Organic light emitting diodes employing phosphorescent metal complexes have been widely studied over the past decade due to their ability to harvest 100% of triplet and singlet electrogenerated excitons and high external quantum efficiencies have been achieved for emission across the much of the visible range. However, achieving high power efficiencies and long device operational stability for deep blue emission remains a challenge. Such emission typically necessitates a high triplet energy which requires very large bandgap host and charge confining materials, significantly increasing the device turn on voltages and potentially reducing device operational stabilities.
Recently, the development of organic emitters exhibiting thermally activated delayed fluorescence (TADF) process has demonstrated the potential to harvest both triplet and singlet excitons due to thermally activated triplet to singlet conversion. Nevertheless, the triplet to singlet conversion process is necessarily energetically unfavorable and the triplet excitons may decay non-radiatively due to the absence of a rapid phosphorescent emission process. Thus, in order to achieve efficient emission the deep blue region, the energy difference between the singlet and triplet energies must be small and the impact of using delayed fluorescent emitters to achieve deep blue emission with lower triplet energy is diminished.
In this presentation we demonstrate the development and characterization of a series of Pd metal complexes capable of simultaneously exhibiting both phosphorescent emission from the triplet state and metal assisted delayed fluorescence (MADF) emission from the higher energy singlet state. Unlike heavy metal free emitters exhibiting TADF, a larger difference between singlet and triplet energies is permitted for emitters exhibiting MADF since any triplet excitons not converted the singlet state can emit through an efficient phosphorescence process, thus preserving the high efficiencies.
These complexes exhibit high photoluminescent emission quantum yields and can be easily tuned to emit across the visible spectrum. Devices employing these complexes demonstrated very high external quantum efficiencies of 22% Peak EQE for the green emitting PdN3N and 26.5% Peak EQE for the blue emitting PdN1N. Furthermore, due to the lower energy of the triplet emission, these blue and yellow-green emitters are compatible with known stable devices structures and long device operational lifetimes were achieved.