Hole mobilities in hydrazone-polycarbonate dispersions

Extensive characterization of the hole mobilities μ in dispersions of p-diethylamino- benzaldehyde-diphenyl hydrazone (DEH) in polycarbonate has been carried out. We report the effect of varying the electric field E, temperature T, and spacing between DEH molecules ρ on μ. These data are analyzed by a procedure that allows proper separation of the functional dependencies of the mobility on E, T, and ρ. It is found that lnμ is proportional to $E^n$($T^{\mathrm{-}1}$-$T_0^{\mathrm{-}1}$), where n=0.5 and $T_0$ is a fitted parameter which decreases with increasing ρ, behavior opposite to the dependence of the glass transition temperature on ρ. These experimental results are not yet understood theoretically. Our procedure for separating the ρ and T dependence is applied to data taken on DEH-polycarbonate and to data taken from the literature on another molecularly doped polymer system, N,N’-diphenyl-N,N’-bis (3-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine (TPD) in polycarbonate. For DEH-polycarbonate, the activation energy is found to be independent of ρ. In contrast, for TPD-polycarbonate the activation energy is strongly dependent on ρ. Our data suggest that small-polaron hopping is occurring in molecularly doped polymers; this different dependence of the activation energy on ρ is consistent with different small-polaron hopping regimes, adiabatic and nonadiabatic, in these two systems.