Hundreds of photo-physical and optical measurements, data from over 2000 devices using almost 50 different materials (most of them synthesised in the project) in various combinations, open scientific discussions and the diligent analysis of results significantly increased our knowledge in the project’s field of research. The drive to make this knowledge available to the whole scientific community is reflected in already 17 publications, and the submission of articles is still ongoing.
Apart from developing and applying a highly sensitive method to determine optical anisotropy, the basic assumptions of commonly used optical orientation measurements have been re-examined. We found that energy transfer processes, neglected up to now, play an important role in the interpretation of such experiments. This demonstrates the strong expertise of Fraunhofer IOF in optical analysis of OLEDs, which will attract further funding and contract research in the future.
We have developed a model to describe transient PL measurements which are routinely used in the field. The model supports various experimental evidence to prove that such measurements have fundamental limitations when it comes to the investigation of energy loss processes, making it necessary to rethink the way how HF is investigated. Nevertheless, our model allows accurate determination of the rISC and FRET rates, which are fundamental parameters governing the efficiency of HF devices. Furthermore, we could show that the rISC rate poses a limitation on how well energy losses can be suppressed by shielding the fluorescent emitter. These results once again prove the University of Durham’s leading position as a research institution in the field of photophysics of organic materials.
Our search for new materials and possibilities to tune performance resulted in nine patent applications. As an example of potential exploitation paths, we found a new class of materials which gives very colour pure deep blue emission. These materials have to be modified further in order to enter the highly attractive market for blue fluorescent emitter materials. On the material side, numerous methods to synthesize TADF and shielded fluorescent emitters were developed and structure-property relationships were identified. Purification protocols have been established in order to obtain high quality materials required for investigation in devices. The result from this work will help to speed up future progress in this area.
A highly efficient blue OLED material set was developed and integrated in a white OLED stack with a reduced number of layers and low driving voltage. Good performance was obtained in both blue and white devices which demonstrates the applicability of the project’s findings, but we acknowledge that performance in terms of colour point and device stability is not yet sufficient for application in consumer products. Nevertheless, the identification of obstacles and limitations is a highly valuable guidance for strategic decisions in further material development activities.
Due to the novelty of some materials, the use of our white stack on a micro display production line was not feasible. So, we established a workflow where coating of the OLED layers is done at Merck while finalising and characterising the wafers at MICROOLED. The resulting performance in the microdisplay device was significantly lower compared to the results obtained with test devices, and additional optimisation of the device and the process flow will be required for industrial application. However, the technical issues could be identified, and the project results serve as basis for further collaboration between the two industrial partners in the field of micro displays.
