This decade, one of the new fields of liquid-crystal (LC) research is LC semiconductors. Recently, studies have been extended to the application of these mesomorphic materials to organic electronic devices such as organic thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs), and so on. Some of the characteristic properties of LC semiconductors, [1,2] such as their self-assembling nature, ambipolar charge transport, electrically inactive domain boundaries, and good solubility in ordinary organic solvents, make them attractive candidates for application in these devices.Crystalline organic semiconductors such as pentacene [3][4][5] and oligothiophenes [6][7][8] have been extensively studied for use in OTFTs because of their relatively high field-effect mobilities. However, pentacene is easily photo-oxidized in air because of its relatively high-lying highest occupied molecular orbital (HOMO) energy level and its narrow band gap. Therefore, novel highly stable crystalline semiconductors have been developed such as fused chalcogenophene compounds [9] and phenanthrene compounds. [10] In general, it is difficult to fabricate OTFTs by using mild solution-processing, because of its significantly low solubility in ordinary organic solvents.
Several poly(N‐phenyl‐2,7‐carbazole)s that have dialkoxy groups at the m‐ and p‐positions (PmpCzDC, PmpPhDC, PmpCBiDC, PmpEHC), a silyl group at the p‐position (PpPhDSiC), and a diphenylamino group (PmPhDAC, PmEHAC) at the m‐position of the N‐phenyl portion are synthesized, and their optical properties are characterized. These polymers have been used as emitting layer materials of organic light‐emitting diode (OLED) devices that have a configuration of ITO/PEDOT(PSS)/polymer/CsF/Al. The OLED devices embedded with PmpCzDC, PmpPhDC, and PmpEHC show intense luminance of about 15 000 cd · m−2 with efficiencies of about 1 cd · A−1, while the devices embedded with PpPhDSiC, PmPhDAC, and PmEHAC show less luminance but retain the color purity of blue emission under a wide range of applied voltages.
The carrier mobility of highly ordered lamellar mesophases was evaluated by a Time-of-Flight (TOF) method for mesogenic 1,4-di(5'-octyl-2'-thienyl)benzene (8-TPT-8) to reveal the fastest drift mobility (0.1 cm(2) V(-1) s(-1) for the lowest temperature mesophase) in calamitic liquid crystals reported to date.
Here we show the simple fabrication of field effect transistor (FET) with a mesophase semiconductor, a derivative of dithienyl naphthalene, which exhibits a fast mobility (10 -1~1 0 -2 cm 2 V -1 s -1 ) of charged carriers in the mesophase. The compound is a mesogen, but with highly ordered layered structure in a triclinic lattice, meaning a 3D-mesophase is formed. The device performance was studied for the transistor mobility, on/off ratio and threshold voltage of device operation, to have 0.14 cm 2 V -1 s -1 , 2 x 10 3 and -27 V at room temperature (in a crystal phase), respectively, even though the thin film active layer (100 nm thick) does have a multi-domain system. However, the XRD studies indicate the uniformly aligned molecules in each domain, of which long axis is inclined to be ca. 27°against the axis perpendicular to the substrate plane. This implies that a self-assembling nature of mesogenic molecules is a certain merit for thin film device fabrication in organic electronics.
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