On the Colour Tuning and Stability of Carbene-Metal-Amide Organic Light-Emitting Diodes
This thesis focusses on the development of a family of light emitting molecules and their incorporation into organic light emitting diodes (OLEDs). OLEDs are currently widely utilised in consumer display products. Blue OLEDs, however, are unable to simultaneous meet the industrial requirements of colour purity, efficiency and device lifetime. Most consumer products choose to compromise on efficiency and hence most commercial blue OLEDs are relatively inefficient compared to their red and green counterparts. The need for an efficient and stable blue emitter is clear and requires investigations into new classes of emitters. One such possibility are carbene-metal-amides (CMAs). CMAs are a family of charge transfer type photoemitters which luminescence through a thermally activated delayed fluorescence mechanism. CMAs have been previously incorporated into highly efficient green OLEDs. In chapter 4, CMAs are first chemically modified through the addition of electron donating and withdrawing groups on the carbazole. The modifications achieve a predictable spectral shift in the luminescence, which can be further tuned by the use of host polarity. Efficient deep blue CMA OLEDs are demonstrated but suffer from exceptionally poor device lifetime. In chapter 5, attempts are made to improve the device lifetime. The CMAs are first chemically modified to improve stability to electro chemical oxidation. Although stability improvements are observed in the cyclic voltammetry, there is no correlating improvement in device lifetime. The CMA material is also purified to try and reduce impurity induced degradation. Again no improvement in device lifetime is achieved. In Chapter 6, an investigation is made into the intrinsic mechanism which drives degradation in CMA OLEDs. Bimolecular exciton-exciton interactions are identified as the primary degradation pathway. Attempts are made to minimise this interaction and lead to some improvement. Ultimately, however, the device lifetime of CMA OLEDs remains extremely short. Future work must find a way to substantially improve the stability of CMAs and may lead to the development of a similar, but new class of emitters.
carbene-metal-amide, CMA, OLED, organic light-emitting diode
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