Dependence of Organic Interlayer Diffusion on Glass-Transition Temperature in OLEDs

Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (T_g) of the material with the lowest T_g. Diffusion occurred when the material with the lowest T_g formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative T_g's. If the supercooled liquid formed at a temperature significantly lower than the T_g of the higher-T_g material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two T_g's were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the "burn in" often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes.