OLED

Effect of Coulomb correlation on charge transport in disordered organic semiconductors

F. Liu, H. van Eersel, B. Xu, J. G. E. Wilbers, M. P. de Jong, W. G. van der Wiel, P. A. Bobbert, and R. Coehoorn

Phys. Rev. B 96, 205203 (2017)

Effect of polaron diffusion on exciton-polaron quenching in disordered organic semiconductors

R. Coehoorn, L. Zhang,  P.A. Bobbert, and H. van Eersel

Phys. Rev. B 95, 134202 (2017)

Exciton-polaron quenching (EPQ) is a major efficiency loss process in organic optoelectronic devices, in particular at high excitation densities. Within commonly used models, the rate is assumed to be given by the product of the exciton density, the polaron density, and a constant EPQ rate coefficient, which is proportional to the polaron diffusion coefficient and an EPQ capture radius. In this work, we study the effects of polaron diffusion on the EPQ rate in energetically disordered materials with a Gaussian density of states using kinetic Monte Carlo simulations, and show that the effective rate coefficient can depend strongly on the polaron concentration and on the electric field. We furthermore find that under realistic conditions, the effective value of the capture radius can exceed the expected value of $\sim 1$ nm by up to two orders of magnitude. To a first approximation, the simulation results can be understood from macroscopic diffusion theory, adapted at finite electric fields to include the observed “polaron wind” effect. However, for strongly disordered systems we find distinct deviations from that theory, related to the very small time and spatial scales involved in the capture process.

Förster-type triplet-polaron quenching in disordered organic semiconductors

R. Coehoorn, P.A. Bobbert, and H. van Eersel

Phys. Rev. B 96, 184203 (2017)

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 ${\mathrm{k\_}}_{\text{TPQ,eff}}$ 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 ${\mathrm{k\_}}_{\text{TPQ,eff}}$ 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.