Dispersive hole transport in organic photorefractive glasses

Abstract

Charge transport is a basic process for the photorefractive (PR) effect and has a strong influence on the grating formation speed. We investigate the transient hole transport in three organic low-molecular PR glasses by the well known time-of-flight technique. We determine the energetic parameters in terms of the empirical Gill formalism. The characteristic depth of the trapping sites correlates with the response time of the PR effect in a holographic experiment. Thus, the introduction of deeper traps, i.e., a broadening of the density of transport states, leads to faster PR response of our systems by spreading the release-time distribution and causing dispersive transport. Consequently not only the absolute value of the mobility but also the transport mechanism -- providing adequate immobilization of the charge carriers -- influences the dynamics of the grating formation. A detailed analysis of the transport mechanism confirms the predictions of the stochastic model of Scher and Montroll: The trace of the current transients indicates dispersive transport and the dependence of the transit time on the applied electric field and the sample thickness obeys the same nonlinear scaling law.