In recent years the development of novel organic semiconductors for optoelectronic applications has attracted a lot of interest both in industry and academics. Especially in the area of organic field-effect transistors (OFETs) and organic light emitting diodes (OLEDs) huge progress has been made. One of the main technological attractions of organic electronics is that the active layers can be deposited at low temperatures by liquid phase techniques. This makes organic semiconductors ideal candidates for low-cost, large-area electronic applications on flexible substrates.[7, 78]
Among the large number of materials investigated, single crystals from fused aromatics like pentacene and rubrene exhibit the highest charge carrier mobilities that have been recorded so far.[25, 39] As these materials suffer from rapid atmospheric degradation and sensitivity to daylight, it is very unlikely that they will be used on a large scale in organic electronics.[41, 42]
Therefore, novel organic semiconductors combining high environmental stability, easy processability and appropriate charge carrier mobilities are still a challenge.
The key parameters that describe the performance of an OLED display are efficiency, color and lifetime.[79] While red and green OLEDs with adequately long lifetimes have been developed for some consumer electronic products, the lifetime of blue OLEDs is still much shorter. The development of a stable blue emitter for OLED applications with a high efficiency is still a key issue in the area of material research.
This thesis addresses two different research issues in the field of organic electronics. New materials based on aromatic amines for OFET and OLED applications are described in this work. First of all aromatic amines with a star-shaped architecture were synthesized and characterized. Their performance as organic p-type semiconductor in OFETs is reported as well as the preparation and optimization of the FET devices. Furthermore a series of novel fused aromatics with carbazole units was prepared. These so-called bisindenocarbazoles were successfully tested as blue emitter in OLEDs. By introducing aromatic side groups to the core molecule, a liquid crystalline derivative was obtained. The influence of these different molecular architectures on morphology, electrochemical stability, HOMO/LUMO levels, thermal and optical properties of the materials will be discussed in detail.
2.1. Aromatic amines with a star-shaped molecular architecture
The first part of the thesis deals with the synthesis and characterization of novel star-shaped molecules based on triphenylamine. Due to their star-shaped architecture, these compounds have almost no tendency to crystallize and therefore form so-called molecular glasses. Today this class of materials is widely used in photocopiers and laser printers. These low molar mass compounds can be processed both from the gas phase and from solution. In both cases homogeneous amorphous films can be obtained from the new star-shaped materials (Figure 2-1, left). In all cases, triphenylamine has been used as core molecule and different carbazole and fluorene side arms were introduced as side arms in order to study the influence on the HOMO and LUMO levels of the target compounds. For efficient charge carrier injection from the gold electrodes of the transistor, a HOMO level of about -5.2 eV is required. The novel materials have been characterized in detail and their OFET performance was investigated.
Figure 2-1. Different molecular architectures: star-shaped design, leading to molecular glasses (left); annelated core molecule with different side group substituents leading to liquid crystalline (LC) phases (right).
2.2. Fused aromatic compounds based on carbazole units
The second part of the thesis describes the development of a new class of fused heterocycles based on 2,7-substituted carbazole units. One of the major outcomes of my diploma thesis is that 2,7-carbazole based compounds are electrochemically unstable and undergo dimerization reactions in the 3- and 6-positions of the carbazole unit. Therefore an appropriate substitution pattern has to be found which could solve this problem. Beside the electrochemical stability, a high environmental stability is required for application in organic electronics. The molecular design should allow the preparation of thin films from the gas phase as well as from solution.
In this thesis, bisindenocarbazoles are introduced as a new class of fused heterocycles (Figure
22 2. Aim of the thesis 2-2). The thermal properties of the bisindenocarbazole can be tailored by introducing different
alkyl side chains in the very last step of the synthesis. As the targed compounds exhibit a stong blue fluorescence they were tested as blue emitter in OLEDs.
Figure 2-2. Chemical structure of the bisindenocarbazole core molecule.
Furthermore a new bisindenocarbazole building block was prepared from which a series of side-chain-substituted derivatives (Figure 2-1, right) were synthesized in order to study the influence on morphology and electrochemical properties.
As described in the introductory chapter, a possible approach to ordered thin films for optoelectronic devices are solution processable liquid crystalline (LC) monodomains. In the case of organic FETs the orientation of liquid crystalline materials has been used to increase the charge carrier mobilities.[56, 57, 80]
By adopting this concept to organic LEDs, it is possible to generate polarized electroluminescence as it was already shown from liquid crystalline polyfluorenes.[81, 82] Due to the rigid rod-like core of the bisindenocarbazole, novel liquid crystalline compounds were obtained by substituting the bisindenocarbazole building block with aromatic side groups (Figure 2-3).
Figure 2-3. MOPAC calculation of a rigid rod-like bisindenocarbazole which exhibits a liquid crystalline phase.
N R
R R R R
R = alkyl