Charge carriers, columnar discotics

In general, for side chain liquid-crystalline polymers, macroscopic molecular alignment is not easy and therefore clear evidence of electronic charge carrier transport was confirmed first in liquid crystals with low molecular weight. In the 1990s, fast electronic conduction was verified in discotic columnar phases of triphenylene derivatives [79,80] and hexabenzocoronene derivatives [81,82] as well as smectic phases of 2-phenylbenzothiazole [83, 84] and 2-phenylnaphthalene derivatives [85], as shown in Fig. 14. Carrier... 

A new class of hexabenzocoronene derivatives, e.g. compounds (Scheme 7.13) can self-organize into liquid crystalline phases composed of molecular stacks that orient spontaneously parallel to the surface [177, 178]. Field effect transistors based on these materials show high charge-carrier mobilities, high on/off ratios, and low tum-on voltages. So far compound 44 exhibits the best field effect transistor properties achieved for a columnar discotic material. Polarized fight microscopy revealed that these materials tend to orient their columns parallel to the surface upon thermal annealing. 

Photoconductive properties are observed for some discotic molecules having abundant it electrons [16-20]. For example, triphenylene derivatives can show high charge carrier mobility in the hexagonal columnar (Colh) LC phase due to their ordered molecular packing [16-20,43-45]. 

Charge carrier transport in mesophases is quite unique compared to that in amorphous and crystalline materials. The mobility hardly depends on temperature at room temperature and above as shown in Fig. 2.6. Furthermore, it hardly depends on electric field either, as shown in Fig. 2.7. This behavior is not limited to one particular class of liquid crystals, but is probably a general characteristic of charge carrier transport in mesophases above room temperature. In fact, this behavior is also found in discotic columnar mesophases [11, 39,40,42]. 

US with a great benefit for device applications requiring large-areas. For the discotic columnar phases, there are few reports of the effect of structural defects on charge carrier transport. Unlike the smectic mesophases, where two dimensional transport takes place, it is likely that structural defects in a column may affect the carrier transport properties seriously, because a carrier has to pass along a column without detouring to adjacent columns. From this point of view, relatively low mobility in the columnar ordered phase of triphenylene derivatives, where the intermolecular distance is as small as 3.5 A, may be explained by the structural defects or disorder of molecular alignment in the columns as described above. 

Where, Ep is the average energy required to produce an electron-hole pair and P is the fraction of charge carriers that survive until the end of the pulse. The values of P are estimated according to a model that describes scavenging of the charge carriers by the aromatic cores of the columnar structures formed by the discotic molecules, see for example ref [14], Since JZftTRMc is measured in three dimensions, the intracolumnar mobility parallel to the columns JZb io is three times as big. 

Van de Craats, A.M., et al. The mobility of charge carriers in aU four phases of the columnar discotic material hexakis(hexylthio)triphenylene. Combined TOF and PR-TRMC results. Adv. Mater. 8(10), 823-826 (1996). doi 10.1002/adma.l9960081012... 

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