Abstract

The charge-carrier mobility of organic semiconductors is a fundamental
material property and one of the central quantities for the optimization
of device performance in, e.g., organic transistors and solar cells.
In order to investigate the intrinsic fundamental (i.e., not device-specific)
charge-transport phenomena in organic solids, molecular crystals are
ideal candidates because of their high degree of structural order.
Nonetheless, even for such ultrapure organic crystals, the theoretical
and numerical description of the charge transport is a highly nontrivial
task due to the strong coupling between the electronic and
vibronic degrees of freedom.

Here, we present a theory for charge transport in organic crystals
which generalizes Holstein's small polaron model to polarons of
arbitrary size and allows to calculate the carrier mobilities using
ab-initio techniques (density-functional theory). The generalized
mobility expression includes both the coherent band transport as well as
the thermally induced hopping on equal footing. As a prototypical example,
the theory is applied to herringbone-stacked crystals where the
temperature dependence of the mobilities is simulated and compared to
experimental data. Finally, the mobility anisotropy is analyzed by a
novel 3D visualization technique for the relevant transport channels.


[1] K. Hannewald et al., Phys. Rev. B 69, 075211 & 075212 (2004).
[2] K. Hannewald and P.A. Bobbert, Appl. Phys. Lett. 85, 1535 (2004).
[3] F. Ortmann, F. Bechstedt, and K. Hannewald, Phys. Rev. B 79, 235206 (2009).
[4] F. Ortmann, F. Bechstedt, and K. Hannewald, New J. Phys. 12, 023011 (2010).
[5] F. Ortmann, F. Bechstedt, and K. Hannewald, Phys. Stat. Sol. B 248, 511 (2011).