Transient photocurrents in a charge-transfer complex of trinitrofluorenone with a carbazole substitutes liquid crystal

We describe transient photoconduction studies on it well known triphenylene discotic liquid crystal molecule modified to incorporate a single carbazole moiety on one side chain and doped with the acceptor, trinitrofluorenone. This material is found to show both electron and hole photocurrents when the transient photoconduction study is carried out in the time of flight geometry although no time of flight transits are observed. Of interest here is the fact that the temperature dependence of the hole and electron photocurrents are radically different, the electron photocurrent being strongly activated while the hole photocurrent decreases with increasing temperature. We suggest a mechanism whereby the electron photocurrent controls the hole photocurrent through recombination. Furthermore, the electron activation energy varies with the applied electric field, indicating the influence of the Poole Frenkel barrier lowering mechanism in the production of the charge carriers. (c) 2005 Elsevier B.V. All rights reserved.