Morphology of OLED Film Stacks Containing Solution-Processed Phosphorescent Dendrimers

Organic light-emitting devices containing solution-processed emissive dendrimers can be highly efficient. The most efficient devices contain a blend of the light-emitting dendrimer in a host and one or more charge-transporting layers. Using neutron reflectometry measurements with in situ photoluminescence, we have investigated the structure of the as-formed film as well as the changes in film structure and dendrimer emission under thermal stress. It was found that the as-formed film stacks comprising poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer/1,3,5-tris­(<i>N</i>-phenylbenzimidazol-2-yl)­benzene (where the host was deuterated 4,4′-<i>N</i>,<i>N</i>′-di­(carbazolyl)­biphenyl or tris­(4-carbazol-9-ylphenyl)­amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris­(<i>N</i>-phenylbenzimidazol-2-yl)­benzene evaporated) had well-defined interfaces, indicating good wetting of each of the layers by the subsequently deposited layer. Upon thermal annealing, there was no change in the poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer interface, but once the temperature reached above the <i>T</i><sub>g</sub> of the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris­(<i>N</i>-phenylbenzimidazol-2-yl)­benzene dissolved. When the film stacks were held at a temperature just above the onset of the diffusion process, they underwent an initial relatively fast diffusion process before reaching a quasi-stable state at that temperature.