Hole Transfer Property at Single Crystal Prepared from Triphenylamine-Derivative Based on Solubility and Supersolubility Curves

Abstract

In this report, we describe anisotropic hole transport in a triphenylamine-derivative single crystal prepared by a solution method with solubility and supersolubility curves. Triphenylamine derivatives are commonly used as a hole-transport material for organic devices. In the electrophotographic photoconductors, the materials are used in an amorphous form, in which charge carrier mobility is in the order of ~104 cm2V-1s-1. For realizing a further high mobility as an organic semiconductor, preparation of a single crystal seems to be a promissing way. In this study, a single crystal of a trihenylamine derivative (TPA : α-Phenyl-4'-[(4-methoxyphenyl)phenylamino]stilbene) was prepared and its anisotropic hole transport property was measured. Firstly, solubility and supersolubility curves were measured employing a tetrahydrofuran (THF) as a solvent. Based on the results, a single crystal preparation was conducted using 45 wt.% TPA solution with a cooling rate of 0.125 °Ch-1 started from 35°C for 48 h, resulted in a crystal of 8 x 0.8 x 0.4 mm. A polarizing microscope observation demonstrated an extinction angle, which ensures that the TPA single crystal is obtained. Next, current-voltage properties of the TPA single crystal were measured in a vacuum chamber connected to a source meter (Keythley 2612A). For the measurement, A set of Au parallel electrodes with a distance of 20 μm were vacuum deposited on the crystal. From the results, long-axis (direction of 8 mm at TPA single crystal) current is larger than the short-axis (direction of 0.8 mm at TPA single crystal) current, which reveals anisotropic hole transport in the crystal is observed. This can be well explained by a molecular orientation of the crystal structure. Figure 1