Plenarvorträge - DPG-Tagungen

Symposium Organic and Hybrid Systems for Future Electronics Donnerstag
charge carriers using a field effect geometry instead offers improved access
to the intrinsic charge transport properties of these materials. We are
investigating single crystals of tetracene and perylene. In order to get a
working FET, the gate isolation has to fulfil some stringent requirements.
We have screened several materials suitable as gate dielectrics. With the
polymer PPX (poly-para-xylylene or parylene), field effect transistors
on tetracene single crystals can be prepared reliably without significant
damage to the delicate crystal surface. The properties of these FET’s
were examined down to liquid helium temperatures, and the influence of
different sample preparation techniques will be discussed. To understand
the pronounced influence of different metals for the source-electrode, we
have measured space charge limited currents (SCLC) on tetracene and
perylene. Trap concentrations as low as 1·10 13 cm −3 in Bridgman-grown
perylene attest the high quality of the crystals used. These SCLC measurements
also show deficient charge carrier injection for many contact
metals, especially in perylene. Therefore, carrier injection was systematically
evaluated, both in the SCLC as well as in the FET geometry.
Supported by the DFG (Sm64/4-1, Ka 427/8).
SYOH 5.68 Do 18:00 B
Morphology-performance relationship in pentacene-based thin
film transistors — •Wolfgang Kalb 1 , Philippe Lang 2 , Mohammad
Motaghi 2 , Matthias Wuttig 1 , and Gilles Horowitz 2 — 1 1.
Physikalisches Institut der RWTH Aachen, Germany — 2 ITODYS / Universite
Paris 7, France
Pentacene-based thin film transistors were fabricated on Si/alumina
substrates with gold top-contact source and drain electrodes. On some
devices, the alumina surface was modified with fatty acid self-assembled
monolayers prior to pentacene deposition. Pentacene layers were grown
by vacuum evaporation at various deposition rates ranging from 0.003A/s
to 3A/s and various film thicknesses. The gate voltage dependent mobility
was extracted from the transfer characteristics with correction for contact
resistance. We find that treating the alumina with fatty acid leads
to a substancial increase in the mobility. The mobility furthermore depends
upon the film thickness. An intermediate deposition rate of 0.1A/s
is found to lead to optimal transistor performance. The growth mechanism
of the pentacene layers was followed by atomic force microscopy.
It is found that the primary growth-stage of pentacene-films leading to
transistors of best performance is characterised by a high nucleation rate.
The size of the grains tends to drastically increase during the deposition
process.
SYOH 5.69 Do 18:00 B
Simulation-Optimization of Ambipolar Organic Field-Effect
Transistors — •Gernot Paasch 1 , Thomas Lindner 1 , and
Susanne Scheinert 2 — 1 IFW Dresden — 2 TU Ilmenau
Ambipolar or double-injection field effect transistors based on amorphous
silicon have been investigated almost two decades ago, but they
did not find significant application. With organic materials as active layer
such devices become of interest due to direct recombination with light
emission. Even advanced existing models based on the Pao-Sah description
neglect both the actual contact properties and the recombination
process. Hence, for analyzing experimental data or optimizing the device
performance detailed numerical simulations have been carried out. Comparison
with experimental data allows for the determination of important
material parameters. Moreover, the simulation reveals the subtle interplay
between the properties of the source and gate contacts, interface
charges, the ratio of the electron and hole mobilities, doping, and recombination
(and the heterojunction in the case of a double layer system).
Operation modes are clarified by inspection of the internal concentration
and field distributions. Further, the simulations result in rules for possible
simple analytical examination of experimental data and for device
optimization.
SYOH 5.70 Do 18:00 B
Effect of Annealing on Characteristic Hole Traps in
Poly(3hexyl-thiophene) Thin Films — •Zivayi Chiguvare,
Jürgen Parisi, and Vladimir Dyakonov — University of
Oldenburg, Faculty of Physics (EHF), 26111 Oldenburg
Bulk transport properties of poly(3hexylthiophene) (P3HT) were studied
by analysing temperature dependent current-voltage characteristics
of the polymer thin films sandwiched between ITO/PEDOT and aluminium
electrodes. It was found that the contacts limit charge injection
under reverse bias, but under forward bias the current is limited by space
charge that accumulates near the hole injecting electrode (ITO/PEDOT)
due to the poor transport properties of P3HT. The forward current density
obeys a power law of the form J ∼ V m , characteristic of SCLC in
the presence of exponentially distributed traps within the band gap. The
energy that characterises the exponential distribution of hole traps increases
with temperature. Hole trap density was found to decrease on
annealing the devices, resulting in more than 2 orders of magnitude increase
in SCLC hole mobility. We attribute this to better chain packing
and thus a large increase in the degree of interchain interactions as compared
to as-cast films.
SYOH 5.71 Do 18:00 B
Influences on the mobility in poly-(3-hexylthiophene) based
organic field effect transistors — •Holger Heil, Roland
Schmechel, and Heinz von Seggern — Insitute of Material Science,
Darmstadt University of Technology, Petersenstr. 23, 64287 Darmstadt,
Germany
Regioregular head-to-tail coupled poly-(3-hexylthiophene) polymers
(P3HT) are one of the most promising candidates for application in organic
field effect transistors (OFETs) due to their self-ordering properties.
The effect of surface treatment as well as mechanical rubbing of
the polymer layer on the morphology, texture and orientation of the
molecules were investigated by means of optical and x-ray diffraction
methods and were correlated to the obtained field effect mobility. It could
be demonstrated that mechanical rubbing along the transport direction
results in an orientation of the polymer chains and enhances the field
effect mobility. But an enhanced crystallinity due to surface treatment
of the insulator surface or drop casting of the polymer results in an even
more increased mobility. The results are discussed with respect to the importance
of inter- and intra-chain charge transfer processes to the overall
charge transport.
SYOH 5.72 Do 18:00 B
Effect of Molecular Weight and Annealing of Poly(3hexylthiophene)s
on the Performance of Organic Field
Effect Transistors — •Achmad Zen 1 , Frank Jaiser 1 , Dieter
Neher 1 , Jens Pflaum 2 , Stephan Hirschmann 2 , Jürgen Rabe 3 ,
Wei Zhuang 3 , Ullrich Scherf 4 , and Udom Asawapirom 4 —
1 University of Potsdam, Institute of Physics, Am Neuen Palais 10,
D-14469 Potsdam — 2 University of Stuttgart, Institute of Physics,
Pfaffenwaldring 57, D-70569 Stuttgart — 3 Humboldt University,
Department of Physics, Newtonstr. 15, D-12489 Berlin — 4 University
of Wuppertal, Macromolecular Chemistry, Gauss-Str.20, D-42097
Wuppertal
Details on the performance of polymer OFETs prepared from P3HT
with different molecular weights are presented. Lowering the molecular
weight of P3HTs dramatically decreases the mobility of the transistors.
This is mainly explained by the ability of the high-molecular weight fractions
to form well-ordered domains with planar backbone conformation
at room temperature and its low density of defects. On the other hand,
we observe a larger degree of crystallization for the low molecular weight
fractions, in accordance to recent publications by others. Based on experiments
performed at elevated temperatures and on annealed samples, we,
however, conclude that crystallinity is not the major factor controlling
the performance of these OFETs.
SYOH 5.73 Do 18:00 B
Dynamic measurements on organic FETs — •I. Hörselmann 1 ,
P. Kornetzky 2 , A. Herasimovich 1 , and S. Scheinert 1 — 1 TU Ilmenau
— 2 IMMS Ilmenau
The application of organic field effect transistors in electronic circuits
requires transistors with frequencies up to 100kHz. One possibility to investigate
the frequency response of the transistors is the measurement of
the transconductance.
For this purpose, we prepared organic FETs using an interdigital pattern
for the source/drain contacts to obtain high w/L ratios. The active
layer made from polyalkylthionphen (P3AT) was spin coated on top of a
silicon wafer with SiO2 as gate insulator.
The measured static current characteristics of the transistor allow an
estimation of the transconductance. At different operating points we
measured the frequency response of these quantity. The results show
the strong influence of the overlap capacities between the gate and
source/drain contacts resulting in a low cut-off frequency. Consequently,
a design using an organic insulator is necessary to realize higher cut-off
frequencies. Additionally, the accuracy was limited by hysteresis effects
already observed during the static measurements.