Flickering polarons extending over ten nanometers mediate charge transport in high-mobility organic crystals

Samuele Giannini, Orestis George Ziogos, Antoine Carof, Matthew Ellis, Jochen Blumberger

Since the invention of quantum mechanics physicists have come to terms with the intuitive fact that electrons can behave either as waves or particles.  Electric conductivity of materials is a good example.  In pure metals conductivity is a manifestation of the wave-nature of electrons whereas in many defective or doped oxides it is due to the particle-nature of electrons - bouncing off one atom and hitting the next like a billiard ball.  Intriguingly, experimental evidence suggests that in certain semiconducting materials made of organic molecules neither picture applies. 

In a recent study from Jochen Blumberger's group, published in Advanced Theory and Simulation, the authors found that in these materials electrons form "flickering" polarons, objects that are "half way" between waves and particles.  They are delocalised over up to 10-20 molecules in the most conductive organic crystals and constantly change their shape and extension under the influence of the thermal motion of the atoms.

The flickering polarons propagate through the crystal by diffusive jumps over several lattice spacings at a time during which they expand to more than twice their average size.  This unexpected result, obtained by advanced non-adiabatic molecular dynamics simulation developed in Jochen's group, is backed up by experimental charge mobility measurements and electron paramagnetic resonance data.

The new physical picture addresses a long-standing question regarding the nature of charge carriers in crystalline organic semiconductors and fills the gap left by traditional transports models.  Jochen's group is now exploring how flickering polarons can be made larger and more wave-like, which would further increase the conductivity of these materials and enable new applications in flexible electronics.



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