Fast Ambipolar Carrier Transport and Easy Homeotropic Alignment in a Metal-Free Phthalocyanine Derivative

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We have investigated charge carrier transport in the various phases of the metal-free phthalocyanine derivative, 1,4,8,11,15,18,22,25-octaoctylphthalocyanine (8H2Pc), by the time-of-flight method. The discotic columnar phase of 8H2Pc easily self-organizes in columns perpendicular to the substrate when cooled from the isotropic phase. We found that well-defined transits were observed, not only for positive carriers, but also for negative carriers. For the columnar rectangular phase at 85°C the hole and electron mobilities were very high at 0.2 and 0.3 cm2/(V$\cdot$s), respectively. These mobilities depend on neither electric field nor temperature. This ambipolar carrier transport was easily observed even in an ambient atmosphere. Interestingly, the mobility in the isotropic phase was of the order of $10^{-3}$ cm2/(V$\cdot$s) and did not depend on temperature, which suggests that the electronic conduction is governed by charge carrier transport in that phase as well.
2005-10-10