Oriented phosphorescent emitters boost OLED efficiency

The orientation distribution of the emissive sites in a phosphorescent Organic LED has been measured utilizing two independent optical methods. In contradiction to common expectations we find a clearly non-isotropic, predominantly parallel emitter orientation in the well-known triplet emitting guest–host system of Ir(MDQ)2(acac) blended in an α-NPD matrix. This result emphasizes the necessity of more sophisticated assumptions on active emitter properties in quantitative optical OLED analysis, and demonstrates a highly promising approach for OLED efficiency optimization.

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► OLED triplet emitter Ir(MDQ)2(acac) blended in α-NPD matrix is studied.
► Polarized emission patterns are analyzed under electrical and optical excitation.
► Unexpected non-isotropic, mainly parallel emitter orientation is found.
► Parallel emitters increase outcoupling efficiency by ∼50% compared to isotropic case.
► Promising approach for efficiency enhancement of phosphorescent guest–host systems is revealed.