Mechanism of delocalisation-enhanced exciton transport in disordered organic semiconductors

Abstract

Large exciton diffusion lengths generally improve the performance of organic semiconductor devices, since they enable energy to be transported farther during the exciton lifetime. However, the physics of exciton motion in disordered organic materials is not fully understood, and modelling the transport of quantum-mechanically delocalised excitons in disordered organic semiconductors is a computational challenge. Here, we describe delocalised kinetic Monte Carlo (dKMC), the first model of three-dimensional exciton transport in organic semiconductors that includes delocalisation, disorder, and polaron formation. We find that delocalisation can dramatically increase exciton transport; for example, delocalisation across less than two molecules in each direction can increase the exciton diffusion coefficient by over an order of magnitude. The mechanism for the enhancement is twofold: delocalisation enables excitons both to hop more frequently and further in each hop. We also quantify the effect of transient delocalisation (short-lived periods where excitons become highly delocalised), and show it depends strongly on the disorder and the transition dipole moments.