Proceedings Article | 30 August 2005
Proc. SPIE. 5940, Organic Field-Effect Transistors IV
KEYWORDS: Semiconductors, Crystals, Ions, Bioalcohols, Liquid crystals, Electron transport, Organic semiconductors, Group IV semiconductors, Temperature metrology, Liquids
We have re-investigated the negative charge carrier transport in discotic columnar phases of triphenylene derivatives and a phthalocyanine derivative by time-of-flight method in order to clarify the intrinsic nature of charge carrier transport in discotic liquid crystals. In a purified hexabutyloxytriphenylene (H4T), in which the fast hole transport was discovered previous reports, the transient photocurrents for negative carriers showed two transits in different time ranges, which were correspond to electron and ionic transports, respectively. The fast mobility corresponded to 10<sup>-2</sup> cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> comparable to the hole mobility reported previously. The fast electron transports were observed in purified hexahexyloxytriphenylene (H6T), hexapentyloxytriphenylene (H5T), and hexahexylthiotriphenylene (HHTT) as well, and the electron mobilities in these materials were 10<sup>-4</sup>, 10<sup>-3</sup> and 10<sup>-1</sup> cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup>, respectively. Furthermore, even in a phthalocyanine derivative that is well known as a typical p-type organic semiconductor, i.e., octaoctylphthalocyanine (8H<sub>2</sub>Pc), a high electron mobility of 0.3 cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> was established, while the highest bulk hole mobility of 0.2 cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> was reported recently.
Therefore, we conclude that the slow negative charge carrier transport reported in discotic liquid crystals previously originates from impurity-induced ionic transport, and that it is very likely for the intrinsic charge carrier transport in liquid crystalline semiconductors to be electronic and ambipolar, while it is very sensitive to the purity.
