Charge-carrier transport in smectic mesophases of biphenyls

Abstract

The charge-transport properties of ω,ω′-alkylalkoxylbiphenyls, one of the simplest calamitic (rodlike) liquid crystals exhibiting smectic mesophases, were investigated using the time-of-flight experiments. A mobility of ∼10−3 cm2∕V s was determined for holes in both smectic B (SmB) and smectic E (SmE) phases. This high electronic mobility, comparable to the highest mobilities in amorphous organic semiconductors, indicates that the self-organization of such simple molecules including small aromatic compounds is quite effective in enhancing the charge transport in organic materials. The mobilities of both smectic mesophases, however, depend on temperatures, which is not the case for other typical smectic liquid crystals reported previously. Interestingly, the mobility in the SmE phase exhibits a Poole–Frenkel type of field dependence. Detailed experimental studies have shown that this field dependence is not due to the trapping effect of carriers caused by chemical contaminants or structural defects such as domain boundaries and dislocation, but arises from the widely distributed density of states with a distribution width of 0.11–0.13 eV for the SmE phase, as estimated based on a two-dimensional Gaussian disorder model. The phase-specific carrier transport in biphenyls may provide us with unique insights leading to a more complete understanding of the charge carrier transport in the mesophase materials.

Liquid Crystalline Ordering and Charge Transport in Semiconducting Materials

“…Even the simplest calamitics, alkylalkoxylbiphenyls, had values of 10 À3 cm 2 Á V À1 Á s À1 in their SmB phases, confirming the effective charge migration due to pronounced organization. [87] On the other hand, due to their low order, nematic [90] Evaluating the results compiled in Table 2, it is evident that the charge carrier mobility increases as expected along with the quality of molecular packing within the smectic layers. Phases with a liquid-like packing (smectic A and C) possess charge carrier mobilities of only 10 À4 cm 2 Á V À1 Á s…”

Charge Generation Measured for Fullerene–Helical Nanofilament Liquid Crystal Heterojunctions

“…Other self-assembling, liquid-crystalline systems have been explored for use in OPV. − One common approach is the use of columnar liquid crystal phases, which show promise with high carrier mobilities, from the large π-π overlap and alignability between electrodes. , However, columnar stacking promotes face to face packing of disk-shaped mesogens (discotics, such as phthalocyanine derivatives), thus limiting the orbital overlap of the mesogen and its electron acceptor, which can limit photoinduced charge transfer. In one recent study a 4.6% power conversion device was fabricated by phthalocyanine and C 60 codeposition …”

Hole transport in the organic small molecule material α-NPD: evidence for the presence of correlated disorder

“…Analyzes of charge transport in small molecule materials, carried out in the framework of the GDM and the CDM but neglecting the charge carrier density dependence, led to Ϸ 0.08-0.15 eV for commonly used OLED materials. [21][22][23][24][25] Neglecting the charge carrier density dependence can be appropriate for the case of injectionlimited transport, in devices with large injection barriers. However, in the absence of large injection barriers the charge carrier density dependence of the mobility and its effect on the current density cannot be neglected.…”