Plenarvorträge - DPG-Tagungen

Symposium Organic and Hybrid Systems for Future Electronics Donnerstag
SYOH 5.74 Do 18:00 B
Spectromicroscopic Characterisation of All Polymer Transistor
Structures — •Klaus Mueller, Yevgen Burkov, and Dieter
Schmeisser — BTU Cottbus, Angewandte Physik-Sensorik, 03013 Cottbus,
P.O.Box 101344
In order to optimize organic field effect transistors, the characterization
of surfaces in terms of their roughness or chemical composition is
very important. We report on high resolution spectromicroscopic mapping
of organic thin film transistors by photoemission electron microscopy
(PEEM). It was shown that PEEM is a useful technique to characterize
the surface morphology (roughness), the chemical homogeneity or
the composition of structures in between the source and drain electrodes.
Mapping of surface potentials, especially at the interface electrode/channel
is possible. We compare different preparation methods and
the characterization at applied voltages is also shown.
The transistors itself were prepared by a new low cost method. The
electrodes for source and drain have been prepared by a plotting method
with colloidal graphite or carbon black as the conducting material. P3HT
was used as active layer and has been prepared by spin coating.
SYOH 5.75 Do 18:00 B
All Polymer Field-Effect-Transistors based on Low-Cost-
Microstructuring — •Klaus Mueller, Ioanna Paloumpa,
Carola Schwiertz, Agnes Jahnke und Dieter Schmeisser —
BTU Cottbus, Angewandte Physik-Sensorik, 03013 Cottbus
Thin film transistors with P3HT as active layer have been prepared by
spin coating. The electrodes for source and drain have been prepared by
a low cost plotting method with colloidal graphite or carbon black as the
conducting material. PEDOT or carbon black was used as gate electrode.
The devices, prepared by different methods and materials have been
characterized by electrical measurements, for example, their transfer and
output characteristics. Furthermore, the influence of different materials
on effects like hysteresis or reproducibility will be compared.
In addition, we describe a project investigating new types of biosensors
with functionalized gate electrodes. Exchanges of dipole moments or
charges by immobilisation of biomolecules will change the electrical characteristic
of the transistor, which leads to a sensing signal proportional
to their amount.
SYOH 5.76 Do 18:00 B
Stability Investigations on Polymer Light Emitting Diodes —
•Dessislava Sainova, Armin Wedel, and Bert Fischer — Fraunhofer
IAP, Geiselberstr. 69, 14 476 Golm
Organic light emitting diodes (LEDs) are an important topic in the
rapidly developing field of flat panel display technologies due to their
application potential as backlights and indicators. Polymer utilization as
emissive LED-layers offers the advantage of low-cost solution processing
on variable substrates.
A significant factor for the successful LED-application is the high operational
stability of the devices. The presentation concerns the stability
investigations of polymer LEDs. The entire LED preparation starting
from the careful substrate cleaning until the encapsulation was made using
clean room and glove box facilities. Optimisation procedures have
been made on different steps of the device preparation e.g. the deposition
conditions of the different layers constituting the devices and the
configuration of the electrodes. The lifetime tests on the encapsulated devices
have been performed in ambient atmosphere under constant current
driving mode of the experimental set up.
SYOH 5.77 Do 18:00 B
Fatigue mechanisms of organic light emitting diodes — •Roland
Schmechel, Frederik Neumann, Yuri A. Genenko, and Heinz
von Seggern — TU-Darmstadt; FB:Material- und Geowissenschaft;
Petersenstr.23; D-64287 Darmstadt
A study on possible electrical fatigue mechanisms in organic light emitting
diodes (OLEDs) is presented. In order to distinguish bulk and interface
properties, electroluminescence (EL) and photoluminescence (PL)
intensities are measured simultaneously during device operation. In addition,
I-V characteristics were recorded before and after fatigue. Unlike an
almost constant PL, a strong decrease in EL is observed indicating an interface
related fatigue mechanism. The I-V characteristics differ strongly
before and after the device operation, especially in the low forward voltage
regime. Whereas the I-V characteristics of the virgin device can be
explained by space charge limited currents and field dependent charge
carrier mobilities, the fatigued, but still operating device has developed
a low Ohmic parallel resistance. At present we conclude that a fatigued
device shows additional conductive pathways through the bulk and a disturbed
charge carrier balance due to reduced injection of charge carriers.
SYOH 5.78 Do 18:00 B
High Efficiency and Low Voltage p-i-n Electrophosphorescent
OLEDs with Double Emission Layer — •Gufeng He, Martin
Pfeiffer, and Karl Leo — Institut fuer Angewandte Photophysik,
Technische Universitaet Dresden, D-01062 Dresden,Germany
We demonstrate high-efficiency and low-voltage organic phosphorescent
light-emitting devices employing a green phosphor,
tris(2-phenylpyridine) iridium [Ir(ppy)3].
The intrinsic emitting layers are sandwiched between two p- and ndoped
layers. The p-i-n structure results in efficient carrier-injection from
both contacts into the doped transport layers and low ohmic losses. Thus,
low operating voltages are obtained compared to conventional undoped
OLEDs.
By doping Ir(ppy)3 into both electron- and hole-transport hosts, a
power efficiency of 70 lm/W and external quantum efficiency of 19.5 at
100 cd/m2 (2.95V). More importantly, the efficiency decays only weakly
with increasing current density (or brightness). A quantum efficiency of
13.5 luminance of around 50,000 cd/m2. This improvement can be attributed
mainly to the confinement of the recombination region to the
interface of the dye doped electron- and hole-transport hosts. Thus, the
influence of electron or blocking layers is reduced as compared to conventional
single-emission-layer structures and charge accumulation at the
interfaces of these blocking layers is avoided.
SYOH 5.79 Do 18:00 B
Dynamics of Charge Carrier Transport and Electroluminescence
in Organic Light-Emitting Diodes — •Anton G. Mückl 1
and Wolfgang Brütting 2 — 1 Experimentalphysik II, Universität
Bayreuth, D-95440 Bayreuth — 2 Experimentalphysik IV, Universität
Augsburg, D-86135 Augsburg
When describing the dynamics of charge carrier transport and recombination
processes in organic light-emitting devices not only the dependence
of the drift mobility of the majority charge carriers on temperature
and applied electric field is important but also the dynamics of the minority
charge carriers should be considered. We have investigated the
mobility of electrons and holes in Alq3 at various temperatures and electric
fields using time resolved electroluminescence measurements on a
specially designed multilayer OLED structure. Both, electron and hole
mobility show a Poole-Frenkel-type dependence on temperature and electric
field, however in contrast to the electron transport the hole transport
is found to be non-dispersive with a temperature independent field enhancement
factor. These results indicate clearly the mechanisms of electron
and hole transport in Alq3 to be of different nature.
We also measured the time-resolved current through unipolar and bipolar
single and double layer OLED structures at various temperatures. The
decay of the current towards the steady-state value will be discussed in
terms of the dynamics of the built-up of space charges and trap filling.
SYOH 5.80 Do 18:00 B
Polymer light-emitting diodes containing dendronized emitters
— •Frank Jaiser 1 , Xiaohui Yang 1 , Dieter Neher 1 , Jianqiang
Qu 2 , and Klaus Müllen 2 — 1 Universität Potsdam, Institut für Physik,
Am Neuen Palais 10, 14469 Potsdam — 2 Max-Planck-Institut für Polymerforschung,
Ackermannweg 10, 55128 Mainz
One approach to the construction of efficient polymer light-emitting
diodes is to dope well-defined emitting dyes into a suitable polymer hosts.
Such dopants can improve LED efficiency by either influencing charge
carrier transport in the material or being a highly-emissive species in the
system. In this respect, various fluorescent and phosphorescent dyes have
been studied in the past. In fact, both types of dyes have been used to
tune the emission color through the visible spectrum.
The understanding of excitation transfer between host and guest
molecules is crucial for the construction of efficient LEDs. Possible transfer
routes are Förster and Dexter transfer or charge trapping on the
guest molecules. Depending on the spatial distance between guest and
host molecules, the different possible transfer mechanisms show vastly
different transfer rates. Dendronization of small molecules is an elegant
way to control this intermolecular distance without changing the electronic
structure of the emitting core. We have studied polymers doped
with different generations of perylene dendrimers. We found that den-