Plenarvorträge - DPG-Tagungen

Halbleiterphysik Dienstag
Infrared spectroscopic ellipsometry (IRSE) for wave numbers from 333
to 4000 cm −1 is demonstrated as useful technique for characterization
of structural, vibrational, and free-charge-carrier properties of conducting
organic thin layers deposited on electrically conducting as
well as on isolating substrates. Pentacene, poly(3,4-ethylenedioxy thiophene)/poly(styrenesulfonate)
(PEDOT:PSS), C60 and polyfluorene are
studied exemplarily. For all materials the infrared dielectric functions
are reported, which can serve as fingerprints for multiple-layer structures
analysis. PEDOT:PSS layers deposited onto n-type and p-type silicon
substrates reveal different conductivity response, indicative for charge
transfer at the semiconductor/polymer interface, whereas the IRSE data
show that the pentacene layers on glass are rendered highly resistive.
HL 23.7 Di 18:15 H13
Ultrafast excitonic relaxation processes in perylene derivatives
— •E. Engel, M. Koschorreck, M. Hoffmann, T. W. Canzler,
T. Hasche, and K. Leo — Institut für Angewandte Photophysik, TU
Dresden, 01062 Dresden, Germany, www.iapp.de
Perylene derivatives are interesting model systems because of
their quasi-1D organic molecular crystal structure, their strong
molecular interaction and their potential for use in low-cost optoelectronic
devices. We investigated polycrystalline thin films of
N-N’-dimethyl-perylene-3,4,9,10-dicarboximide (MePTCDI) and
perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA). Using ultrafast
pump-probe spectroscopy, we temporally resolved the relaxation process
of free excitons from the absorbing states down to the indirectly emitting
states. The time constant, measured by photoinduced absorption, is
about 30fs. Furthermore, we investigated signal contributions from
stimulated emission and ground state bleaching. The relaxation
processes are discussed within the framework of an exciton band
structure model[1].
HL 24 Photovoltaik I
[1] M. Hoffmann, Z. G. Soos and K. Leo, Nonlinear Optics 29, 227
(2002)
HL 23.8 Di 18:30 H13
Ultrafast Relaxation between Davydov Components in
Crystalline N-N’-dimethyl-perylene-3,4,9,10-dicarboximide
(MePTCDI) — •Marco Koschorreck, Egbert Engel,
Karl Leo, and Michael Hoffmann — Institut für Angewandte
Photophysik, TU Dresden, 01062 Dresden, Germany, www.iapp.de
We investigated polarization anisotropies in polycrystalline films of the
organic semiconductor N-N’-dimethyl-perylene-3,4,9,10-dicarboximide
(MePTCDI) by means of pump-probe and time-resolved luminescence
measurements. The timescale of the observed processes range from tens
of femtoseconds to several picoseconds.
On the shorter timescale (50fs), the equilibration process of the initially
differently populated Davydov components was observed. The
textbook model for polarization anisotropies was extended to the case
of two coupled Davydov components. Through this it is possible to
predict the remaining anisotropy after this very fast relaxation process.
The experimental results agreed well with the theoretical values. The behavior
at longer timescales (100ps) could be explained by exciton diffusion
between randomly oriented crystallites. While crossing the crystallite
boundaries, the excitons lose their polarization memory and the polarization
anisotropy goes to zero. Temperature dependent measurements
indicate an incoherent thermally activated hopping process.
HL 23.9 Di 18:45 H13
N,N’-perarylated -2,2’-bithiophene - as a new typ of hole transporting
materials — •Olaf Zeika — Novaled GmbH 01069 Dresden
Zellescher Weg 17
Synthesis, characterisation and possible use in OLED
Zeit: Dienstag 17:45–19:15 Raum: H15
HL 24.1 Di 17:45 H15
Doping experiments with HfSxSe2−x — •Steffen Duhm, Ralf
Severin, Alicia Krapf, Christoph Janowitz, and Recardo
Manzke — Institut f. Physik, Humboldt-Unversität zu Berlin,
Newtonstr. 15, 12489 Berlin
HfSxSe2−x is an indirect semiconductor of the group of transition metal
dichalcogenides, the band-gap of which ranges from 1.1 eV (HfS2) to 2
eV (HfSe2) with a nearly linear dependence on x. Due to this property it
can be used as a source material for a two junction cell (ideal band-gap
1.1 eV and 1.8 eV). In the present work, niobium doped HfS1Se1 (bandgap
∼ 1.5 eV) crystals with different doping concentrations were grown
by chemical vapour transport (CVT). The crystals were characterized by
energy dispersive X-ray analysis (EDX) and absorption measurements.
Despite the indirect band-gap of HfSxSe2−x the absorption coefficient is
very high and enables thin-film solar cells. The quasi two-dimensional
character of HfSxSe2−x prevents surface states and dangling bonds. As
the material is thus highly suitable for a Schottky contact, the work
function was measured using a Kelvin probe.
HL 24.2 Di 18:00 H15
Investigation of the chemical and electronic properties of the
ZnO/Cu(In,Ga)(S,Se)2 interface in thin film solar cells — •G.
Storch 1 , L. Weinhardt 1 , C. Heske 1 , E. Umbach 1 , S. Visbeck 2 ,
T.P. Niesen 2 , and F. Karg 2 — 1 Experimentelle Physik II, Universität
Würzburg, Am Hubland, D-97074 Würzburg — 2 Shell Solar, 81739
München
Thin film solar cells based on Cu(In,Ga)(S,Se))2 (CIGSSe) commonly
contain a CdS buffer layer between the absorber and the (i-ZnO/n-ZnO)
window layer. For environmental reasons, it is desirable to replace the
Cd-containing buffer, either by alternative materials or by direct deposition
of the window layer on the absorber. However, depositing the standard
window layer directly on the absorber results in poor efficiencies.
In contrast, the use of (Zn,Mg)O instead of i-ZnO leads to cells showing
satisfactory performance. Until today, the reason for this discrepancy is
unknown. As a first step towards an understanding, we have investigated
the i-ZnO/CIGSSe interface by using X-ray photoelectron spectroscopy
(XPS), X-ray emission spectroscopy (XES) and UV-photoelectron spectroscopy
(UPS) combined with inverse photoemission (IPES). The result-
ing chemical and electronic picture of the interface, including the band
alignment in both the valence and conduction band, will be discussed and
compared to results from related interfaces (e.g., i-ZnO/CdS) as well as
alternative preparation methods.
HL 24.3 Di 18:15 H15
Band alignment at the i-ZnO/CdS interface in Cu(In,Ga)(S,Se)2
thin film solar cells — •L. Weinhardt 1 , C. Heske 1 , E. Umbach 1 ,
S. Visbeck 2 , T.P. Niesen 2 , and F. Karg 2 — 1 Exp. Physik II, Universität
Würzburg — 2 Shell Solar GmbH, Munich
In solar cells based on Cu(In,Ga)(S,Se)2 (CIGSSe) one potential target
for further optimization of cell performance is the commonly used ZnO
window layer. Today, an n-ZnO/i-ZnO structure is generally used, separated
from the CIGSSe absorber by a CdS buffer layer. Apart from important
bulk parameters of the ZnO layer, such as window transmission and
resistivity, particularly the properties of its interface to the CdS buffer
layer represent a crucial component for the optimization of solar cells. For
a further improvement of this interface a detailed understanding of the
chemical and electronic properties is needed. We have thus investigated
the ZnO/CdS interface using X-ray photoelectron spectroscopy to probe
its chemical properties, and we have used UV and inverse photoemission
for a direct determination of the band gaps and band alignments at the
heterojunction. The results will be discussed in view of a level alignment
model for the complete CIGSSe thin film solar cell device, taking into
account the previously found electronic and chemical structure of the
CdS/CIG(S)Se interface [1,2] as well as new results for absorbers with
varied sulfur content at the surface.
[1] L. Weinhardt et al., Proc. 17th EPSEC Munich 2001, p. 1261.
[2] M. Morkel et al., Appl. Phys. Lett. 79 (2001) 4482.
HL 24.4 Di 18:30 H15
Formation mechanism of the metastable defect in Cz-silicon solar
cells — •Karsten Bothe, Rudolf Hezel, and Jan Schmidt
— Institut für Solarenergieforschung Hameln/Emmerthal (ISFH), Am
Oherberg 1, D-31860 Emmerthal
The efficiency of solar cells made on boron-doped oxygen-contaminated
crystalline silicon decreases under illumination or minority-carrier injection
in the dark by up to 10 % relative. This performance degradation has