We present an ab initio computational study of triplet exciton diffusion in four phosphorescent emitters commonly used in organic light-emitting diodes (OLEDs). By kinetic Monte Carlo simulations, triplet diffusion lengths… Click to show full abstract
We present an ab initio computational study of triplet exciton diffusion in four phosphorescent emitters commonly used in organic light-emitting diodes (OLEDs). By kinetic Monte Carlo simulations, triplet diffusion lengths are obtained for these emitters in neat films and as a guest in two different hosts. The triplet transfer rates governing the diffusion contain a transfer integral factor that includes both F\"orster and Dexter contributions and a Franck-Condon weighted density of vibrational states that includes the coupling to all intramolecular vibrations in a fully quantum mechanical way. We find that at guest concentrations around 10 mol% the F\"orster transfer contribution is most important. At larger concentrations of about 30--40 mol% the Dexter contribution becomes dominant. We show that obtaining the triplet transfer rates by the semiclassical Marcus theory yields diffusion lengths that are too short and that using a simple cubic lattice in combination with the often used Miller-Abrahams rates instead of using a real morphology with the ab initio rates leads to an underestimation of the diffusion lengths due to transfers down in energy that are too slow.
               
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