Charge Mobilities in Organic Semiconducting Materials Determined by Pulse-Radiolysis TimeResolved Microwave Conductivity: (pi)-Bond-Conjugated Polymers versus (pi)-(pi)-Stacked Discotics

A review is given of the one-dimensional, intrachain and intracolumnar, charge mobilities, (sigma)(mu)1D, determined for (pi)-bond-conjugated polymeric and for (pi)-(pi)stacked columnar discotic materials using the pulse-radiolysis time-resolved microwave conductivity technique. The largest values, on the order of 10 cm^2/(V s), are found for single-crystal polydiacetylenes polymerized either thermally or with low doses of radiation. Much lower values of (sigma)(mu)1D, covering the range from 0.009 to 0.125 cm^2/(V s), are found for solution-synthesized conjugated polymers for which six different backbone structures have been investigated. This is attributed mainly to their complex morphology and the resulting static disorder in the backbone structure. The highest mobilities for this class of material, ca. 0.1 cm^2/(V s), are found for liquid crystalline derivatives of polyfluorene and poly(phenylenevinylene). Larger mobilities are found for discotic materials, with maximum values close to 1 cm^2/(V s) in both the crystalline solid and liquid crystalline phases. This is attributed to their self-organizing nature and hence higher degree of structural order, which compensates for the weaker electronic coupling between monomeric units in the discotics compared with the covalently bonded conjugated polymers.