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 alpha-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.