Proceedings of nanoGe Spring Meeting 2022 (NSM22)
DOI: https://doi.org/10.29363/nanoge.nsm.2022.141
Publication date: 7th February 2022
The transport of charges and excitons is well understood in two extremes: in highly ordered materials, transport is by band conduction, while in highly disordered ones, it is by hopping. Many organic semiconductors fall in the intermediate regime between band transport and hopping, making either set of approximations inaccurate. In particular, intermolecular couplings mean that there is usually some delocalisation across multiple molecules (or segments of polymers), while disorder ensures that this effect is spatially limited. Theoretically describing the movement of partially delocalised carriers and excitons is difficult, because it depends on a complicated interplay of energetic disorder, quantum-mechanical couplings, and polaron formation.
We report delocalised kinetic Monte Carlo (dKMC), a new computational method that is able to describe the motion of partially delocalised charges and excitons in all regimes of disorder [1]. We implement numerical innovations that allow us to work in three dimensions, a regime that had proven too complicated for all comparable approaches. dKMC reveals new, basic physics of transport in organic semiconductors and explain why mobilities predicted by traditional kinetic Monte Carlo are usually too low. In particular, delocalisation over just a few molecules can increase mobilities by orders of magnitude.
We also extend dKMC to describe charge separation at a heterojunction [2], the first approach to do so that includes all the necessary ingredients: delocalisation, disorder, and polaron formation. We show that delocalisation can play a decisive role in charge separation, with even modest delocalisation able to double internal quantum efficiencies.
[1] Balzer, Smolders, Blyth, Hood, and Kassal, Chem. Sci. 12, 2276 (2021).
[2] Balzer and Kassal, arXiv:2108.05032 (2021).
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