Modeling and simulation of electronic and excitonic emission properties in organic host-guest systems

We propose electronic and excitonic emission models for the quantitative simulation of organic light emitting diodes comprising host-guest system in emission layer. The electronic model contains the thermionic injection at the electrodes, the charge transport with the field- or doping-dependent mobility, and Langevin's recombination. The excitonic emission model includes radiative (or nonradiative) decay, exciton diffusion, singlet-triplet intersystem-crossing, exciton annihilation, and polaron quenching. Forster and Dexter energy transfers as well as exciton blocking are also added to the excitonic emission model for the simulation of phosphorescent host-guest system. When these models are applied to a series of tris(2-phenylpyridine) iridium (Ir(ppy)(3))-doped 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) devices with various doping concentration, the simulated electronic and excitonic emission properties are in good agreement with experimental results. The real devices show gentle efficiency roll-off at high Ir(ppy)(3) concentration and current efficiency reversion between 10% and 15% doping concentrations at over 20 mA cm (2), which are successfully realized by the proposed models in which triplet-triplet annihilation in host sites is considered. It is also numerically demonstrated that exciton confinement in emitting la yer by controlling the energy, lifetime, and diffusion of exciton results in improvement of luminance efficiency of the devices. (C) 2009 Elsevier B.V. All rights reserved.