Investigating the molecular orientation of Ir(ppy)₃ and Ir(ppy)₂(acac) emitter complexes by X-ray diffraction

We study thermally evaporated thin films of Ir(ppy)₃ and Ir(ppy)₂(acac) by means of grazing incidence X-ray diffraction (GIXRD) and grazing incidence wide-angle X-ray scattering (GIWAXS). Ir(ppy)₃ and Ir(ppy)₂(acac) are both widely used as phosphorescent green emitter molecules in organic light-emitting diodes (OLEDs) and it was previously found that differences in their average transition dipole orientation affect the light extraction efficiency in OLEDs. Here we show that in pure films both materials form crystalline grains and that these grains exhibit a preferred orientation with respect to the substrate. When doped into an amorphous host, both the orientation and formation of the crystallites remain nearly unchanged for the concentration range accessible with GIXRD and GIWAXS. This is remarkable given that the transition dipole moments have found to be oriented only for Ir(ppy)₂(acac) but isotropic for Ir(ppy)₃. Analysis of the crystallite size indicates that the tendency to form crystallites is stronger for Ir(ppy)₃ than for Ir(ppy)₂(acac). From a comparison of the thin-film diffraction data of Ir(ppy)₃ to its powder pattern, we infer that Ir(ppy)₃ molecules are oriented with their permanent dipole moment roughly parallel to the substrate. Our findings will guide the further understanding of the mechanisms controlling transition dipole orientation and may thus lead to further improvements in device efficiency.