The decisive role of delocalisation in organic electronics

Abstract

The movement of charge and energy is one of the most fundamental processes in materials science, underpinning technologies such as solar cells, light-emitting diodes, batteries, and electronics. Transport is well understood in both highly ordered materials (band conduction) and highly disordered ones (hopping conduction). However, in moderately disordered materials—including many organic semiconductors—transport lies between these two well understood extremes in the intermediate regime, where the movement of carriers is not well understood. Accurately modelling conduction in this intermediate regime is difficult because describing wavefunction delocalisation requires a fully quantum-mechanical treatment, which is challenging in disordered materials that lack periodicity. Therefore, most models of transport in disordered organic semiconductors assume the carriers are localised to individual molecules and move by hopping from one to another. In developing dKMC, this thesis shows that the fundamental physics of transport in moderately disordered materials is that of charges hopping between partially-delocalised electronic states. The approach is the first to treat, in three dimensions, all the processes crucial in organic semiconductors: disorder, delocalisation, and polaron formation. As a result, it can treat the intermediate transport regime between band and hopping conduction. dKMC reveals that delocalisation, an often-neglected quantum effect, plays a crucial role in improving all of the fundamental processes in organic photovoltaics. The mechanism by which delocalisation improves all of these fundamental processes is essentially the same: by enabling polaron states to hop further and faster. Therefore, dKMC reveals the importance of including delocalisation in future models of charge transport in organic electronics, as well as designing future materials and devices that can facilitate carrier delocalisation.