Plenarvorträge - DPG-Tagungen

Symposium Organic and Hybrid Systems for Future Electronics Donnerstag
plane components of the dielectric function the average tilt angle of the
F16PcVO molecular planes with respect to the substrate plane was found
to be 56 0 for fused silica substrates and between 0 0 and 3 0 for KBr substrates.
SYOH 5.29 Do 18:00 B
Strongly Enhanced Thermal Stability of Organic Semiconductor
Thin Films Induced by Aluminum Oxide Capping Layers
— •S. Sellner 1,2 , A. Gerlach 3 , F. Schreiber 3 , M. Kelsch 1 , N.
Kasper 1 , H. Dosch 1,2 , S. Meyer 4 , J. Pflaum 4 , M. Fischer 5 , and B.
Gompf 5 — 1 MPI für Metallforschung, Stuttgart, Germany — 2 Institut
für Theoretische und Angewandte Physik, Universität Stuttgart, Germany
— 3 Physical Chemistry Laboratory, Oxford University, UK — 4 III.
Physikalisches Institut, Universität Stuttgart, Germany — 5 I. Physikalisches
Institut, Universität Stuttgart, Germany
We show that the thermal stability of thin films of the organic semiconductor
diindenoperylene (DIP) can be substantially increased by an
aluminium oxide capping layer (50 nm). The DIP films do not only stay
on the substrate (oxidized silicon), but even remain crystalline on a time
scale of ≥ 1 hour more than 250 K above the desorption temperature
for uncapped films. The results of in-situ X-ray diffraction are supported
by thermal desorption spectroscopy. We argue that this very effective
enhancement of the thermal stability compared to uncapped and also
metal-capped organic layers is related to the low mobility of aluminum
oxide and the relatively well-defined as-grown interfaces with limited interdiffusion.
We discuss possible mechanisms for the eventual breakdown
at high temperatures.
SYOH 5.30 Do 18:00 B
Direct and inverse photoemission spectroscopy on organic
semiconductors in the pristine and reduced state — •Torsten
Schwieger 1 , Martin Knupfer 1 , Weiying Gao 2 , and Antoine
Kahn 2 — 1 Leibniz Institute for Solid State and Materials Research
Dresden, D-01069 Dresden, Germany — 2 Department of Electrical
Engineering, Princeton University, Princeton, NJ 08544, USA
Phthalocyanines and α-NPD are organic semiconductors which are
commonly used in organic electronic devices like transistors and diodes.
We intercalated the organic semiconducting film with potassium in order
to investigate the impact of negative charge to the electronic structure
of the organic system. A combined direct and inverse photoemission
spectroscopy study of the occupied and unoccupied states of the organic
semiconductors ZnPc and α-NPD in the pristine and K-intercalated state
is presented. A combination of PES and IPES allows the determination
of the transport gap in organic materials both in the undoped and doped
state. The observed splitting of the lowest unoccupied molecular orbitals
upon potassium intercalation leads to an evaluation of the size of correlation
effects in both molecular systems. These effects are discussed in
detail.
As expected, the Fermi level is found to shift towards the vacuum level
upon intercalation. However, the results clearly demonstrate that the
Fermi level in potassium intercalated organic semiconductors cannot a
priori be assumed to be pinned at the onset of the lowest unoccupied
molecular orbital in all cases.
SYOH 5.31 Do 18:00 B
Band gap engineering of insulating films due to defects and adsorption
of organic molecules — •C. Tegenkamp and H. Pfnür —
Institut Für Festkörperphysik, Appelstr.2 D-30167 Hannover,Germany
For possible applications of organic molecules in electronic circuits,
the functionality of a chemically designed molecule can be significantly
changed due to fundamental interactions of the molecule with a surface
after adsorption. Generally, the strength of the chemical bond should depend
on serveral parameters like substrate, orientation of the surface and
defects.
As an example of functional organic molecules, we will present results
about the problem of contact charging between insulators. Therefore, we
have studied the adsorption of OH-substituted benzoic acids on insulating
films like NaCl(100) and KCl(100) in the multilayer, monolayer
and submonolayer regime using photoelectron- and electron energy loss
spectroscopy. The experiments were performed on both defect–free surfaces
and on surfaces decorated with anion vancancies. In accordance
with DFT claculations, there are specific molecular orbitals which are
in resonance with the valence band structure of the alkali halides. Interestingly,
defects like anion vancancies only strengthen the bond by
polarization effects. A reactivity of these centers in form of a dissoci-
ation of the molecules as for water has not been found. Although the
molecules are chemisorbed in presence of defects, this is not reflected in
energetic shifts of molecular orbitals of the functional acid- and phenoli
OH–groups, which are most important for our model of contact charging.
Our experimental results are supported by simple DFT calculations.
SYOH 5.32 Do 18:00 B
Electronic properties of organic/organic semiconductor interfaces
CuPC/C60 — •Olga Molodtsova, Torsten Schwieger,
and Martin Knupfer — Leibniz Institute for Solid State and Materials
Research Dresden, D-01069 Dresden, Germany
Phthalocyanines and C60 are typical organic materials which are often
used in organic electronic devices such as diodes, transistors or solar
cells. Especially the electronic structure of their interface in solar cells is
of great interest for the functionality of these devices.
In this contribution we present a study of the interface properties of the
molecular organic semiconductor copper Phthalocyanine (CuPc) on C60
using photoemission (PES) and near-edge X-ray absorption spectroscopy
(NEXAFS). As well we present a study of C60/CuPc-interfaces with different
orientation of the molecules (standing on polycrystalline gold and
lying on gold single crystals). We discuss important electronic parameters
like electron and hole injection barriers and chemical interactions at this
organic/organic interface. Further we will adress the electronic properties
of mixed C60/CuPc-films.
SYOH 5.33 Do 18:00 B
Photo Electron Spectroscopy of Silicon-Organic Pigment Interfaces
— •Ulrich Weiler 1 , Thomas Mayer 1 , Ralf Hunger 1 ,
Wolfram Jaegermann 1 , Christian Kelting 2 , and Derck
Schlettwein 2 — 1 Technische Universität Darmstadt, FB Materialund
Geowissenschaften, FG Oberflächenforschung — 2 Universität
Oldengurg, Physikalische Chemie 1
One of the prerequisites to obtain a photovoltaically active composite
material uniting high light absorption of an organic dye with good charge
separation and transport properties of Si is the specific alignment of the
dye frontier orbitals versus the Si bands: in order to inject both, the
photo excited electrons from the dye LUMO into the Si conduction band
and the holes from the dye HOMO into the Si valence band, the LUMO
should energetically be situated just above the CB and the HOMO just
below the VB. According to the Anderson model, ZnPc should match
perfectly this requirement. As model systems for the composite, the interface
properties of ZnPc and related organics adsorbed on different Si
surfaces have been characterized with photoelectron spectroscopy. Experimental
results of ZnPc on H-terminated Si are in good agreement
to the Anderson model implying that no additional dipoles are formed
at the interface. In addition, the experimental set-up of our deposition
system for silicon organic compounds will be explained on our poster. In
order to conserve the organic dyes during growth of the compound, Si
will be deposited from a remote hot wire source by CVD and the dyes
will be co-sublimated from a second source.
SYOH 5.34 Do 18:00 B
Energy level alignment at organic interfaces: Interface induced
gap states and charge neutrality levels — •Thorsten U. Kampen
and Dietrich R. T. Zahn — Institut für Physik, TU Chemnitz,
D-09107 Chemnitz
The energy level alignment at organic interfaces determines the efficiency
of charge injection into organic films. Quite often the vacuum
level alignment rule has been used to determine the energy level alignment
at organic interfaces. Here, barrier heights can simply be calculated
using the ionisation potentials or electron affinities of semiconductors materials
and work functions of metals. For organic interfaces a conclusion
has been reached that in general the vacuum levels do not align. At intimate,
abrupt, and defect free interfaces of inorganic semiconductors
interface induced gap states are the primary mechanism determining the
energy level alignment. These interface states derive from the bulk states
and their character changes across the band gap from more acceptorlike
closer to the conduction band to predominantly donor-like nearer
to the valence band. The branch point where the character changes has
the significance of a charge neutrality level. This work shows that the
concept of interface induced gap states may also be applied to organic
interfaces. The charge neutrality levels of PTCDA, DiMe-PTCDI, and
CuPc are found to be 1.96 eV, 1.93 eV and 0.51 eV above the HOMO,
respectively.