Förster-type triplet-polaron quenching in disordered organic semiconductors

Abstract

Triplet-polaron quenching (TPQ) is a major cause of the efficiency loss at large current densities in phosphorescent organic light-emitting diodes. The nature of the interaction process is presently not well understood. In this paper, we study TPQ due to Förster-type triplet-polaron interactions in energetically disordered organic semiconductors with a Gaussian polaron density of states. A continuum theory, which neglects the spatial inhomogeneity and energetic disorder, is from a kinetic Monte Carlo approach shown to correctly predict that the effective steady-state TPQ rate coefficient k TPQ,eff depends only sensitively on the polaron diffusion in a rather narrow range of diffusion coefficients. However, in this regime, significant discrepancies between the two approaches are found, in particular for realistic values of the TPQ-Förster radius, around 3 nm, and for systems with strong energetic disorder. Both approaches show that k TPQ,eff is not constant but can depend on the polaron density and the electric field. Various methods for deducing the TPQ mechanism from experiment are discussed, including an approach which utilizes the shape of the time-dependent photoluminescence after pulsed illumination.

Sub–turn-on exciton quenching due to molecular orientation and polarization in organic light-emitting devices

“…This view is also supported by kinetic Monte Carlo simulations, which found that holes in a CBP:Ir(ppy) 3 device spend >95% of the time on Ir(ppy) 3 molecules ( 42 ). The typical Förster radius for energy transfer from a phosphorescent emitter to its cation has been estimated at 2 to 3 nm ( 40 ), similar to the hole distribution width extracted here. If Förster-type EPQ is active here, then a narrower charge distribution could be consistent with our results.…”

“…It is also possible that the distributed charge density predicted by our model is an artifact of its simplified description of EPQ. EPQ may occur either by nearest-neighbor interactions or by long-range Förster transfer (6,40), which requires overlap between the emission spectrum of the emitter and the absorption spectrum of the cation. Here, the holes responsible for quenching most likely reside on Ir(ppy) 3 molecules, as the Ir(ppy) 3 + cation absorbs near 600 nm (41) and Ir(ppy) 3 has a shallower highest occupied molecular orbital energy than CBP, TPBi, and 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), leading to hole trapping.…”

Effect of exciton diffusion on the triplet-triplet annihilation rate in organic semiconductor host-guest systems

“…Similarly, the loss due to TPQ is expressed using the expression dT /dt = −k TPQ nT , with n the polaron volume density. From KMC simulations, we have shown that these descriptions can indeed provide a proper description of the local rate, but not under all conditions [12][13][14]. For Förster-type TTA, k TT is not a constant but increases at high triplet densities [12].…”

P‐184: Distinguished Poster: Coupled 3D Master Equation and 1D Drift‐Diffusion Approach for Advanced OLED Modeling

“…Note that as an approximation polaron quenching as well as annihilation processes are modeled purely locally. Alternatively, these processes can also be modeled using long-range Förster type interactions [10,11].…”