Charge carrier mobility is at the center of organic electronic devices. The strong couplings between electrons and nuclear motions lead to complexities in theoretical description of charge transport, which pose a major challenge for the fundamental understanding and computational design of transport organic materials. This tutorial review describes recent progresses in developing computational tools to assess the carrier mobility in organic molecular semiconductors at the first-principles level. Some rational molecular design strategies for high mobility organic materials are outlined.

The mechanism of charge transport in organic materials is still controversial from both experimental and theoretical perspectives. At room temperature, molecular deformations interact strongly with the charge carrier both through intermolecular and intramolecular phonons, suggesting a thermally activated hopping mechanism as described by the Marcus electron transfer theory. However, several experimental measurements have indicated that the electronic transport behaves in a "bandlike" manner, as indicated by a decrease in mobility with increasing temperature, in contradiction to the Marcus description. Bandlike first-principles calculations based on the Holstein-Peierls model tend to overestimate the charge mobility by about 2 orders of magnitude. Here, a hopping model is derived that not only quantitatively describes the charge mobility but also explains the observed bandlike behavior. This model uses the quantum version of charge-transfer theory coupled with a random-walk simulation of charge diffusion. The results bridge the gap between the two extreme mechanisms. This first-principles method predicts the room-temperature hole mobilities to be 2.4, 2.0, and 0.67 cm(2)/V s, for rubrene, pentacene, and tetracene, respectively, in good agreement with experiment