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Control of the long-range self-organization of polycyclic aromatic hydrocarbons for device applications [Elektronische Ressource] / Wojciech Pisula

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Control of the Long-Range Self-Organization of Polycyclic Aromatic Hydrocarbons for Device Applications Dissertation zur Erlangung des Grades “Doktor der Naturwissenschaften” am Fachbereich Chemie und Pharmazie der Johannes Gutenberg-Universität in Mainz Wojciech Pisula geb. in Cieplice Slaskie Zdroj Mainz 2005 Tag der mündlichen Prüfung: 28.09.2005 II III Contents Contents Introduction........................................................................................................................................1 Chapter 1 Supramolecular Organization of Discotic Liquid Crystals......................................6 1.1. Discotic Liquid Crystals............................................................................................................ 6 1.2. Mechanism of the Charge Carrier Transport ........................... 11 1.3. Electronic Devices based on Organic Matierials ................................................ 12 1.4. Homeotropic Alignment ................................................................................. 14 1.5. Alignment Techniques ........................................................... 16 1.5.1. Single Crystal Growth.............................................................................. 16 1.5.2. Vacuum Deposition................................................................

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Published 01 January 2005
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Control of the Long-Range
Self-Organization of Polycyclic Aromatic
Hydrocarbons for Device Applications







Dissertation zur Erlangung des Grades
“Doktor der Naturwissenschaften”

am Fachbereich Chemie und Pharmazie
der Johannes Gutenberg-Universität in Mainz



Wojciech Pisula
geb. in Cieplice Slaskie Zdroj
Mainz 2005


















Tag der mündlichen Prüfung: 28.09.2005


II

















III Contents

Contents

Introduction........................................................................................................................................1
Chapter 1 Supramolecular Organization of Discotic Liquid Crystals......................................6
1.1. Discotic Liquid Crystals............................................................................................................ 6
1.2. Mechanism of the Charge Carrier Transport ........................... 11
1.3. Electronic Devices based on Organic Matierials ................................................ 12
1.4. Homeotropic Alignment ................................................................................. 14
1.5. Alignment Techniques ........................................................... 16
1.5.1. Single Crystal Growth.............................................................................. 16
1.5.2. Vacuum Deposition.......................................................................................................... 16
1.5.3. Simple Solution Processing ..................................................................... 17
1.5.4. Langmuir-Blodgett Technique...................................................................... 18
1.5.5. Epitaxy Growth on PTFE Alignment Layers................................................................... 20
1.5.6. Alignment from the Isotropic Phase ............ 22
1.5.7. Alignment from Solution along a Concentration Gradient.............................................. 23
1.6. Alignment of HBC molecules.............................................................................. 23
Chapter 2 Materials and Alignment Techniques..............................................27
2.1. Materials.................................................................................................................................. 27
2.2. Zone Processing Methods ............................................................................... 31
2.2.1. Filament Extrusion................................................................................... 32
2.2.2. Zone-casting..................................................................................................................... 33
2.2.3. Zone-crystallization ........................................................ 36
Chapter 3 Relation between the Molecular Architecture and the Supramolecular
Organization in Bulk..............................................................................................................38
IV Contents
3.1. Relationship between Core Size, Side Chain Length and the Supramolecular
Organization in the Hexagonal Mesophase.......................................................................... 38
3.2. Investigation of the Supramolecular Structure of Dove-Tailed HBCs ................................... 52
3.2.1. Alkyl Side Chains with Branching at the β-position .................................... 52
3.2.2. Branched Side Chains bearing Ether Linkages........................................ 61
3.3. Examples for pronounced Supramolecular Order................................................ 67
3.4. Control of the Superperiodicity along Columnar Structures .......................... 71
3.4.1. Hexa-peri-hexabenzocoronenes.................................................................... 72
3.4.2. Extended Aromatic Cores of C96 ................................... 85
Chapter 4 Self-organization in Solution and in Solution Processed Thin Films....................87
4.1. Solution Processing of Crystalline Hexa-peri-hexabenzocoronenes ................... 88
4.1.1. Hexakis-dodecyl-hexa-peri-hexabenzocoronene ............................................................. 88
4.1.1.1. Self-Aggregation in Solution and in Drop-Cast Films of HBC-C ...... 88 12
4.1.1.2. Zone-casting of HBC-C .......................................................................................... 91 12
4.1.2. Dove-Tailed Hexa-peri-hexabenzocoronenes................................................................ 123
4.2. Solution Processing of Non-Crystalline Discotics................. 129
Chapter 5 Control of the Thermal Behavior and Processing from the Isotropic Phase .....141
5.1. Control of the Isotropization Temperature by the Introduction of Dove-Tailed Side
Chains ................................................................................................................................. 141
5.2. Morphology Formation (edge-on arrangement) during Crystallization from the
Isotropic Phase of Alkyled HBCs....................................................................................... 145
5.2.1. Processing from the Isotropic Phase ....................................................... 154
5.2.1.1. Zone-crystallization ................................................................................................ 154
5.2.1.2. Effect of the Curvature on the Self-organization from the Isotropic Phase............ 157
5.3. Control of the Thermal Behavior by Asymmetrical Substitution and the Zone-
crystallization of “unwrapped” HBC.................................................................................. 161
5.4. Control of the Homeotropic Alignment of HBCs......................................... 169
5.4.1. HBCs substituted by Dove-Tailed Side Chains with Ether Linkages............................ 170
V Contents

5.4.2. Homeotropic Alignment of All-Hydrocarbon HBCs..................................................... 178
5.4.3. Binary Mixtures of Discotics with Different Molecular Architecture.......... 187
5.4.4. Outlook for TOF Measurements on Homeotropically Aligned Samples ...................... 197
Conclusions.....................................................................................................................................200
Experimental Appendix.........................................................................................202
References .......................................................................................................................................205
List of Publications & Presentations..........................................................................................223














VI
Index of Abbreviations
AFM atomic force microscopy
CD circular dichroism
DSC differential scanning calorimetry
FET field-effect transistor
FFT fast Fourier transformation
g gram
HBC hexa-peri-hexabenzocoronene
HDMS hexamethyldisilazane
HOPG highly oriented pyrolytic graphite
HR-TEM high-resolution transmission electron microscopy
ITO indium-tin oxide
LB Langmuir-Blodgett
LC liquid crystal
LED light emitting diode
NMR nuclear magnetic resonance
POM polarized optical microscopy
PAH polycyclic aromatic hydrocarbons
Pc phthalocyanine
PTFE poly(tetrafluoroethylene)
THF tetrahydrofuran
TOF time-of-flight
UV-Vis ultraviolet/visible
WAXS wide-angle X-ray scattering
XRD X-ray diffraction
VII




























VIII Introduction
Nowadays, people are surrounded by a variety of different and complex electronics with
integrated circuits which are established in plenty of areas. These electronics are integrated in
devices which are of essential importance for our daily life and improve substantially the
peoples standard of living. But due to the rapid technological progression and the introduction
of novel electronics, our close environment is exposed to permanent changes.
Most of the technology is based on metals and inorganic semiconductors. Undoubtedly,
these materials possess excellent conductive properties, but nevertheless other materials with
novel properties are intensively investigated which could fulfill the great demand for further
device miniaturization on the one hand and allow low cost production of these devices on the
other. Following these requirements organic materials appear to be currently the most
potential candidates for the partial replacement of metals and silicon in field-effect transistor,
light-emitting diodes or photovoltaic devices. Furthermore, organic semiconductors are very
promising due to their low cost and their easy processibility. Both academic and industrial
research put considerable effort in the investigation of the suitable organic materials for
electronic devices. For instance, the number of scientific publications concerning organic
compounds exploited in electronic devices increased extremely since the beginning of the
90’ies, whereby many of the reports were placed in high-ranking journals. Recently, Siemens
announced the successful construction of a photovoltiac cell based on organic compound with
an efficiency of more than five percent being a breakthrough in this field. These two examples
should emphasize the great impact of the field of organic semiconductors. Thereby, the most
essential parameter is the charge carrier mobility in these materials, which determinates in a
high degree the successful application of the compounds.
One can distinguish between two technological progresses; on one hand the ongoing
circuits miniaturization leads to molecular devices (molecular electronics) which consists of
only one single molecule. These devices are characterized by a very high electronic
efficiency. For instance, a single carbon nanotube exploited in a FET device showed recently
1 This value is significantly higher in comparison to the mobilities up to 79,000 cm²/Vs.
2common MOSFETs (1000 cm²/Vs) based on the inorganic counterpart silicon. On the other
hand there is a great demand of producing cheap electronics (plastic electronics) in mass
production for one-way use which is not possible by using inorganic materials. For the latter
kind of application the electronic performance is secondary, since device mobilities up to 1.0 2 Introduction
cm²/Vs are necessary in order to replace amorphous silicon, which is mainly used today in
cheap devices.
One concept for the fabrication of low-cost devices is to apply organic materials,
consisting of molecules which show the tendency to self-assembly. These molecules which
can be regarded as single building blocks, self-organize via specific intermolecular
interactions into complex supramolecular structures. One group of this kind of materials is
polycyclic aromatic hydrocarbons (PAHs) consisting of a planar aromatic core which can
be substituted in order to control the bulk properties. Due to π-interactions between the
aromatic cores these molecules self-organize into supramolecular columnar structures along
which a one-dimensional charge carrier migration takes place. Local charge carrier mobilities
of up to 1.1 cm²/Vs for hexa-peri-hexabenzocoronene derivatives have been observed making
these compounds very promising for practical application.
However, the transfer of these properties into electronic devices is difficult and requires
a detailed investigation of adequate processing techniques. In devices the most essential
requirement for an undisturbed one-dimensional charge migration along the columns is a high
long-range order in the active layer which is deposited between the electrodes. Local defects
at domain boundaries in unoriented layers can trap the charge carriers and decrease
considerably the device performance leading to significantly lower charge carrier mobilities.
Thus, the development of appropriate processing techniques became an essential
challenge for the fabrication of unperturbed long-range oriented organic semiconductors. This
close relationship between supramolecular structure and electronic properties has been
investigated impressively for thiophene oligomers and polymers or phthaloyanines. However,
no breakthrough could be achieved in the design of a processing technique, so that the
performance of the devices based on these organic compounds is still under the desired
properties for practical application. The most important requirement is surely the fabrication
of highly ordered samples resulting in excellent device performance. Today no processing
technique could completely satisfy this demand. Vacuum deposition is an exception which
however is not in agreement with the next requirement of simplicity in manufacturing and low
cost fabrication. An additional need from the industrial point of view is the production of
large ordered areas, which is mainly possible by solution processing.
One reason for the general lack of success in processing of organic materials might be
the insufficient consideration of the intrinsic behavior of the compounds in solution or in the
isotropic phase for the processing procedure. Transfer of knowledge between the synthesis
and the final application is only possible by the establishment of a materials scientist as a link
which can give a feedback to the supramolecular chemistry. The morphology formation is a