Thomas Mayer

This document is part of Part 4 'Adsorbate Species on Surfaces and Adsorbate-Induced Surface Core Level Shifts' of Subvolume A 'Adsorbate Layers on Surfaces' of Volume 42 'Physics of Covered Solid Surfaces' of Landolt-Börnstein - Group III Condensed Matter.

ABSTRACT Molecular layer deposition (MLD) is a solvent-free technique to prepare organic molecular layers with a similar level of control and perfection as that known from inorganic films grown by atomic layer deposition (ALD). In both, MLD and ALD, the deposition process relies on the self-limiting chemical reaction of two precursors on the surface of the substrate. A plethora of MLD processes has been demonstrated.[1] Among them, hybrid materials made from typical metal-organic precursors and alcohols, so called metalcones, have received considerable attention.[2] However, there is only very limited work on MLD of molecular systems which provide functionalities like charge transport or luminescence.[3; 4] In our work, a general avenue for the controlled preparation of conformal and highly luminescent monolayers of metal chelate complexes is presented, among them tris(8-hydroxyquinolinato)aluminium (Alq3) and bis(8-hydroxyquinoline)zinc (Znq2). Alq3 is an extremely important model compound for organic electronic applications - it has been the electron transporter and luminescent material in the first efficient hetero-structure organic light emitting diode (OLED). The controlled formation of Alq3 or Znq2 monolayers is achieved by functionalization of the substrate with amino groups which serve as initial docking sites for trimethyl aluminum (TMA) or diethyl zinc (DEZ) molecules which datively bind to the amine (see figure).[5] Subsequent exposure to 8-Hydroxyquinoline (8-HQ) results in the self-limiting formation of highly luminescent monolayers on arbitrary surfaces, e.g. curved 3D objects or highly porous silica aerogels. The growth mechanism is studied by in-situ quartz crystal microbalance and ex-situ by highly sensitive (time-resolved) optical absorption/emission spectroscopy and photo-electron spectroscopy. Applications of these thin conformal luminescent layers will be discussed. [1] S.M. George, B. Yoon, R.A. Hall, A.I. Abdulagatov, Z.M. Gibbs, Y. Lee, D. Seghete, B.H. Lee, in, Atomic Layer Deposition of Nanostructured Materials, Wiley-VCH Verlag GmbH & Co. KGaA, 2011, pp. 83-107. [2] B.H. Lee, B. Yoon, A.I. Abdulagatov, R.A. Hall, S.M. George, Advanced Functional Materials 23 (2013) 532-546. [3] O. Nilsen, K.R. Haug, T. Finstad, H. Fjellvåg, Chemical Vapor Deposition 19 (2013) 174-179. [4] O. Nilsen, K. Klepper, H. Nielsen, H. Fjellvaåg, ECS Transactions 16 (2008) 3-14. [5] A. Räupke, et al., ACS Applied Materials & Interfaces 6 (2014) 1193-1199.

ABSTRACT A common phenomenon of organic solar cells (OSCs) incorporating metal-oxide electron extraction layers is the requirement to expose the devices to UV light in order to improve device characteristics - known as the so-called "light-soaking" issue. This behaviour appears to be of general validity for various metal-oxide layers, various organic donor/acceptor systems, and regardless if single junction devices or multi stacked cells are considered. The requirement of UV exposure of OSCs may impose severe problems if substrates with limited UV transmission, UV blocking filters or UV to VIS down-conversion concepts are applied. In this paper, we will demonstrate that this issue can be overcome by the use of Al doped ZnO (AZO) as electron extraction interlayer. In contrast to devices based on TiOx and ZnO, the AZO devices show well-behaved solar cell characteristics with a high fill factor (FF) and power conversion efficiency (PCE) even without the UV spectral components of the AM1.5 solar spectrum. As opposed to previous claims, our results indicate that the origin of s-shaped characteristics of the OSCs is the metal-oxide/organic interface. The electronic structures of the TiOx/fullerene and AZO/fullerene interfaces are studied by photoelectron spectroscopy, revealing an electron extraction barrier for the TiOx/fullerene case and facilitated electron extraction for AZO/fullerene. These results are of general relevance for organic solar cells based on various donor acceptor active systems.

ABSTRACT A new perylene diimide derivative, namely N,N'-diallyl-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarboxylic acid diimide (phenoxy-allyl-PTCDI, abbreviated PA-PTCDI), is introduced. The investigations presented in this paper aim at finding a molecule for use as a sensitzer in thin film silicon solar cells in order to enhance efficiency. The synthesis is described along with optical and electrochemical measurements of PA-PTCDI in solution. A good agreement is found between the measured data and theoretical calculations. The molecule is characterized further by optical and photoemission data on thin films, which also show that the dye can be sublimed in vacuum. The interface between the dye and silicon is investigated on the model system Si(111):H with synchrotron-induced photoemission spectroscopy. The result is an electronic lineup with the gap centers of silicon and PA-PTCDI almost at identical positions and thus very similar band discontinuities from the lowest unoccupied molecular orbital (LUMO) to the conduction band as well as from the highest occupied molecular orbital (HOMO) to the valence band. This clearly permits a transfer of photogenerated electrons and holes from PA-PTCDI to silicon. The experimental valence band discontinuity matches very well the value calculated for a very similar PTCDI molecule.

In this study we report on new concepts to generate light emission in organic thin film transistors. The initial physical understanding of light emission from tetracene based field-effect transistors was proposed to be originated from a strong underetching of the drain and source electrodes. This underetched electrodes in combination with the evaporated tetracene is thereby believed to generate a virtual OLED at the drain electrode. Accumulated holes have to leave the gate oxide interface to reach the drain electrode by crossing the bulk of the organic semiconductor. Light then occurs by injection of electrons in a large electric field in the bulk. Today's transistors do not show the underetching anymore but are still emitting light only at the drain electrode, again supporting the initial interpretation of a defect state at the edge of the drain electrode. In this context the question how electrons can overcome a potential barrier of 2.7 eV is still open. Therefore an investigation of the gold tetracene interface by UPS and XPS techniques has been started and preliminary data indicate the unexpected result that the barrier for electrons is comparable to that for holes. In a further step the generation of an ambipolar transistor by interface doping with calcium was tried and an n-type pentacene transistor could be fabricated but the strategy failed for tetracene. Finally an electrochemical interface doping was performed by the application of Lithium triflate in PEO to a thin interface layer between gate oxide and tetracene. This leads to light emission but unfortunately also to the loss of the gate voltage influence. Based on these results a possible strategy will be presented.

Die Verbindung unterschiedlicher Material-klassen zu funktionellen Kompositen soll den Gestaltungsspielraum z.B. der optoelek-tronischen Materialeigenschaften erweitern. Wir untersuchen in zwei Projekten, inwieweit sich organische Farbstoffmoleküle in anorga-nischen Halbleiterschichten als zusätzliche Absorber von Sonnenlicht eignen. Dadurch könnte zum einen die benötigte Dicke und damit der Materialeinsatz verringert wer-den. Perspektivisch soll zum anderen der ausgenutzte Spektralbereich erweitert wer-den. Hohe theoretische Wirkungsgrade bei geringem Materialeinsatz motivieren die ersten Schritte auf dem Weg zu Solarzellen aus organisch/anorganischen Kompositen. Organic/inorganic composites for photovol-taic application combining different materi-al classes to functional composites shall open a wide range of tuning e.g. optoelectronic material properties. In two projects we inve-stigate the possibilities of incorporating organic dye molecules into inorganic semi-conductors as addi...

Measurements of the beam energy of electron storage rings based on the resonant spin depolarization method usually take the riseof the count rate of electrons scattered by the spin depen-dent Touschek effect as the indication of the depolarization. We present experimental results showing that the rise of Touschek scattering due to depolarization may as well be monitored by the change of the lifetime of the beam. At BESSY I the lifetime is determined by current measurements taken with a commercially available high precision Parametric Current Transformer (PCT). We present our experiences with a newly installed PCT showing a high signal-to-noise ratio and a low offset drift. The presented method in monitoring the depolarization offers a simple way to high precision energy determination at low emittance storage rings, where Touschek scattering is the dominant contribution to the lifetime.

The parameters necessary to dimension an equipment for the measurement of the beam energy of a storage ring via the resonance depolarisation method are collected. The lay out of the set up for BESSY II is given in comparison to the operational system at BESSY I.

The semiconductor/electrolyte interface is modeled in ultraviolet radiation through coadsorption of electrolyte components (Br2, Na, and H2O) on Van der Walls surfaces and analyzed by photoemission. By spectroscopy of valence band area and redox level, the line structures of substrate band adsorbate emissions, their energetic positions, and relative shifts were evaluated as a function of the adsorption mode (molecular or dissociative) and the electrical potential above the surface. It is found that Br2 does not adsorb on InSe but the reaction is activated by X-ray irradiation; in order to explain this difference, the electronic and spatial structure of substrate surfaces was discussed. It is concluded that Br2 acts on nonreactive substrates as an acceptor and increases electron affinity. It is observed that H2O molecularly adsorbs at 100K in WSe2, InSe, and GaSe; the coadsorption of Br2 and H2O shows that the acceptor character of Br2 is stronger than the donor character of H2O.

Semiconductor/electrolyte model interfaces are prepared in UHV by coadsorbing the redox active electrolyte species Br2 and the solvent H2O onto chemically inert van der Waals (0001) surfaces of WSe2. Only a small fraction of the adsorbed Br2 is ionosorbed due to charge exchange with the semiconductor bulk; most of it and H2O are molecularly adsorbed. Thus the observed changes of band bending and work function are related to charge transfer processes at the interface to achieve electronic equilibrium. The value of band bending obtained for the model electrolyte is in reasonable agreement to the equivalent Volta potential of the electrolyte contact. For the Br2/H2O coadsorption system it is only determined by Br2 and not affected by coadsorbed H2O. In addition, the relative density of state distribution of the semiconductor and the model electrolyte can directly be determined from the electron distribution curve of the valence band region and compared to theoretical expectations of el...