Jun-ichi Hanna

Liquid crystals for organic thin-film transistors

Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200 °C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10 cm2 V−1 s−1) after thermal annealing at 120 °C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

High ambipolar carrier mobility in self-organizing terthiophene derivative

Dialkylterthiphene derivatives exhibiting self-organization of molecular alignment were synthesized and their electrical properties were studied. It was found that these materials exhibited very fast electron transport in smectic mesophases as well as hole transport, both of which were independent of electric field and temperature. The mobility for electrons and holes was 5×10−4 cm2/V s in smectic C (SmC) and was increased up to 2×10−3 cm2/V s and even more up to 1×10−2 cm2/V s in accompany with upgrade of molecular alignment from SmC to SmF and from SmF to SmG, respectively. This abrupt increase in the mobility to the extent of about one order of magnitude corresponds to that in 2-phenylnaphthalene derivatives at the phase transition from SmA to SmB and from SmB to SmE whose molecular alignments are in good correlation with those of SmC, SmF, and SmG, respectively.

Very high time-of-flight mobility in the columnar phases of a discotic liquid crystal

1,4,8,11,15,18,22,25-Octaoctylphthalocyanine shows the highest time-of-flight (“long-range”) hole mobility so far reported for the columnar phase of a discotic liquid crystal. Unlike most other high long-range mobility columnar discotics, there is no clear evidence from x-ray diffraction of high order.

Fast ambipolar carrier transport in smectic phases of phenylnaphthalene liquid crystal

The carrier transport of a new calamitic liquid crystal, i.e., 2-(4′-octylphenyl)6-dodecyloxynaphthalene (8-PNP-O12) was investigated by the time-of-flight technique. The fast ambipolar electronic conduction was observed in two smectic phases of 8-PNP-O12. The carrier mobilities were determined to be 1.6×10−3 cm2/V⋅s in the lower temperature smectic phase (Sm1) between 79 and 100 °C and to be 2.5×10−4 cm2/V⋅s in the higher temperature phase (Sm2) between 100 and 121 °C, regardless of carrier signs, i.e., electron and hole. The slower ambipolar transport was observed in isotropic phase as well, whose mobility was 8×10−5 cm2/V⋅s. These mobilities were independent of applied electric fields and temperature.

Anomalous high carrier mobility in smectic E phase of a 2-phenylnaphthalene derivative

Ambipolar carrier transport properties in different phases of a liquid-crystalline photoconductor, 2-(4-octylphenyl)–6-n-butoxynaphthalene, were investigated by a time-of-flight technique. Carrier mobilities were increased stepwise when phase transition took place as the temperature decreased. The smectic E phase in the range of 55–125 °C exhibited nondispersive ambipolar carrier transport with an anomalous high carrier mobility of 1.0×10−2 cm2/V s, while the smectic A phase between 125 and 129 °C had similar carrier transport with a smaller mobility of 4×10−4 cm2/V s. In contrast to the crystalline phase, structural defects in the smectic E phase, which were obvious under microscopic observation with polarized illumination, did not deteriorate the carrier transport properties.

A novel preparation technique for preparing hydrogenated amorphous silicon with a more rigid and stable Si network

A novel preparation technique, termed ‘‘chemical annealing,’’ was developed with the aim of making a stable and rigid Si network structure. The hydrogen content (CH) in the films and the optical gap could be reduced gradually without any change in substrate temperature by alternating deposition of a hydrogenated amorphous silicon layer several tens of A thick and treatment with atomic hydrogen. These films showed CH of 1.5–10 at. %, and exhibited high photoconductivities in the level of 10−5–10−4 S/cm. In the films with CH of 3 at. % or less, in particular, improvement was observed in stability against illumination with light. Their photoconductivity remained at about 65% of the initial value even after illumination with white light (AM1, 100 mW) for 60 h. In addition, time‐of‐flight experiments revealed a significant enhancement in hole drift mobility to a value of 0.2 cm2/V s at 300 K.

Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films

Figure 1 . Chemical structures of the liquid crystalline materials used in the present experiments, i.e., ω , ω ’-dioctylterthiophene (8-TTP-8) and 2,7-didecylbenzothienobenzothiophene (C 10 -BTBT). The research of materials for organic fi eldeffect transistors (OFETs) has been extended to the exploration for new materials suitable for the fabrication of polycrystalline thin fi lms by solution processes such as spincoating and ink-jet techniques instead of costly vacuum evaporation techniques. In fact, solution-processed polycrystalline thin fi lms from precursor materials, [ 1 , 2 ] such as 6,13-bis(triisopropylsilylethynyl)-pentacene, [ 3 ] and alkylated oligothiophene, [ 4–6 ] hexabenzocoronene, [ 7 ] and benzothienobenzothiophene (BTBT) [ 8 ] derivatives, attained high fi eld-effect transistor (FET) mobility over 1 cm 2 V − 1 s − 1 comparable to that of vacuum evaporated fi lms of pentacene. [ 9 ]

Highly Oriented ZnO Films Prepared by MOCVD from Diethylzinc and Alcohols

Alcohols have been shown to be useful as oxidizing agents of diethylzinc for the growth of ZnO films by MOCVD. C-axis oriented ZnO films have been deposited on glass substrates at 300°C. The problem of a premature reaction, which is observed between diethylzinc and oxygen or water vapor, is shown to be overcome when alcohols are used instead. The formation, stability and decomposition of the precursor of MOCVD, e.g., adducts, are discussed on the basis of a mass-spectroscopy analysis.

Polarized light emission from a calamitic liquid crystalline semiconductor doped with dyes

Liquid crystal cells containing a homogeneously aligned calamitic liquid crystalline semiconductor, 2-(4-octylphenyl)-6-dodecyloxynaphthalene, doped with dyes such as 3-(2-benzothiazolyl)7-diethylaminocoumarin, emitted a visible light when a dc bias was applied. This light emission occurred irrespective of phase, and polarized light emission was well established in the Smectic B phase. The unique features of the calamitic liquid crystalline semiconductors are also discussed from a practical viewpoint.

Preparation of photoconductive a-SiGe alloy by glow discharge

Abstract Systematic investigations had been made of preparing a-SiGe alloy with high photoconductivity. The a-SiGe alloy prepared by glow discharge from gaseous mixture of GeF4SiF4H2 showed a satisfactory photoresponse and an effective doping feasibility to control its Fermi-levels, resulting from a great reduction in the amount of the dangling bonds.