Plenarvorträge - DPG-Tagungen

Symposium Organic and Hybrid Systems for Future Electronics Donnerstag
dronization does not help to suppress luminescence quenching in thin
films but decreases the probability of direct charge trapping on the dye.
SYOH 5.81 Do 18:00 B
Polymer electrophosphorescence with high power conversion
efficiencies — •Xiaohui Yang 1 , Frank Jaiser 1 , Dieter Neher 1 ,
Dirk Hertel 2 , Thomas Däubler 2 , and Klaus Bonrad 2 —
1 University of Potsdam, Institute of Physics, Am Neuen Palais 10, 14469
Potsdam, Germany — 2 Schott Spezialglas GmbH, Hattenbergstraße 10,
55122 Mainz, Germany
We demonstrate efficient single-layer polymer phosphorescent lightemitting
devices based on a green-emitting iridium-complex blended
into the polymer host poly (N-vinyl carbazole), co-doped with electrontransporting
and hole-transporting molecules. Efficiencies and current
densities of the devices increase with increasing iridium-complex concentration.
The devices with high iridium complex concentration can be
operated at relatively low voltages, resulting in a large power conversion
efficiency of up to 19.2 lm/W at luminance efficiencies exceeding 30 cd/A.
The overall performances of these devices are approaching those of small
molecule multilayer devices, which suggests that efficient electrophosphorescent
devices with acceptable operating voltages can be achieved in very
simple device structure fabricated by spin-coating.
SYOH 5.82 Do 18:00 B
Investigation of electric field in organic thin film devices by
electroabsorption spectroscopy — •Wenge Guo, Jens Drechel,
Michael Hoffmann, Martin Pfeiffer, and Karl Leo — Institut
fuer Angewandt Photophysik, Dresden,Germany
Electroabsorption (EA) spectroscopy is a non-destructive technique
used to study the internal electric field distribution in organic light emitting
diodes and organic solar cells. In EA measurement, an electric field
modulation is applied to the device and a synchronous change in the optical
absorption coefficient is detected. The change in the amplitude of
the optical absorption coefficient at the fundamental frequency ? of the
applied electric field is proportional to the static internal electric field
in the device. As a starting point, we conduct electroabsorption measurements
on polycrystalline MePTCDI and PTCDA thin single layer
devices. EA spectra at 1? and 2? are obtained. In EA measurements at
1?, the EA signal vanishes as applied DC bias compensate the build-in
potential due to the work function difference of the two electrodes. From
this, a reasonable built-in potential (0.5 0.7eV) is observed. Since the
EA response at 1? is linear with the applied DC bias in single layer devices,
this method can be used further for characterization of static field
distribution in multilayer devices.
SYOH 5.83 Do 18:00 B
Linewidth-limited energy transfer in single conjugated polymer
molecules — •John Lupton 1 , Juergen Mueller 1 , Florian
Schindler 1 , Ullrich Scherf 2 , and Jochen Feldmann 1 —
1 Photonics and Optoelectronics Group, Sektion Physik, LMU Muenchen
— 2 FB Chemie, Universitaet Wuppertal
Single molecule fluorescence spectroscopy is a powerful tool, which allows
a direct distinction between homogeneous and inhomogeneous spectral
broadening. We apply this technique to gain fundamental insight into
energy transfer processes in conjugated polymers relevant to molecular
electronics. At low temperatures we are able to identify individual homogeneously
broadened chromophore units on the polymer chain. Using
time resolved and polarisation sensitive fluorescence spectroscopy we image
the intramolecular transfer of excitation energy from higher energy
segments on the chain to lower energy segments. As the temperature is
raised we find that the emission lines of individual chromophore units
are broadened. Concomitantly, the average polarisation anisotropy decreases,
suggesting that excitation energy is distributed across different
chromophore units. Studies of the single chain fluorescence as a function
of time show a pronounced blinking behaviour of the multichromophoric
chain at room temperature, but not at low temperatures, demonstrating
that chromophore coupling is controlled by temperature and spectral
linewidth (Mueller et al., Phys. Rev. Lett. 2003). High resolution fluorescence
spectroscopy also enables us to gain insight into the strength
and nature of vibrational coupling and structural relaxation on a single
molecule level.
SYOH 5.84 Do 18:00 B
Time resolved temperature measurements using molecular
thermometers — •Joachim Stehr, Gunnar Raschke,
Thomas A. Klar, John M. Lupton, and Jochen Feldmann
— Photonics and Optoelectronics Group, Department of Physics and
CeNS, Ludwig-Maximilians-Universität, Amalienstrasse 54, 80799
Munich
Platin octaethyl porphyrin (PtOEP) shows a photoluminescence (PL)
spectrum dominated by a red band at 650 nm and a thermally activatable
band at 540 nm. Therefore, this molecule can be used as a molecular
thermometer by taking the ratio between these two emission bands. A
resolution of 0.25 K has been demonstrated [1]. We want to use these
molecular thermometers to observe heating and cooling dynamics at the
surface of micro and nano objects. Heating is accomplished either by
pulsed electrical excitation of thin wires or by pulsed optical excitation
of nanoparticles. The optical excitation of the PtOEP molecules is delayed
with respect to the heating pulses. Towards the realisation of such
a molecular thermometer we also investigate the internal dynamics of the
PtOEP molecules following excitation.
[1] J. M. Lupton, Appl. Phys. Lett. 81, 2478 (2002)
SYOH 5.85 Do 18:00 B
A Spectroscopic Investigation Using Model Oligofluorenes —
•Chan Im 1 , Chunyan Chi 1 , Panagiotis E. Keivanidis 1 , Andreas
Ziegler 1 , Sigrud Höger 1 , Gerhard Wegner 1 , Jung-Ho Jo 2 , Ji-
Hoon Kang 2 , and Do-Young Yoon 2 — 1 Max-Planck Institute for
Polymer Research, Ackermannweg 10, 55128 Mainz, Germany — 2 Seoul
National University, Department of Chemistry, Seoul, South-Korea
Polyfluorene derivatives are very promising electroluminescent materials
due to their highly effcient electroluminescence with blue emission
color. Unfortunately, there is still a well-known problem that a broad
green emission band appears and grows up during the operating of light
emitting devices. While many groups have shown that ketonic defects on
polymer backbones are mainly responsible for this green band, there are
still many questions which should be answered as for the population of
these green emissive states. To understand the nature of such ketonic defect
states in detail, we have decided to study a model system including
a number of oligofluorenes both with and without an embedded ketone
unit. After synthesis of highly pure oligofluorenes, delayed fluorescence
measurements were performed at various temperatures either as films or
as dilute solutions to estimate photoluminescent properties for long lived
emission bands, e.g. the green band. Additionally, fast spectroscopy using
a streak camera within a time range of nanoseconds was carried out in
order to trace blue singlet emission decay behavior. Those detailed spectroscopic
results show significantly different photoluminescence behavior
between oligofluorenes with various sidechain groups especially in solid
state.
SYOH 5.86 Do 18:00 B
Towards polymer spintronics: Manipulating the triplet state in
conjugated polymers — •M. Reufer 1 , J. M. Lupton 1 , J. Feldmann
1 , and U. Scherf 2 — 1 Photonics and Optoelectronics Group,
Physics Department, University of Munich, Germany — 2 Fachbereich
Chemie, Universität Wuppertal, Gauss-Str. 20, 42097 Wuppertal, Germany
Organic semiconductors are characterised by strong exchange interactions,
which typically lead to a large energetic splitting of the singlet
and triplet states. The presence of both of these levels is crucial to the
photophysics of molecular semiconductors and particularly relevant to
the operation of light-emitting diodes. Lupton et al. recently presented
a novel technique to visualise triplet excitations in conjugated polymers
using a new class of materials exhibiting strong room temperature phosphorescence
due to trace amounts of metallic impurities [1]. Here we
demonstrate that we can manipulate the triplet state by applying external
perturbations in the form of electrical or optical fields. We can
directly monitor and time resolve triplet creation and annihilation due
to electric field induced carrier dissociation and recombination. As the
conjugated polymers used exhibit strong optical gain, we are also able
to use stimulated emission to deplete the singlet state before intersystem
crossing to the triplet state occurs. This allows us to image and
time resolve the process of spin-forbidden intersystem crossing and the
associated spin flip in organic semiconductors directly.
[1] Lupton et al., Phys. Rev. Lett. 89, 167401 (2002)