High Carrier Mobility up to 0.1 cm² V^–1 s^–1 at Ambient Temperatures in Thiophene‐Based Smectic Liquid Crystals

Abstract

plastics or metal foils. This is associated with the possibility of realizing efficient and reliable TFTs exhibiting high mobility, and opens new doors to so-called ªinvisible electronic circuitsº which will be highly important for the next generation of invisible and flexible electronics; e.g., as switching for addressing organic light-emitting matrices. ExperimentalThe ZnO films (doped and undoped) were deposited onto sodalime glass substrates by radiofrequency (rf, 13.56 MHz) magnetron sputtering using a ceramic-oxide target of ZnO from Super Conductor Materials Inc. with a purity of 99.99 % and 2 in (5.08 cm) in diameter. The sputtering was carried out at room temperature, and the argon deposition pressure was 0.15 Pa. The distance between the substrate and the target was 10 cm, and the rf power was changed between 50 W and 175 W. The deposition rate was varied between 15± 30 nm min ±1 . The film thickness was measured using a surface profilometer (Dektak 3 from Sloan Tech). Electrical resistivity was measured as a function of temperature in the range of 300±500 K using thermally evaporated aluminum electrodes in a coplanar configuration. X-ray diffraction measurements were performed at room temperature in air using the Cu Ka line (Rigaku DMAX III-C diffractometer). Surface morphologies were analyzed using a field-emission scanning electron microscope (Hitachi S-1400). The optical transmittance measurements were performed with a Shimadzu UV-vis 3100 PC double-beam spectrophotometer in the wavelength range from 200 nm to 2500 nm. Organic semiconductors were first utilized for the industrial fabrication of organic photoconductive coatings for xerographic photoreceptors (OPCs), [1] and have found recent application in organic light-emitting diodes (OLEDs).[2]Recently, increasing attention has been paid to organic fieldeffect transistors (OFETs).[3] For OPCs and OLEDs, thin films of amorphous organic semiconductors are used, which are characterized by a low carrier mobility, 10 ±6 to 10 ±3 times smaller than those of crystalline materials. [4] In the OFET applications, however, the amorphous materials are not available because the field-effect transistor requires a high mobility for fast switching or a high current density. Hence, polycrystalline materials, e.g., vacuum-evaporated pentacene, have been studied extensively for high mobility thin-film transistors (TFTs). In fact, a high mobility of up to 8 cm 2 V ±1 s ±1 has already been achieved. [5] In practical applications of polycrystalline materials, however, a new problem of ªgrain boundariesº that originates from molecular alignment in crystalline materials, and which is not the case in amorphous materials, emerges: the grain boundaries cause electrically active localized states and spoil long-term stability by adsorbing ambient contaminants. Therefore, its control is a key issue in realizing practical applications of polycrystalline materials. On the other hand, there is an approach that avoids this boundary effect. In the last ten years, it has been rev...

High mobility ambipolar charge transport in a cross-linked reactive mesogen at room temperature

“…They exceed those measured in cross-linked networks from nematic mesogens ͑ϳ10 −4 cm 2 V −1 s −1 ͒, 4 those measured in smectic mesogens dispersed in a host network ͑ϳ10 −3 cm 2 V −1 s −1 ͒, 5 and those previously reported for cross linked films by the authors. 6 Although these values are not as large as those for single-molecular crystals, 7 or those in nonreactive liquid crystal smectic mesophases, 8 or indeed columnar mesophases, 9 the formation of a solid, stable, insoluble, and semiconducting layer, will allow easy incorporation of these materials in actual device structures ͑field-effect transistor, organic light-emitting device͒, in much the same manner as the polymeric materials are currently used. The mobility data for the various materials is summarized in Table I.…”

Development of Liquid-Crystalline Semiconductors with High Carrier Mobilities and Their Application to Thin-film Transistors

“…41,42 Since the 1990s, transport of electronic charge carriers has been studied in the discotic columnar phases of hexaalkyltriphenylene, 30,31 hexaalkylthiotriphenylene, 32 hexaalkoxyhexabenzocoronene, 43 and perylene tetracarboxylate derivatives. 44 Transport of electronic charge carriers has also been confirmed in calamitic systems such as the smectic phases of 2-phenylbenzothiazole, 33 2-phenylnaphthalene, 34 oligothiophene, [45][46][47] and bisthienylbenzene derivatives. 48 The molecular structures of typical liquid-crystalline semiconductors are shown in Figure 1.…”

“…[30][31][32][33][34][45][46][47] This behavior is markedly different from that of amorphous organic semiconductors, in which the carrier mobility strongly depends on the electric field strength and temperature.…”

“…48 The molecular structures of typical liquid-crystalline semiconductors are shown in Figure 1. The carrier mobilities were determined by the TOF technique, [30][31][32][33][34][45][46][47][48] microwave absorption measurements, 43 and space-charge-limited current meaurements. 44 The drift mobility in these phases, which was determined to be in the range of 10 À4 -10 À1 cm 2 V À1 s À1 by the TOF technique, was found to be strongly affected by the structure of the liquid crystal phase.…”

Temperature-Independent Hole Mobility in Field-Effect Transistors Based on Liquid-Crystalline Semiconductors

“…Asymmetric structures of these compounds should inhibit the crystallization and result in the appearance of the metastable LC phases at room temperature [6], [9].…”