Dynamics of exciton-exciton annihilation

Frenkel-excitons are the basic neutral electronic excitations in organic systems. Their behavior is crucial for the performance of photonic devices based on organic materials. Already at moderate excitation densities the interaction between the excitons has a strong impact on their dynamics. In particular it results in annihilation events which reduce the exciton lifetime. The interaction results from Coulomb coupling and the annihilation process exhibits similarities to interatomic Coulombic decay. We investigate by ultrafast absorption spectroscopy the exciton dynamics in an organic model system which consists of dye molecules as active sites and a polymer matrix as host. The dipole approximation and transition rates based on Förster theory are applied to simulate the migration of the excitons as well as their interaction. It turns out that energetic disorder has a strong influence on the dynamics and has to be taken appropriately into account. To this end a formulation for the Förster rate in the presence of inhomogeneous broadening is developed. Since for the investigated system the relevant transition strengths and densities of states can be characterized by spectroscopic means a parameter free test of the applied theory is possible.