Atomic Resolution Microscopy Reveals Cluster, Charge Transport, and Diffusion Metrics in Guest:Host Blend Films

Quantitatively determining the 3D arrangement of guest molecules in a guest:host emissive layer remains a challenge, particularly for beam-sensitive organic materials. By extracting 3D coordinates from depth-sectioned HAADF-STEM stacks of Ir(ppy)2(acac):TCTA and Ir(ppy)3:TCTA vacuum-deposited guest:host blend films, we enable direct access to molecular-scale morphology in beam-sensitive materials. We find that guest aggregation is significantly stronger than expected from random distributions, with the difference diminishing only at high concentrations. Coupling these morphologies to master equation transport modeling reveals that aggregation alters transport pathways and reduces mobility. Additionally, thermal motion of guest molecules between the image frames provides an estimate of molecular diffusion, linking structural and dynamic behavior.