Hole transport in isopropyl carbazole—polycarbonate mixtures

Abstract

The hole mobility μ has been measured in dispersions of N-isopropyl carbazole in polycarbonate for relative concentrations c between 100% (PVK) and 2%. At sufficiently high temperature (T) and field (F), transport is essentially coherent. For c<40%, μαc ⁿ where n≃4, i.e. transport is non-percolative. For c>10%, μαR ²exp(− 2αR), where R is the average carbazole spacing and α≃0·62× 10⁸cm⁻¹, almost independent of T and F. For c<5% and high T and F, μ becomes substantially greater than predicted by the above relation. At constant T, logμ is proportional to F ^1/2 rather than to F. At constant F, an approximate fit to a relation μα exp [−(T ₀/T)²] is obtained, but systematic deviations suggest an apparently field-dependent activation energy of about 0·35 eV at F = 0. These observations are interpreted in terms of field-enhanced polaron tunnelling, leading, in first approximation, to a Poole-Frenkel-like behaviour of μ.