Molecular orientation in organic semiconductor is an important factor influencing electrical and optical properties. In organic light emitting diodes (OLEDs), it has long been recognised that orienting the transition dipole moment of an emitter along the horizontal direction (parallel to the substrate) can enhance the outcoupling efficiency beyond that achieved under isotropic orientation, as demonstrated in polymer-based and vacuum evaporated fluorescent molecule-based OLEDs. Nonetheless, the orientation of the transition dipole moments of iridium complexes used as phosphorescent emitters in efficient OLEDs is typically considered to be isotropic because they are near-globular and small enough to have configurational diversity in their orientational states. Therefore, it is believed that the theoretical EQE limit of phosphorescent OLEDs is 25~30%. Recently, however, some heteroleptic iridium complexes (HICs) have been reported to have transition dipole moments oriented preferentially along the horizontal direction. The outcoupling efficiency of the emitted light from the horizontally oriented emit-ting dipoles in an OLED can reach 45% which is much higher than isotropically oriented transition dipoles. Unfortunately, the origin of the preferred orientation of some phosphorescent dyes and factors influencing the orientation of the transition dipole moments is not fully understood yet.

© 2014 Optical Society of America

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