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

Abstract

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 $\ensuremath{\sim}1$ 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.