Exciton transport in amorphous polymers and the role of morphology and thermalisation

Abstract

Understanding the transport mechanism of electronic excitations in conjugated polymers is key to advancing organic optoelectronic applications, such as solar cells, organic light-emitting diodes and flexible electronics. While crystalline polymers can be studied using solid-state techniques based on lattice periodicity, the characterisation of amorphous polymers is hindered by an intermediate regime of disorder and the associated lack of symmetries. To overcome these hurdles we have developed a reduced state quantum master equation approach based on the Merrifield exciton formalism. This new approach allows us to study the dynamics of excitons’ centre of mass and charge separation (CS), going beyond the standard model of charge-neutral Frenkel excitons. Using this model we study exciton transport in conjugated polymers and its dependence on morphology and temperature. Exciton dynamics consists of a thermalisation process, whose features depend on the relative strength of thermal energy, electronic couplings and disorder, resulting in remarkably different transport regimes. By applying this method to representative systems based on poly(p-phenylene vinylene) (PPV) we obtain insight into the role of temperature and disorder on localisation, CS, non-equilibrium dynamics, and experimental accessibility of thermal equilibrium states of excitons in amorphous polymers.