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Even after over 30 years’ development and successful commercialization of organic light emitting diodes (OLEDs), very interesting and important innovations have occurred in OLEDs in recent years. Recently reported thermally activated delayed fluorescence (TADF) materials, utilizing intra- and inter-molecular charge transfer processes upon excitation, is an example which can harvest triplet excitons or exciplexes to light. Another example is the demonstration of external quantum efficiencies (EQEs) of over 30%, approaching almost 40% without any external light extraction structures. This high efficiency was realized based on the development of highly oriented phosphorescent or TADF emitters parallel to the substrate (horizontal direction), combined with advanced device structures. Improving efficiency is important because it is directly related to the reduction of power consumption and device lifetime.
In this presentation, recent development of high efficiency OLEDs based on phosphorescent, TADF and fluorescent emitters will be discussed. We will show that EQEs over 50% is achievable without any extra light extracting structure and over 80% with light extraction structures if OLEDs are fabricated using organic semiconductors with the low refractive index and fully horizontal emitting dipoles. Finally, we will also discuss on the possibility of realizing high efficiency blue OLEDs with long lifetime, one of the most important issues in OLEDs.
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