with SOFCs lies in the thickness of the electrolyte. An increase in efficiency can be achieved by utilizing an electrolyte which is as thin as possible while still being pin-hole free, thus avoiding short-circuiting. Using ALD to deposit these kinds of materials it is possible to achieve very thin pin-hole free films even on porous materials. A process for depositing WOx was developed using the precursors (tBuN)2(Me2N)2W and H2O. ALD growth occurred from 275C – 325C, while CVD-growth was observed at 350C. It should be noted that the CVD films are still highly uniform, but have much higher growth rates and contain N. LWO thin films have been deposited by combining the processes for deposition of La2O3 (La(thd)3 + O3), and WOx ( (tBuN)2(Me2N)2W + H2O). The stoichiometry of the deposited material should have a La:W ratio in the 5.2 – 5.8 region. A 16:1 pulse relationship between La:W yielded the proton conducting phase of LWO, with an atomic stoichiometry of La:W = 5.6. All LWO samples have low roughness with Rq = 1-2 nm. XRD of heat treated samples confirms the presence of LWO.

Authors : M.Coll, J.Gazquez, A. Palau, X.Obradors and T. Puig
Affiliations : Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193 Bellaterra (Barcelona) Spain

Resume : CeO2 is a very attractive material widely known for its use in catalysis, energy storage systems, and gas sensing, caused by the presence of oxygen vacancies and facile conversion between Ce3+ and Ce4+. For most of these applications, slight changes in film thickness, composition, stoichiometry, ion motion or structural defects can dramatically affect its properties. The surface self-limiting Atomic Layer Deposition (ALD) characteristic growth offers great potential to ensure a nanoscale control of CeO2 properties over other deposition techniques such as chemical solution deposition, pulsed laser deposition or sputtering. For the first time, we prepared highly epitaxial as-deposited ALD-CeO2 thin films on several single crystal substrates (LAO, STO, YSZ) at temperatures below 300ºC obtaining a growth per cycle of ≈ 0.2Å/cycle. This extremely low growth rate has been identified as a key parameter to ensure epitaxial growth at these low temperatures. X-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), atomic force microscopy (AFM) and advanced scanning transmission electron microscopy (STEM) confirmed the formation of ultrasmooth, pure and epitaxial (00l) CeO2 films with a thickness range of 2 to 10 nm. Additionally, further tuning of ALD conditions for CeO2 deposition allowed us to obtain conformal coatings on 3D substrates such as vertically aligned carbon nanotubes and TiO2 nanotubes for photochemical and catalytic applications.

Authors : G. Luka 1; L. Wachnicki 1; B. S. Witkowski 1; R. Jakiela 1; M. Godlewski 1,2
Affiliations : 1. Institute of Physics, Polish Acad. of Sciences, Warsaw, Poland 2. Dept. of Mathematics and Natural Sciences College of Science, Cardinal S. Wyszynski University, Warsaw, Poland

Resume : Transparent conductive oxides (TCOs) are nowadays widely investigated mainly due to a possible replacement of indium tin oxide (ITO) as a transparent electrode material. Among various TCOs, zinc oxide, especially doped with group III elements (Al, Ga) is very promising for such applications. Recently, it was found that titanium dioxide films in a crystalline structure of anatase and doped with niobium (TiO2:Nb) reveal very low resistivities, ~10-4 Ohmcm, comparable to those for ITO films. Transparent conductive TiO2:Nb films that show good optical and electrical properties grown by magnetron sputtering or pulsed laser deposition were already reported. Till now, however, there are very few papers reporting TiO2:Nb growth by atomic layer deposition (ALD). In this work, firstly, we concentrate on the ALD growth conditions that lead to anatase TiO2 growth. Next, we investigate the Nb doping mechanism during the TiO2:Nb deposition. Based on secondary ion mass spectroscopy, x-ray diffraction and scanning electron microscopy, we check the uniformity of Nb distribution as well as the possible formation of Nb foreign phases in TiO2:Nb films depending on the Nb content. Finally, we show the optical and electrical properties of ALD-grown TiO2:Nb films and indicate the optimal growth conditions leading to the TiO2:Nb films with the lowest resistivities. This work was partially supported by the European Union within the European Regional Development Fund, through the Innovative Economy grant (POIG.01.01.02-00-108/09).

Authors : Kristin Bergum, Helmer Fjellvåg, Ola Nilsen
Affiliations : Centre of Materials Science and Nanotechnology, University of Oslo

Resume : The demand for cheap, reliable transparent conductive oxides (TCOs) has increased due to highly competitive solar cell and electronic markets striving to replace the current industrial standard ITO (Indium Tin Oxide). The interest is both on performance and cost - the price of Indium is rising, and better-performing devices would yield a precious advantage. Doped ZnO is widely predicted to be the successor to ITO as, when doped, it has properties that are comparable to that of ITO. Many electronic devices have a low thermal budget and a demand for reproducible high conformality across the sample. Atomic Layer Deposition (ALD) is therefore ideal. The inherent layer-like structure of ALD depositions can however be problematic. As these are deposited at low temperature there is often little diffusion and cyclic inhomogenieties may occur in the doping distribution. We have therefore investigated some of the known ways to counter this issue without annealing, such as changing the precursor and using surface inhibition. A significant variation in resistivity was evident depending on the film thickness in titanium doped zinc oxide. Increasing the thickness from 100 to 200 nm decreases the specific resistivity by one order of magnitude. The lower resistivity stems from an increase in mobility – with almost one order of magnitude difference for the samples with low Ti content, while the carrier concentrations change minimally. The lowest resistivities are in samples with 1-3at% Ti.

Authors : Robert L.Z. Hoye, Kevin P. Musselman, David Munoz-Rojas, Giorgio Ercolano, Judith L. MacManus-Driscoll
Affiliations : Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K.; Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0H3, U.K.

Resume : The commercial application of new generation solar cells depends on the use of low-cost, atmospheric-based, fast, low-temperature and roll-to-roll compatible deposition methods. One such method is Atmospheric (or Spatial) Atomic Layer Deposition (AALD). This implements Atomic Layer Deposition in atmospheric conditions by separating the precursors in space rather than in time. One area we have worked on to show the benefits of using AALD for solar cells is the Mg-doping of ZnO for hybrid solar cells with poly(3-hexylthiophene-2,5-diyl) (P3HT) as the p-type light absorbing material. The uniform 50 nm AALD ZnMgO films were deposited in under 7.5 minutes. By varying the carrier nitrogen gas flow rate through the precursors, fine control was achieved over the film composition, with the Mg content varied from 0% to 53%. This resulted in the band gap being increased over a wide range, from 3.3 eV to 5 eV. Through the significant increase in the conduction band level, the open-circuit voltage of ZnMgO-P3HT devices increased from 300 mV to 800 mV. This resulted in the maximum power conversion efficiency obtained being 270% larger than in undoped ZnO-P3HT devices. The significant improvement in device performance and easy fabrication of the Mg-doped ZnO make the AALD process extremely attractive for the commercial application of new generation solar cells.

Authors : Ricardo M. Silva(1), Patrícia A. Russo()2, Rui F. Silva(1), Nicola Pinna(3)
Affiliations : (1) Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal. (2) Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal. (3) Department of Chemistry, University of Humboldt, 12498 Berlin, Germany.

Resume : Vertically-aligned carbon nanotube (VACNT) forests grown on metallic substrates are of great interest as high surface area electrodes for applications such as supercapacitors [1]. Direct growth on metal substrates by thermal chemical vapor deposition (TCVD) on a prior ex-situ catalyst (i.e. iron oxide nanoparticles) layer has proven to be challenging due to phenomena like catalyst coarsening and alloying to the metal substrate. At high temperatures (i.e. 750ºC), the small catalyst nanoparticles tend to agglomerate into larger ones through atomic interdiffusion, which is known as “Ostwald ripening”. Efforts have been done in solving this problem by improving the thermal stability of the catalyst nanoparticles through different approaches, although an efficient and direct solution to catalyst agglomeration is still open. Atomic layer deposition (ALD) is a technique to deposit conformal thin films on surfaces regardless of whether the materials are flat or possess high-aspect ratio features, with atomic thickness control through self-limiting surface reactions [2]. ALD of metal oxides such as alumina (Al2O3) has been reported as a technique for stabilization of the catalyst nanoparticles used in catalytic reactions [3]. In the present work an ultrathin Al2O3 layer was deposited on monodisperse catalytic iron oxide nanoparticles by ALD in order to prevent their agglomeration and coalescence at high temperatures, and also to act as a diffusion barrier in the growth of VACNTs. Inconel 600, a nickel-based super-alloy that keeps its microstructural integrity even at high temperatures, was chosen as substrate material, ensuring the formation of VACNTs forest-metal contacts [4]. References: [1] C. Du, J. Yeh, N. Pang, Nanotechnology, 2005, 16, 350-353 [2] M. Knez, K. Nielsch, L. Niinistö, Adv. Mater. 2007, 19, 3425-3438 [3] J. Lu, B. Fu, M.C. Kung, G. Xiao, J. W. Elam, H.H. Kung, P. C. Stair, Science, 2012, 335, 1205-1208 [4] S. Talapatra, S. Kar, S. K. Pal, R. Vajtai, L. Ci, P. Victor, M. M. Shaijumon, S. Kaur, O. Nalamasu, P. M. Ajayan, Nature Nanotechnology, 2006, 1, 112-116

Authors : J. Haeberle1, H. Gargouri2, F. Naumann2, M. Arens2, S. Brizzi1, K. Henkel1, M. Tallarida1, D. Schmeißer1
Affiliations : 1 Brandenburg Technical University, Applied Physics and Sensors, K.-Wachsmann-Allee 17, 03046 Cottbus, Germany 2 Sentech Instruments GmbH, Schwarzschildstraße 2, 12489 Berlin, Germany

Resume : Aluminum oxide (Al2O3) deposited by atomic layer deposition (ALD) has been demonstrated as surface passivation layer in solar energy conversion devices [1, 2]. The applied thickness ranges from only one layer to several tens of nanometer [2]. Al2O3 can be deposited by thermal ALD and plasma enhanced ALD (PEALD) using trimethylaluminium (TMA, Al(CH3)3) as MO-precursor. For low temperature applications PEALD is preferred over thermal ALD [2]. We report about our results of producing Al2O3 using thermal ALD and PEALD in the SENTECH SI ALD LL system. The layers were deposited in the temperature range between room temperature and 200°C. We discuss ellipsometric data (thickness, refractive index, growth rate) as well as X-Ray Photoelectron Spectroscopy (XPS) results. The distribution of refractive index and thickness over 4” wafers will be shown. The XPS data are analyzed in terms of O/Al ratios and carbon contaminations. Comparing the 200°C process we observe similar refractive indices and O/Al ratios but increased growth rates and reduced carbon contaminations in the PEALD samples. This work is supported by BMBF program “Zukunft des Technologietransfers in strukturschwachen Regionen” (grant numbers 03IN2V4A, 03IN2V4B). [1] F. Le Formal, N. Tétreault, M. Cornuz, T. Moehl, M. Grätzel, K. Sivula: Chem. Sci. 2 (2011) 737. [2] G. Dingemans, W. M. M. Kessels: J. Vac. Sic. Technol. A 30 (2012) 040802.

Authors : Antonio Notarnicola, Tobias Grünzel, Julien Bachmann
Affiliations : University of Erlangen Chemistry; University of Hamburg Physics

Resume : Elemental silicon has the potential to be used as negative electrode material in lithium ion batteries with a fivefold larger capacity than the current standard material, graphite. However, the volume changes taking place upon charge and discharge typically cause mechanical tensions and eventually fractures in the solid, which precludes practical application. We propose silicon nanotube arrays as a platform in which the hollow central cavity serves to alleviate this problem. We demonstrate the thermal reduction of silicon oxide films, deposited using ALD, by lithium vapors under argon. The reaction proceeds quantitatively and homogeneously to pure silicon. It is also applicable to arrays of silica nanotubes created in porous anodic alumina. The tubular morphology is maintained, so that the tubes can be electrically contacted and tested electrochemically in a lithium-containing electrolyte. Their cyclic voltammograms display the features associated with the incorporation and removal of lithium ions.

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Solar cells II: Passivation : Radoslaw Chmielowski
Authors : W.M.M. Kessels, G. Dingemans, S. Smit, J.A. van Delft, D. Garcia-Alonso
Affiliations : Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Resume : The unique features of atomic layer deposition (ALD) can be employed to face processing challenges for various types of next-generation solar cells. Hence, ALD for photovoltaics (ALD4PV) has attracted great interest in academic and industrial research in recent years. In this presentation, the recent progress of ALD nanolayers applied to various solar cell concepts and their future prospects are reviewed [Van Delft et al., Semicond. Sci. Technol. 27, 074002 (2012)]. The presentation will focus particularly on the application of ALD oxides (Al2O3, SiO2, SiO2/Al2O3 stacks) for the passivation of surfaces of high-efficiency silicon solar cells [Dingemans et al., J. Vac. Sci. Technol. A 30, 040802 (2012)]. The role of chemical passivation (reduction of defect density) and field-effect passivation (electrostatic shielding of charge carriers) will be elucidated. It will be shown that the level and the polarity of the field-effect passivation by SiO2/Al2O3 stacks can be tuned precisely by controlling the fixed charge density in the stack through the variation of the SiO2 interlayer thickness in the range of 1 -10 nm. Moreover, the formation of passivating contacts by ALD Al2O3/ZnO and ALD Al2O3/ZnO:Al stacks will be discussed. Finally, the upscaling of the ALD to high-throughput solar cell production will be addressed [Kessels and Putkonen, MRS Bulletin 36, 907 (2011)].

Authors : Christoph Hossbach1, Hannes Klumbies2, Claudia Richter3, Claudia Keibler4, Marion Geidel1, Frederik Nehm2,3, Lars Müller-Meskamp2, Uwe Schröder3, Siegfried Menzel5, Matthias Albert1, and Johann W. Bartha1
Affiliations : 1 TU Dresden, Institute of Semiconductors and Microsystems, 01187 Dresden, Germany; 2 TU Dresden, Institut für Angewandte Photophysik, 01187 Dresden, Germany; 3 Namlab gGmbH, 01187 Dresden, Germany; 4 Fraunhofer COMEDD, Maria-Reiche-Str. 2, 01109 Dresden, Germany; 5 Leibniz Institute for Solid State and Materials Research, 01171 Dresden, Germany

Resume : Organic electronic devices like light-emitting diodes (OLED) or photovoltaic cells (OPV) show a high moisture sensitivity and need permeation barriers with water-vapor transmission rates (WVTR) of 10-4 to 10-6 g/m?/d. Atomic layer deposition (ALD) allows the low-temperature deposition of virtually pinhole-free inorganic thin films of barrier materials like Al2O3, TiO2, Si3N4, ZrO2 or HfO2. Such films can be combined to nano-laminates, which allow the clogging of diffusion pathways as well as the tuning of mechanical properties and therefore an improved barrier performance. By using molecular layer deposition (MLD) the inorganic films can be complemented with additional elastic layers of organic or hybrid organic/inorganic materials. These can improve the elasticity of the overall laminate, which is important for the encapsulation of flexible devices and substrates. In our presentation we will show selected results of our experiments on the direct and non-direct encapsulation with Al2O3\TiO2 and Al2O3\alucone thin film laminates. The emphasis will be on the barrier properties of these laminates and their dependencies on selected film and process parameters. Beyond this, we will give a summary of process development, metrology issues as well as the film nucleation on PEN foils.

Authors : Yanlin Wu, Hannes Wedemeyer Jan Michels, Jan Mock, Radoslaw Chmielowski, Stéphane Bourdais, Takuma Muto, Mikio Sugiura, Fritz Dildey, Timon Kampschulte, Gilles Dennler, Julien Bachmann
Affiliations : University of Erlangen Chemistry; University of Hamburg Physics; University of Applied Sciences Hamburg Environmental Engineering; IMRA Europe SAS

Resume : Extremely thin absorber (ETA) solar cells are nanostructured all-solid photovoltaic devices in which light absorption occurs in a thin intrinsic layer. In this work, ETA cells are built in which an Sb2S3 light absorber coating is created by atomic layer deposition (ALD). A model system based on flat layers of ITO, TiO2, Sb2S3, CuSCN and Au is assembled first. The external quantum efficiency (EQE) spectrum follows the absorption spectrum of Sb2S3. The EQE depends on the light absorber thickness and on the light intensity in a very non-linear manner. In nanostructured cells based on colloidal, nanocrystalline TiO2, we show subsequently that Sb2S3 can be deposited by ALD homogeneously along the depth axis and that it is free of oxide. Furthermore, the Sb2S3 layer thickness can be optimized systematically. In our geometry, we find an optimal thickness of 10 nm, which yields efficiencies of up to 2.6%.

Authors : G. Sarau1, M. Latzel1, O. Feddersen-Clausen2, and S. Christiansen1
Affiliations : 1. Max Planck Institute for the Science of Light, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany 2. modularflow, Geisbergstr. 17, 50939 Cologne, Germany

Resume : A mini-ALD chamber prototype has been developed by modularflow. Its overall dimensions are 6.5x7.5x1.5 cm2, while the reactor covered by a glass sheet has a diameter of only 2 cm enabling the use under a Raman spectrometer. To the best of our knowledge, this represents the smallest ALD reactor ever. In addition to the size, the innovation consists in using lasers to both promote surface chemical reactions and in-situ Raman and/or photoluminescence characterization during the ALD growth. Moreover, the reactor can be heated up to 300 oC providing a unique combination of thermal and light induced ALD for testing novel precursors, processes, and materials for energy applications. The flexibility for new experiments is further increased by the much less time and effort needed to clean this system as compared to existing ALD systems. The deposition area is controlled through the size of the laser beam that can be varied from (sub-)micrometers to milimeters by scanning the standard micro-spot using two orthogonal piezo-driven mirrors resulting in a macro-spot. In this way, a mask-free ALD deposition is performed. The growth rate, thickness uniformity, and conformality with respect to the laser parameters are investigated. We study the photoinduced deposition of Cu, Ag, and TiO2. The two metals are used as electrode materials in fuel cells as well as for solar fuel generation (e.g. CO2 or water splitting), while TiO2 is used for photochemical and dye-sensitized solar cells.


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