Most models of charge transport in disordered organic semiconductors assume the carriers are localised to individual molecules and move by hopping from one to another. Here I introduce delocalised kinetic Monte Carlo (dKMC), the first three-dimensional treatment of partially delocalised carrier motion in disordered materials, showing the critical role delocalisation plays in charge transport [1], charge separation [2], exciton motion [3], and exciton dissociation [4].

dKMC 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, noise, and polaron formation. As a result, it can treat the intermediate transport regime, between band and hopping conduction. Applying dKMC to carrier transport reveals that even a small amount of delocalisation can increase carrier mobilities by an order of magnitude, explaining why it is often underestimated by conventional hopping models [1].

Secondly, dKMC resolves how charges in organic photovoltaics overcome their significant Coulombic attraction (an order of magnitude greater than the available thermal energy) and separate efficiently. Even small amounts of delocalisation can produce large enhancements in the efficiency at which charges separate, even if they start out in a thermalised CT state [2]. Importantly, delocalisation does not enhance efficiency by the commonly assumed mechanism of reducing the Coulombic attraction; instead, the enhancement is a kinetic effect produced by the increased overlap of electronic states [2].

Thirdly, applying dKMC to exciton motion shows that even a small amount of delocalisation can increase the exciton diffusion constant by over two orders of magnitude [3]. The mechanism for the enhancement is twofold: delocalisation enables excitons both to hop faster and further in each hop [3]. We also quantify the effect of transient delocalisation, and show it depends strongly on the strength of disorder and dipole moments [3].

Finally, I will also report recent work on using dKMC to study how excitons dissociate in organic photovoltaics. dKMC reveals that even small amounts of delocalisation can boost exciton dissociation efficiencies by over an order of magnitude. The mechanism of these delocalisation enhancements is surprising, explaining important fundamental physics about the role of delocalisation and interfaces in efficient dissociation.