Effect of matrix anisotropy on theapparent emitter orientation inOrganic LED

Spontaneously aligning emitter molecules can improve the external efficiency of OLED devices and have beenreportedfor different types of emittersdoped into suitable hosts. Similar to the morphological alignment of the active molecules, a birefringence of the host affects the apparent emitter orientation. In order to investigate this effect experimentally, Ir(ppy)3is co-evaporated into the emitting layer of OLED using hosts with different birefringence properties. Analyzing the electroluminescence emission patterns quantitatively reveals the apparent orientation distribution of the emitters. The results emphasize that host birefringence can be used to suppress perpendicular emitter contributions while amplifying the impact of the horizontal dipoles, thus leading to an enhanced forward emission.