1-, 2- and 3-dimensional phenylene-based materials for optoelectronic applications

A series of oligo-phenylene dendronised conjugated polymers was prepared. The divergent synthetic approach adopted allowed for the facile synthesis of a range of dendronised monomers from a common intermediate, e.g. first and second generation fluorene. Only the polymerisation of the first generation and alkylarylamine substituted dendronised fluorene monomers yielded high molecular weight materials, attributed to the low solubility of the remaining dendronised monomers. The alkylarylamine substituted dendronised poly(fluorene) was incorporated into an organic light emitting diode (OLED) and exhibited an increased colour stability in air compared to other poly(fluorenes). The concept of dendronisation was extended to poly(fluorenone), a previously insoluble material. The synthesis of the first soluble poly(fluorenone) was achieved by the incorporation of oligo-phenylene dendrons at the 4-position of fluorenone. The dendronisation of fluorenone allowed for a polymer with an Mn of 4.1 x 104 gmol-1 to be prepared. Cyclic voltammetry of the dendronised poly(fluorenone) showed that the electron affinity of the polymer was high and that the polymer is a promising n-type material. A dimer and trimer of indenofluorene (IF) were prepared from the monobromo IF. These oligomers were investigated by 2-dimensional wide angle x-ray spectroscopy (2D-WAXS), polarised optical microscopy (POM) and dielectric spectroscopy, and found to form highly ordered smetic phases. By attaching perylene dye as the end-capper on the IF oligomers, molecules that exhibited efficient Förster energy transfer were obtained. Indenofluorene monoketone, a potential defect structure for IF based OLED’s, was synthesised. The synthesis of this model defect structure allowed for the long wavelength emission in OLED’s to be identified as ketone defects. The long wavelength emission from the indenofluorene monoketone was found to be concentration dependent, and suggests that aggregate formation is occurring. An IF linked hexa-peri-hexabenzocoronene (HBC) dimer was synthesised. The 2D-WAXS images of this HBC dimer demonstrate that the molecule exhibits intercolumnar organisation perpendicular to the extrusion direction. POM images of mixtures of the HBC dimer mixed with an HBC with a low isotropic temperature demonstrated that the HBC dimer is mixing with the isotropic HBC.

Synthesis of hexa-peri-hexabenzocoronenes - from building blocks to functional materials

Discotic hexa-peri-hexabenzocoronene (HBC) derivatives have attracted intensive scientific interest due to their unique optoelectronic properties, which depends, to a large extend, upon the attached functional groups. The presented work covers the synthesis of novel HBC building blocks and new HBC derivatives as functional materials. The traditional preparation of HBC derivatives requires elaborate synthetic techniques and tremendous effort. Especially, more than 10 synthetic steps are usually necessary to approach HBCs with lower symmetries. In order to simplify the synthetic work and reduce the high costs, a novel synthetic strategy involving only four steps was developed based on 2,3,5,6-tetraphenyl-1,4-diiodobenzene intermediates and palladium catalyzed Suzuki cross coupling reactions. In order to introduce various functionalities and expand the diversity of multi-functionalizations, a novel C2v-symmetric dihalo HBC building block 2-47, which contains one iodine and one bromine in para positions, was prepared following the traditional intermolecular [4+2] Diels-Alder reaction route. The outstanding chemical selectivity between iodo and bromo groups in this compound consequently leads to lots of HBC derivatives bearing different functionalities. Directly attached heteroatoms will improve the material properties. According to the application of intramolecular Scholl reaction to a para-dimethoxy HPB, which leads to a meta-dimethoxy HBC, a phenomenon of phenyl group migration was discovered. Thereby, several interesting mechanistic details involving arenium cation intermediates were discussed. With a series of dipole functionalized HBCs, the molecular dynamics of this kind of materials was studied in different phases by DSC, 2D WAXD, solid state NMR and dielectric spectroscopies. High charge carrier mobility is an important parameter for a semiconductive material and depends on the degree of intramolecular order of the discotic molecules in thin films for HBC derivatives. Dipole – dipole interaction and hydrogen bonds were respectively introduced in order to achieve highly ordered supramolecular structure. The self-assembly behavior of these materials were investigated both in solution and solid state. Depending upon the different functionalities, these novel materials show either gelating or non-linear optical properties, which consequently broaden their applications as functional materials. In the field of conceivable electronic devices at a molecular level, HBCs hold high promise. Differently functionalized HBCs have been used as active component in the studies of single-molecular CFET and metal-SAMs-metal junctions. The outstanding properties shown in these materials promise their exciting potential applications in molecular devices.

Studies of degradation of organic solar cell materials using electrical scanning probe microscopy

Intense research is being done in the field of organic photovoltaics in order to synthesize low band-gap organic molecules. These molecules are electron donors which feature in combination with acceptor molecules, typically fullerene derivarntives, forming an active blend. This active blend has phase separated bicontinuous morphology on a nanometer scale. The highest recorded power conversionrnefficiencies for such cells have been 10.6%. Organic semiconductors differ from inorganic ones due to the presence of tightly bonded excitons (electron-hole pairs)resulting from their low dielectric constant (εr ≈2-4). An additional driving force is required to separate such Frenkel excitons since their binding energy (0.3-1 eV) is too large to be dissociated by an electric field alone. This additional driving force arises from the energy difference between the lowest unoccupied molecular orbital (LUMO) of the donor and the acceptor materials. Moreover, the efficiency of the cells also depends on the difference between the highest occupied molecular orbital (HOMO) of the donor and LUMO of the acceptor. Therefore, a precise control and estimation of these energy levels are required. Furthermore any external influences that change the energy levels will cause a degradation of the power conversion efficiency of organic solar cell materials. In particular, the role of photo-induced degradation on the morphology and electrical performance is a major contribution to degradation and needs to be understood on a nanometer scale. Scanning Probe Microscopy (SPM) offers the resolution to image the nanometer scale bicontinuous morphology. In addition SPM can be operated to measure the local contact potential difference (CPD) of materials from which energy levels in the materials can be derived. Thus SPM is an unique method for the characterization of surface morphology, potential changes and conductivity changes under operating conditions. In the present work, I describe investigations of organic photovoltaic materials upon photo-oxidation which is one of the major causes of degradation of these solar cell materials. SPM, Nuclear Magnetic Resonance (NMR) and UV-Vis spectroscopy studies allowed me to identify the chemical reactions occurring inside the active layer upon photo-oxidation. From the measured data, it was possible to deduce the energy levels and explain the various shifts which gave a better understanding of the physics of the device. In addition, I was able to quantify the degradation by correlating the local changes in the CPD and conductivity to the device characteristics, i.e., open circuit voltage and short circuit current. Furthermore, time-resolved electrostatic force microscopy (tr-EFM) allowed us to probe dynamic processes like the charging rate of the individual donor and acceptor domains within the active blend. Upon photo-oxidation, it was observed, that the acceptor molecules got oxidized first preventing the donor polymer from degrading. Work functions of electrodes can be tailored by modifying the interface with monomolecular thin layers of molecules which are made by a chemical reaction in liquids. These modifications in the work function are particularly attractive for opto-electronic devices whose performance depends on the band alignment between the electrodes and the active material. In order to measure the shift in work function on a nanometer scale, I used KPFM in situ, which means in liquids, to follow changes in the work function of Au upon hexadecanethiol adsorption from decane. All the above investigations give us a better understanding of the photo-degradation processes of the active material at the nanoscale. Also, a method to compare various new materials used for organic solar cells for stability is proposed which eliminates the requirement to make fully functional devices saving time and additional engineering efforts.

Local dynamics and bending mechanics of mesostructured materials

Die vorliegende Arbeit behandelt die Anwendung der Rasterkraftmikroskopie auf die Untersuchung mesostrukturierter Materialien. Mesostrukturierte Materialien setzen sich aus einzelnen mesoskopen Bausteinen zusammen. Diese Untereinheiten bestimmen im Wesentlichen ihr charakteristisches Verhalten auf äußere mechanische oder elektrische Reize, weshalb diesen Materialien eine besondere Rolle in der Natur sowie im täglichen Leben zukommt. Ein genaues Verständnis der Selbstorganisation dieser Materialien und der Wechselwirkung der einzelnen Bausteine untereinander ist daher von essentieller Bedeutung zur Entwicklung neuer Synthesestrategien sowie zur Optimierung ihrer Materialeigenschaften. Die Charakterisierung dieser mesostrukturierten Materialien erfolgt üblicherweise mittels makroskopischer Analysemethoden wie der dielektrischen Breitbandspektroskopie, Thermogravimetrie sowie in Biegungsexperimenten. In dieser Arbeit wird gezeigt, wie sich diese Analysemethoden mit der Rasterkraftmikroskopie verbinden lassen, um mesostrukturierte Materialien zu untersuchen. Die Rasterkraftmikroskopie bietet die Möglichkeit, die Oberfläche eines Materials abzubilden und zusätzlich dazu seine quantitativen Eigenschaften, wie die mechanische Biegefestigkeit oder die dielektrische Relaxation, zu bestimmen. Die Übertragung makroskopischer Analyseverfahren auf den Nano- bzw. Mikrometermaßstab mittels der Rasterkraftmikroskopie erlaubt die Charakterisierung von räumlich sehr begrenzten Proben bzw. von Proben, die nur in einer sehr kleinen Menge (<10 mg) vorliegen. Darüberhinaus umfasst das Auflösungsvermögen eines Rasterkraftmikroskops, welche durch die Größe seines Federbalkens (50 µm) sowie seines Spitzenradius (5 nm) definiert ist, genau den Längenskalenbereich, der einzelne Atome mit der makroskopischen Welt verbindet, nämlich die Mesoskala. In dieser Arbeit werden Polymerfilme, kolloidale Nanofasern sowie Biomineralien ausführlicher untersucht.rnIm ersten Projekt werden mittels Rasterkraftmikroskopie dielektrische Spektren von mischbaren Polymerfilmen aufgenommen und mit ihrer lokalen Oberflächenstruktur korreliert. Im zweiten Projekt wird die Rasterkraftmikroskopie eingesetzt, um Biegeexperimente an kolloidalen Nanofasern durchzuführen und so ihre Brucheigenschaften genauer zu untersuchen. Im letzten Projekt findet diese Methode Anwendung bei der Charakterisierung der Biegeeigenschaften von Biomineralien. Des Weiteren erfolgt eine Analyse der organischen Zusammensetzung dieser Biomineralien. Alle diese Projekte demonstrieren die vielseitige Einsetzbarkeit der Rasterkraftmikroskopie zur Charakterisierung mesostrukturierter Materialien. Die Korrelation ihrer mechanischen und dielektrischen Eigenschaften mit ihrer topographischen Beschaffenheit erlaubt ein tieferes Verständnis der mesoskopischen Materialien und ihres Verhaltens auf die Einwirkung äußerer Stimuli.rn