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New fluorene based materials for organic electronics
(2005)
- 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.
