Mechanistic description of the efficiency loss in organic phosphorescent host-guest systems due to triplet-polaron quenching

In this study we demonstrate how a mechanistic description can be obtained of the interplay of all processes that give rise to the efficiency loss due to triplet polaron quenching (TPQ) in phosphorescent host-guest systems such as used in organic light-emitting diodes (OLEDs). We study unipolar devices with an emissive layer consisting of m-MTDATA:Ir(ppy)(2)(acac), in which excitons on the phosphorescent Ir(ppy)(2)(acac) molecules are quenched by holes on the m-MTDATA host. The final TPQ-process is disentangled from all other relevant processes, such as polaron and exciton diffusion and field-induced exciton dissociation, by carrying out a combination of electrical, photoluminescence (PL) and field-induced dissociation experiments. The analysis is supported by carrying out kinetic Monte Carlo simulations. We find that a conventional approach, within which the loss is phenomenologically quantified using a rate coefficient, cannot consistently describe all experimental results. For a wide temperature range a fair mechanistic description of all results is obtained when using a TPQ Forster radius of 3.8 nm and a triplet exciton binding energy of 0.9 eV.