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The efficiency of organic light emitting diodes has been significantly improved during the last several years by developing phosphorescent and TADF emitters with high horizontal emitting dipole orientation and high PLQY along with the development of device structure with excellent charge balance to get EQE reaching almost 40%, corresponding to almost the theoretical limit of the efficiency. The most important remaining issue now is the development of stable deep blue OLEDs. In this talk, we will firstly present a theoretical approach to understand the degradation process of phosphorescent OLEDs followed by the three strategies to realize deep blue OLEDs including the phosphorescent dye sensitized fluorescent OLEDs, deep blue TADF OLEDs using emitters with narrow emission spectrum, high PLQY and horizontal emitting dipole orientation, and finally deep blue OLEDs utilizing triplet-triplet annihilation (TTA) process based on anthracene derivatives with the efficiencies overcoming the conventional theoretical limit.
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