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Book of Abstracts

1st

Winter School of

Organic Electronics

from materials to applications

Heidelberg (Germany), 9-12 December 2010

1st Winter School of Organic Electronics, Heidelberg (Germany), 9-12 December 2010

About the Winter School

The objective of this winter school is to bring together researchers from academia and

industry in order to exchange ideas and knowledge in the field of organic electronics.

The school takes place in the framework of the excellence cluster Forum Organic Electronics.

This activity is funded by the Ministry of Education and Research. It involves 26 enterprises,

universities and research centers in the Rhine-Neckar Metropolitan Region.

The winter school is part of the Organic Electronics Education (OEE) project coordinated by

Heidelberg University (www.il.uni-heidelberg.de/oe).

More information about the 1st Winter School can be found on following website:

www.il.uni-heidelberg.de/oe/winterschool2010

Organising Committee (Heidelberg University)

Olga Franco

Norbert Gretz

Florian Lohmann

Alexa Poehnert

Scientific Committee

Olga Franco (Heidelberg University)

Wolfgang Kowalsky (TU Braunschweig)

Michael Kröger (Heidelberg University)

Uli Lemmer (Karlsruhe Institute of Technology)

Keynote Lecturers

Donal Bradley (Imperial College London)

Uwe Bunz (Heidelberg University)

David Jones (University of Melbourne)

Hagen Klauk (MPI for Solid State Research)

Wolfgang Kowalsky (TU Braunschweig)

Leeor Kronik (Weizmann Institute of Science)

Uli Lemmer (Karlsruhe Institute of Technology)

Christian Lennartz (BASF SE)

Henning Sirringhaus (University of Cambridge)

Contact Email

[email protected]


TABLE OF CONTENTS

Keynote lectures

Research area A: Advanced materials for organic electronics

Charge transport physics of high mobility organic semiconductors

H. Sirringhaus LA – 1

Materials Design for OPV

D. Jones LA – 2

Large Heteroacenes and cruciforms

U. Bunz LA – 3

Research area B: Organic electronic devices: OLED, OPV, OTFT

Title to be announced

D. Bradley

Light management in organic optoelectronic devices

U. Lemmer LB – 1

Low-voltage organic thin-film transistors

H. Klauk LB – 2

Research area C: Modelling of organic electronic devices & Organic

lasers

Density functional theory for organic semiconductors

L. Kronik LC – 1

Modelling charge transport in amorphous organic semiconductors

C. Lennartz LC – 2

Organic lasers

W. Kowalsky LC – 3

Contributed talks


Transition metal oxides in organic light emitting diodes

Sami Hamwi et al. TA – 1

Electrical field assisted growth and manipulation of surface for -conjugated polymers in

ionic liquids

S. Ahmad et al. TA – 2

Visualizing photovoltaic nanostructures with analytical electron microscopy

M. Pfannmöller et al. TA – 3

Photoconductivity in squaraine: PCBM lateral near-infrared detector

A. Iacchetti et al. TA – 4


A New Measurement standard for organic thin tilm transistors

S. Hengen et al. TB – 1

Stacked organic light emitting diodes for lighting applications

C. Diez et al. TB – 2

Novel multilayer electrodes for semitransparent solar cells

H. Schmidt et al. TB – 3

Organic semi-transparent and tandem solar cells

A. Colsmann et al. TB – 4



lasers

Excited interface states in organic solar cells: A computational study

A. Fuchs et al. TC – 1

Quantum-chemical description of exciton-interaction

B. Lunkenheimer et al. TC – 2

Charge transport simulation in amorphous organic materials

F. May et al. TC – 3

Modeling of the transient photoresponse of organic solar cells including trap states

S. Kettlitz et al. TC – 4

Towards electrically driven organic lasers: materials and devices

T. Rabe et al. TC – 5

Organic lasers for plastic lab-on-a-chip systems

C. Vannahme et al. TC – 6

Plastic lasers for optical spectroscopy

S. Klinkhammer et al. TC – 7

Posters


Novel solution-processible semiconducting polymers: design, synthesis and performance in

organic field-effect transistors

T. Bilkay et al. PA – 1

Tetraazaperopyrenes – An interesting class of functional dyes

S. Geib et al. PA – 2

IR studies on the growth process of organic materials and their mixture

T. Glaser et al. PA – 3

Local curing method for solvent based polymer materials

M. Janka et al. PA – 4

Spectroscopic investigation of photocatalyst for CO2 reduction to methanol

A. Keese et al. PA – 5

Coating and drying of small molecule based OLEDs

K. Peters et al. PA – 6

Photocatalytic CO2 reduction on dye sensitized TiO2

L. Pöttinger et al. PA – 7

Physical phase plates in transmission electron microscopy

K. Schultheiss et al. PA – 8

Infrared spectroscopic ellipsometry of high-performance organic semiconductor thin films

J. Trollmann et al. PA – 9

An experimental and theoretical study of novel luminescent di-, tri- and tetranuclear

copper-triazole complexes

D. M. Zink et al. PA – 10


Field dependence of mobility in organic thin films

M. Al Helwi PB – 1

Microwave annealing of polymer solar cells with various transparent anode materials

H. Flügge et al. PB – 2

Organic semiconductors for thermoelectric applications

A. Gall et al. PB – 3

Nano-morphology studies of highly efficient organic solar cells by low-energy electron

transmission microscopy

M.F.G. Klein et al. PB – 4

Triplet excitons in fluorescent host/guest system and its implications on organic solid state

lasers

M. Lehnardt et al. PB – 5


Liquid-processed continuously tunable organic DFB-lasers

X. Liu et al.

PB – 6

Polymer solar cells with power conversion efficiencies approaching 6%

F. Nickel et al.

PB – 7

Transient photovoltaic characteristics of inverted polymer solar cells employing

titaniumoxide interlayers

S. Schmale et al.

PB – 8

High-performance organic thin-film transistors based on small molecules

S.-L. Suraru et al. PB – 9

Transparent POLYMER electrodes for efficient organic photovoltaic devices

H. Vogeler et al. PB – 10

Enhancement of top emission for organic light-emitting devices by nano-aggregated

outcoupling layer

Z. Wang et al. PB – 11


lasers

Theoretical study of triplet excimers in organic semiconductors

M. Pabst et al. PC – 1


Keynote Lectures


LA – 1

Charge transport physics of high mobility organic semiconductors

Henning Sirringhaus

A Cavendish Laboratory, University of Cambridge, Madingley Road,

Cambridge CB3 0HE, UK

Conjugated organic semiconductors offer new opportunities for the controlled manufacturing

of active electronic circuits by a combination of solution processing and direct printing. We

will review recent insights into the device and charge transport physics of both conjugated

polymers as well as solution-processible small molecule organic semiconductors, with a

particular focus on the microscopic processes that limit the field-effect mobility in these systems.


LA – 2

Materials Design for OPV

David Jones

A Victorian Organic Solar Cell Consortium (VICOSC)

Bio21 Institute, University of Melbourne

30 Flemington Road,

Parkville, Victoria 3010

Band-gap engineering, interface engineering, charge mobility, morphology control or device

architecture. What is important? Where do you start in designing new materials for organic

photovoltaics? Solubilised graphitic small molecules, for example dibenzochrysenes and

hexa-benzocoronenes, have demonstrated excellent charge mobility, morphology control and

the potential for band-gap engineering and these will be used as a model to examine the

development of new classes of active PV materials. How do we speed up the discovery of

new materials and how do we bypass the linear testing of these materials? What possibilities

for new device architectures do new deposition techniques offer and can we rapidly examine

the “device space” to discover the optimum device architectures? Not all the answers are known for all the areas but we will examine what can be done and what might be possible.


LA – 3

Large Heteroacenes and Cruciforms

Uwe Bunz

Institute of Organic Chemistry, Heidelberg University, Heidelberg, Germany

We discuss the synthesis and the properties of novel nitrogen containing heteropentacenes.

We have developed a Pd-catalyzed process that makes these materials accessible in excellent

yields. Cyclic voltammetry shows that the tetraazapentacenes are easily reduced and display

absorption features that are red-shifted from those of the regular pentacenes. The packing of

these heteropentacenes will be discussed


LB – 1

Light management in organic optoelectronic devices

U. Lemmer

Light Technology Institute, Karlsruhe Institute of Technology (KIT),

Karlsruhe, Germany

With the internal quantum efficiencies of the best phosphorescent emitters reaching values of

close to 100% the optimization of light extraction has become a pivotal task for further

improvement of OLEDs. This is particularly true for future energy efficient illumination devices based white OLEDs.

The lecture will give an overview on the different concepts for light management in OLEDs.

Guided mode and surface plasmon extraction in OLEDs by the incorporation of

nanostructures in or close to the organic thin film stack will be discussed. Focusing on

techniques that allow for large area structuring, we use laser interference lithography and

plasma etching to fabricate periodically nanostructured OLEDs and compare his approach to nonperiodic nanoparticle scattering layers.


LB – 2

Low-Voltage Organic Thin-Film Transistors

Hagen Klauk*

Max Planck Institute for Solid State Research, Stuttgart, Germany

Organic thin-film transistors (TFTs) are of interest for applications that require electronic

functionality with low or medium complexity distributed over large areas on unconventional

substrates, such as flexible plastic film. Generally these are applications in which the use of

silicon devices and circuits is technically or economically not feasible, such as flexible

displays[1]

and large-area sensors[2]

. Active-matrix displays based on high-efficiency organic

light emitting diodes (OLEDs)[3]

require TFTs that can be operated with voltages of about

3 V. A promising approach to low-voltage organic TFTs are gate dielectrics based on a thin,

plasma-grown aluminum oxide layer in combination with an organic self-assembled

monolayer, providing a dielectric thickness of about 5 nm and a capacitance close to 1 F/cm2

[4]. The static and dynamic performance of organic p-channel TFTs is already sufficient for

OLED displays with VGA resolution, where the TFTs operate with frequencies of a few tens

of kilohertz. Increasing the performance of organic TFTs into the Megahertz regime requires

scaling of the lateral TFT dimensions to 1 m and below[5]

. Advances are also required in the

environmental stability of organic TFTs[6]

and in the development of high-mobility organic

n-channel TFTs for low-voltage, low-power organic complementary circuits[7]

.

[1] http://www.sony.net/SonyInfo/News/Press/201005/10-070E/index.html

[2] T. Someya, Y. Kato, T. Sekitani, S. Iba, Y. Noguchi, Y. Murase, H. Kawaguchi, T.

Sakurai, Proceedings of the National Academy of Sciences 2005, 102, 12321

[3] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys.

Lett. 2004, 85, 3911

[4] T. Sekitani, T. Yokota, U. Zschieschang, H. Klauk, S. Bauer, K. Takeuchi, M. Takamiya,

T. Sakurai, T. Someya, Science 2009, 326, 1516

[5] F. Ante, F. Letzkus, J. Butschke, U. Zschieschang, K. Kern, J. N. Burghartz, H. Klauk,

International Electron Devices Meeting, 2010

[6] U. Zschieschang, F. Ante, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T.

Sekitani, T. Someya, K. Kern, H. Klauk, Adv. Mater. 2010, 22, 982.

[7] W. Xiong, U. Zschieschang, H. Klauk, B. Murmann, IEEE International Solid-State

Circuits Conference 2010, 134.

* [email protected]

Keywords: organic thin-film transistors, organic complementary circuits

Figure: Photograph and transfer function of a 6-bit analog-to-digital converter fabricated

on a glass substrate using low-voltage organic TFTs in a complementary circuit design.


LC – 1

Density functional theory for organic semiconductors

Leeor Kronik

Department of Materials and Interfaces,

Weizmann Institute of Science, Rehovoth 76100, Israel

E-mail: [email protected]

The combination of density functional theory (DFT) with powerful spectroscopic tools, e.g.,

photoemission spectroscopy or absorption spectroscopy, is an important approach to

elucidating the electronic structure of materials. In recent years, it has become a particularly

popular tool for studying organic semiconductors and their interfaces with inorganic

substrates – topics of great importance in organic electronics. Here I review our recent

progress in understanding the strengths, limitations, and true predictive power of such

analyses. In particular, the consequences of self-interaction and derivative discontinuity errors

and the importance of long-range exchange and correlation are analyzed. I then show how this

allows for the a priori selection and incorporation of the correct "physical ingredients" into

(typically) orbital-dependent functionals. Finally, via judicious comparison with a variety of

pertinent experimental results on prototypical organic electronic molecules and structures, I

show that this approach results in quantitatively accurate calculations of properties often

considered to be "too difficult for standard DFT".


LC – 2

Modelling charge transport in amorphous organic semiconductors

Christian Lennartz

BASF SE, Ludwigshafen, Germany ;Innovation Lab GmbH, Heidelberg, Germany

BodyText[1]

. Despite of the great research progress in the field of Organic light emitting

diodes (OLED) as well as organic solar cells (OPV) a molecular scale insight into the basic

processes like charge transport, charge recombination/separation, exciton migration etc. is

still in a very early stage. However in order to design new materials in a rational fashion and

to address problems like degradation mechanisms this type of understanding is crucial. In this

lecture first steps in that direction with respect to charge transport will be illustrated.

Charge transport in amorphous organic materials is generally believed to take place via

charge carrier hopping between adjacent molecules. We therefore use Marcus theory based

models to describe the impact of the chemical structure on charge transport. The procedure

includes generation of film morphologies by force field techniques, calculation of the

monomolecular and bimolecular quantum chemical transport parameters and finally the

charge transport simulation by kinetic Monte Carlo simulations. This approach will be

illustrated by the analysis of charge transport in Alq3. Specifically we will analyse differences

in electron and hole mobilities as well as their field dependencies in terms of molecular

parameters. As an extension to our previous models [1] [2], the influence of outer sphere

reorganisation energies as well as site energies including polarisation effects will be discussed.

[1] Kwiatkowski, J. J., Nelson, J., Li, H., Bredas, J. L.,Wenzel, W., Lennartz, C.; Physical

Chemistry Chemical Physics 10 (2008), 1852

[2] Nagata, Y., Lennartz, C., Journal of Chemical Physics 129 (2008), 34709

* E-mail: [email protected]


LC – 3

Organic Lasers

Wolfgang Kowalsky

Institut für Hochfrequenztechnik, TU Braunschweig, Schleinitzstrasse 22, 38106 Braunschweig,

Germany

Compared to well established liquid based dye lasers, amplifying media based on amorphous

organic thin films allow the realization of versatile, cost effective, and compact lasers. Aside

from that, the materials involved are organic semiconductors, which in principle allow the

fabrication of future electrically driven organic laser diodes. A highly promising, low-loss

resonator geometry for these lasers is the distributed feedback (DFB) structure, which is based

on the periodic modulation of the refractive index in the waveguide on the nanometer scale.

By variation of the grating period one may tune the laser emission within the gain spectrum of

the amplifying medium. Organic lasers will be demonstrated spanning the entire spectral

region from 360 nm to 715 nm. Tuning ranges as large as 115 nm in the red spectral region

and more than 30 nm in the UV render these novel lasers highly attractive for various

spectroscopic applications [1]. As the grating period is typically between 100 nm and 400 nm

the DFB resonators are fabricated by electron beam lithography. These gratings may,

however, be used as masters to obtain an arbitrary amount of copies by nanoimprint

lithography into plastic substrates [2]. Therefore these lasers are very attractive even for single-use applications.

Today, the key challenge in the field is the realization of the first electrically driven organic

laser. Key pre-requisites are highly efficient amplifying material systems which allow for low

threshold operation and charge transport materials that bring about the stability to sustain the

necessary current densities, several orders of magnitude higher than in OLEDs.

[1] T. Riedl, T. Rabe, H. H. Johannes, W. Kowalsky, J. Wang, T. Weimann, P. Hinze, B. S.

Nehls, T. Farrell und U. Scherf: “Tunable organic thin-film laser pumped by an inorganic

violet diode laser “, Appl. Phys. Lett. 88, 2006, p. 241116.

[2] T. Rabe, K. Gerlach, T. Riedl, H.-H. Johannes, W. Kowalsky, J. Niederhofer, W. Gries, J.

Wang, T.,Weimann, P. Hinze, B. Nehls, T. Farrell und U. Scherf: “Quasi-continuous-

wave operation of an organic thin-film distributed feedback laser", Appl. Phys. Lett. 89,

2006, p. 081115.

* E-mail of the corresponding author: [email protected]

Keywords: organic laser, DFB resonator, optical gain


Contributed Talks


TA – 1

Transition Metal Oxides in Organic Light Emitting Diodes

Sami Hamwi*1, Michael Kröger

2, Jens Meyer

3, Thomas Winkler

1, Thomas Riedl

4, Antoine

Kahn3, Wolfgang Kowalsky

1

1Institute of High-Frequency Technology, Technical University of Braunschweig, Braunschweig,

Germany 2InnovationLab GmbH, Heidelberg,Germany

3Department of Electrical Engineering, Princeton University, Princeton, USA

4Institute of Electronic Devices, University of Wuppertal, Wuppertal, Germany

The development of OLEDs towards higher efficiency and lifetime is strongly driven by the

prospect of low-cost production of large-area applications in the future. The introduction of

transition metal oxides (TMOs) in OLEDs is regarded as a promising concept for further

improving their properties due to their technological compatibility with organic layers and

their high thermal stability. The first results from the insertion of TMOs in OLEDs indicate

their versatile application as neat functional layers and electrochemical dopants of organic

semiconductors. On the other hand, the knowledge of their electronic properties and the mode

of operation in OLEDs is very limited so far.

Thus, we will present some physical correlations arising from the application of TMOs in

organic light emitting diodes. First, the electronic structure of molybdenum oxide (MoO3) and

tungsten oxide (WO3) is analyzed by photoelectron spectroscopy and Kelvin probe. It is

demonstrated that both TMOs exhibit comparably deep-lying energy levels[1,2]

. Moreover,

their electronic structure indicates them as intrinsically n-doped semiconductors. Then, the

concept of electrochemical doping via MoO3 as p-type dopant of organic wide band gap

semiconductors will be discussed. The focus is on the determination of the doping

concentration dependent charge carrier densities. Measurement techniques, known from the

world of inorganic semiconductors, such as capacitance-voltage measurements on metal-

insulator-semiconductor structures or the direct analysis of the space charge region in doped

semiconductors at the metallic electrode are applied for the determination of the densities and

compared with each other. The study reveals an approximately linear increase of the charge

carrier density with higher doping concentrations as a general result[3]

. At the same time, the

doping efficiency of MoO3 is unexpectedly low and under five percent on average. Finally,

the concept of stacking several light emitting units on top of each other by using TMOs within

charge generation layers (CGL) as the interconnecting units of the stacked OLEDs will be

analyzed. We will reveal that the actual charge generation in interconnecting units using

TMOs occurs at the interface between the TMO and the adjacent hole transporting layer[4]

.

[1] M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, A. Kahn, Org. Electron. 2009,

10, 932.

[2] J. Meyer, M. Kröger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, A. Kahn, Appl. Phys.

Lett. 2010, 96, 193302.

[3] S. Hamwi: Transition Metal Oxides in Organic Light Emitting Diodes, Dissertation,

Cuvillier Göttingen 2010, ISBN: 978-3-86955-484-6.

[4] S. Hamwi, J. Meyer, M. Kröger, T. Winkler, M. Witte, T. Riedl, A. Kahn, W. Kowalsky,

Adv. Funct. Mater. 2010, 20, 1762

* [email protected]

Keywords (transition metal oxide, electronic structure, electrochemical doping, charge

generation layer)


TA – 2

Electrical field assisted growth and manipulation of surface for

-conjugated polymers in ionic liquids

Shahzada Ahmad*,Hans-Jurgen Butt

Max Planck Institute for Polymer Research, Ackermannweg 10

D-55128 Mainz, Germany

Room temperature ionic liquids are molten salts composed of organic ions and acts as ready

to use electrolytes, can produce unparallel structural variation with robust cycling life when

use a media for electro-synthesis. Ionic liquids are electrochemically stable and now regarded

as ‘‘Holy Grail’’ of solvent to cater the need of future electrochemical challenges. As a

consequence they are ideal solvents for a range of electrochemical processes that are

otherwise difficult or impossible to study. Many different materials can be electrodeposited

from ionic liquids including metals and conducting polymers in a cost effective way directly

on the electrodes. These materials are useful for energy storage and conversion devices,

catalysts and actuators.

The realization of long-lived -conjugated polymer electrochemical devices remains a

difficult goal because of performance limitations that include poor environmental stability and

electrochemically cycling between oxidation states of the electrolytes. By the use of ionic

liquids as growth media one can easily alter the growth conditions to produce grains with

diameters in the nanometer scale. Ionic liquids are excellent media for electropolymerization

of conducting polymers, and can exercise strong influence on chemical or physical properties,

which can orchestrate the formation of nanoporous films with uniform and adherent layers on

any conducting electrode [1-2].

For example poly(3,4-ethylenedioxythiophene) [PEDOT] nanoporous layers

electropolymerized in ionic liquids allows the fabrication of platinum free highly efficient

dye-sensitized solar cells, at a fraction of the cost of platinized electrode. Thin layers of

PEDOT have reasonably high electrocatalytic properties and low charge transfer resistance

than platinum along with very high cycling life. This synthesis route can be easily exploit to

fabricate PEDOT and provide an avenue for applications requiring stable redox polymers in

energy devices [3,4].

[1] S. Ahmad, M. Deepa, S. Singh, Langmuir 2007 23, 11430.

[2] M. Deepa, S. Ahmad, Eur. Pol. J. 2008, 44, 3288.

[3] S. Ahmad, J.-H Yum, Z. Xianxi, M. Grätzel, H.-J. Butt, M. K. Nazeeruddin, J. Mater.

Chem., 2010, 20, 1654

[4] S. Ahmad, J.-H Yum, H.-J. Butt, M. K. Nazeeruddin, M. Grätzel, ChemPhysChem, 2010,

13, 2814.

* E-mail: [email protected],[email protected]

Keywords: Ionic Liquids, Conducting polymers, Electropolymerization, Scanning Probe

Microscopy


TA – 3

Visualizing photovoltaic nanostructures with analytical electron

microscopy

M. Pfannmöller*1

, H. Flügge2, G. Benner

3, T. Rabe

2, W. Kowalsky

2, R. R. Schröder

1

1 CellNetworks, Heidelberg University, Heidelberg, Germany, 2 Institute for High-Frequency

Technology, TU Braunschweig, Braunschweig, Germany, 3 Carl Zeiss NTS, Oberkochen, Germany

Improving efficiency of bulk-heterojunctions (BHJ) depends largely on their 3D structure at

the nanoscale. Attempts to visualize thin films of BHJs have been made using transmission

electron microscopy (TEM). It provides nanometer resolution and the possibility for

tomographic 3D reconstructions[1]

. However, so far bright field contrast was achieved by

settings that destroy the inherent resolution and, hence, reliability of image interpretation[2]

.

Furthermore, image contrast can result from additional thickness variations. Another method

to distinguish materials is to exploit their spectral characteristics by visualizing electronic

excitations[2]

. Using ultra-thin films and adapted data analysis, different optical excitations

can directly be linked to structure.

We show that using electron energy-loss spectroscopy, optical spectra of photovoltaic

acceptor and donor materials can be observed in analytical TEM. Modern microscopes allow

spectral analysis of samples down to ~1 eV[3]

. We investigated the spectral features of bare

films of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and poly(3-hexyl-thiophene)

(P3HT), and an annealed BHJ of both materials (ratio 1:1). Spectra are shown in Fig. 1A and

reveal, for PCBM and P3HT, fine-structured electronic excitation peaks. Peaks around 2.5 eV

and 3.5 eV for P3HT and PCBM, respectively, fit to optical measurements. The BHJ

spectrum, at first glance, appears as a composition of both pure film spectra. A superposition

of P3HT and PCBM according to 1:1 ratio is clearly different from the experimental spectrum

of the BHJ. This implies that even band bending effects of interfaces are detectable with this

type of spectroscopy. Using these analytical techniques, materials in BHJs as well as bulk and

interfaces can be identified by spectral features. In Fig. 1B a normal bright field image of a

BHJ is given. We show areas separated into PCBM, P3HT and interface according to spectral

information (Fig. 1C).

Fig. 1: Spectra of P3HT, PCBM, BHJ and a superposition of P3HT and PCBM at a ratio of 1:1 (A), bright field

image of a BHJ at focus (B), map of P3HT, PCBM and interface as determined from spectral features (C).

[1] S. van Bavel, E. Sourty, G. de With, S. Veenstra, J. Loos, J. Mat. Chem. 2009, 19, 5388.

[2] A. A. Herzing, L. J. Richter, I. M. Anderson, J. Phys. Chem. C, Article ASAP.

[3] R. F. Egerton, Rep. Prog. Phys. 2009, 72, 016502.


Keywords: Bulk heterojunction, Morphology, Electron energy-loss spectroscopy


TA – 4

Photoconductivity in Squaraine:PCBM lateral near-infrared detector

A. Iacchettia,b*

, M. Bindaa, D. Natali

a,b, L. Beverina

c, M. Sasso

c, M. Sampietro

a,b

aPolitecnico di Milano, Dip. di Elettronica e Informazione, Milano, Italia; b) Center for Nano Science

and Technology of IIT@PoliMi, Milano, Italia; c) Università di Milano-Bicocca, Dipartimento di

Scienze dei Materiali e INSTM, Milano, Italia.

This work is about the realization and characterization of organic photodetectors based on a

small molecule from the squaraine family blended with PCBM, to detect near-infrared (NIR)

radiation. The importance of NIR light detection ensues from the manifold applications

involving it, like optical communications, remote control, biomedical sensing and imaging.

We have already demonstrated a photodetector based on these materials with good efficiency (EQE = 3.5%), high response speed (10 MHz) and able to stand ambient air operation

1.

This study investigates the working regime of these photodetectors, to infer information

useful to their improvement, by studying the dependence of photocurrent on applied voltage,

incident power density and electrode distance. The device structure is lateral, with the organic

film spin-coated onto a glass substrate with prelitographed metal electrodes. This structure is

advantageous in that it has small capacitance, does not need transparent conductor, and it is

easy to realize.

The external quantum efficiency decreases with the incident optical power following a

sublinear power law over four decades. This reveals that a loss mechanism, dependent on the

incident optical power, is present in the form of either a reduction of the active area (due to

space charge effects2 or an enhancement of the recombination due to the increased carrier

concentration. In order to discriminate between the two cases we investigate the scaling of

photocurrent on the channel length. We obtain that photocurrent is higher on shorter devices,

that excludes the first hypothesis and is in accordance with the latter one. Hence devices act

as photoconductors, where, despite unbalanced electron/hole mobilities, neutrality is

maintained by injecting contacts. The sublinear behavior of the photocurrent on the optical power turns out to be related to an exponential distribution of recombination centers.

In addition, detailed photocurrent scaling analysis is carried out, revealing the presence of

contact resistances, as confirmed by their independent extraction from thin film transistor structures.

[1] M. Binda, T. Agostinelli, M. Caironi, D. Natali, M. Sampietro, L. Beverina, R. Ruffo, F.

Silvestri, Org. Elec. 2009, 10, 7.

[2] A. Rose, Concepts in photoconductivity and allied problems, Intescience. 1963.

* [email protected]

Keywords: Organic Photodetector, Near-infrared, Photoconductivity, Light intensity dependence

The figure on the left represents photodetector EQE with light intensities at different voltage bias.

The right one shows photocurrent values for three device lengths at 30 V applied voltage.


TB – 1

A New Measurement Standard for Organic Thin Film Transistors

S. Hengen1, S. Pankalla

2, D. Spiehl

2, E. Dörsam, M. Glesner

2 and J. Giehl

1

1, University of Applied Sciences, Mannheim, Germany

2, Darmstadt University of Technology, Darmstadt, Germany

Organic thin film transistors (OTFT) start to be widely used for flexible and low cost

applications. In order to minimize production costs and maximize yield, model based

simulations have to be performed to evaluate performance and functionality. We have

performed electrical measurements at solution processed OTFTs fabricated using spin

coating, flexo-printing and vacuum deposition techniques in order to test our model and the applicability of the model parameters we used.

We present measurement techniques leading to a measurement standard for printed OTFT.

The properties of the materials used for OTFTs and the influence of electric measurements are

incorporated in the presented standard as well as the requirements static and dynamic models

have on the measurement. It turned out that data collected via electrical characterisation are

extremely sensitive to the measurement timing and the electrical stress applied to the device.

Therefore, a strict routine is the only way for repeatable measurements and comparable data.

The actual routine delivers data for multiple gate-source and drain-source voltages. All

measurements have been performed in forward and reverse directions to investigate hysteresis

effects.

The aim of this measurement standard is the extraction of material and device parameters for

electrical circuit simulation and more over to ensure comparability of different manufacturing

techniques. This standard allows exchange of measurement data between project partners

independently of the fabricated material (i.e. semiconductor, dielectric) and electric measurement equipment.

A fully automated equipment to fulfil this standard has been build up for characterising

solution-processed and printed OTFTs on a reference substrate[1]

. Large quantities of devices

should be analyzed to get insight to the quality of the fabrication process and to find out the

influence of process parameters on the OTFTs.

The measured OTFTs have been built by mass printing techniques on the mentioned reference

substrates. The substrates consist of a PEN foil with evaporated source/drain contacts of gold

hereon. A top-gate structure is realized by applying an organic semiconductor layer that is

printed by the use of flexography. One of the biggest challenges for printing was the very low

shear viscosity of the semiconductor fluid[2]

.

[1] S. Pankalla, T. Hollstein, M. Glesner, Proceedings of the LOPE-C 2010

[2] Handbook of print media: technologies and production methods / ed. Helmut Kipphan,

Springer, 2001

* [email protected]

Keywords: organic, printing, characterisation, simulation


TB – 2

Stacked Organic Light Emitting Diodes for Lighting Applications

C. Diez1,2

, S. Seidel1, and W. Brütting

2

1.Osram Opto Semiconductors, Regensburg, Germany

2. University of Augsburg, Germany

Organic light emitting diodes (OLEDs) have attracted increasing attention in recent years

since they are strong candidates for solid state light sources. The reported key parameters like

the efficiencies and lifetimes of such devices have successively been improved. The peak

efficiency, however, is often achieved at low current densities, and hence low brightness, due

to exciton quenching by triplet-triplet annihilation and triplet-polaron quenching at high

current levels [1].

For solid state lighting applications it is essential to obtain high efficiencies at high luminance

levels, too. Therefore OLEDs have to be operated at high current densities without leading to

accelerated degradation effects and therefore reduced brightness. Additionally, single OLED

stacks on very large active areas are not radiating homogeneously.

One possible approach to circumvent this issue is vertical stacking of OLEDs in a bottom

emitting configuration. Stacked OLEDs consist of several OLED units, connected in series to

each other by doped organic pn-junction layers as suggested by L.S. Liao et al. [2]. These

charge generation layers (CGLs) can be described similarly to inorganic pn-junctions

operated under reverse bias. Tunnelling occurs from the HOMO level of the p-doped layer to

the LUMO level of the n-doped layer [3].

It is shown that the best approach for lighting applications regarding efficiency and

uniformity is stacking of a fluorescent blue unit together with a phosphorescent yellow

building block which is formed of red and green emission layers. The properties of the yellow

building block can be improved by a variation of the emitter concentrations and the emission

layer thicknesses. In this context energy transfer and quenching mechanisms between the red

and green emission layers are investigated.

Additionally, a mixed matrix approach for the red and green emission broadens the

recombination zone leading to balanced devices which do not change colour with increasing

current density. After the optimization it can be shown that the lifetime of the yellow building

block can be more than doubled.

[1] M.A. Baldo, C. Adachi, and S. R. Forrest, Phys. Rev B 62, 10967, (2000).

[2] L. S. Liao, K. P. Klubek, and C. W. Tang, Apl. Phys. Lett. 84, 167-169, (2004).

[3] M. Kröger, S. Hamwi, J. Meyer, T. Dobbertin, T. Riedl, W. Kowalsky, H.-H. Johannes,

Phys. Rev B 75, 235321, (2007).

* [email protected]

Keywords: Solid state lighting, stacking, charge generation layers, yellow building block


TB – 3

Novel multilayer electrodes for semitransparent solar cells

H. Schmidt*1, T. Winkler

1, F. Nikolayzik

1, I. Baumann

1, T. Riedl

2, H. Flügge

1, S. Schmale

1, T.

Rabe1, S. Hamwi

1, W. Kowalsky

1

1 Institut für Hochfrequenztechnik, TU Braunschweig. Braunschweig, Germany,

2 Institute of Electronic Devices, University of Wuppertal, Wuppertal, Germany

We will report on novel transparent multilayer electrodes prepared by rf sputtering of zinc tin

oxide (ZTO) and thermal evaporation of silver (Ag) as contact for organic electronics.

Specifically we study the electrical and optical properties of the novel multilayer electrode

and use it as top contact for semitransparent bulk hetero junction (BHJ) [regioregular of

poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester] solar cells with an

inverted device architecture.

Recently, films of transition metal oxides (TMO) as tungsten oxide (WO3) or molybdenum

oxide (MoO3) have been evidenced to work as efficient buffer layers to prevent organic layers

from damages due to the sputter deposition process of the top electrode 000. These highly

transparent TMOs can be thermally evaporated on top of organic layers without introducing

damages and have widely been used for organic solar cells [4] due to their favorably

electronic structure [5]. Efficient semitransparent organic solar cells with rf sputtered indium

tin oxide (ITO) top contact using a TMO buffer layer have been shown [1]. However the

room temperature sputtered ITO as top electrode has a small conductivity as annealing or

other post treatment of the ITO is not possible without damaging the organic device.

Here we use a novel multilayer electrode deposited at room temperature consisting of

ZTO/Ag/ZTO (ZAZ). This multilayer has similar optical properties in the VIS range

compared with ITO, though a significantly higher conductivity. We will show how varying

the thickness of the ZTO layers and the thin silver film influence electrical and optical

properties of these novel multilayers. Furthermore we will compare semitransparent organic solar cells using ITO or ZAZ as top electrode cells with a cell area >2.25 cm .

The authors gratefully acknowledge financial support by the German Federal Ministry for

Education and Research BMBF (Grant No. FKZ 13N10316-EPIO).

[1] H. Schmidt, H. Flügge, T. Winkler, T. Bülow, T. Riedl, and W. Kowalsky, Appl. Phys.

Lett. 2009, 94, 243302.

[2] J. Meyer, T. Winkler, S. Hamwi, S. Schmale, H. -H. Johannes, T. Weimann, P. Hinze, W.

Kowalsky, and T. Riedl, Adv. Mater., 2008, 20, 3839.

[3] H. Schmidt, T. Winkler, M. Tilgner, H. Flügge, S. Schmale, T. Bülow, J. Meyer, H. -H.

Johannes, T. Riedl, and W. Kowalsky, Proc. SPIE 2009 7416, 741611.

[4] V. Shrotriya, G. Li, Y. Yao, C. -W. Chu, and Y. Yang, Appl. Phys. Lett., 2006, 88,

073508

[5] M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, Appl. Phys. Lett.

2009, 95, 123301.

* [email protected]

Keywords: semitransparent, plastic solar cell, multilayer electrodes, transparent conducting

oxides


TB – 4

Organic semi-transparent and tandem solar cells

A. Colsmann, A. Pütz, M. Reinhard, F. Nickel, H. Do, U. Lemmer

Light Technology Institute, Karlsruhe Institute of Technology (KIT)

Engesser Strasse 13, 76131 Karlsruhe, Germany

Due to the spectrally limited absorption of organic semiconductors, organic photovoltaic

devices are promising candidates for integrated semi-transparent photovoltaics. Possible

applications cover façade integration, window shadowing, off-grid solutions and autonomous

signage. In this work we present semi-transparent polymer solar cells comprising the highly

efficient low-bandgap co-polymer PSBTBT with an absorption extending to the near infrared.

The devices fabricated in this work exhibit comparable electronic properties upon the

utilization of a transparent sputtered zinc oxide cathode or an opaque metal cathode,

respectively. Moreover, transmitted light through devices PSBTBT exhibits excellent color

rendering properties, making these solar cells suitable for window shadowing.

Further investigations were carried out in order to replace the cathode with a transparent

polymeric electrode with respect to future fully printable device architectures. In order to

apply cathodes from low work function materials like zinc oxide or conductive polymers like

PEDOT:PSS, it is mandatory to incorporate n-doped functional interlayers between the

cathode and the active layer. N-doped interlayers are well known for vacuum-sublimed

devices. However, there are only few reports for solution processable counterparts.

Extrinsically indium or aluminum doped zinc oxide nanoparticles are promising materials in order to form such an n-doped buffer layer between the cathode and the absorbing layer.

Understanding semi-transparent organic solar cells further allows for the fabrication of fully

solution processable polymer tandem solar cells.

Keywords: Solar cells, semi-transparent, tandem solar cells, polymer

Winter School of Organic Electronics, Heidelberg (Germany), 9-12 December 2010

TC – 1

Excited interface states in organic solar cells: A computational study

Andreas Fuchs*1, Christian Lennartz

1

1BASF SE, Department of Computational Chemistry, Ludwigshafen, Germany

Organic solar cells can be built from thin films (typically 100 nm) of organic semiconducting

dyes acting as donors (polymers or small-molecule compounds) and acceptor molecules with

high electron affinities (fullerene derivates or other acceptors such as TTF, (F4-)TCNQ,…).

Light is converted into electric energy via excitation of the dye, migration of the excited state

(exciton) to the dye/acceptor interface and separation of the exciton into free charge carriers

which can be extracted at the electrodes. Achieving efficient charge photogeneration has long

been recognized as a vital challenge for molecular-based solar cells[1]

. However, electron

transfer across the interface does not necessarily directly generate free charge carriers. The

electron and hole pair are still exhibiting significant Coulomb attraction, resulting in the formation of electrostatically bound interfacial electron hole pairs.

We will present a combined QM/MM approach for the calculation of the energy of the

charge-separated state using combinations of dye- and acceptor-molecules at their respective

interface. The expected ground state polarity of the interface can be changed by variation of

the dye/acceptor combination. We will start from an unpolar interface (Pentacene:C60), where

no charge transfer is expected. The Pentacene/PTCDA and TTF/TCNQ interfaces will be

examined as model systems where partial or integer charge transfer from donor to acceptor

can be expected due to the increasing interface polarity. We will further address the impact of

molecular orientation on the energetics as well as different QM approaches (TD-DFT, CIS , CC2 and MR-CI)

[2].

[1] T. M. Clarke and J. R. Durrant, Chem. Rev. 2010, Article ASAP, DOI: 10.1021/cr900271s

[2] M. Linares, D. Beljonne, J. Cornil, K. Lancaster, J.-L. Brédas, S. Verlaak, A. Mityashin,

P. Heremans, A. Fuchs, C. Lennartz, J. Idé, R. Méreau, P. Aurel, L. Ducasse, F. Castet, J. Phys. Chem. 2010, 114, 3215

* [email protected]

Keywords: Organic Solar Cells, Interface Properties, Excited States


TC – 2

Quantum-chemical description of exciton-interaction

Bernd Lunkenheimer*1 and Andreas Köhn

1

Theoretische Chemie, Institut für Physikalische Chemie, Universität Mainz,

Jakob-Welder-Weg 11, D-55128 Mainz, Germany

Modern organic light-emitting diodes (OLEDs) play an increasing role in research and

industry because of their promising optical properties. We are interested in getting a basic

atomistic understanding of excitons in OLEDs. For this purpose our aim is to simulate

exciton-diffusion in layers of typical OLED-materials with the help of kinetic Monte Carlo

(KMC)-methods. Crucial for this is the evaluation of the electronic coupling matrix element between two excited molecules.

Often the exciton-interaction between two molecules is described via coupling of locally

excited states (LE, Förster/Dexter-approach) which is limited to weak interactions. We will

present a model that extends the conventional approximations with charge-transfer (CT)-

contributions to make it suitable for stronger interactions where overlap plays an increasing

role. We tested a pilot-implementation of this model for the pyrrole-dimer. Furthermore we

will show some first results of our investigations calculating the coupling elements for

Aluminium-tris-(8-hydroxy-quinoline) which is a prototypical OLED-material.

* [email protected]

Keywords : exciton, quantum-chemistry, rate-description, Förster/Dexter-theory


TC – 3

Charge Transport Simulation in Amorphous Organic Materials

Falk May1, and Denis Andrienko

1

Max-Planck-Institute for Polymer Research, Mainz, Germany

In the last few years organic materials have become a promising alternative to their inorganic

counterparts in various applications for example in field effect transistors, photovoltaics and

light emitting diodes. The advantage of using organic materials is due to a shear abundance of

compounds that can be specially designed to meet different purposes, e.g. flexibility, light

weight, or processability leading to low production cost. However, for many applications that

demand a high mobility of charge carriers (>1cm2/Vs) as for large area displays, organic

materials are still lacking behind their inorganic counterparts.

Understanding the dynamics of charge carriers (holes and electrons) in organic materials is

therefore a key question in the field, that we are currently addressing with the help of

computer simulations. Our long term goal is to understand the underlying processes and be able to help in designing new materials with improved mobilities

[1].

In order to do so we use a multi-scale approach starting from the description of (semi)-

classical atomic interactions to generate a realistic morphology by molecular dynmics. At a

higher length-scale we then describe charge carrier hopping through Marcus rates[2]

by

including molecular interactions based on a quantum chemical calculations. On the largest

length-scale we use kinetic Monte Carlo simulations to finally obtain charge carrier

mobilities.

Figure[3]

: a)

chemical structure of hexa-benzo-coronene (HBC). b) simulated atomistic morphology. c)

HOMO orbitals of HBC for description of hole transport.

[1] X. Feng, V. Marcon, W. Pisula, M. R. Hansen, J. Kirkpatrick, F. Grozema, D. Andrienko,

K. Kremer, K. Mullen, Nature Materials, 421-426, 2009

[2] R.A. Marcus, Review of Modern Physics 65, 599610, 1993

[3] A. Troisi, D. L. Cheung, D. Andrienko Physical Review Letters,102, 116602, 2009

* [email protected]

Keywords: Mobility Charge Transport Simulation


TC – 4

Modeling of the transient photoresponse of organic solar cells including

trap states

Siegfried Kettlitz, Nico Christ*, Uli Lemmer

Light Technology Institute (LTI), Karlsruhe Institute of Technology (KIT), Germany

In addition to experimental investigation of the properties of organic semiconductor devices,

numerical device simulation is an interesting tool to obtain detailed insights into the inner

processes of such devices. Research activities in modeling and simulation of organic

photodetectors and organic solar cells have concentrated so far on describing the steady state

of the current-voltage characteristics. These steady state investigations suffer from the

superposition of different carrier transport processes. In contrast, the investigation of time-

dependent current density measurements allows for the distinction of several physical

processes as they become dominant at different field strengths, at different current densities or

at different time intervals. The comparison of experimentally measured and numerically

modeled transient photoresponses of organic solar cells after a short laser pulse irradiation

using a blend of P3HT:PCBM as organic absorber has been discussed recently[1]

.

Based on the promising accordance of the measured and simulated results we extended our

numerical drift-diffusion model to account for the influence of trap states in the blend. By

detailed investigation of the current density characteristics at different laser pulse intensities

and different applied voltages we directly observe the carrier capture and release dynamics.

Furthermore, we account for strong local field strength variations playing a major role in the

trapping and detrapping process of charge carriers. By fitting to experimental results we can

estimate both the relevant electron and hole trap states at the same time.

The excellent accordance of the simulated and measured results over a large variation range

of the parameters laser pulse intensity, applied voltage and organic solar cell diameter allows

for a quantitative understanding of the trap related carrier dynamics.

[1] Nico S. Christ, Siegfried W. Kettlitz, Sebastian Valouch, Simon Züfle, Christian Gärtner,

Martin Punke, and Uli Lemmer. Nanosecond response of organic solar cells and

photodetectors. J. Appl. Phys., 2009, 105, 10

* Corresponding Author: [email protected]

Keywords: Organic photodetector, exponential trap distribution, numerical simulation


TC – 5

Towards electrically driven organic lasers: Materials and devices

T. Rabe*1, M. Lehnhardt

1, T. Riedl

2, W. Kowalsky

1

1 Institut für Hochfrequenztechnik, TU Braunschweig, Schleinitzstrasse 22, 38106 Braunschweig,

Germany.

2 Institute of Electronic Devices, University of Wuppertal, Rainer-Gruenter Str. 21, 42119 Wuppertal,

Germany.

Optically pumped thin film lasers based on organic gain materials have been studied

extensively within the last two decades [1]. Properties like large tunability, substrate

independence as well as low cost device fabrication make them very attractive as coherent light sources for lab on chip applications in biomedicine or sensing.

Since organic gain materials are semiconductors as well, an organic laser diode seems to be

straight forward, especially considering the fast progress on other fields of the organic

optoelectronic like OLEDs and OPV. However, up to date no organic laser diodes could be

demonstrated.

Organic laser diodes are based on thin film waveguiding, making the laser extremely sensitive

to loss mechanism within the device. Contact absorption, charge carrier (polaron) absorption,

and absorption due to triplet states pose the main challenges [2]. Furthermore, the predicted

threshold current densities exceed 100-1000 A/cm . This causes extremely high charge carrier

and triplet densities, which inhibit the realization of an organic laser diode so far.

In this contribution fully contacted device structures with extremely low waveguide losses

will be discussed. Furthermore, a special measurement technique is presented allowing a very

sensitive determination of charge induced absorption [3]. Here, an exemplary study of

2,2´,7,7´,-tetrakis(N,N-diphenylamine)-9,9´,-spiro-bifluorene (Spiro-TAD) reveals a very low

polaron absorption cross section of p < 2,6 x10-18

cm2 for 560 nm < < 660 nm.

[1] V. G. Kozlov, V. Bulovic, P. E. Burrows, R. Forrest, Nature 1997, 389, 362.

[2] N. Tessler, D. J. Pinner, V. Cleave, D. S. Thomas, G. Yahioglu, P. Le Barny, R. H.

Friend, Appl. Phys. Lett. 1999, 74, 2764.

[3] T. Rabe, P. Görrn, M. Lehnhardt, M. Tilgner, T. Riedl, W. Kowalsky, Phys. Rev. Lett.

2009, 102, 137401.


Keywords: organic laser diode, optical waveguide, optical gain, polaron absorption


TC – 6

Organic Lasers for Plastic Lab-on-a-Chip Systems

Christoph Vannahme*1,2

, Sönke Klinkhammer2,1

, Mads Brøkner Christiansen,3 Falko

Brinkmann4, Steven Lenhert

5,4, Anders Kristensen

3, Uli Lemmer

2, Timo Mappes

1

1Institute for Microstructure Technology, Karlsruhe Institute of Technology,

76128 Karlsruhe, Germany, 2Light Technology Institute, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany,

3Department of Micro and Nanotechnology, Technical University of Denmark, DTU Nanotech, DTU-

building 345 east, DK-2800 Kongens Lyngby, Denmark, 4Institute of Nanotechnology, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany and

Physikalisches Institut, Westfälische Wilhelms-Universität, and Center for Nanotechnology

(CeNTech), 48149 Münster, Germany, 5Department of Biological Science and Integrative NanoScience Institute, Florida State University,

Tallahassee, Florida 32306-4370, USA

Lab-on-a-chip (LOC) systems enable screening and bio-medical or chemical point-of-care

analysis. Optical sensing promises a high sensitivity and a reduced response time.

Particularly, laser light can excite fluorescent markers very efficiently without spectral

overlap with the marker emission. On the other hand, marker free detection can be achieved

by utilizing optical transducers. Plastic integrated photonic LOC systems benefit from the low

costs of raw material and feasibility of using mass production technologies to fabricate them.

Here, we present the integration of organic semiconductor lasers by nanoimprinting a

distributed feedback (DFB) grating and thermal evaporation of an organic semiconductor

material. Waveguides, guiding visible light are defined by deep ultraviolet exposure of

poly(methyl methacrylate) (PMMA). Efficient end-fire coupling is achieved by introducing a

topographical step between the upper edges of the active region and the passive waveguide[1]

.

Additionally, the integration of optofluidic dye lasers into plastic foil based LOC systems is

shown[2]

. These lasers, though having a large threshold in comparison to organic

semiconductor laser, exhibit large output energies of up to 1 J per pulse. Finally, integrated

detection schemes on plastic LOC systems are discussed, focusing on excitation of fluorescence

[3, 4] and lipid multilayer waveguide grating couplers

[5].

[1] C. Vannahme, S. Klinkhammer, A. Kolew, P.-J. Jakobs, M. Guttmann, S. Dehm,

U. Lemmer, T. Mappes, Microelectron. Eng. 2010, 87, 693-695.

[2] C. Vannahme, M. Brøkner Christiansen, T. Mappes, A. Kristensen, Opt. Express 2010, 18,

9280-9285.

[3] T. Mappes, S. Lenhert, O. Kassel, C. Vannahme, M. Schelb, J. Mohr, Digest of the

IEEE/LEOS Summer Topical Meetings 2008, 215-216.

[4] S. Klinkhammer, T. Woggon, C. Vannahme, T. Mappes, U. Lemmer, Proc. SPIE 2010,

7722, 77221I.

[5] S. Lenhert, F. Brinkmann, S. Walheim, T. Laue, C. Vannahme, S. Klinkhammer,

S. Sekula, M. Xu, T. Mappes, T. Schimmel, H. Fuchs, Nat. Nanotechnol. 2010, 5, 275-

279.

* [email protected]

Keywords: Organic DFB lasers, Optofluidics, Lab-on-a-Chip, Nanoimprint


TC – 7

Plastic Lasers for Optical Spectroscopy

Sönke Klinkhammer1,2

, Thomas Woggon2,1

, Xin Liu1, Christoph Vannahme

2,1, Timo Mappes

2,

Uli Lemmer1

1 Lichttechnisches Institut (LTI) and Center for Functional Nanostructures (CFN), Karlsruhe Institute

of Technology, 76128 Karlsruhe, Germany;

2 Institut für Mikrostrukturtechnik (IMT), Karlsruhe Institute of Technology, 76128 Karlsruhe,

Germany

Organic semiconductors are of increasing interest in the field of optoelectronics. Their

spectrally broad emission range in the visible along with high energy conversion efficiencies

and ease of thin film fabrication make them ideal candidates as gain material in broadband

laser sources [1]. As resonators, distributed feedback (DFB) structures consisting of a surface-

corrugated substrate are well established as they come along with high efficiencies, a high

modal selectivity and small linewidths [2]. A thin film of the organic gain material on top of

the grating structure forms a slab waveguide in which the guided mode experiences

diffraction according to the Bragg formula las = 2 neff/m. The lasing wavelength is

controlled by the grating period as well as the effective refractive index of the guided mode.

Due to the flexibility of thin film fabrication and imprinting techniques, these optically

pumped lasers can either be used as tunable free space light sources or be integrated into photonic lab-on-chip devices (LOCs).

Here, we present schemes to continuously tune organic lasers [3]. By employing a simple

optical setup, we are able to probe samples with spectroscopic features within the emission

range of our DFB laser and demonstrate the feasibility of such a tunable device for

spectroscopic analysis applications [4].

[1] I.D.W. Samuel, G.A. Turnbull, Chem. Rev. 107, 1272 (2007)

[2] C. Karnutsch, C. Pflumm, G. Heliotis, J. deMello, D.D.C. Bradley,J. Wang, T.

Weimann, V. Haug, C. Gärtner, U. Lemmer, Appl. Phys. Lett. 90, 131104 (2007)

[3] S. Klinkhammer, T. Woggon, U. Geyer, C. Vannahme, S. Dehm, T. Mappes, U.

Lemmer, Appl. Phys. B: Lasers and Optics 97, 787-791 (2009)

[4] T. Woggon, S. Klinkhammer, U. Lemmer, Appl. Phys. B: Lasers and Optics 99, 47-51

(2010)

* [email protected]

Keywords: organic semiconductor laser, organic DFB laser, tunable laser, transmission

spectroscop


Posters


PA – 1

Novel Solution-Processible Semiconducting Polymers: Design, Synthesis

and Performance in Organic Field-Effect Transistors

T. Bilkay, K. Schulze, T. Egorov-Brening, and S. Janietz

Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, D-14476 Potsdam, Germany

Organic semiconductors on flexible substrates offer the potential as a low cost alternative to

amorphous silicon in various electronic applications. Particular attention is focused on soluble

semiconductors for organic field-effect transistors (OFETs). OFETs based on solution-

processible polymers as well as small molecules have obtained impressive improvements in

performance in the recent past.[1]

Key classes of materials for this purpose are soluble oligoacenes, oligo- and polythiophenes

and their respective copolymers. In this context poly(3-hexylthiophene)s (P3HT) and

pentacenes have received much attention because of their environmental and thermal stability,

good solubility, microcrystalline structure and charge-carrier mobility.[2]

In this study, the synthesis of polythiophene via Grignard metathesis developed by

McCulloch[3]

with hydrophilic and hydrophobic side chains and their OFET-performence are

investigated. Well defined copolymers, namely poly(3-[hexyl-co-3,6-dioxaheptyl]-

thiophenes) in different molar ratios (1:1, 1:2 and 2:1) were synthesized. In addition to the

statistical copolymers a new block copolymer poly(3-[hexyl-block-3,6-dioxaheptyl]-

thiophenes) with a block ratio of 25:75 was synthesized in the same manner by sequentially

adding two monomers. The compounds were characterized via Gel permeation

chromatography, 1H-Nuclear magnetic resonance-, absorption-spectroscopy, Differential

scanning calorimetry and Cyclic voltammetry. It is known that self-assembly in poly(3-

hexylthiophene) (P3HT) polymers results in a lamellae structure with two-dimensional

conjugated sheets formed by interchain stacking. The 3,6-dioxaheptyl functionalized P3HT

does not influence the microcrystalline structure in the Copolymers. A microphase separation

was observed in the diblock copolymer. This new class of thiophenes shows hole mobilities in

the range of 10-2

cm2/Vs in OFETs. Devices based on this new copolythiophenes and

poly(methyl methacrylate) as a dielectric show high air stability over several months and

without losses in the OFET performance.

Literature:

[1] S. Allard, M. Forster, B. Souharce, H. Thiem, U. Scherf, Angew. Chem. 2008, 120, 2.

[2] J. E. Anthony, Angew. Chem. 2008, 120, 460.

[3] R. D. McCulloch, Adv. Mater. 1998, 10, 93.

*[email protected]


PA – 2

Tetraazaperopyrenes – An Interesting Class of Functional Dyes

Sonja Geib,a Susanne Martens,

a Till Riehm,

a Manfred Matena,

b Thomas A. Jung

c

and Lutz H. Gadea*

a, Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270,

69121 Heidelberg, Germany.

b, Institut of Physics, University of Basel, Klingelbergstraße 82, 4056 Basel, Switzerland.

c, Laboratory for Micro- and Nanostructures, Paul-Scherrer-Institute, 5232 Villingen, Switzerland.

We recently devised efficient syntheses for 4,9-diaminoperylene-quinone-3,10-diimine

(DPDI) as well as tetraazaperopyrenes (TAPP) derived from this building block.[1]

TAPP was found to form a closed-packed assembly on Cu(111), whereas the intermolecular

interactions are based on van-der-Waals-forces. After annealing at 150 °C, a metal

coordinated rectangular network was obtained which was found to be commensurate to the

underlying Cu surface. In this case the organic molecules were coordinated to Cu atoms

through the nitrogen atoms. A second annealing step at temperatures >240 °C activated the

molecules on the surface and led to covalently linked polyaromatic chains.[2]

2,9-disubstituted TAPP-derivatives display characteristic absorbance and emission behaviour.

Going from neutral to acidic media led to a bathochromic shift of the absorption band

depending on the degree of protonation of the molecules. Whereas the neutral form only

showed weak fluorescence, quantum yields of up to 50% were determined in trifluoroacetic or

sulphuric acid.[3]

[1] a) K. W. Hellmann, C. H. Galka, I. Rüdenauer, L. H. Gade, I. J. Scowen, M. McPartlin, Angew.

Chem. Int. Ed. 1998, 37, 1948; b) L. H. Gade, C. H. Galka, K. W. Hellmann, R. M. Williams, L. De

Cola, I. J. Scowen, M. McPartlin, Chem. Eur. J. 2002, 8, 3732.

[2] a) M. Matena, T. Riehm, M. Stöhr, T. A. Jung, L. H. Gade, Angew. Chem. Int. Ed. 2008, 47, 2414.

b) M. Matena, M. Stöhr, T. Riehm, J. Björk, S. Martens, M. S. Dyer, M. Persson, J. Lobo-

Checa, K. Müller, M. Enache, H. Wadepohl, J. Zegenhagen, T. A. Jung, L. H. Gade, Chem.

Eur. J. 2010, 16, 2079. [3] a) T. Riehm, G. De Paoli, A. E. Konradsson, L. De Cola, H. Wadepohl, L. H. Gade, Chem. Eur. J.

2007, 13, 7317. b) S. Martens, T. Riehm, S. Geib, H. Wadepohl, L. H. Gade, submitted for

publication.

* [email protected]


PA – 3

IR studies on the growth process of organic materials and their mixture

T. Glaser*1, M. Binder

1, C. Lennartz

2, C. Schildknecht

2, A. Pucci

1

1Kirchhoff Institute for Physics, University of Heidelberg, Heidelberg, Germany

2BASF SE, Ludwigshafen, Germany

In this study, we show how infrared spectroscopy can be used for in-situ monitoring the

growth of organic materials. The materials, in this case a phosphorescent emitter that was

developed for organic light emitting devices and a suitable host material, were deposited by

vapour sublimation under ultra-high vacuum conditions. Comparing the experimental spectra

of thin layers of the pure materials to quantum chemical calculations and measurements on

the pristine powders shows that the molecules were not destroyed during evaporation.

Embedding the emitter into the host gives rise to only small changes in the relative intensity

of the IR absorption bands due to the change in background polarisability, but significantly

effects the photoluminescence.

* [email protected]

Keywords: Infrared spectroscopy


PA – 4

Local curing method for solvent based polymer materials

M. Janka1, S. Tuukkanen

1, T. Joutsenoja

1, Donald Lupo

1

1 Tampere University of Technology, Department of Electronics, Tampere, Finland

Solution processable polymer materials allow the use of low cost manufacturing methods

such as printing and spin coating. These methods offer a possibility to use flexible and

lightweight plastic substrates. Despite their advantages, low cost processes cannot always

provide the level of resolution and registration required for instance in electronics. Here, we

report a method for aligning solvent based polymer films by local heating.

Organic polymers are widely investigated because of their promising properties: good

processability as solvent based inks, low cost, and low curing temperature. Conventionally

organic polymers are cured globally by heat or electromagnetic radiation. In several

applications higher resolution or alignment accuracy is needed. Initial results indicate that

local heating can provide such resolution and registration in cost-effective manner.

* [email protected]

Keywords: Local curing, polymer


PA – 5

Spectroscopic investigation of photocatalyst for CO2 reduction to

methanol

Keese, Anna1, 2

, PD Dr. Patrick Koelsch*1, 2

1Department of Applied Physical Chemistry, University of Heidelberg,

Im Neuenheimer Feld 253, 69120 Heidelberg, Germany;

2Institute of Toxicology and Genetics (ITG), Karlsruhe Institute of Technology,

Campus North, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

As a consequence of the steadily increasing concentration of CO2 in the atmosphere and its

potential link to global warming, research activities involving CO2 reduction scenarios are

strongly growing over the last years. By not only reducing CO2 emissions but also converting

CO2 into value-added products such as fuels, the emissions of CO2 could potentially be used

to bridge the gap between fossil energy resources and cost-effective alternative energy

resources within the next 30 years. One intrinsic difficulty in CO2 reduction is its inertness

which requires substantial amounts of energy for conversion. Our approach is to use sunlight

radiation to excite dye-functionalized TiO2 nanoparticles, which in turn decompose CO2 into

e.g. methanol within a photocatalytic process. In order to optimize the efficiency and

selectivity of the photocatalyst through the investigation of the catalysts’ surface under

reaction conditions the technique of IR-visible sum-frequency-generation (SFG) spectroscopy

is used. This highly surface sensitive method provides insights into the reaction mechanism at

the molecular level. For the SFG experiments, a home-built photocatalytic flow cell is used.

With this cell it is possible to pump a CO2- saturated aqueous solution underneath the

photocatalyst which is adsorbed to an optically transparent hemicylindrical CaF2 prism. It also

enables simultaneous irradiation of the sample area from the bottom through a fiber port and

recording of an SFG spectrum in situ. The temperature can be controlled by a cooling circuit.

Preliminary SFG spectra of the dyes bound to a thin TiO2 film in the CH-stretching and lower

wavenumber regions show obvious features indicating that some dye molecules are adsorbed

on the film’s surface. This suggests that SFG spectroscopy can contribute to the

understanding of the binding pattern of the dye to the TiO2 film and thus, to the understanding

of the photocatalyst’s role in the CO2 reduction process. Hence, SFG is paving the way for

effective tuning of the reaction selectivity.

*[email protected]

Keywords: SFG spectroscopy, CO2 reduction, photocatalysis


PA – 6

Coating and drying of small molecule based OLEDs

K. Peters*, P. Scharfer, W. Schabel

Institute of Thermal Process Engineering, Thin Film Technology (TFT)

Karlsruhe Institute of Technology (KIT), Germany

Research on electronic devices based on organic materials, like organic light-emitting diodes

(OLEDs), has gained much attention due to the potential for cheap and ultrathin illumination

sources with high viewing angle and color range. Charges are induced from the electrodes

into an organic semiconducting layer and recombine to form an exciton. The relaxation from

the excited to the ground state results in the emission of light. In multilayer devices it is

possible to improve the carrier injection efficiency from the electrodes into the light-emitting

layer, ensuring the emission color purity, but increasing cost and complexity. The structure

may include specific layers for hole or electron injection (HIL/EIL), transport (HTL/ETL) and

blocking. As OLED material, low and high molecular weight compounds can be used.

Currently small molecule based OLEDs show the best performance for applications in the

lighting market. SMOLEDs are commonly fabricated via vacuum deposition processes, which

include an additional purification and allow the multi-layer preparation without serious

problems. Unfortunately, the sublimation technique is relatively expensive and limited to

vaporizable materials. Alternatively, solution processes allow continuous, low cost and large-

scale production with less material usage, but there are still challenges to face, like the

solubility and coatability of the materials and the formation of homogeneous thin films.

In the project “Print-OLED” within the BMBF cluster of excellence “forum organic

electronics in the metropolitan region Rhine-Neckar” the preparation of solution processed

SMOLEDs is investigated. The goal of the project is a highly efficient monochrome

multilayer OLED. Different materials with different solvents or solvent mixtures are

investigated, regarding the solubility and coatability. Primarily, the hole-injection-layer is

coated from solution on glass substrates with a transparent conducting layer and different

deposition methods for thin, homogeneous and structured films are evaluated. Special

attention will be drawn to the drying process of the casted solutions, since this is an important

step during the production. The exact knowledge of the drying process is not only required for

the construction of dryers in an economical production, first results also denote a strong

influence on the formation of the film (homogeneous material spreading, dewetting, surface

structure, alignment of the molecules). The missing layers and the electrodes are applied via

vacuum deposition and hence the consequent influence of the solution processed layers on the

electronic properties and electroluminescence of the device can be tested

* [email protected]

Keywords: OLED, coating, drying


PA – 7

Photocatalytic CO2 reduction on Dye sensitized TiO2

Leo Pöttinger *1, Anna Keese

2, Thomas Bürgi

1

Physikalisch-Chemisches Institut Universität Heidelberg, Im Neuenheimer Feld 253, 69120

Heidelberg, Germany, Institut für Toxikologie und Genetik, Forschungszentrum Karlsruhe, Hermann-

von-Helmholtz Platz , 76344 Eggenstein-Leopoldshafen, Germany

Reducing the greenhouse gas CO2 and the substitution of fossil fuels are challenging tasks for

the future. The photocatalytic recycling of CO2 to hydrocarbons with sunlight as renewable

energy is therefore investigated. Attenuated total reflection infrared spectroscopy (ATR-IR)

spectroscopy was used to probe a thin layer of porous TiO2. In the first step “in-situ”

adsorption experiments of CO2 and dyes with exposure to UV and visible light were

performed. Furthermore metal co-catalysts were added to enhance the photocatalytic activity.

The ATR-IR measurements show stable carbonates on the TiO2 nanoparticles after purging

the CO2 saturated aqueous solution with the pure water (MilliQ). After irradiation with UV

light the carbonate bands vanish. The same characteristics can be observed with dye sensitised

TiO2 and visible light exposure. The increase of new bands was observed and could be a hint

of intermediate products in the multiple electron process.

Furthermore the adsorption of dyes on TiO2 was measured by ATR-IR spectroscopy. A 0.5

mmol/l solution of different dyes in solvents as acetonitrile or THF was purged over the

porous TiO2 surface. The perylene dyes show no significant difference to the pure powder spectra whereas an opening of the anhydride group is proposed

[1].

[1] Tomas Edvinsson, Chen Li, Neil Pschirer, Jan Schöneboom, Felix Eickemeyer, Rüdiger

Sens, Gerrit Boschloo, Andreas Herrmann, Klaus Müllen, and Anders Hagfeldt, The

Journal of Physical Chemistry C 2007 111 (42), 15137-15140.

* [email protected]

Keywords: Photocatalysis, Dye, TiO2, IR-spectroscopy


PA – 8

Physical phase plates in transmission electron microscopy

K. Schultheiss*1,2

, B. Gamm3, D. Gerthsen

3, R.R. Schröder

1,2

1Innovationlab, Heidelberg, Germany,

2Bioquant, University of Heidelberg, Heidelberg, Germany,

3Laboratorium für Elektronenmikroskopie, Karlsruhe Institute of Technology, Karlsruhe, Germany

Studying the morphology of polymer compounds is an important goal in organic electronics.

Using a transmission electron microscope, one of the main issues is to obtain enough visible

contrast from different amorphous materials with only small density variations. The mainly

light elements lead only to small electron phase shifts which can conventionally only be made

visible by defocusing the objective lens on the expense of the image sharpness.

By means of a physical phase plate (PP), which can be inserted in the back focal plane of the

objective lens, either the scattered or unscattered electrons can be additionally phase shifted

by /2. This leads to a contrast enhancement for weak phase objects similar to phase-contrast

light microscopy. In transmission electron microscopy (TEM) there are several concepts of

phase shifting devices discussed and realized. One suggestion of Boersch [1]

was to use an

electrostatic potential to create a phase shift, which has been realized recently in different PP

designs [2-5]

.

By applying a voltage on the PP an arbitrary positive or negative phase shift can be induced to achieve optimal object contrast without defocusing the image.

Fig. 1 shows the example of a thin amorphous carbon film with density fluctuations

conventionally imaged and with positioned PP and different applied voltages.

without phase plate with phase plate 0V 0.4 V -0.4 V

Fig. 1: TEM images of an amorphous carbon film without and with phase plate at different applied voltages.

[1] H. Boersch Z. Naturforsch. 1947, 2a, 615.

[2] K. Schultheiß, F. Pérez-Willard, B. Barton, D. Gerthsen, R.R. Schröder, Rev.Sci. Instr.

2006, 77, 033701.

[3] R. Cambie, K.H. Downing, D. Typke, R.M. Glaeser, J. Jin, Ultramicroscopy 2007, 107,

329.

[4] R.R. Schröder, B.Barton, H.Rose, G. Benner, Proc. Microscopy Conference 2007, 8.

[5] K. Schultheiss, J. Zach, B. Gamm, M. Dries, N. Frindt, R.R. Schröder, D. Gerthsen,

Microsc. Microanal. 2010, doi:10.1017/S1431927610093803.

* [email protected]

Key words: transmission electron microscopy, phase plate, phase contrast


PA – 9

Infrared Spectroscopic Ellipsometry of

High-Performance Organic Semiconductor Thin Films

Jens Trollmann, Robert Lovrin i and Annemarie Pucci

Kirchhoff-Institut für Physik der Universität Heidelberg

The exploration of efficient organic semiconductor materials usable in field effect transistors

is steadily being driven forward. Especially solution-processable and therefore printable

materials are very promising to afford low cost, large-area electronic products on flexible

substrates producible at moderate temperatures. Within the production process many steps are

necessary which might influence the chemical composition of the material and thereby the

electrical performance of the device. Chemical information can be obtained from

spectroscopic measurements in the mid infrared (fingerprint region).

We perform variable angle infrared spectroscopic ellipsometry to determine the dielectric

function of organic semiconductor layers in the spectral region 333 5900 cm1 by means of a

commercial IR-ellipsometer (Woollam IR-VASE). In addition to the chemical composition of

the thin film the orientation of the molecules within the layer is investigated. These studies are

backed by AFM and IR-transmittance measurements as well as DFT calculations. In this

work, we will present results obtained on a novel high-performance air-stable n-type

semiconductor.

Work supported by BMBF: Infrarotspektroskopie an Materialien und Schichtsystemen;

part of the leading edge cluster Forum Organic Electronics (project POLYTOS).


Keywords: Infrared Ellipsometry, Organic Semiconductor, POLYTOS


PA – 10

An experimental and theoretical study of novel luminescent di-, tri- and

tetranuclear copper-triazole complexes

D. M. Zink*1,2

, T. Baumann2, M. Nieger

3, E. C. Barnes

4, W. Klopper

5, S. Bräse

1

1Institute of Organic Chemistry, KIT, Karlsruhe, Germany;

2cynora GmbH, KIT-Hightech Inkubator,

www.cynora.de; 3Department of Chemistry, University of Helsinki, Helsinki, Finland;

4 Institute of

Nanotechnology, Karlsruhe Institute of Technology, KIT, Germany; 5Institute of Physical Chemistry,

Karlsruhe Institute of Technology , KIT; Germany

1. Introduction

Luminescent metal complexes have recently been in the focus of many scientific

investigations, owing to their potential use as organic light emitting diodes,[1]

or solar cells.[2]

Highlighting their application as organic light emitting diodes, a great number of these

luminescent complexes, have been shown to significantly increase the luminosity of the

diode, such as in the case of triplet emitters. Although luminescent complexes of triazole

ligands with copper[3]

are well-known, in contrast, luminescent complexes with ClickPhos

ligand have not yet been reported, but nevertheless exhibit great potential due to their simple and economical preparation,

[4,5] good optical characteristics, as well as their modal structure.

2. Synthesis and characterization of ligands and complexes

For the synthesis of the different di-, tri- and tetranuclear copper-triazole complexes, we used

a modular ligand system based on ClickPhos-like 1,4,5-trisubstituted triazoles. The selection

of the ligands, which can be synthesized in an easy-to-handle and straightforward one-pot

reaction, was based on two central considerations: those of electronic effects and substitution

pattern. Therefore, a whole series of phosphinotriazolyl ligands featuring of a variety of aryl and alkyl substituents have been synthesized. Treatment of these potential bidentate ligands with copper salts yielded a series of new emissive metal complexes possessing three different structural motifs in term of di-, tri- and tetranuclear copper triazole complexes.

Their three-dimensional structures were determined by X-ray structure analysis and revealed

either a charged trinuclear copper complexe or two different neutral di- and tetranuclear

copper complexes (Figure XXX). DFT calculations and experimental geometries of the

complexes are in fair agreement, and provide further information about the influence of the ligand structure on the coordination mode.

Both the absorption and the corresponding emission peak positions of these complexes were measured, for which a rational explanation is provided by the HOMO/LUMO correlation via the assistance of computational approaches. DFT calculations show that the emission of the new complexes are assignable to MLTC excited states from the polynuclear copper core to the triazole ligands. We anticipate that such a modular design strategy might help for the easy preparation of highly efficient and color tunable group 11 fluorescent dyes. * [email protected] Keywords: luminescent triazole-copper complexes

[1] Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. Rev. 2006, 250, 2093.

[2] O’Regan, B.; Grätzel, M. Nature 1991, 353, 737.

[3] Manbeck, G. F.; Bennessel, W. W.; Evans, C. M.; Eisenberg, R. Inorg. Chem. 2010, 49,

2834.

[4] Krasinski, A.; Fokin, V. V.; Sharpless, K. B. Org. Lett. 2004, 6, 1237.

[5] Liu, D.; Gao, W.; Dai, Q.; Zhang, X. Org. Lett. 2005, 7, 4907.


PB – 1

Field dependence of mobility in organic thin films

Mustapha Al Helwi

BASF SE, Joint Innovation Lab (JIL), Ludwigshafen, Germany

A simple yet powerful tool, which is the Admittance Spectroscopy (AS) is applied in order to

understand the field dependence of mobility in thin films of disordered organic small

molecules. A Monte Carlo simulation shows a negative field dependence of mobility (NFDM)

at low field regime when the sample thickness is small [1]. First experimental observations

using AS are in excellent agreement with the Monte-Carlo calculation. In this poster, we

discuss the measurement procedure and first results obtained for a single layer device at

different temperatures.

Figure: Overview of the measurement method.

[1] Raj Mohan, Manoranjan P. Singh, M.P. Joshi, Org. Electr. 2010, 11, 1642-1648.

[email protected]

Mobility, admittance spectroscopy, OLED, organic thin film


PB – 2

Microwave annealing of polymer solar cells with various transparent

anode materials

H. Flügge1, H. Schmidt

1, T. Riedl

2, S. Schmale

1, T. Rabe

1, J. Fahlbusch

1, M. Danilov

3, H.

Spieker4, J. Schöbel

1, W. Kowalsky

1

1 Institut für Hochfrequenztechnik, TU Braunschweig. Braunschweig, Germany, 2 Institute of

Electronic Devices, University of Wuppertal, Wuppertal, Germany,3 Institut für

Faserverbundleichtbau und Adaptronik, Deutsches Zentrum für Luft-und Raumfahrt e.V, Germany, 4

Institut für Elektromagnetische Verträglichkeit, TU Braunschweig. Braunschweig, Germany

Microwave irradiation has been presented as an easy and low-cost method to promote and

control the morphology of bulk- heterojunction [regioregular of poly(3-hexylthiophene):

(6,6)-phenyl C61 butyric acid methyl ester] (P3HT:PCBM) polymer photovoltaic devices

[1,2]. In this work, highly efficient solar cells with power conversion efficiencies (PCE) of up

to 3 % were achieved through annealing with a commercially available microwave oven (2.45

GHz), similar to those of devices produced via conventional annealing on a hotplate .

Concomitantly, as the main benefit the annealing time could be substantially reduced.

Here, we examine the effect of the multimode microwave irradiation on the individual layers

of the solar cell architecture. The temperature of ITO coated glass and PET substrates is

evidenced to increase with increasing microwave irradiation power. On the contrary, no

change of temperature was found for single P3HT:PCBM, PEDOT:PSS and neat substrates

without ITO. Thus, ITO thermal heating through microwave irradiation was identified as the

cause for the annealing of the polymer solar cells [3].

Owing to the cost and limited availability of ITO, alternative anode materials are vigorously

pursued. At present it is not clear if these materials allow for similar microwave annealing of

organic solar cells. A detailed study of various transparent anode materials (ITO, PEDOT:PSS

and ZnO) and their response to multimode microwave irradiation will be presented. Based on

this study, strategies for the optimized microwave annealing of polymer bulk- heterojunctions

will be discussed and prospective benefits for production will be evaluated.

[1] C. J. Ko, Y.K. Lin, F.C. Chen, Adv. Mater. 2007, 19, 3520

[2] O. Yoshikawa, T. Sonobe, T. Sagawa and S. Yoshikawa, App. Phys. Lett. 2009, 94,

083301

[3] H. Flügge, H. Schmidt, T. Riedl, S. Schmale, T. Rabe, J. Fahlbusch, M. Danilov, H.

Spieker, J. Schöbel, W. Kowalsky, App. Phys. Lett. 2010, 97, 123306


Keywords: organic solar cells, annealing, electromagnetic wave absorption, anodes


PB – 3

Organic Semiconductors for Thermoelectric Applications

M. SCHOLDT*, H. DO, J. LANG, A. GALL**, A. COLSMANN, U. LEMMER, et. al

KIT, formerly named Universität Karlsruhe (TH),Light Technology Institute,

Engesserstr. 13, 76131 Karlsruhe, Germany

Fraunhofer-Institute for Physical Measurement Techniques IPM, Thermoelectric Systems,

Heidenhofstraße 8, 79110 Freiburg,Germany

The thermoelectric performance of thin films fabricated from two commercially available,

highly conductive polymer formulations based on poly (3,4-

ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was investigated. In order to

enhance the electrical conductivity, the high-boiling solvent dimethyl sulfoxide (DMSO) was

added. By changing the content of DMSO the electrical conductivity was increased by a

factor of two without changing the Seebeck coefficient or the thermal conductivity. We

achieved ZT = 9.2 x 10-3

at room temperature upon the addition of 5 vol.% DMSO to the

PEDOT:PSS formulation.

* [email protected]

** [email protected]

Keywords: thermoelectric generators, organic semiconductors, PEDOT:PSS


PB – 4

Nano-morphology studies of highly efficient organic solar cells by low-

energy electron transmission microscopy

M.F.G. Klein*1, M. Pfaff

2, M. Reinhard

1, J. Czolk

1, S. Valouch

1, E. Müller

2, A. Colsmann

1, D.

Gerthsen2, U. Lemmer

1

1 Light Technology Institute, Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe, Germany

2 Laboratory for Electron Microscopy, Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe,

Germany

The efficiency of polymer based bulk heterojunction (BHJ) solar cells strongly depends on

the film morphology of the absorption layer. The film morphology itself is influenced among

others by the processing conditions, the choice of the solvent system and temperature

treatment[1]

. In order to investigate and optimize the active layer morphology, it is

indispensable to image the morphology in the range of 10 nm. Atomic force microscopy

(AFM) and transmission electron microscopy (TEM) can be utilized for this purpose.

However, both methods exhibit some severe limitations. AFM measurements are restricted to

the surface, TEM imaging suffers from low contrast between acceptor- and donor-dominant

regions in the absorption layer. To overcome these limitations, we use an advanced electron microscopy technique.

In this work we show, that a scanning electron microscope operated in the transmission mode

at low energies (15 keV and below) which is equipped with a high-angle annular dark-field

(HAADF) detector enables imaging of the nano-morphology of BHJ solar cells. HAADF

scanning transmission electron microscopy (STEM) at low energies is highly sensitive with respect to the small atomic-number and density differences.

As a model system we investigated poly(3-hexylselenophene) (P3HS) based OSCs due to the

high Z-number of selenium[2]

. Low-kV-STEM images revealed an obvious change in

morphology after thermal annealing which we then analyzed quantitatively by a power

spectral density (PSD) analysis. We attribute the observed change to a crystallization of the

polymer. This finding is in good agreement with UV-VIS spectroscopy, transient

photocurrent measurements and current voltage-curves[3,4]

.

[1] B. Schmidt-Hansberg, M.F.G. Klein, K. Peters, F. Buss, J. Pfeifer, S. Walheim, A. Colsmann, U. Lemmer, P. Scharfer, W. Schabel, J. Appl. Phys. 2009, 106, 124501.

[2] M.F.G. Klein, M. Pfaff, J. Czolk, M. Reinhard, S. Valouch, E. Müller, U. Lemmer, A.

Colsmann, D. Gerthsen, submitted to Adv. Func. Mat. 2010.

[3] N.S. Christ, S.W. Kettlitz, S. Valouch, S. Züfle, C. Gärtner, M. Punke, U. Lemmer, J. Appl. Phys. 2009, 105, 104513.

[4] K.J. Ihn, J. Moulton, P. Smith, J. Poly. Sci. Part B. Polym. Phys. 31 (1993), p. 735

* [email protected]

Nano-morphology, P3HS, Scanning Transmission Electron Microscope (STEM), High-Angle

Annular Dark-Field (HAADF)


PB – 5

Triplet excitons in fluorescent host/guest system and its implications on

organic solid state lasers

Marcus Lehnhardt*1, Torsten Rabe

1, Thomas Riedl

2, Wolfgang Kowalsky

1

1 Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Schleinitzstr. 22, 38106

Braunschweig, Germany.

2 Institute of Electronic Devices, University of Wuppertal, Rainer-Gruenter Str. 21, 42119 Wuppertal,

Germany.

Triplet related absorption and quenching effects in fluorescent organic materials are not only

interesting from the fundamental point of view but have also significant implications for

various device applications. In organic light emitting diodes triplet annihilation effects have

been evidenced in several reports to strongly reduce the device efficiency [1,2]

. Recently, the

generation of additional singlet excitons in fluorescent organic light emitting diodes via

triplet-triplet annihilation has been reported in order to excel the theoretical quantum

efficiency limit [3]

. In case of organic solid state lasers the accumulation of triplet states has

been shown to be a major obstacle to prevent continuous wave operation or even electrically

pumped lasing [4, 5]

. We study the triplet related absorption in fluorescent host materials and in

host/guest systems. To observe the non-radiative triplets in fluorescent materials we use a new

measurement technique based on a waveguide structure to achieve high sensitivity. The

triplets are generated via intersystem crossing by pumping with a violet laser diode.

The measurements were carried out in amorphous thin films at room temperature, under

conditions of high relevance for applications. Here we will present our results for the triplet

lifetime, the absorption spectra and the triplet-singlet annihilation in fluorescent host/guest

systems. We observed a significantly increased triplet lifetime in the host/guest systems

compared to the pristine materials caused by a reduction of triplet-triplet annihilation, which

can be an effect of general character. Furthermore we will discuss the role of triplet excitons

in an organic solid state laser based on the all polymer host/guest system poly[(9,9-di-n-

octylfluorenyl-2,7-diyl)-alt-benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) doped with poly[2-

methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) in detail. For this

exemplary material system we show the paramount impact of triplets on the dynamics diode

pumped organic solid state lasers.

[1] Y. Zhang, M. Whited , M. E. Thomson, S. R. Forrest, Chem. Phys. Lett. , 2010, 495, 165.

[2] S. Reineke, K. Walzer, K. Leo, Phys. Rev. B, 2007, 75, 125328.

[3] Y. Luo, H. Aziz, Adv. Func. Mater, 2010, 20, 1285.

[4] M. Lehnhardt, T. Riedl, T. Weimann, W. Kowalsky, Phys. Rev. B. 2010, 81, 165206.

[4] C. Gärtner, C. Karnutsch, U. Lemmer, C. Pflumm, J. Appl. Phys., 2007, 101, 023107.

* [email protected]

Keywords: triplet excitons, organic solid state laser, fluorescent host/guest systems, triplet

lifetime, diode pumped laser


PB – 6

Liquid-Processed Continuously Tunable Organic DFB-Lasers

Xin Liu, Sönke Klinkhammer, Uli Lemmer

Lichttechnisches Institut (LTI) and Center for Functional Nanostructures (CFN), Karlsruhe Institute

of Technology, 76128 Karlsruhe, Germany

Organic materials play a significant role in the development of optoelectronic devices. There

is currently great interest in fabricating lasers with organic semiconductors, which are

beneficial not only because they have high energy conversion efficiencies within the visible

part of the electromagnetic spectrum, but also because they can be processed by inexpensive

liquid-based techniques such as spin coating, doctor blading, ink-jet printing [1] or screen

printing [2]. We employ the method of “horizontal dipping” [3,4] to fabricate vertically

emitting distributed feedback (DFB) lasers [5], whose emission wavelengths can be

continuously tuned in the whole visible range. According to the Bragg formula

m laser = 2 neff the laser output wavelength can be controlled by changing the refractive

index of waveguide layer, for example by tuning the thickness of the waveguide layer. This

can be realized with “horizontal dipping”.

We present our work on coating various organic laser materials with this method. Multiple

continuously tunable DFB lasers with various emission wavelengths in the visible range were

fabricated. Their emission characteristics upon optical excitation are presented.

[1] S. Chang, J. Liu, J. Bharathan, Y. Yang, J. Onohara, J. Kido, Adv. Mater. 11, 734 (1999)

[2] F. Pschenitzha, J. C. Sturm, Appl. Phys. Lett. 74, 1913 (1999)

[3] B. Park and M. Y. Han, Opt. Express 17(24), 21362–21369 (2009).

[4] Chun Ge, Meng Lu, Xun Jian, Yafang Tan, and Brian T. Cunningham, Opt. Express 18,

12980-12991 (2010)

[5] H. Kogelnik, C. V. Shank, Appl. Phys. Lett. 18, 408 (1971)

Keywords: distributed feedback laser, organic laser, tunable laser, thin film deposition


PB – 7

Polymer solar cells with power conversion efficiencies approaching 6%

F. Nickel, A. Pütz, M. Klein, A. Colsmann, U. Lemmer

Light Technology Institute, Karlsruhe Institute of Technology (KIT)

Engesser Strasse 13, 76131 Karlsruhe, Germany

The prospect for fabricating fully printable, low cost polymer solar cells is currently attracting

a lot of attention from researchers and industry. So far there are only a few materials available

that are suitable for the fabrication of devices exhibiting power conversion efficiencies

exceeding 5%. A major limitation of organic photovoltaic devices is the spectral limitation of

the polymer absorption to the visible light. Therefore it is vitally important to investigate

polymers with an extended absorption to the infrared. One of the most promising approaches

towards highly efficient solar cells is the use of co-polymers, where alternating donor- and

acceptor-like monomers account for an effective reduction of the band-gap and subsequently

an absorption in the infrared. The concept of co-polymers can also be used to combine the

properties of different units resulting in higher stability and performance.

In this work we present polymer:fullerene solar cells comprising poly[(4,4’-bis(2-

ethylhexyl)dithieno[3,2-b:2’,3’-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl]

(PSBTBT) or Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-

benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) blended with [6,6]-phenyl C61-

butric acid methyl ester ([60]PCBM) or [6,6]- phenyl C71-butric acid methyl ester (PC[70]BM) with power conversion efficiencies approaching 6%.

Besides compulsory current density-voltage characteristics we also investigated the quality of

these solar cells by means of photoluminescense studies and external quantum efficiency measurements.

Keywords: Solar cells, block co-polymers, quantum efficiency


PB – 8

Fig 0: Decay of FF after UV exposure in

different ambients

Transient photovoltaic characteristics of inverted polymer solar cells

employing titaniumoxide interlayers

S. Schmale1, H. Schmidt*

1, K. Zilberberg

2, T. Riedl

2, H. Flügge

1, T. Rabe

1, S.Hamwi

1 and W.

Kowalsky1

1 Institute of High-Frequency Technology, Technical University of Braunschweig, Braunschweig,

Germany, 2 Institute of Electronic Devices, University of Wuppertal, Wuppertal, Germany

We will present organic bulk heterojunction [regioregular of poly(3-hexylthiophene): (6,6)-

phenyl C61 butyric acid methyl ester] solar cells with an inverted device architecture

employing titania interlayers as electron selective layers prepared by atomic layer deposition

(ALD) or wet processing.

The current-voltage characteristics of pristine devices feature a pronounced S-shape with low

filling factors (FF) as well as high series resistances Rs and consequently low efficiencies,

which has been found by several groups [1][2][3]. Upon illumination with ultraviolet (UV)

light, the S-shape withers and a significant increase of the FF and decrease of the (Rs) is

found, whereas the open circuit voltage (Voc) remain unchanged. We proved this alteration to

be transient by cutting off UV-light and observing a

decay of the FF. We study the impact of various

ambient conditions (air, vacuum, oxygen) on the

dynamics of the decay of the FF after UV

illumination (Fig 1). Furthermore we carried out

Kelvin probe measurements on titania surfaces

before and after UV exposure. The interaction of

oxygen and titania is evidenced as the dominant

mechanism [4]. Exposure to UV-light after decay

increases the FF again similar to the first UV

illumination, which clearly indicates a reversible

process relating to the interaction between oxygen

and the titania interlayer. ALD encapsulated devices

are shown to retain high FF after UV illumination

regardless of the ambient conditions.

[1] T. Kuwabara, T. Nakayama, K. Uozumi, T. Yamaguchi, and K. Takahashi, Sol. Energy

Mater. Sol. Cells, 2008, 92, 1476

[2] C. S. Kim, S. S. Lee, E. D. Gomez, J. B. Kim, and Y.-L. Loo, Appl. Phys. Lett. 2009, 94,

113302

[3] R. Steim, S. A. Choulis, P. Schilinsky, and C. J. Brabec, Appl. Phys. Lett. 2008, 92,

093303

[4] H. Schmidt, K. Zilberberg, S. Schmale, H. Flügge, T. Riedl, and W. Kowalsky, Appl.

Phys. Lett. 2010, 96, 243305.

* [email protected]

Keywords: plastic solar cell, titania, atomic layer deposition


PB – 9

High-Performance Organic Thin-Film Transistors Based on Small

Molecules

S.-L. Suraru1

, M. Stolte1, J. H. Oh

2, Z. Bao

2, J. Brill

3, M. Könemann

3, J. Qu

3,

U. Zschieschang4, H. Klauk

4, F. Würthner*

1

1Institut für Organische Chemie and Röntgen Research Center for Complex Material Systems,

Universität Würzburg, Am Hubland, 97074 Würzburg, Germany, 2Department of Chemical

Engineering, Stanford University, CA 95305, USA, 3InnovationLab GmbH, Speyerer Straße 4, 69115

Heidelberg, Germany and BASF SE, 67056 Ludwigshafen, Germany, 4Max Planck Institute for Solid

State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

The development of organic thin film transistors (TFTs) has emerged to a great field of

research, strongly connected to the finding of new powerful organic semiconductor materials.

Herein we report the synthesis of new -conjugated small molecules exhibiting excellent field

effect mobilities and high on/off ratios in air.

Thus easily accessible dichlorinated naphthalene tetracarboxylic diimides (NDIs) were

synthesized and their optical and electrochemical properties were investigated. These

compounds show n-type behavior with outstanding field-effect mobilities of up to 1.3 cm

2 V

-1 s

-1 and on/off ratios of 10

7.[1,2]

[1] J. H. Oh, S-L. Suraru, W-Y. Lee, M. Könemann, H. W. Höffken, C. Röger, R. Schmidt, Y.

Chung, W-C. Chen, F. Würthner, Z. Bao Adv. Funct. Mater. 2010, 20, 2148–2156.

[2] M. Stolte, S.-L. Suraru, F. Würthner, J. H. Oh, Z. Bao, J. Brill, M. Könemann, J, Qu, U.

Zschieschang, H. Klauk Proc. of SPIE, 2010, 7778, 777804-1.

* [email protected]

Keywords: organic semiconductor, thin film transistor, naphthalene diimide


PB – 10

Transparent POLYMER Electrodes for Efficient Organic Photovoltaic

Devices

H. Vogeler, A. Pütz, H. Do, M. Reinhard, A. Colsmann, U. Lemmer

Light Technology Institute, Karlsruhe Institute of Technology,

Engesser Strasse 13, 76131 Karlsruhe, Germany, E-mail: [email protected]

Due to recent reports about organic bulk-heterojunction photovoltaic devices with power

conversion efficiencies exceeding 8%, polymer solar cells made a giant leap towards

economically interesting optoelectronic devices. In contrast to their inorganic counterparts,

organic molecules exhibit a spectrally limited absorption, making them a predestined material

class for semi-transparent optoelectronic devices. Possible applications for semi-transparent

solar cells range from façade integration and window shadowing to integrated power sources

for off-grid energy systems. Autonomous signage is a further field of application among many

others. One of the expected advantages of organic photovoltaic devices is the potential for

significant fabrication cost reduction through printing processes. Subsequently, it is favorable

to use printable, i.e. solution processable, functional layers instead of vacuum deposited films.

In this work we present highly efficient organic photovoltaic devices ( 3.5%) based on a

poly-(3-hexylthiophene-2,5-diyl) : bis [6,6]-phenyl C61-butric acid methyl ester

(P3HT:bisPC60BM) bulk heterojunction incorporating solution processable polymer anodes

from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). We further

investigated organic photovoltaic devices utilizing PEDOT:PSS cathodes. Since PEDOT:PSS

has a high work function, proper energy alignment at the cathode interface becomes vitally

important. Subsequently, an additional, intentionally n-doped electron transport layer was

incorporated into the device architecture below the cathode. Among other n-doped organic

material systems we favoured a lithium doped bathophenanthroline (BPhen:Li) layer followed

by nominal 3 nm gold on top of a poly-(3-hexylthiophene-2,5-diyl):PC60BM bulk

heterojunction before cathode deposition. The polymer cathode then was spincast on top of

the layer stack. Fortunately, BPhen:Li exhibits a different surface energy as compared to

P3HT:PCBM leading to an exclusive wetting of PEDOT:PSS on BPhen:Li and thus to an in

situ self-patterning of the polymer cathode. While the BPhen:Li interlayer accounts for an

ohmic contact to the PEDOT:PSS cathode, gold with its high density of states is needed in

order to convert the electron current in BPhen:Li to a hole current within the p-type

PEDOT:PSS, i.e. an efficient lossless charge recombination from electrons in BPhen:Li and

holes in PEDOT:PSS.

[1] A. Colsmann, F. Stenzel, G. Balthasar, H. Do, and U. Lemmer, “Plasma patterning of poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) anodes for efficient polymer solar cells”, Thin Solid Films 517

(2009) 1750

[2] H. Do, M. Reinhard, H. Vogeler, A. Pütz, M.F.G. Klein, W. Schabel, A. Colsmann, and U. Lemmer,

„Polymeric anodes from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) for 3.5% efficient organic

solar cells“, Thin Solid Films 517 (2009) 5900

[3] F. Nickel, A. Puetz, M. Reinhard, H. Do, C. Kayser, Alexander Colsmann, and Uli Lemmer, „Cathodes

comprising highly conductive poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) for semi-transparent

polymer solar cells”, Org. Electron. 11, 535 (2010)

[4] A. Colsmann, J. Junge, T. Wellinger, C. Kayser, and U. Lemmer, „Optimization of electron transport

and cathode materials for efficient organic solar cells”, SPIE-Eur. Conf. Proc., Strasbourg 6192 (2006) 6195


PB – 11

Enhancement of top emission for organic light-emitting devices by nano-

aggregated outcoupling layer

Ziyao Wang *

1, Zhijian Chen

2, Lixin Xiao

2, Qihuang Gong

2

1Lichttechnisches Institut, Karlsruher Institut für Technologie, Karlsruhe, Germany,

2State Key

Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing, People’s

Republic of China

A stable self nano-aggregated bathocuproine (BCP) film was fabricated and introduced atop

of a conventional organic light emitting diode for enhancing top emission. As shown in Fig. 1,

it leads to a 2.7–2.1-fold enhancement on top emission at applied voltage from 4 to 9 V which

is much larger than the 1.5–1.3-fold enhancement for a device overlaid with an amorphous

BCP film. The effective outcoupling of this method probably arises from surface plasmon

modes being scattered by only the nanostructured surface. Moreover, this method nearly

preserves the original electroluminescent spectra and has no damage on electrical properties.

Fig. 1. Top electroluminescence (EL) enhancement for the EL device with amorphous

(square) and nano-aggregated (circle) BCP film compared with the EL device without BCP

film as a function of voltage. The inset shows a schematic drawing of the EL device as well as

a photo of top emission from bare Ag cathode and from nano-aggregated BCP film.

* [email protected]

Keywords: Nanostructures, Enhanced top emission, Organic light-emitting devices


PC – 1

Theoretical study of triplet excimers in organic semiconductors

Mathias Pabst,* Bernd Lunkenheimer, Andreas Köhn

Institut für Physikalische Chemie, Johannes-Gutenberg-Universität Mainz, Mainz, Germany

75% of the excited states in organic electronic devices are triplets. For phosphorescent

devices triplet states are required while for fluorescent devices they appear as competitors to

the desired singlet states. Thus for the design of new efficient materials a theoretical understanding of their properties is crucial.

In neighboring molecules, especially for closely overlapping pi-electron systems, strong

interactions may occur in the excited state leading to excimers, i.e. excited states of two

molecules of the same species which have properties significantly different from the excited

states of a single molecule. In typical organic semiconductor devices the conditions for

excimer formation are usually fulfilled and thus a detailed investigation of these is important

for the atomistic understanding of macroscopic properties.

We present here our findings on triplet excimers [1,2] obtained by ab initio calculations on two aromatic systems serving as prototypes for organic semiconductors.

[1] M. Pabst, A. Köhn, J. Phys. Chem. 2010, 114, 1639.

[2] M. Pabst, A. Köhn, in preparation

* [email protected]

Keywords: Triplet; Excimers; Theory; Coupled-Cluster.

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