Electron vs. hole transport in organic materials

A general observation is that the mobility for electron transport in organic materials used in electronic and photonic devices can be orders of magnitude smaller than the mobility for hole transport. A rough explanation for this is given in a useful review by Bredas et al. The figure below is the basis for the discussion of how for a pair of adjacent molecules the splitting of HOMO's is larger than that of LUMO's.


This splitting is proportional to the matrix element (t in a Huckel model) between orbitals on different molecules. This is larger for the HOMO's because the wavefunction has fewer nodes than the LUMO does and so the intermolecular overlap is less.
The mobility is proportional to t^2 (see the review by Horowitz) and so is much smaller for electron than hole transport.

However, this is not the end of the story since one also needs to consider effects such as whether impurities in an actual material are more effective at trapping charge carriers for electrons or for holes. Such issues are discussed in this Nature paper concerning n-type organic FET's.