10 search hits
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Neue Materialien auf Basis arylsubstituierter 1,3,5-Triazine für blau phosphoreszierende organische Leuchtdioden
(2012)
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Andrea Jahreis
- Als Beleuchtungstechnologie der Zukunft stellen organische Leuchtdioden (OLEDs) eine effiziente Alternative zur Glühbirne dar und eröffnen darüber hinaus aufgrund ihrer Eigenschaft als Flächenstrahler gänzlich neue Design- und Anwendungsmöglichkeiten. Weißes Licht in OLEDs wird meist durch Kombination von Emittern der drei Primärfarben rot, grün und blau erzeugt. Für rot und grün sind bereits zahlreiche stabile Matrix-Emitter-Systeme sowie Transport- und Blockermaterialien bekannt. Stabile blaue Emitter und zu diesen kompatible Materialien sind dagegen rar und stehen deshalb im Fokus der Materialentwicklung. Diese Arbeit befasst sich mit der Synthese und Charakterisierung neuer niedermolekularer Verbindungen auf Basis arylsubstituierter 1,3,5-Triazine als Elektronenleiter, Lochblocker oder Matrixmaterial für OLEDs mit blauen Phosphoreszenzemittern. Für den Einsatz in diesen müssen die Materialien ein komplexes Anforderungsprofil erfüllen: neben der thermischen und chemischen Stabilität ist bei Verwendung von blauen Phosphoreszenzemittern vor allem ein ausreichend hohes Triplettniveau eine wichtige Voraussetzung. Die im Rahmen dieser Arbeit hergestellten Aryl-1,3,5-triazine können aufgrund der Verknüpfung von Diphenyltriazin als Grundeinheit mit elektronisch unterschiedlichen Substituenten über verschiedene Brückeneinheiten in drei Materialklassen eingeteilt werden. Die symmetrisch aufgebaute Verbindungsklasse der arylsubstituierten Bis-1,3,5-triazine wurde durch die Verknüpfung zweier Diphenyltriazineinheiten über verschiedene aromatische Brücken erzielt und basiert auf der Struktur des literaturbekannten Elektronenleiters 4,4‘-Bis-[2-(4,6-diphenyl-1,3,5-triazinyl)]-1,1‘-biphenyl, der eine Biphenylbrücke - und damit ein für blau nicht ausreichendes Triplettniveau - besitzt. Durch Einführung neuer Brückeneinheiten sollte eine Verringerung der Konjugation und somit eine Anhebung des Triplettniveaus erreicht werden, um die Bis-1,3,5-triazine auch in Kombination mit blauen Phosphoreszenzemittern verwenden zu können. Hierfür wurden drei verschiedene Konzepte verfolgt: Eine Verdrillung des Biphenyls durch Methylgruppen in 2- und 2‘-Position, eine meta-Anknüpfung über einen Phenylring sowie die Verwendung von Dibenzofuran als Heteroaromaten mit geringerer Konjugation als Biphenyl. Die Synthese der Bistriazine erfolgte in zwei Schritten. Zunächst wurde in einer Ringschlussreaktion aus den jeweiligen Disäurechloriden der Brückeneinheit und dem gewünschten Benzonitrilderivat das 3,5-Diaza-pyryliumsalz gebildet. Die anschließende Ammonolyse lieferte schließlich das entsprechende Bistriazin. Durch Alkylsubstitution der äußeren Phenylringe konnte die Löslichkeit der Materialien stark verbessert werden. Alle Verbindungen zeichnen sich durch hohe thermische Stabilität und eine für den späteren Aufdampfprozess vorteilhafte Sublimationsneigung aus. Durch die Substitution mit tert-Butylgruppen gelang es, die Kristallisationsneigung zu unterdrücken und amorphe Filme mit hohen Glasübergangstemperaturen bis zu 182 °C zu erhalten. Die größte Verringerung der Konjugation konnte für die Derivate mit meta-Anknüpfung erreicht wird. Die dafür maßgeblichen optischen Bandlücken lagen zwischen 3,9 eV und 4,1 eV. Die Konjugation bestimmt ebenso das Triplettniveau, das für einen effizienten Betrieb der OLEDs höher liegen muss als das Triplettniveau des Emitters. Die Bistriazine erreichten Triplettenergien bis zu 2,84 eV und sind damit ausreichend für die Verwendung mit blauen Phosphoreszenzemittern. Die Stabilität der Materialien unter dem Einfluss von Elektronen wurde sowohl im Cyclovoltammetrie-Experiment als auch im „Single-Carrier“-Bauteil nachgewiesen. Für die Klasse der silylsubstituierte Phenyl-1,3,5-triazine wurde zur Verbesserung der morphologischen Eigenschaften und der Löslichkeit eine der Diphenyltriazineinheiten der Bistriazine durch eine Triphenylsilylgruppe ausgetauscht. Im Vergleich zu den Bistriazinen konnte die Löslichkeit der Materialien deutlich verbessert und die Kristallisationsneigung gesenkt werden. Die Verbindungen bilden stabile amorphe Filme und weisen Glasübergangstemperaturen bis 115 °C auf. Ihre Triplettenergien von bis zu 2,91 eV liegen über denen der Bistriazine und sind daher auch in Kombination mit tiefer blauen Phosphoreszenzemittern verwendbar. Durch wiederholte Reduktionen im Cyclovoltammetrie-Experiment konnte gezeigt werden, dass es sich bei dieser Klasse ebenfalls um stabile Elektronenleiter handelt. Die donorsubstituierten Phenyl-1,3,5-triazine bilden durch Kombination von Diphenyltriazin als Akzeptor und Phenylcarbazol als Donor ein bipolares Matrixmaterial. Die beiden Einheiten sind dabei nicht konjugativ über eine Etherbindung verknüpft, um ein Absinken der Triplettenergie zu vermeiden. Die Materialien dieser Klasse zeigen von allen untersuchten Verbindungen in dieser Arbeit die geringste Neigung zur Kristallisation. Sie bilden stabile amorphe Filme mit Glasübergangstemperaturen bis 90 °C aus. Die erfolgreiche Trennung des Ladungstransports auf separierten Molekülteilen wurde in der Cyclovoltammetrie und in quantenmechanischen Rechnungen bestätigt. In fluoreszenzspektroskopischen Messungen konnte das Vorliegen von Carbazolexcimeren nachgewiesen werden. Daher konnten die Materialien nicht als Matrix in blau phosphoreszierenden organischen Leuchtdioden eingesetzt werden. Abschließend wurde ein Material aus der Klasse der Bistriazine als Elektronenleiter und Blocker für Löcher und Exzitonen in OLEDs mit blauen Phosphoreszenzemittern eingesetzt. Für das beste Bauteil mit optimierter Ladungsträgerbalance wurden eine externe Quantenausbeute von 14,7 % (bei 300 cd/m²) und eine maximale Leuchtdichte von 113.000 cd/m² erzielt.
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Novel Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes
(2011)
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Pamela Schrögel
- Organic light-emitting diodes (OLEDs) have been commercially used in full-colour active matrix (AMOLED) displays for a couple of years. Only recently, a new application of OLEDs in the field of lighting has been opened up. For white emission monochrome systems of the three primary colours red, green and blue need to be combined. The major issue from the materials’ point of view is still the lack of stable host-emitter systems for blue emission. This thesis deals with the development of new host materials for blue phosphorescent emitters. The host material has to meet a complex profile of requirements. As most crucial feature the triplet energy of the host material has to exceed the triplet energy of the emitter. An increase of triplet energy of the host material is achieved by reducing the conjugated π-system in the host molecule. This thesis describes three synthetic approaches to high triplet energies by confining the π-conjugation: by introducing torsion in the molecular structure, by choosing a meta-linkage and by a non-conjugated linkage. The first and second approach was applied to carbazole-based host materials, whereas the third was demonstrated on phosphazene-based host materials. In the first approach, the molecular structure of a well-known carbazole-based host material, 4,4’-bis(carbazol-9-yl)-2,2’-biphenyl (CBP), was optimised by introducing torsion via methyl or trifluoromethyl substituents in the 2- and 2’-positions of the central biphenyl moiety to yield twisted CBP-derivatives. By confining the conjugated system in combination with selective methyl substitution a series of host materials with superior thermal and photophysical properties was obtained. Compared with the triplet energy of 2.58 eV for CBP, high triplet energies of 2.95 eV could be realised for the twisted CBP-derivatives. In addition, appropriate substitution of the crystalline CBP results in amorphous materials with high glass transition temperatures of up to 120°C. In cyclic voltammetry the electrochemical properties were studied. Here, it was found that the systematic variation of the substitution patterns enables fine-tuning of the energetic positions of the HOMO and LUMO. This helps to avoid injection barriers at materials’ interfaces in the OLED device. By blocking the activated sites in the host molecules a stability of the electrochemically oxidised species against dimerisation could be demonstrated. In the second approach, the conjugation in the same parent carbazole-based compound CBP was reduced by choosing a meta-type of linkage instead of the common para-linkage of the carbazole substituents to the central biphenyl unit. As a result of the meta-linkage, triplet energies of more than 2.90 eV were achieved. No further increase in triplet energy was observed by introducing additional torsion in the molecular structure as described in the first approach. Moreover, the thermal properties were optimised by selective methyl substitution to yield host materials with glass forming properties and high glass transition temperatures of up to 120°C. All host materials were tested in a comparative OLED device study in combination with a phosphorescent emitter with saturated blue emission. For the best host material of this series an external quantum efficiency of 9.7 % and a high brightness of 10 800 cd/m2 were achieved. Both series of carbazole based host materials – the twisted and the meta-linked CBP-derivatives – were synthesised by Ullmann reaction of a dihalogenated biphenyl unit with two (substituted) carbazole units under classic conditions. Noteworthy is the intermediate 5,5’-diiodo-2,2’-dimethyl-biphenyl – a simple and versatile building block in the synthesis of materials with confined conjugation. The synthesis by direct iodination of 2,2’-dimethylbiphenyl, to the best of our knowledge, has not been described in literature before. In the third approach, the class of low molecular weight phosphazenes, which is less described in the context of OLED-materials, was chosen as hosts for blue phosphorescent emitters. As a common characteristic all host materials consist of a six-membered ring of alternating phosphorus and nitrogen atoms. Each phosphorus atom bears two aromatic substituents attached via a non-conjugated linkage. Depending on the type of linkage to the central phosphazene core two sets of host materials can be distinguished: phenoxy substituted phosphazenes with phosphorus-oxygen bonds and phenyl substituted phosphazenes with phosphorus-carbon bonds. The phenoxy substituted derivatives were synthesized by nucleophilic substitution of the chlorine atoms in hexachlorocyclotriphosphazene with phenolates as nucleophils whereas the phenyl substituted derivatives were prepared by cyclocondensation of three equivalents of phosphinic amides. Due to their superior thermal properties compared to the phenoxy substituted series the phenyl substituted phosphazenes are better suited for the use in OLED devices. They exhibit particularly high triplet energies of up to 3.4 eV. Thus, they can be combined with deep blue phosphorescent emitters. Another specialty of the phenyl substituted phosphazenes is a balanced charge carrier transport characteristic. To conclude, each of the three presented approaches yields host materials with triplet energies high enough for a combination with blue phosphorescent emitters. Regarding the morphological stability the extensively studied carbazole based host materials exceed the novel phosphazene based host materials.
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Designing novel host materials for blue phosphorescent organic light-emitting diodes
(2009)
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Michael Rothmann
- The overall efficiency of an organic light-emitting diode (OLED) is always limited to the efficiency of its individual components. The most important component is the emission layer, where excitons are formed and light is generated. This thesis deals with the improvement of one class of OLED component, namely host materials for blue phosphorescent emitters. Three generations of 1,3,5-triazine-based materials with varying donor-substituents are presented in this work. In the first generation carbazole units are bound to the triazine core. The second generation consists of triazines with diarylamino substituents. The third generation compounds are a combination of disubtituted triazines from the first two generations and a phenoxy-carbazole unit. While the first and second generation comprise substituents that are directly bound to the triazine core, in the third generation triazines the phenylcarbazole-donor is attached via a nonconjugated ether bond. Within each generation various properties are tailored to fulfill the complex profile of requirements for host materials. Known nucleophilic substitution reactions were further improved to enable the efficient synthesis of novel host materials in very high purity and high yields. The sequential replacement of the chlorines of cyanuric chloride is dependent on temperature, actual ring substitution and the nature of the nucleophile. Effective methods were developed to yield asymmetrically substituted triazines in a controlled manner. The thermal properties, including the thermal stability to enable the processing by vapor deposition and the glass forming properties to result in a morphological stability of prepared thin emission layers, were controlled by systematic investigation of different substitution patterns. Thus, glass transition temperatures up to 170 °C are presented. Studies of the long term stability of amorphous host films, carried out for several materials, revealed its importance for long term efficient devices. The electrochemical properties of the novel compounds were investigated by cyclic voltammetry to study the energetic position of the HOMO and the LUMO as well as the stability of the material upon oxidation and reduction. Using this method the injection properties of the materials were determined. The blocking of activated positions resulted in reversible redox behavior. Furthermore the ionization potential was decreased for the third generation triazines to yield an improved hole injection into these materials. Additionally computational calculations were carried out to understand and further improve the energy levels by substituent exchange. This led directly to the development of bipolar host materials with separated hole and electron transport units within one molecule. Furthermore single carrier devices were fabricated to demonstrate the benefits of the transport bipolar characteristics. For the efficient operation of a device the triplet energy of the host material has to be higher compared to the emitter. First generation triazines exhibit triplet energies up to 2.96 eV and therefore enable the use of light and middle blue phosphorescent emitters. Second generation triazines comprise exceptionally high triplet energies up to 3.24 eV. These are amongst the highest values reported in the literature and facilitate the use of deep blue phosphorescent emitters. For hosts of the third generation the triplet energy depends on the choice of the triazine moiety. They are therefore suited for light and deep blue emitter. Extensive photo physical characterizations of all materials have been carried out in solutions, neat films and doped films. Energy transfer experiments with several emitters additionally gained valuable information about the compatibility of host and guest molecules All generations of triazines are tested as host material in OLEDs. The optimization of the device configurations was carried out by combinatorial evaporation. The sequential adaption of layer thickness and composition helped to improve the device performance. The stepwise optimization of the host material properties resulted in an enduring progression concerning the luminance and efficiency. For the third generation triazines 11.5 % external quantum efficiency and a high brightness of 33000 cd/m2 were achieved.
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Tailoring Fluorene-based Oligomers for micron and sub-micron sized Photopatterning
(2009)
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Esther Scheler
- This thesis describes the work on tailor-made synthesis, characterization and application of well-defined fluorene oligomers for photoptatterning. Two types of fluorene oligomers are presented: pure fluorene oligomers and fluorene cooligomers incorporating various comonomers for adjusting the conductive properties towards electron and hole conduction. Since possible applications for these materials feature organic light emitting diodes and organic field effect transistors we focused on the preparation of well-defined and defect free oligomers and the preservation of their electro-optical properties during photopatterning. Further on the requirements for material synthesis are easy procedures and large quantities. Therefore we developed an approach, which produces large quantities combined with the adjustment of the desired properties in one single step. The synthetic strategy throughout the thesis comprises the addition of an endcapping species in aryl-aryl polymerization reactions. The tailormade endcapper fulfils three tasks at once, the control of the molecular weight, the introduction of polymerizable acrylate moieties and the avoidance of undesired endgroups. As aryl-aryl coupling methods the nickel catalyzed Yamamoto and palladium catalyzed Suzuki condensations were applied. The completeness of the aryl-aryl coupling and the endcapping was proven with Maldi-ToF mass spectrometry. With this approach the properties of the oligomers can be easily adjusted in view of the optimization of their photopolymerization behaviour. The first oligomer series deals with the effect of the molecular weight on the properties and photopatterning behaviour of pure fluorene oligomers. The molecular weights were controlled by the amount of functionalized endcapper, which carried the polymerizable acrylate groups. As coupling method the Yamamoto coupling was applied. The molecular weights defined the temperature range of the nematic mesophases. An increase of the average chain length leads to higher transition temperatures (e.g. clearing temperatures) and to better film forming properties. The photopolymerization is usually performed in the nematic state to achieve a sufficient mobility of the acrylates. The irradiation conditions had major consequences on the preservation of the characteristic electro-optical properties of the fluorenes, the harsher the conditions the higher was the probability to destroy the chemical structure by photooxidation. Further on since each chain only carries two acrylate functionalities attached to the endcappers the total number of acrylates is different for high molecular weight and low molecular weight mixtures. The lowest molecular weight mixture contains the most acrylates and shows the lowest transition temperature, which leads to the best micron sized photopatterns. The second generation of pure fluorene oligomers demonstrate how different contents of polymerizable groups affect the photopolymerization behaviour. Here the molecular weights were kept constant around 5000 g/mol by equal amounts of endcapper and the acrylate groups were introduced by the fluorene monomers. The Yamamoto coupling was used and upon cooligomerization with a non-acrylate fluorene monomer the acrylate content was changed from 10% to 100%. The photopolymerization times strongly depend on the acrylate content, the 100% acrylate oligomer could be photopatterned in 30 seconds, whereas the 80% and 60% mixtures needed 2-5 minutes. In the best case crosslinking is 20 times faster than found for the preceding series, which ensures the preservation of the electro-optical properties. With the highest acrylate content a photocrosslinking even at room temperature became possible. The third oligomer series describes the incorporation of various comonomers such as TPD and bithiophene via Yamamoto reaction. Taking the knowledge of the two preceding generations into account we exploited the acrylate monomer from series two and introduced 30% comonomer. This ensured a sufficient content of acrylates for a fast photopatterning and enough comonomer for a shift of the electronic properties. We found that the electronic structure of the comonomer strongly affected the behaviour in the Yamamoto reaction. The HOMO and LUMO energy values were shifted towards hole or electron conduction. The photopatterning conditions were similar as found for the corresponding pure oligofluorenes with a 60% acrylate content. 2-5 minutes exposure produced highly emissive micro patterns. Thus a change of the electro-optical properties does not affect the photopolymerization behaviour and vice versa. Since we found differences in the incorporation of comonomers during Yamamoto coupling we applied the Suzuki coupling, which ensures an alternately linkage of monomers. Here protective groups had to be used since the Pd-catalyst does not tolerate acrylate functionalities. We found that the comonomers were incorporated quantitatively, but the major difficulty proved to be the polymeranalogous reactions following the polycondensation. The energy values were shifted towards electron and hole conduction and the photopolymerization behaviour was similar to the Yamamoto oligomers. An exposure time of 2-5 minutes produced patterns with a maximum resolution of 700 nm. To conclude the endcapping strategy combined with the Yamamoto coupling is a most effective tool for the adjustment of properties within one single step. The acrylate content as well as the molecular weight can be precisely tuned, which allows a good control of the photopatterning properties. In some cases e.g. with electron withdrawing comonomers the Suzuki cross coupling is the method of choice.
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Kombinatorische Untersuchung und Optimierung von organischen Multischichtleuchtdioden
(2006)
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Markus Bäte
- Ein Schwerpunkt dieser Arbeit lag auf der Weiterentwicklung und Verbesserung der an diesem Lehrstuhl vorhandenen Aufdampfanlage zur Herstellung von komplex aufgebauten Multischichtleuchtdioden. Um eine verbesserte Kontrolle der Schichtdicken und der Aufdampfrate zu ermöglichen wurden im Rahmen der Doktorarbeit in der Aufdampfkammer drei widerstandsgeheizte Quellen durch drei Effusionsquellen ersetzt. An diese drei Effusionsquellen wurden zusätzliche Sensoren, zur Bestimmung der Aufdampfrate und Schichtdicke, angebracht. Um den Gradienten in der Schichtdicke, der, abhängig von der Quellenposition, während des Aufdampfprozesses entsteht, zu reduzieren, wurde der vorhandene feststehende Maskenschlitten durch einen Drehteller mit Maskeneinheit ersetzt. Dieser Drehteller rotiert während des Aufdampfprozesses über den Quellen. Er bietet alle kombinatorischen Möglichkeiten des Maskenschlittens zur Sektoren- und Gradientenerzeugung. Mit dem Drehteller konnte der gerätebedingte Gradient der aufgedampften Schichten auf bis unter 7 % reduziert werden. Ein weiteres Ziel, die Leuchtdioden vor Luftsauerstoff und Feuchtigkeit zu schützen und damit die Lebensdauer zu erhöhen, wurde verwirklicht durch den Anbau einer Inertgaskammer an die Aufdampfanlage, in der fertig gestellte Leuchtdioden verkapselt werden können. Schließlich wurde, für eine schnelle Charakterisierung kombinatorischer Sektorenbibliotheken, eine neue Apparatur, der FLASHScan® 530, in Betrieb genommen. Mit diesem Gerät ist es möglich sowohl Absorptions- als auch Lumineszenzspektren von kombinatorischen Sektorenbibliotheken schnell und effizient durchzuführen. Im Rahmen dieser Arbeit konnte gezeigt werden, dass es mit der weiterentwickelten Aufdampfanlage möglich ist effektiv organische Leuchtdioden herzustellen und, insbesondere, dass mittels neu entwickelter kombinatorischer Sektorenbibliotheken in einem Versuch verschiedene Parameter, wie z.B. Schichtabfolge, variiert und die Sektoren miteinander verglichen werden können. Neben der Weiterentwicklung der Aufdampfanlage lagen weitere Schwerpunkte auf der Untersuchung und Optimierung von grün, rot und blau emittierenden Substanzen in organischen Leuchtdioden.
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New Carbazole Based Materials for Optoelectronic Applications
(2006)
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Martin Sonntag
- The motivation for this thesis was the synthesis, characterization and the testing of new, environmentally stable materials based on aromatic amines for OFET and OLED applications. The preparation of high quality thin films from solution as well as from the gas phase was an another important issue. The first part of this thesis deals with star-shaped molecular glasses with triphenylamine as core molecule. Substituted fluorene and carbazole units were attached to the core molecule as side arms via a trifold Suzuki cross coupling reaction. The target compounds were highly purified by medium pressure liquid chromatography (MPLC) as purity is an important prerequisite for organic materials to be used for optoelectronic applications. Before the new materials were finally tested in transistor devices, a suitable surface treatment of the OFET substrates was developed. By introducing self-assembled monolayers, prepared from hexamethyldisilazane (HMDS), on top of the SiO2 insulator layer of the FET substrates, the field-effect mobility was increased by at least one order of magnitude. Furthermore it was possible to improve on/off-ratios as well as turn on voltages. In conclusion hole carrier mobilities up to 3 x10-4 cm2/Vs and on/off-ratios of 10000 were achieved from the new star-shaped compounds. The performance of the devices was not affected by a four month storage period in air and daylight. The second part of this thesis describes the synthesis and characterization of a new class of fused heterocycles based on carbazole units. For this issue a series of bisindenocarbazoles is introduced as a new class of fused heterocycles. A synthetical procedure was designed which allows to tailor the thermal properties of the target compounds by introducing different alkyl substituents in the very last step of the synthesis. Yields up to 50 % can be obtained after six synthetical steps including purification of the intermediates. CV measurements showed the electrochemical stability of the novel compounds. Altogether five bisindenocarbazoles with different alkyl substitution patterns have been prepared and characterized. Their morphology varies from highly crystalline materials with short alkyl side chains to amorphous molecular glasses if longer or branched alkyl groups are attached to the core. As the bisindenocarbazoles exhibit a bright blue fluorescence together with high quantum yields, they were tested as blue emitter for OLED applications. In typical setups for blue light emitting LEDs, the blue emitter is doped into a wide band gap host material in order to avoid quenching of the electroluminescence and to adjust the energy levels of the different materials used in the setup. For this issue CBP, mCP and TCTA were tested as matrix materials together with a bisindenocarbazole as emitter. A combinatorial evaporation setup was used for the preparation of the OLED devices in order to dope the different host systems by co-evaporation of the bisindenocarbazole dye. This deposition method also allows the variation of the film thicknesses of the charge transport layers in a single experiment. By using this device architecture a deep-blue emission from the bisindenocarbazole dye at CIE color coordinates of x = 0.19 and y = 0.17 was obtained at a hole blocking layer thickness of 40 nm. Luminance values up to 200 cd/m2 were achieved with this series of devices. The turn on of the light emission was observed at 5 V. These very first results show that the bisindenocarbazole is a promising new blue fluorescent emitter for organic LEDs. Due to the rigid rod-like core of the bisindenocarbazole it was possible to obtain a novel derivative exhibiting a broad nematic mesophase by extending the core with aromatic side groups. By using a suitable alignment method, it is possible to obtain well ordered LC monodomains from which increased charge carrier mobilities can be obtained. The nematic LC phase was characterized with polarizing microscopy (POM) and small angle X-ray scattering (SAXS).
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New fluorene based materials for organic electronics
(2005)
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Heiko Thiem
- The major topic of this PhD thesis is the synthesis of a number of conjugated polymers and low mass model compounds from fluorene and bithiophene building blocks. Such materials are attractive candidates for application in organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). The first part of my thesis deals with the synthesis of reactive mesogens with conjugated fluorene units and acrylate end groups as photopolymeriseable species. They were prepared by Suzuki cross coupling reactions with the use of a monofunctional endcapper. Careful MALDI-TOF and GPC analysis show that this procedure leads to fluorene oligomers which are terminated with two acrylate units. This synthetic route drastically reduces the number of steps compared to the synthesis of monodisperse fluorene trimers or pentamers and makes fluorene reactive mesogens available in a gram scale. The materials show broad nematic mesophases. The clearing temperatures can be shifted from 100 °C to 310 °C by changing the molecular weight of the oligomers. The orientation of the crosslinked reactive mesogens on top of rubbed polyimide layers results in a maximum orientation ratio of 15/1 parallel and perpendicular to the rubbing direction. A major advantage of reactive mesogens as materials for OLEDs and OFETs is their ability to be photocrosslinked. By this procedure the liquid crystalline orientation is permanently fixed by a densely crosslinked network. In order to apply reactive mesogens it is necessary to understand the process of photocrosslinking in detail. This process was investigated by PhotoDSC measurements on two different mesogens, one with a nematic and one with a smectic mesophase. The kinetics of the photopolymerisation reactions and the total conversion in dependence from initiator concentration of the reactive mesogen with the nematic mesophase were measured. It becomes clear that the reaction rate and the total conversion decrease with smaller amounts of photoinitiator. Nevertheless a conversion of 75 % can be realised with only 0.01 weight % of initiator, which is much less than is usually used in photopolymerisations. We were able to polymerise the other reactive mesogen in the smectic mesophase. Nevertheless this phase is at lower temperatures compared to the isotropic phase, the polymerisation kinetics is faster and a maximum conversion of 75 % is reached. Apart from the work on reactive mesogens new materials for the use in OFETs are part of this thesis. Since the HOMO levels of pure fluorene compounds are too low for an efficient charge carrier injection in OFETs fluorene and bithiophene building blocks have been combined. For the use in OFETs the alternating polymer containing 9,9-dioctylfluorene and bithiophene monomers (F8T2) is one of the best investigated polymers. To overcome the problem of the very high transition temperature from the nematic in the isotropic phase which hinders an efficient orientation a synthesis of oligomers based on the F8T2 structure but with a lower molecular weight was developed, to obtain a new class of stable and solution processable materials for OFET applications. The molecular weight can be tuned by the amount of the monofunctionalised 2-bromofluorene which acts as an endcapper. Like F8T2, all oligomers show broad nematic phases. The clearing temperature can be tailored by varying the molecular weight of the oligomers between 80 °C and 288 °C. A plot of the reciprocal polymerisation degree versus the clearing temperature of these oligomers with different degree of polymerisation results in a clearing temperature of 366 °C for the ideal polymer. The better control of purity compared with polymers or oligomers and the well defined structure renders small molecules attractive candidates for the use in OFETs. A number of mixed fluorene-bithiophene trimers with different alkyl side chains in the 9 position have been synthesised. The characterisation by cyclic voltammetry results in a HOMO level of – 5.3 eV. This is exactly the desired level for efficient hole injection from the gold or conducting polymer electrodes in an OFET. We have tested three of the trimers in OFETs and found that the mobility varied from 10exp.-5 cm2/Vs in an amorphous film to 3x10exp.-3 cm2/Vs in a polycrystalline material. To demonstrate the stability of this material, measurements were performed after three months storage under ambient conditions. The mobility was the same as in a freshly prepared device. Almost no hysteresis can be detected even after a few months at ambient conditions. The preserved high mobility is another hint, that the molecules are much more stable than pure thiophene materials.
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New Methods for the Investigation of Organic Thin-Film Devices
(2004)
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Helmut Hänsel
- We have developed new techniques for the investigation of organic thin-film devices and have focussed on properties on the molecular scale as well as on macroscopic properties of organic devices. Scanning probe techniques were used to obtain spatially resolved information on morphology and electro-optical properties. Structural changes in composite-based devices were found to have an important influence on device performance. Furthermore, two modes of electroluminescence detection have been developed. Local luminescence detection in the optical near-field by a scanning near-field optical microscope allowed us to monitor the light emission around a dark spot with a resolution better than 134 nm and to observe the electrode ablation. Finally, we have established a new scanning probe technique, named SELM, "Scanning Electroluminescence Microscopy". The simultaneous detection of a PtIr-tip-induced electroluminescence and shear force allows us to distinguish between topography and conductivity. This technique has revealed a strong spatial variation in the electro-optical properties of Alq3 films on ITO substrates. The existing combinatorial preparation method has been supplemented by a variable testing setup that permits the simultaneous investigation of 64 devices under nearly identical conditions. Both OLEDs and photovoltaic cells have successfully been tested over more than 300 hours of continuous operation so that it was possible to study the influence of material combinations and layer thicknesses on the performance and on the degradation of the devices. Variable-angle spectroscopic ellipsometry has been used for the optical characterisation of materials and an automation has been provided for the analysis of combinatorially prepared device arrays. Furthermore, a Mathematica program has been developed for the theoretical description of the short-circuit current in photovoltaic cells. By this means it was possible to explain in detail the observed performance enhancement in heterojunction solar cells, induced by an additional TiO2 layer. The optical and electronic contribution could only be identified by the variation of both layer thickness and device type. The strength of the setup presented is its ability to produce and to test devices under nearly identical conditions and to yield reliable data, which in turn can be used to test physical models. Finally, we have addressed the degradation process of OLEDs. The experiments have shown that inert gas plays an essential role in protecting against degradation, not only by the exclusion of reactive species but also by its heat-transport capabilities. These investigations are only just beginning and further combinatorial studies paired with AFM measurements are currently being developed.
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Neue Materialien für optoelektronische und elektronische Bauteile
(2004)
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Klaus Kreger
- Die vorliegende Arbeit beschäftigte sich mit der Synthese von organischen Verbindungen auf der Basis von Fluorenen und Carbazolen, welche als aktives Material in Halbleiterbauteilen, wie organische Leuchtdioden (OLED) oder organische Transistoren (OFET) eingesetzt wurden. Molekular einheitliche, niedermolekulare Verbindungen können in hochreiner Form erhalten werden. Sie besitzen keine Endgruppen und zeigen keine Verteilung von physikalischen Eigenschaften. Die Verbindungen sollen eine stabile nichtkristalline Morphologie besitzen. Verbindungen mit sternförmiger Architektur neigen häufig zur Glasbildung. Unter solchen so genannten organischen Gläsern versteht man niedermolekulare Verbindungen, die in der Lage sind eine stabile amorphe Phase auszubilden. Solche trigonalen sternförmigen Verbindungen bestehen aus einem starren konjugierten Kern und einer entsprechenden Anzahl an Flügelgruppen. Als Flügelgruppen kamen verschiedene 9-alkylierte Fluoren- und Carbazolbausteine zum Einsatz. Als Kerne wurden Triphenylamin, Triphenylbenzol, Benzol, Tristhienylbenzol und Triphenylen verwendet. Der Schlüsselschritt der Synthese ist eine Suzukireaktion. Die meisten Verbindungen bilden amorphe Phasen, wobei die Glasübergangstemperaturen zwischen 57-158°C variieren. Die amorphe Phase ist dabei ausgesprochen lagerstabil. Die energetischen Lücken liegen zwischen 3,0-3,5 eV. Solche Verbindungen zeigen eine intensiv blaue bis violettblaue Fluoreszenz. Die HOMO-Niveaus variieren zwischen -5 eV und -6 eV. Aminderivate eignen sich als Lochleiter sowie als blaue Emitter. Verbindungen mit tiefliegenden HOMO-Niveaus und großen energetischen Lücken können als Lochblocker fungieren. Daneben wurden konjugierte organische Materialien synthetisiert, die eine hochgeordnete Phase aufweisen. Hierfür kamen Flüssigkristalle in Frage. Neben der Synthese von Flüssigkristallen, deren Phase durch Unterkühlung eingefroren werden kann, wurden auch so genannte Reaktivmesogene hergestellt. In diesem Fall wird die momentane morphologische Ordnung mittels einer photochemischen Reaktion permanent in einem Netzwerk fixiert. Des Weiteren galt darauf zu achten, dass die Materialien bevorzugt Löcher leiten. Stäbchenförmige Verbindungen, wie z.B. ein 2,7-Bis(carbazol-2-yl)-fluoren zeigen bereits ein flüssigkristallines Verhalten. Der Zugang zu solchen Verbindungen wurde via Suzukireaktion realisiert. Eine Methoxygruppe in der 7-Position des Carbazols ermöglichte nach einer Etherspaltung den weiteren Aufbau zum Reaktivmesogen. Mit gezielten repetiven Suzukireaktionen war auch der Zugang zu linearen 2,7-verknüpften Pentameren mit unterschiedlichen Abfolgen von Fluoren- und Carbazolbausteinen möglich. Solche Verbindungen können dabei eine flüssigkristalline Phasenbreite von mehr als 250°C besitzen. Schließlich wurden N-Arylierte Tercarbazole hergestellt. Solche Tercarbazole sind thermisch extrem stabil und besitzen stabile amorphe Phasen mit Glasübergangstemperaturen bei 103°C bzw. 190°C. Deren HOMO-Niveaus berechnen sich zu –5.2 eV, was etwas tiefer als ein entsprechendes N-alkyliertes 3,6-verknüpftes Tercarbazol ist. Die N-Arylgruppe ist offensichtlich nicht an der Konjugation beteiligt. Mehrschicht-OLEDs wurden mittels Aufdampfprozesse hergestellt. Dabei fungierte ein sternförmiges Triphenylbenzolderivat als Lochblocker. Ein sternförmiges Aminderivat wurde als Lochleiter und als Emitter eingesetzt. Das effizienteste Device wurde durch Aufdampfen von Stufengradienten ermittelt. Eine 5 nm dicke Lochblockerschicht ist ausreichend, um den Löcherstrom an der Grenzfläche zu stoppen. Die spannungsunabhängige Emission mit den CIE-Koordinaten x = 0.154 und y = 0.146 stammt ausschließlich vom Lochleiter. Die Leuchtdichten betragen ~400 cd/m2 bei 10V. Die max. Effizienzen liegen bei ~2 cd/A und die Lichtausbeuten bei ~0,8 lm/W. Eine Reihe der Materialien wurden bei Philips (Eindhoven) und Merck (Southhampton) auf ihre Transistoraktivität untersucht. Von der Mehrzahl der Verbindungen konnten dabei homogene Filme aus Lösung erhalten werden. Dabei hat sich gezeigt, dass das Einsetzen eines Stromes wesentlich von einem ohmschen Kontakts mit der Gold-Elektrode abhängig ist. Ebenso erhöhen unzureichende Filmbildungseigenschaften die Onset-Spannung und führen zu niedrigen Drainströmen. Beides kann durch das Verwenden von Carbazolbausteinen anstelle des Fluorens und mit der Wahl von geeigneten Alkylketten vermieden werden. Die Orientierbarkeit und die Strukturierbarkeit von Reaktivmesogenen wurde demonstriert. Allerdings sinkt dabei die Beweglichkeit nach der Vernetzung deutlich. Insbesondere sternförmige Verbindungen auf der Basis von Triphenylaminen zeigen ein Einsetzen des Stromes nahe bei 0V. Solche amorphe Filme besitzen eine Beweglichkeit von mehr als 10-4 cm2/Vs in nichtoptimierten Devices. Die On/Off-Verhältnisse betragen 104-105. Als wesentliche Vorzüge solcher aromatischen Amine sind die geringen Hystereseffekte und die exzellente Lagerstabililtät an Luft zu nennen.
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Synthesis, characterisation and application of low molecular weight and polymeric 1,3-di-2-thienylbenzo[c]thiophenes
(2004)
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Roman Kisselev
- The synthesis and characterisation of the new class of compounds, dithienylisothianaphthene phenyldiamines (DTITNPDs) is described. These bifunctional hole transport dyes combine well-known hole-transport property of triaryl amines and thiophenes as well as low band gap nature of isothianaphthene (ITN) moiety. The synthetic strategy is chosen in such a way to obtain low molecular weight and polymeric DTITNPDs. Low molecular weight DTITNPDs are synthesised by Pd-catalysed amination of dibromo dithienylisothianaphthene with secondary amines. On the other hand, poly(DTITNPD)s are obtained via polycondensation of diiodo dithienylisothianaphthene and bis(secondary amine)s using a modified Ullmann reaction. The multi-step syntheses of dibromo dithienylisothianaphthene and novel diiodo dithienylisothianaphthene are optimised. The preparation of new bis(secondary amine)s are also described. Moreover, the influence of substituents on optical, electro-chemical and thermal properties of DTITNPDs and poly(DTITNPD)s is also investigated. The low molar mass (monomers) and polymeric DTITNs are also obtained. These compounds also possess hole transport property of thiophene and low band gap nature of ITN. Poly(DTITN)s are synthesised from corresponding monomers using FeCl3 oxidative polymerisation. Multi-step syntheses of DTITN monomers are also presented here. The main highlight of this work is the realisation of solution processable and film-forming and air-stable poly(DTITN) and poly(DTITNPD)s in addition to the model compounds DTITNs and DTITNPDs. The model compounds, polymeric DTITNPDs as well as poly(DTITN)s are characterised by means of 1H-NMR-, FT-IR-, MS- and UV-Vis- spectroscopy. Their thermal and electro-chemical behaviour is studied using TGA, DSC and CV measurements. All intermediates, synthesised in this work are also fully characterised by spectroscopic methods discussed above, except UV-Vis spectroscopy. Novel DTITNs and DTITNPDs show good thermal and electro-chemical stability as well as ability to form smooth thin films. DTITNPDs show lower band gap, solubility in common organic solvents and better thermal stability compared to DTITNs. For the application in organic electro-optical devices materials with improved optical and charge transport properties are required. Moreover, these hole transport dyes should match the energy levels (HOMO/LUMO) of the electron transport partner for efficient charge transfer/injection. In this respect, the main attention is placed on variation of energy levels in synthesised molecules by structure modification. The structure modification in DTITN usually changes the LUMO level in the molecule. In contrast to the DTITN, the introduction of different substituents into diphenylamine allows manipulation of HOMO level in DTITNPDs. Thus, the combination of DTITN and triarylamines leads to DTITNPD, where the values of both energy levels can be varied. Novel DTITNPDs exhibit smaller band gap compared to DTITNs. The better delocalised HOMO level in the DTITNPDs compared to DTITNs leads to novel hole transport dyes with Eg less than 1.8 eV. The low molar mass DTITNPDs are tested in plastic solar cells and multi-layer solar cells in combination with electron transport perylene bisimide derivatives and fullerene (C60). The poly(DTITN) is used in plastic solar cell in combination with a soluble fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). All of these compounds show good performance in solar cells. External quantum efficiency (IPCE spectrum) for the last solar cell shows a maximum of 40 % at 350 nm and a maximum of 15 % at 540 nm, at maximum wavelength of absorption. A promising result is obtained using low molar mass DTITNPD as red-emitter in OLEDs. When the red emitter doped in Alq3 at concentration of 1 % is used as emitting layer in OLED, the pure red electroluminescence with maximum brightness of 13830 cd/m2 at operating voltage of 12 V is observed. This device exhibits a high efficiency of 3.8 cd/A at 6 V bias, emitting bright red electroluminescence with CIE coordinates of x = 0.66 and y = 0.34, closely resembling the desired standard red colour (NTSC standard: x = 0.67 and y = 0.34) set for RGB displays.
