Plenarvorträge - DPG-Tagungen

Halbleiterphysik Montag
laubt. Wir finden für Füllfaktoren ν < 2 und bei resonanter Anregung des
untersten Landaulevels ein Quantumbeating, welches auf die kohärente
Kopplung der untersten beiden Landaulevel, vermittelt durch Magnetoplasmonen,
zurückgeführt werden kann. Durch die Möglichkeit die 2D-
Ladungsträgerdichte der Probe über ein Gate zu variieren, sind wir in der
Lage SR-FWM auch im Übergangsbereich vom 2D-Elektronengas zum
exzitonischen Regime durchzuführen.
Diese Arbeit wird von der DFG im Rahmen der Projekte SCHU1171/1
und SCHU1171/2 gefördert.
HL 12.95 Mo 16:30 Poster A
Optical properties of organic – inorganic mixed layers in dependence
on preparation conditions — •Marieta Levichkova,
Jacky Assa, Hartmut Fröb, and Karl Leo — Institut für
Angewandte Photophysik, TU Dresden, 01062 Dresden, Germany,
www.iapp.de
Thin films of organic dyes incorporated in silicon dioxide matrix are
obtained by co-evaporation of both host and dye in high vacuum. The
technique allows preparation of mixed layers independent on dye’s solubility
in the matrix. This in principle possible variation of dye concentration
provides the opportunity to investigate the behavior of even very
diluted solid solutions compared to pure dye layers. Luminescence and
absorption spectroscopy are applied to study the optical properties of
the layers. Significant alterations in absorption and luminescence spectra
in dependence on dye concentration are explained in consequence
of aggregation effects and energy transfer. It is demonstrated that the
evaporation geometry and preparation conditions offer an effective way
to reduce the possibilities for non-radiative transitions thus increasing
the photoluminescence quantum yield.
HL 12.96 Mo 16:30 Poster A
Investigation of charge carrier dynamics in polymer/fullerene
solar cells by means of photoinduced reflection/absorption spectroscopy
— •Christian Arndt 1 , Uladzimir Zhokhavets 1 , Martina
Mohr 1 , Gerhard Gobsch 1 , Maher Al-Ibrahim 2 , and Steffi
Sensfuss 2 — 1 Institute of Physics, Ilmenau Technical University, 98684
Ilmenau, Germany — 2 TITK Institute Rudolstadt, Department Functional
Polymer Systems, 07407 Rudolstadt, Germany
The discovery of a photoinduced charge transfer between conjugated
polymers and fullerenes led to the development of bulk heterojunction
polymer/fullerene solar cells. This kind of organic solar cells currently
offers an efficiency up to 3.5%. In order to improve this efficiency, a detailed
knowledge of the generation and recombination dynamics of the
charge carriers is essential.
The photoinduced absorption spectroscopy is a powerful tool to investigate
the generation and recombination dynamics of polarons. Up to now,
most of this spectroscopic studies were done on thin films, whereas little
is known about the behaviour of the photoinduced charge carriers in the
real device. In this work, we investigated the generation and recombination
dynamics of polarons in bulk heterojunction MDMO-PPV/PCBM
solar cells on flexible substrates, which give AM 1.5 efficiencies up to 3%
by means of photoinduced reflection/absorption spectroscopy. Due to the
HL 13 Hauptvortrag Bayer
reflection geometry of the photoinduced absorption experiment we were
able to study the generation and recombination dynamics of polarons
within these devices.
HL 12.97 Mo 16:30 Poster A
Exciton transport in PTCDA layers studied by time-resolved,
temperature dependent luminescence quenching experiments.
— •R. Schüppel, K. Leo, and M. Hoffmann — Institut für Angewandte
Photophysik, TU Dresden, 01062 Dresden, Germany
For opto-electronic applications of organic semiconductors, the properties
of the photo-excited states (excitons) have to be understood in detail.
In particular, the transport properties of excitons, which are described in
terms of diffusion, are significant for the optimization process of devices,
e.g. solar cells. We performed luminescence quenching experiments with
thin vapor-deposited films of PTCDA (perylene-3,4:9,10-tetracarboxylicdianhydride),
which are partially covered by a quenching layer, in our case
TiOPc (titanyl-phthalocyanine). From the time-resolved PTCDA luminescence,
we can distinguish between primarily excited states (lifetime
τa 1ns). The transport, which
is accessible from the luminescence quenching by a variation of the layer
thickness, shows no temperature dependence for the short-living states.
The transport in the emitting states is thermally activated for high temperatures
(T>60K), which suggests incoherent hopping transport. For
low temperatures, we observe no temperature dependence, which can be
explained by several transport mechanisms.
HL 12.98 Mo 16:30 Poster A
Determination of the excitonic dispersion in α-PTCDA with
electron energy loss spectroscopy and optical spectroscopy —
•Reinhard Scholz, Igor Vragović, and Michael Schreiber —
Institut für Physik, Technische Universität Chemnitz
In the present work, recent experimental findings concerning electron
energy loss spectroscopy, linear optical spectroscopy and low temperature
photoluminescence are interpreted with model calculations based on the
transfer of Frenkel excitons. It can be shown in detail that the exciton
transfer between non-degenerate vibronic sublevels results in deviations
of the excitonic dispersion from a simple cosine-shape [1]. Moreover, these
off-diagonal parts of the exciton transfer have a strong influence on the
oscillator strength of the vibronic sublevels: In optical absorption at the
Γ-point of the Brillouin zone, the coupling strength is redistributed to
the upper levels [2], whereas at the surface of the Brillouin zone, it accumulates
in the vibronic ground state. The wave vector dependence of
the lineshape observed with electron energy loss spectroscopy (EELS)
reflects this behaviour [3]. The photoluminescence band dominating at
low temperatures results from a vertical recombination process starting
from the minimum of the lowest branch of the excitonic dispersion [1].
[1] I. Vragović and R. Scholz, Phys. Rev. B 68, 155202 (2003).
[2] I. Vragović, R. Scholz, and M. Schreiber, Europhys. Lett. 57, 288
(2002).
[3] I. Vragović, Frenkel exciton model of excitation and recombination
processes in crystalline α-PTCDA, Dissertation, Technische Universität
Chemnitz (2003).
Zeit: Dienstag 09:30–10:15 Raum: H15
Hauptvortrag HL 13.1 Di 09:30 H15
Quantum dots: Building blocks of quantum devices? — •M.
Bayer — Universität Dortmund, Experimentelle Physik II, Otto-Hahn
Str.4, 44227 Dortmund
Quantum dots have attracted considerable attention recently for their
potential to implement modern fields of physics such as quantum optics
or quantum information processing in a solid state environment. While
quite some progress has been made to manifest this potential, still a lot of
open questions remain. In this contribution we will try to present some of
these dot applications and discuss related problems and strategies how
to overcome them. In quantum optics, for example, single dots might
be used as light sources for single photons or entangled photons. For
these purposes a high degree of control of the light-matter interaction
is required, which can be obtained by placing the dots in a resonator.
The current status of these activities will be considered. For quantum
information applications quantum bits with controllable interactions between
them are needed, in order to realize gates. Such gates might be
obtained by coupling of dots. Results of studies on demonstration of coherent
coupling, on control of coupling and on decoherence of excitons
will be presented.