Seeing is believing: experimental 3D mapping of iridium(III) complexes in light-emitting guest:host blends

The optical and electronic properties of organic semiconductor thin films are intimately coupled to their morphology at the atomic level. Atomic level morphology of non-crystalline systems is difficult to probe experimentally and hence molecular dynamics (MD) simulations have been used to provide a means to examine the morphologies of evaporated amorphous thin films with unrivalled spatial resolution. These simulated films have been used to undertand the experimentally measured properties such as charge mobility and photoluminescence quantum yield. However, MD simulations occur on timescales much faster than that used in experiment. In this presentation we report high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with depth sectioning to reconstruct the three-dimensional distribution of the emissive layers composed of iridium(III) complexes blended into an amorphous host. We will show that the results validate the morphology of blend films created using MD simulations which mimic the evaporation film forming process and are also consistent with experimentally measured charge transport and photophysical properties.