Morphology of OLED Film Stacks Containing Solution-Processed Phosphorescent Dendrimers

Abstract

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(N-phenylbenzimidazol-2-yl)benzene (where the host was deuterated 4,4'-N,N'-di(carbazolyl)biphenyl or tris(4-carbazol-9-ylphenyl)amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris(N-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 T of the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris(N-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.

Diffusion in Organic Film Stacks Containing Solution-Processed Phosphorescent Poly(dendrimer) Dopants

“…This continued until the film temperature reached 119 °C, at which point a distinct intermediate layer (IML) with SLD between those of the light-emitting and TPBi layers was observed. This is similar to the behavior that has been previously reported for films containing solution-processed light-emitting dendrimer blends …”

“…The film exhibited no significant changes in luminescence up to 130 °C and then began to show a decrease in PD1 emission intensity at around 140 °C, which was the temperature at which a significant decrease in SLD was observed. This is similar to that reported for a solution-processed dendrimer: d -CBP blend film deposited on PEDOT:PSS, where a decrease in PL intensity was observed on thermal annealing . The decrease in PL could be due to PEDOT:PSS acting as a quencher or protons from the PEDOT:PSS layer diffusing into the emissive layer causing degradation of the iridium­(III) complexes.…”

“…It is also possible that the higher SLD layer at the air interface is due to a change in density, with a higher density crust forming on top of the film. While it is not possible to determine with certainty which of these possibilities is responsible for the SLD gradient, it is clear that there is a gradient, which is different from that reported for solution-processed dendrimer films, in which a uniform distribution was observed . The fact that vertical phase separation has been observed for the poly­(dendrimer) containing film is not unprecedented as vertical phase separation of small molecule:polymer blends has been previously reported in the literature. − For example, Shin et al .…”

“…We have also found that the as-formed multilayer films deposited by evaporation have well-defined interfaces, but under thermal stress, the layers begin to intermix once the glass transition temperature ( T g ) of the material in a layer has been exceeded. , Similarly, for thin film stacks containing solution-processed light-emitting dendrimers, we observed that, above the T g of the host:dendrimer layer, the layer behaved as a supercooled liquid into which the adjacent charge transporting layer dissolved . The time dependencies of these processes were also studied, and it was observed that initial rapid diffusion occurred followed by a quasi-stable state, when the temperature was held at a point just above that at which the diffusion process began …”

Solubility, Solubility Modeling, and Antisolvent Precipitation of 1,3-Bis(9-carbazolyl)benzene in Organic Solvents

“…Organic light-emitting diodes (OLEDs) are now widely used in the fields of display and illumination and are rapidly developing in the direction of flexibility, low cost, industrialization, and controllable size. − For the mainstream demand, solution-processed technology is being paid attention to, such as screen printing, inkjet printing, , spin coating, and contact printing . The solubility of the OLED functional layer materials in organic solvents is an important parameter of the solution-processed technology. − Most of the conversional OLED materials are poorly soluble in organic solvents, which are difficult to be formulated as inks for printing. According to the previous research in nanoscience and nanotechnology, the organic particles with reduced size in nanoscale usually exhibit unique thermodynamic and kinetic characteristics that the bulk materials display. − In particular, the solubility of organic particles can be enhanced when the particle size decreased on the basis of the Ostwald–Freundlich equation .…”