Triplet exciton diffusion in metalorganic phosphorescent host-guest systems from first principles

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 Förster 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 Förster 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.

Three-Dimensional Modeling of Organic Light-Emitting Diodes Containing Molecules with Large Electric Dipole Moments

“…[22], with a Förster radius R F,diff of 1.5 nm, a Dexter prefactor (rate in the limit of zero distance) of k D,0 = 2.5 × 10 10 s −1 , and an exciton wave-function decay length of λ exc = 0.3 nm. For the 6.5 mol% CBP:Ir(ppy) 2 (acac) systems studied, Förster transfer is the most important process [55]. The Förster radius is close to the value of 1.6 nm that follows from a recent quantum-chemical calculation study [40].…”

Kinetic Monte Carlo Study of Triplet-Triplet Annihilation in Conjugated Luminescent Materials

“…For host-guest systems, the Eindhoven group have investigated the diffusion of triplets in detail by using transient PL measurements coupled with KMC simulations, and have analyzed the relative contribution of singlestep Förster-type triplet-triplet interactions and the multistep triplet diffusion processes to the mechanism of TTA in phosphorescent host-guest systems [1,32,[40][41][42][43]. The results from the Eindhoven group stimulated us to consider the contribution of dipole-dipole-coupling-induced long-range Förster rate to the triplet hopping in addition to the short-range Dexter rate [1,[40][41][42].…”

Accurate and fast master equation modeling of triplet-triplet annihilation in organic phosphorescent emission layers including correlations

“…We neglect in this work energetic disorder, so that there is no energy difference between triplets on different guest sites. Explicit calculations for Ir-cored phosphorescent emitters yield an approximately Gaussian triplet energy disorder with a standard deviation of about 0.05 eV [17], which can be considered, at room temperature, as weak disorder. We consider the following processes: (i) Radiative decay of triplets on the guest sites with a rate k r = 1/τ , where τ is the radiative triplet lifetime, which is taken to be equal for all sites.…”

“…For a typical guest concentration c g = 10% the KMC simulations take for a single run with R F,diff = 1.5 nm about a factor 3 longer than the SA calculations, while this factor is an order of magnitude higher for R F,diff = 3.0 nm. For R F,diff = 1.5 nm, which may be considered as a realistic value for isoenergetic transfer between Ir-cored phosphorescent emitter molecules [17], KMC simulations for a typical number of 100 runs take at least two orders of magnitude longer than SA calculations. A further gain of three orders of magnitude can be obtained by making use of the PA, although this can slightly affect the accuracy; see Sec.…”

“…TTA can also occur by Dexter-type processes. However, these processes are only important at very high guest concentrations [17] that are not relevant for efficient phosphorescent OLEDs because of concentration quenching. FIG.…”