  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.
