Impact of thermal transport parameters on the operating temperature of OLEDs

Excess heat in organic light emitting diodes (OLEDs) that is produced during their operation may accelerate their degradation and may cause an inhomogeneous brightness distribution, in particular in large area OLEDs. Assessing the quantitative impact of heat excess is difficult, because all decisive processes related to charge transport and emission via charge recombination are thermally activated. For example, electric currents that are elevated due to larger temperatures cause additional Joule heating and increase, hence, the device temperature even further. Here we establish how parameters responsible for heat transport, i.e., the thermal conductivity of the organic layers and the heat transfer coefficient between the device surface and the environment govern the temperature inside the OLED. We establish, relying on three-dimensional drift-diffusion simulations that self-consistently couple thermally-activated charge transport and heat transport, that the thermal conductivity of organic layers is not a bottleneck for heat transport, because the encountered layer thicknesses in realistic device geometries prevent heat accumulation. The heat transfer to the ambient environment is the key parameter to dissipate excess heat from the device. Intentionally elevated operating temperatures, that may improve the OLEDs electric performance, are not necessarily beneficial, as any increase in operating temperature decreases the device stability. The thermal effects being decisive for the OLED temperature occur in device layers beyond the electrically active region. We propose analytical expressions that relate the temperature in the device for a given point of operation to the heat transfer to the environment and the substrate.
Comment: 19 pages, 5 figures