In this project we study the dynamics of excitons and interplay between different exciton species as well as between excitons and charge carriers, both in the bulk of individual layers (intra-molecular) and at interfaces of multilayer organic semiconductor devices (inter-molecular). We will extend the established 1D drift-diffusion approach [1] and combine it with a novel 3D Master equation model [2] as well as compare it with 0D analytical formulas.

In a first part of this project we are using advanced simulations to better understand the influence of different exciton quenching processes, such as triplet-polaron quenching (TPQ) and triplet-triplet annihilation (TTA), on the efficiency of an OLED device for different emitter concentrations.

Figure 2 shows the excited states lifetime τ* data (directly proportional to the efficiency of an OLED) of a green phosphorescent OLED stack with an emitter concentration of 2% and 16%. From fitting of the data with 0D analytical formulas the underlying exciton quenching mechanism for the reduction of the excited states lifetime at increasing current densities is either TPQ or TTA.

With the combination of 3D Master equation and 1D drift-diffusion approach it should be possible to set up an electro-optical model that is able to reproduce both cases with one set of parameters and thus allows us to understand the exciton processes in this OLED better.

In the end it should be possible to predict the optimum emitter concentration for the highest efficiency and the lowest efficiency roll-off at high current densities.