Single-layer method for quantifying the triplet exciton diffusion coefficient in disordered organic semiconductor materials

We propose an easy-to-use method for determining the triplet exciton diffusivity in disordered organic semiconductors using single-layer films containing a small concentration of Ir-cored guest molecules. The carefully selected guest molecules are used to create in a time-resolved photoluminescence (PL) experiment triplet excitons that are initially located on the guest molecules and that as a result of weak triplet confinement subsequently diffuse through the host material, thereby enhancing the PL efficiency loss resulting from triplet-triplet annihilation (TTA). The diffusion coefficient is obtained using a simple analytic model that takes all contributions to the enhanced TTA-rate into account. The model is validated using Kinetic Monte Carlo simulations. We demonstrate the method by application to four materials that are frequently used as a host in the emissive layer of organic light-emitting diodes. By combining these materials with chemically different emitter molecules, the consistency of the method is shown. The observed strong material dependence of the diffusion coefficient is explained qualitatively from the triplet exciton transition density distributions on the molecules studied that are obtained from Density Functional Theory calculations.