New hole drift mobility measurements for the molecularly doped polymer DEH (p-diethylaminobenzaldehyde-diphenyl hydrazone) in polycarbonate and literature data for TPD (N,N'-diphenyl-N,N'-bis( 3-methylphenyl)-[ 1,l '-biphenyll-4,4'-diamine) in polycarbonate are analyzed by a new procedure which separates the functional dependence of the mobility on temperature and molecular spacing p. Our analysis indicates that the dependence of the mobility on p arises from different sources. In DEH: polycarbonate it is due to an overlap integral, as normally expected. In TPD: polycarbonate it is due entirely to the dependence of the activation energy on p, a significant clue to the underlying microscopic hole hopping mechanism. These data are consistent with the small polaron hopping theory in two regimes, non-adiabatic hopping in DEH:polycarbonate and adiabatic hopping in TPD : polycarbonate.
Charge transport in amorphous organic systems is interesting to study because mobility data raise questions about our understanding of simple hopping models. Molecularly doped polymers are ideal model organic systems because the critical variables that should control the hopping mechanism can be varied over wide ranges. These include the spacing between molecules p, the electric field E (fields as high as 100 V/urn are typically achieved), and temperature T. The form of the mobility p is predicted by all theoretical analyses to be [ l-3]