Charge transport transient photocurrent measurements

The photoconductivity in polyacetylene, the simplest conjugated polymer, has been the subject of intense investigations [103 106]. Transient photoconductivity measurements on a picosecond time scale have been carried out [107-112]. These ultrafast methods are a powerful tools to investigate the transport properties as well as the recombination kinetics of charged excitations. It was found [107] that the photocurrent in trans-polyacetylene consists of two components a fast component which relaxes on a picosecond time scale and for which a carrier mobility of about 1 cm V s was reported [110,111] and a slow component with earner lifetimes up to seconds. 

Nevertheless, we were not able to carry out TOP measurements with calamitic monomers, because the dark currents in the liquid crystalline phases were too high. As mentioned before, such measurements are possible with the cyclic tetramers. The transient photocurrents of the tetrasiloxanes 19, 21, 22 illustrate that the carrier transport is totally dispersive, i. e. dominated by deep traps in which the charge carriers are captured. This is a typical behaviour of amorphous polymers. No transit time could be detected and no statements about the carrier mobility can be made for the rod-like mesogens. 

Figure 11-7. The structure of homeotropically aligned columnar phase (left) an ideal transient photocurrent in a TOF measurement in a trap-free sample (right). The fast mode is electronic conduction including electron and hole mobilities the slow mode is ionic charge transport...

As described in the previous section, both electronic and ionic conductions take place in mesophases when the material contains trace amounts of chemical impurities. These types of conduction exhibit different charge carrier transport properties, as demonstrated by the transient photocurrents the different mesophases measured under the same conditions as shown in Fig. 2.10. In these transient photocurrents, the fast transit times are shifted to shorter times as the molecular order in the mesophases is increased from SmA to SmE and from Coin to the plastic phase, while the slow transits stay in the same time range of 1,000 (xs irrespective of the mesophase in both smectics and discotics. Figiue 2.11 shows Arrhenius plots of the mobilities for fast and slow transits of the 2-phenylnaphthalene derivative 8PNP-012. The mobility for the fast transit hardly depends on the temperature, while the mobility for the slow transit does depend on temperature, with an activation energy... 

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