Proceedings of 13th Conference on Hybrid and Organic Photovoltaics (HOPV21)
Publication date: 11th May 2021
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. I this talk, I will report the development of 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 both conduction and charge separation in organic photovoltaics.
dKMC shows that the fundamental physics of transport in moderately disordered materials is that of charges hopping between partially delocalised electronic states [1]. Our 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 is able to treat the intermediate transport regime, between band conduction 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 the underestimation by conventional hopping models [1].
I will also report recent work on using dKMC to resolve the question of how charges in organic photovoltaics are able to overcome their significant Coulombic attraction and separate efficiently. The low dielectric constants in organic semiconductors produce Coulombic attractions an order of magnitude greater than the available thermal energy. Delocalisation has been suggested as an explanation, because it could increase the initial separation of charges in the CT state. By applying dKMC to the problem, we overcame the considerable computational hurdle of tracking the correlated quantum-mechanical motion of two delocalised particles. We find that even small amounts of delocalisation, across less than two molecules, can produce large enhancements in the efficiency at which charges separate, even if they start out in a thermalised CT state. Importantly, these delocalisation enhancements are a kinetic effect, rather than the common hypothesis that delocalisation increases efficiency by reducing the Coulomb attraction.
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International Conference on Hybrid and Organic Photovoltaics
International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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