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∆V C q th eff 12 −2 p ( VGS = 0V) = = 3. 9 * 10 cm , (2) where q is the elementary charge. This figure is equal to nt, as long as the negative charges are trapped directly at the dielectric/semiconductor interface. The lack in hysteresis, however, observed for the p-type OFET drain current characteristic, indicates (1) the absence of hole traps and (2) the absence of recombination of the mobile holes with the trapped electrons. This leads to the conclusion, that the negative charges are not localized directly at the semiconductor dielectric interface, but in a near surface layer of the PMMA dielectric, isolated from mobile holes in the transistor channel. We have demonstrated the feasibility to convert unipolar n-type pentacene OFETs into ptype OFETs by a modification of the polymer dielectric through a UV treatment in air. The change in OFET polarity is due to a large positive threshold voltage shift of about 60 V, resulting from trapped negative charge carriers within the UV-treated PMMA layer. The proposed modification of the dielectric / semiconductor interface allows for the development of organic complementary circuits, since the introduced p- and n-channel transistors exhibit comparable current characteristics in terms of mobilities (≈ 0,1 cm 2 /Vs), on / off ratios (≈ 10 4 -10 5 ) and current maxima, as derived from the respective transfer characteristics. References [1] C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater.14 (2002) 99. [2] M. Ahles, R. Schmechel, and H. v. Seggern, Appl. Phys. Lett. 85 (2004) 4499. [3] T. Yasuda, T. Goto, K.Fujita, and T. Tsutui, Appl. Phys. Lett. 85 (2004) 2098. [4] A. Torikai, M. Ohno, and K. Fueki, J. Appl. Polym. Sci. 41 (1990) 1023. [5] L.-L. Chua, J. Zaumseil, J.-F. Chang, E. C.-W. Ou, P. K.-H. Ho, H. Sirringhaus, and R. H. Friend, Nature 434 (2005) 194. [6] R. Schmechel, M. Ahles, and H. v. Seggern, J. Appl. Phys. 98 (2005) 084511. - 72 -
Synchrotron Induced Photoelectron Spectroscopy at the Solid- Liquid Interface of Dye Sensitized Solar Cells Konrad Schwanitz, Eric Mankel, Ralf Hunger, Thomas Mayer, and Wolfram Jaegermann At BESSY we run the experimental station SoLiAS, dedicated to solid-liquid interface analysis. SoLiAS allows for the transfer of wet chemically prepared surfaces to the ultra high vacuum without contact to ambient air. In addition in situ (co)adsorption of volatile solvent species onto liquid nitrogen cooled samples is possible. SoLiAS proves to be very useful in analyzing the chemical and electronic structure of solid-liquid interfaces e.g. of dye sensitized solar cells. A monolayer of Ru(N3)-dye was adsorbed from ethanol solution under clean N2 atmosphere in an UHV-integrated electrochemical cell (EC). Acetonitrile or benzene were adsorbed from the liquid in the EC or in situ from the gas phase. Ex situ sintered nanocrystalline anatase substrates as well as in situ deposited polycrystalline TiO2 samples were used. Distinct reversible changes occur in synchrotron induced photoelectron valence band and core level spectra when the solvent is adsorbed to pristine and dye covered TiO2 substrates. Based on the experimental results the alignment of electronic states and a model on the dye-solvent interaction have been deduced. The valence band maximum of nc-TiO2 is found at EB = 3.6 eV binding energy while the fundamental gap is 3.2 eV only. Surface gap states related to Ti 3+ 3d 1 orbitals are found with a maximum at EB = 1.3 eV and in addition just below the Fermi level. In the rigid band model these states are assigned to occupied conduction band states but may be due to substochiometric TiO2-x. Adsorption of acetonitrile is accompanied by quenching of the surface gap states (Fig. 1). This finding is confirmed in Ti2p core orbital spectra by the quenching of the Ti 3+ low binding energy shoulder of the Ti 4+ bulk emission. Coadsorption of acetonitrile (Fig. 2) shifts the dye HOMO by 150 meV from 2.0 eV binding energy to 2.15 eV. Fig. 1: Intensity normalized (O2p valence band) SXPS spectra of the gap states of nanocrystalline anatase TiO2 in the course of acetonitrile adsorption and desorption. Fig. 2: SXPS spectra of the gap states of nanocrystalline anatase TiO2 in the course of Ru(N3)-dye adsorption and acetonitrile coadsorption and desorption. - 73 -
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ANNUAL REPORT 2 0 0 6 FACULTY OF MA
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Annual Report 2006 Faculty 11 Mater
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Preface The year 2006 of the Depart
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difference of numerical and analyti
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Institute of Materials Science Phys
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Research Projects Bulk Amorphous Al
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el Dsoki, C.; Landersheim, V. ; Kau
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Das, J.; Kim, K. B.; Xu, W.; Wei, B
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Staff Members Head Prof. Dr. Jürge
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Zhou L.; Rixecker G.; Aldinger F.;
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concerned with the synthesis and ch
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Darmstadt University of Technology
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Geomaterials Science Geomaterial Sc
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Gregori, G.; Kleebe H.-J.; Sorarù,
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Hessische Industriemüll GmbH). Due
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Technical Personnel Josef Kolb, Dip
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Research Projects Environmental sca
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Fig. 1: Aerial photo of the Lake Li
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Lithological Map of the Japanese Ar
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tower stripping, and therefore much
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Diffusion and reaction in micro- an
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Rift dynamics, uplift and climate c
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Geology, land use change and erosio
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nearly half a century has been perf
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The Effect of LiF Addition on the S
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In case of the undoped sintered B6O
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Microstructures in Omphacite and Ot
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Modelling phase-assemblage diagrams
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As an example, Fig. 2 shows the che
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MATERIALS SCIENCE Physical Metallur
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