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Title: Conducting liquid crystals
Author: Nanan, Suwat
ISNI:       0000 0004 2674 8471
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2009
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In this thesis, the physical properties of disco tic liquid crystals have firstly been studied but the main focus concerns an investigation of photoconduction properties of liquid crystals using the Time-ofFlight (ToF) method. Photoconduction ofHAT6, HAT6-NOz and 8H2Pc have been reinvestigated. These materials show good ToF signals and the transit times can obviously be determined. The mobility of each compound agrees well with that reported in the literature. HAT6 shows temperature independence of hole mobility over the entire mesophase range. In contrast, the hole mobility of HAT6-N02 increases with increasing temperature and this supports the hopping conduction model. Interestingly, 8H2Pc shows very short transit time and provides a high hole mobility value (10-' cm2y-'s'). The hole mobilities of HHTT in the Colh phase determined from various systems are in reasonable agreement with those reported in the literature. Increasing the temperature results in decreasing of the hole mobility. This suggests a band conduction mechanism. The laser power strongly influences both the shape and the transit time of the ToF signals obtained from HHTT. This behaviour is ascribed to a space charge effect and measured signals agree qualitatively well with those simulations previously run by Leon Lever using a Monte Carlo method. HTTT shows an Onsager-type mechanism of photocharge generation. At low light intensities the quantum yield is in the range of 10-4. The ToF photoconduction ofH7T shows good transit shapes and a hole mobility of 2.5xl0's cm2V's' was found. The addition of an extra hexyloxy (C6H130) group into the HAT6 structure results in a decrease of the transition temperature and a lowering of the el~ctronic conduction. The ToF photoconduction of quinone-HAT6 shows good ToF signals having the transit times in the range of hundreds of microsecond. A hole mobility value of 8.1 x 1 0-scm2V-' s-' was obtained. Chare carrier transport of Octa(5-methylhexyl)phthalocyanine has been investigated by the ToF method and it shows good ToF signals. A very high hole mobility value of 0:14 cm2V-'s-' was obtained. This value is slightly higher than that observed in octaoctylphthalocyanine (8H2Pc) (10-' cm2V-'s-'). ToF photoconductions ofHABT8 and chloroquinone-HAT6 were attempted. Unfortunately, the loglog plots of the ToF signals obtained from both materials did not show clear transit times. As a result, detennination of the hole mobility over the meso phase range is hard to achieve. A charge injection study of some triphenylene-based discotic liquid crystals has been made. Unfutunately, neither Schottky model nor Bardeen model can be used to explain the barrier heights at the metallDLC interfaces. This suggests .that the alternative model is needed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available