Plenarvorträge - DPG-Tagungen

Symposium Organic and Hybrid Systems for Future Electronics Donnerstag
blends. Even on nominally homogeneous m-LPPP films, we observe pronounced
spatial fluctuations in both magnitude and dynamics of the ultrafast
picosecond decay of the photoinduced absorption on sub-micron
length scales. In this paper, we introduce our novel experimental technique
and discuss the implications of our findings.
SYOH 2.3 Do 15:15 H37
Morphology-dependent energy transfer within polyfluorene
thin films — •Anna Köhler, Amena L.T. Khan, Paiboon
Sreearunothai, Laura M. Herz, and Michael J. Banach —
Cavendish Laboratory, University of Cambridge, Cambridge CB3 OHE,
U.K.
Polyflourenes are a class of blue luminescent organic semiconductors
with photophysical properties that vary strongly with the morphology of
the film. In Poly (9,9-dioctylfluorene) (PFO), distinct phases of molecular
order have been identified such as the disordered glassy phase and
crystalline alpha and beta-phases where the PFO chains are considered
to be planar. We present a detailed study of the interrelationship between
morphology and photophysics in thin films of PFO. Small amounts of the
crystalline beta-phase dominate the emission characteristics. We demonstrate
that the formation of the two phases during spin-coating can be
controlled reliably through the choice of the boiling point and solubility
parameter of the solvent. A Franck-Condon analysis shows that the planar
chains are characterized by a low Huang-Rhys parameter, consistent
with well delocalised excited states, while the non-planar chains of the
glassy phase show high values for the Huang-Rhys parameter. The close
intermixing of the two phases and the large spectral overlap of emission
from the glassy phase and absorption from the b-phase leads to efficient
transfer on a sub-picosecond timescale with a large Förster radius of
about 8 nm. As a result, the steady state emission spectra show a significant
fraction of emission from the beta-phase if only a few percent of
the chains are planarized.
SYOH 2.4 Do 15:30 H37
Up-conversion Fluorescence in Polyfluorenes doped with
Metallated Macrocycles — •Stanislav Baluschev 1 , Panagiotis
Keivanidis 1 , Tzenka Miteva 2 , Gabriele Nelles 2 , Ulrich
Scherf 3 , Akio Yasuda 2 , and Gerhard Wegner 1 — 1 Max-Planck-
Institute for Polymer Research, Ackermannweg 10, 55128 Mainz —
2 Sony International (Europe), MSL, Hedelfinger Str. 61, 70327 Stuttgart
— 3 Bergische Universität Wuppertal, Fach. Chemie, Gauss Str. 20,
42119 Wuppertal
In our research towards polymer lasers we achieved for the first time
quasi-cw up-conversion in doped polyfluorenes (PFs). Metallated macrocycles
as porphyrins and phthalocyanines were used as dopands (sensitisers).
Upon excitation with 532 nm the characteristic emission of the
PF (420-480 nm) was detected together with the sensitisers emission.
The system was optimised for dopands type, doping level and for the
side-chain pattern of the PFs. The macroscopic temperature was controlled
and the efficiency of the sensitiser excitation was optimised. As a
result, up-conversion at pumping densities as low as a few kWcm −2 were
achieved to be compared very favourably with the GWcm −2 necessary
for similar magnitude two-photon excited fluorescence in PFs.
From our results it is evident that the up-converted PF emission is
a consequence of an energy transfer process from the sensitisers to the
PF. We suggest that a mechanism involving a sequential multiphoton
absorption in the macrocycles may govern this energy transfer process.
SYOH 3 Electronic Structure and Transport Properties
Zeit: Donnerstag 16:00–16:45 Raum: H37
SYOH 3.1 Do 16:00 H37
Electronic transport in tetracene single crystals: A benchmark
for thin film devices — •J. Pflaum, J. Niemax, A.K. Tripathi,
Chr. Herb, and Chr. Ender — 3. Phys. Inst., Universität Stuttgart,
70550 Stuttgart
Although a major contribution in the field of molecular electronics is
attributed to organic thin film devices, the intrinsic electronic properties
of organic materials without the influence of structural disorder or contact
effects can be studied only for single crystals. We have carried out transport
studies on tetracene single crystals prepared by different growth
techniques in combination with chemical analysis by gas chromatography.
Comparing the charge carrier transport, different temperature dependences
of the hole mobility together with different mobility values at
room temperature (RT) were observed. For crystals grown by a modified
Bridgman setup the hole mobility of ∼ 0.1cm 2 /Vs at RT is much smaller
than that of ∼ 1cm 2 /Vs obtained for crystals grown by sublimation in an
H2 vapor stream. The result can be explained by different concentrations
and energies of the occurring traps related to the respective preparation
method. Supported by the DFG (OFET-Schwerpunktprogramm).
SYOH 3.2 Do 16:15 H37
Dynamics of Hot Electrons in PTCDA Thin Films — •Steffen
Berger, Alexander Mönnich, Jens Wüsten, Martin Aeschlimann,
and Christiane Ziegler — University of Kaiserslautern, Department
of Physics,
Charge carrier dynamics are of importance for many applications of organic
semiconductors. We therefore studied the electron dynamics in 100
nm thin 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) films
without and with dopants by means of time-resolved two-photon photoelectron
spectroscopy (TR-2PPE) on a femtosecond scale.
As light source a titanium-sapphire laser system with a repetition rate
of 80 MHz and a pulse length of 20 fs was used. Frequency doubling
gives a photon energy of 3.1 eV, which allows to examine the life time of
electrons in PTCDA with energies of 1.3-3.0 eV above the Fermi level.
By deposition of alkali-metals such as Na the conductivity of the
n-type organic semiconductor 3,4,9,10-perylene-tetracarboxylic dianhydride
(PTCDA) can be increased by several orders of magnitude. In TR-
2PPE we observed an increase in the life time of electrons with energies
> 1.2 eV above EF after Na reaction. This can be explained by results
from inverse photoemission spectroscopy which reveals that the density
of states at 1.2 eV above EF is reduced by the same process, i.e. there
are less unoccupied states in which the hot electrons can relax.
SYOH 3.3 Do 16:30 H37
Influence of Defect States on the Electronic Transport of Pentacene
Thin Film Transistors — •Pravesh Kumar 1 , Dietmar
Knipp 1,2 , Armin R. Volkel 2 , and Veit Wagner 1 — 1 International
University Bremen, School of Science and Engineering, Germany — 2 Palo
Alto Research Center, Palo Alto, USA
The influence of acceptor and donor like defect states on the current
voltage characteristics of polycrystalline pentacene thin film transistors
(TFTs) were explored. The thermally evaporated pentacene films
were prepared on organic and inorganic dielectrics. Careful control of
the preparation conditions leads to TFTs with very similar mobility of
0.4 cm 2 /Vs and on/off ratio > 10 8 on thermal oxide, silicon nitride and
poly-vinyl phenol (PVP) dielectrics. To gain insides in the electronic
transport behavior of the organic TFTs we used a one-dimensional transistors
model. The model assumes an exponential distribution of defect
states in the bandgap [1]. The experimental data can be described by a
steep exponential distribution of donors close to the valence band and a
shallower distribution of acceptors deeper in the bandgap. The influence
of different preparation conditions of the pentacene film and the organic
dielectric PVP will be discussed in greater detail by a comparison of
experimental and simulation data. Dihexylquaterthiophene (DH4T) was
also used to fabricate the thin film transistors. The mobility and the on/
off ratios for pentacene and DH4T thin film transistors were compared.
[1] A.R. Volkel, R.A. Street, D. Knipp, Phys. Rev. B66, 195336 (2002).