Separate charge transport pathways determined by the time of flight method in bimodal polytriarylamine

Abstract

Time of flight photocurrent transient studies on thin films of bimodal polytriarylamine (PTAA) show two distinct and separate arrival times for hole transport in the same sample at a single field. The corresponding mobilities differ by two orders of magnitude, typically μfast∼10−3 cm2 V−1 s−1 and μslow∼10−5 cm2 V−1 s−1 at room temperature, and are measured parametric in electric field and temperature. The mobility data are analyzed using the correlated disorder model by Novikov, yielding a fitting parameter set. The two conduction paths are believed to come about as a result of phase segregation between the shorter and longer polymer chains with the shorter chains giving rise to the faster conduction pathways (as confirmed by results obtained for monomodal, shorter, and longer chain PTAA, by sample thickness scaling of the photocurrents and by reversal of the illuminated electrode). Separate arrival times are also obtained in a blend of the two short and long chain monomodal polymers. The phase separation within the film is inferred by the appearance of two glass transition temperatures using dynamic mechanical thermal analysis.