Transient Photoconductivity in Poly( N -vinylcarbazole)

Transport of holes through thin films of poly(N-vinylcarbazole) was studied using transient photoconductivity techniques. The photogeneration depends on the electric field with quantum efficiency of generation approaching a value of 0.1 at an applied electric field of 108 V/m. The motion of carriers cannot be characterized by a drift mobility but can be fully represented by an “effective transit time” given by (Teff)−1 = (mτ0)−1exp(γE1/2 − ε0 / kT), where m = d / d0 (d being the thickness of the film and d0 a characteristic length), τ0 is a characteristic release time from a localized state, E is the applied electric field, ε0 is an activation energy, k is the Boltzmann constant, and T is the absolute temperature. This relationship suggests a model in which holes jump from one localized state to the next one, the “effective transit time” being the sum of release times from the localized states. Bulk trapping occurs at low fields, and the release from these traps depends on both temperature and electric field.