Dispersive hole transport in organic photorefractive glasses

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 w ell known time-of-flight technique. We determine the energetic parameters i n terms of the empirical Gill formalism. The characteristic depth of the tr apping sites correlates with the response time of the PR effect in a hologr aphic experiment. Thus, the introduction of deeper traps, i.e., a broadenin g of the density of transport states, leads to faster PR response of our sy stems by spreading the release-time distribution and causing dispersive tra nsport. Consequently not only the absolute value of the mobility but also t he transport mechanism - providing adequate immobilization of the charge ca rriers - influences the dynamics of the grating formation. A detailed analy sis of the transport mechanism confirms the predictions of the stochastic m odel of Scher and Montroll: The trace of the current transients indicates d ispersive transport and the dependence of the transit time on the applied e lectric field and the sample thickness obeys the same nonlinear scaling law . [S0163-1829(99)02147-5].