In my project I was exploring the use of charge transfer and exciplex based emitters with 100% efficiency through the use of thermally activated delayed fluorescence (TADF) in OLEDs. These stable emitters remove the need to use iridium in OLED devices. This was not a trivial matter due to the way an OLED works. Electron–hole recombination in the organic emitter layer creates 25% singlet excitons that decay radiatively, producing light directly, and 75% non-radiative triplet excitons. Thus, to make an efficient OLED, these triplets must in some way be converted into singlets. Heavy metal complexes have efficiently intermixed singlet and triplet states, via effective heavy atom spin orbit coupling, yielding near 100% light emission by phosphorescence. Without this ‘triplet harvesting’, a fluorescent OLED would be limited to a maximum 25% internal quantum efficiency. My ideas were to use exciplex phenomena, which was previously discovered in solution and never used in solid state emitters. Exciplex effects were considered as undesirable by many scientists, viewed exclusively as a loss process diminishing OLED efficiency. However, thanks to my investigations, exciplexes have now been shown to provide possibilities to make a wide range of emitters. This is due to the fact that the wavelength of the emission in such systems is not dependent on the band-gap value of a single compound, but the HOMO-LUMO offset between donor and acceptor molecules. This means, I was able to design a wide range of stable exciplex emitters in a wide range of emitting wavelengths.
In this project I was responsible for designing, fabricating and characterization of OLED devices, as well as searching for new compounds and new exciplex systems for analysis. Thanks to my wide knowledge of electrochemical processes and spectroscopic analysis I was able to use this in a search for new co-compounds for exciplex layers. In my project I was able to design and form exciplex based OLED devices, which had 100% interconversion. I was able to demonstrate that an exciplex emitter can yield efficient OLEDs, >19% EQE (almost 100% efficiency). In conclusion, I was able to form several highly efficient, heavy atom free OLED devices based on TADF and exciplex phenomena.
