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 ln{μ} is proportional to Eⁿ(T^-1-T₀^-1), where n=0.5 and T₀ 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.