Liquid Crystalline Semiconductors Research Articles

対称・非対称ジオクチルチエニルナフタレン(8TNAT8)混合物の液晶性と電界効果トランジスタ特性

対称・非対称ジオクチルチエニルナフタレン(8TNAT8)混合物の液晶性と電界効果トランジスタ特性

Nanostructured liquid-crystalline semiconductors – a new approach to soft matter electronics

In contrast to solid-state organic semiconductors, superstructures can be constructed in liquid-crystalline (LC) phases, using chirality, hybridization with nanofibers, and nanosegregation.

オリゴシロキサン鎖を有する液晶性ペリレンテトラカルボン酸ビスイミド誘導体の分子凝集構造と電子機能

Liquid-crystalline semiconductors exhibit good carrier transport properties as well as solution-processability. The applications to field-effect transistors, light-emitting diodes, and solar cells have been studied.Liquid-crystalline perylene tetracarboxylic bisimide derivatives bearing oligosiloxane moieties were designed. They exhibit columnar phases at room temperature and do not crystallize even when they are cooled to −100°C. The mesomorphic columnar structures are stabilized by nanosegregation promoted by the interaction between the oligosiloxane moieties. The electron mobility exceeds 0.1 cm2V−1s−1 at room temperature in the ordered columnar phase. The analysis of the electron transport characteristics in the columnar phases indicates closed aggregation of the π-conjugated cores, resulting in high electron mobilities.

Solution-processed bilayer photovoltaic devices with nematic liquid crystals

The cross-linking of polymerisable liquid crystalline semiconductors is a promising approach to solution-processable, multilayer, organic photovoltaics. Here we demonstrate an organic bilayer photovoltaic with an insoluble electron-donating layer formed by cross-linking a nematic reactive mesogen. We investigate a range of perylene diimide (PDI) materials, some of which are liquid crystalline, as the overlying electron acceptor layer. We find that carrier mobility of the acceptor materials is enhanced by liquid crystallinity and that mobility limits the performance of photovoltaic devices.

Monodomain planar alignment of 1,4,8,11,15,18,22,25-octahexylphthalocyanine by melt growth method

The monodomain growth of a columnar liquid crystalline (LC) semiconductor by a melt growth method has been studied. The domain growth of the LC phase of 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2) was observed by generating a temperature gradient on a cell at the temperature of LC-isotropic phase equilibrium. The large-area monodomain C6PcH2 in planar alignment was obtained by the method. The optical and electrical anisotropies in the oriented C6PcH2 were investigated and discussed by taking the column alignment into consideration.

High electron mobility in a columnar phase of liquid-crystalline perylene tetracarboxylic bisimide bearing oligosiloxane chains

A liquid-crystalline (LC) semiconductor based on perylene tetracarboxylic bisimide (PTCBI) bearing four 1,1,1,3,3-pentamethyldisiloxane chains was synthesized. This LC PTCBI derivative exhibits ordered columnar phases and the low temperature columnar phase is retained below room temperature. The electron mobility in the ordered columnar phase of this compound at room temperature exceeds 0.1 cm2 V−1 s−1. Moreover, this compound is soluble in various organic solvents, except alcohols, and thin films in which the columnar aggregates are aligned parallel to the substrates are produced by spin-coating. In particular, LC thin films in which the columnar stacks are uniaxially oriented can be produced by a friction transfer method.

Influence of polymorphism on charge transport properties in isomers of fluorenone-based liquid crystalline semiconductors

We measured the charge carrier mobilities for two isomers of fluorenone-based liquid crystalline organic semiconductors from their isotropic down to crystalline states through one or two mesophases. Improved charge transport properties of melt-processed crystalline films were obtained for the isomer exhibiting a highly ordered mesophase below its disordered smectic phase.

A Possibility of 2-Dimensional Transport of Charged Carriers in Columnar Phases of Liquid Crystalline Semiconductors

1,4,8,11,15,18,22,25-hexahexylphthalocyanine was studied on the carrier mobility characteristics by a Time-Of-Flight (TOF) technique. The compound exhibits an ambipolar nature of charge transport and the carrier mobilities for hole and electron were determined to be 0.3 and 0.2 cm2 V−1 s−1 in the hexagonal disordered columnar (Colhd ) mesophase and 0.3–1.4 and 0.4–0.5 cm2 V−1 s−1 in the crystal. These values were obtained for poly-domain films. A possibility of 2-dimensional transport of carriers was discussed in terms of the molecular structure and orderings in both Colhd and crystal phases.

Mobility enhancement in solution-processable organic transistors through polymer chain alignment by roll-transfer printing

A roll-transfer printing method has been proposed for the uniaxial alignment of the liquid crystalline polymer semiconductor of poly(3,3″′-didodecylquarterthiophene) (PQT). It is shown that the roll-transfer printing of PQT at its liquid crystal phase temperatures yields highly aligned films in which the PQT molecules are aligned parallel to the roll-transfer direction and oriented perpendicular to substrates. The average field-effect mobility in the alignment direction of 0.17 cm 2/Vs is 8 times higher than that in the perpendicular direction. The alignment of polymer chains is attributed to the enhancement of shear-stress effects during roll-transfer printing in the liquid crystal phase.

Electron- or Hole-Transporting Nature Selected by Side-Chain-Directed π-Stacking Geometry: Liquid Crystalline Fused Metalloporphyrin Dimers

Novel liquid crystalline (LC) semiconductors were prepared from the copper complex of a fused porphyrin dimer as the electroactive core by attaching to its periphery dodecyl and semifluoroalkyl side chains site-specifically (P≡P(hetero)) and semifluoroalkyl side chains alone (P≡P(homo)). The former and latter formed rectangular columnar and orthorhombic LC mesophases, respectively, where the stacking geometries of the π-conjugated core are quite different from one another. Although the π-electronic properties of the core units in P≡P(hetero) and P≡P(homo) in solution are substantially identical to one another, transient photocurrent profiles of their LC states under time-of-flight conditions clearly showed that P≡P(hetero) behaves as an n-type semiconductor, whereas P≡P(homo), in contrast, behaves as a p-type semiconductor.

Hole Transport of a Liquid-crystalline Phenylterthiophene Derivative Exhibiting the Nematic Phase at Ambient Temperature

Abstract A liquid-crystalline phenylterthiophene derivative which exhibits a nematic phase at room temperature has been prepared. The hole mobility in the nematic phase at room temperature reaches 4 × 10−4 cm2 V−1 s−1. This value is two orders of magnitude larger than those of amorphous organic semiconductors and comparable to those in the smectic A or C phases of liquid-crystalline semiconductors.

Ordered Materials for Organic Electronics and Photonics

We present a critical review of semiconducting/light emitting, liquid crystalline materials and their use in electronic and photonic devices such as transistors, photovoltaics, OLEDs and lasers. We report that annealing from the mesophase improves the order and packing of organic semiconductors to produce state-of-the-art transistors. We discuss theoretical models which predict how charge transport and light emission is affected by the liquid crystalline phase. Organic photovoltaics and OLEDs require optimization of both charge transport and optical properties and we identify the various trade-offs involved for ordered materials. We report the crosslinking of reactive mesogens to give pixellated full-colour OLEDs and distributed bi-layer photovoltaics. We show how the molecular organization inherent to the mesophase can control the polarization of light-emitting devices and the gain in organic, thin-film lasers and can also provide distributed feedback in chiral nematic mirrorless lasers. We update progress on the surface alignment of liquid crystalline semiconductors to obtain monodomain devices without defects or devices with spatially varying properties. Finally the significance of all of these developments is assessed.

Diethynylbenzene-Based Liquid Crystalline Semiconductor for Solution-Processable Organic Thin-Film Transistors

We report here the synthesis and characterization of novel diethynylbenzene-based liquid crystalline semiconductor (P1) for organic thin-film transistors (OTFTs). Compound P1 was synthesized by the Sonogashira coupling reaction between 2-bromo-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene and 1,4-bis(dodecyloxy)-2,5-diethynylbenzene. Top contact OTFTs were fabricated by spin casting with 2 wt% solution of P1 in chloroform and their best performance, which exhibited a hole mobility of 4.5 x 10(-5) cm2/Vs, was showed after annealing of the films at liquid crystalline temperature. Time-of-flight (TOF) mobility measured at liquid crystalline phase was observed to be 1.5 x 10(-6) cm2/Vs for both positive and negative carriers. These results indicate that the liquid crystallinity helps to improve the molecular packing and enhance charge mobility for P1. These advantages can be applicable to design and construct solution-processable OTFT materials for electronic applications.

Organic Semiconductors: Impact of Disorder at Different Timescales

The charge transport in organic materials, from molecular crystals to polymers, is determined by their degree of disorder. The dynamic disorder in ideal molecular crystals at room temperature and the static disorder in disordered polymers are just two limiting cases of the timescale of the fluctuations in the electronic Hamiltonian caused by nuclear motions. In fact, a very large number of important materials (e.g. liquid crystalline semiconductors) are actually in an intermediate regime where the disorder is neither purely static nor purely dynamic. This Minireview discusses the recent contribution of computational chemistry (molecular dynamics and quantum chemistry) to the characterization of these transport regimes and outlines the theoretical methods that can be used to relate the system characteristics to the measurable mobility.

Alignment Change of Discotic Liquid Crystal Domains with Wavelength Tunable CO<sub>2</sub> Laser Irradiation

Infrared-induced alignment change with wavelength tunable CO2 laser irradiation for columnar liquid crystal domains was investigated for a triphenylene-based discotic liquid crystal. A uniformly aligned alignment change of domains was observed when a plused linearly polarized infrared laser light corresponding to the wavelength of the aromatic C-O-C stretching vibration band (9.65µm) was irradiated while it was not re-aligned uniformly with continious wave irradiation. The results strongly imply that the infrared irradiation is a possible technique for device fabrication by use of columnar mesophase as a liquid crystalline semiconductor.

Flexible field-effect transistors from a liquid crystalline semiconductor by solution processes

We have fabricated the flexible field-effect transistors based on the liquid crystalline phenylterthiophene derivative via a solution process. A dielectric layer of polyvinyl alcohol (PVA) are deposited on a polyimide sheet substrate by a spin-coating method and then the thin film of 5-propyl-5′′-(4-pentylphenyl)-2,2′:5′,2′′-terthiophene is fabricated from its chlorobenzene solution on the dielectric layer by spin-coating or solution sheared deposition method. The fabricated devices exhibit good electrical performance with a hole mobility of up to 0.05 cm 2 V −1 s −1 as well as excellent mechanical flexibility. The performance does not degrade at the external strain reached 3%. Our results demonstrate that liquid crystalline semiconductors are promising materials for low-cost flexible electronic devices.

Synthesis of liquid crystalline benzothiazole based derivatives: A study of their optical and electrical properties

Two new donor–acceptor type liquid crystalline semiconductors based on benzothiazole have been synthesized. Their structural, photophysical and electronic properties were investigated using X-ray diffraction, atomic force microscopy, cyclic voltammetry, UV–Vis, photoluminescence, and Raman spectroscopy. The liquid crystalline behaviour of the molecules was thoroughly examined by differential scanning calorimetry (DSC) and optical polarizing microscope. The DSC and thermogravimetric analysis (TGA) show that these materials posses excellent thermal stability and have decomposition temperatures in excess of 300 °C. Beyond 160 °C both molecules show a smectic A liquid crystalline phase that exists till about 240 °C. Field-effect transistors were fabricated by vacuum evaporating the semiconductor layer using standard bottom gate/top contact geometry. The devices exhibit p-channel behaviour with hole mobilities of 10 −2 cm 2/Vs.

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

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

Thin-film transistors based on liquid-crystalline tetrafluorophenylter thiophene derivatives: thin-film structure and carrier transport

The authors fabricated thin films by solution processes using liquid-crystalline (LC) semiconductors, 5-alkyl-5’’-(4-hexyltetrafluorophenyl)-2,2’:5’,2’’-terthiophene ( 2–5). Films of 5-propyl-5’’-(4-hexyltetrafluorophenyl)-2,2’:5’,2’’-terthiophene ( 2) show similar molecular packing as their non-fluorinated counterparts. However, the degree of molecular packing ordering from X-ray diffraction measurement is higher, and the films exhibit a more crystal-like structure. Moreover, fluorination has a remarkable effect on their mesomorphic behaviors. Films of 2 consist of large size LC domains (in the range of 100 μm) at room temperature. Thin-film transistors (TFTs) of 2 show p-type operation with good hole mobility up to 0.027 cm 2/Vs as well as improved operation stability under ambient conditions and high on/off ratio. Tetrafluorophenyl substitution leads to lowering of HOMO energy by 0.15 eV for 2 and 0.35 eV for 5, resulting in operation stability. Variable-temperature current-voltage measurements indicate intrinsic carrier transport in films of 2.

Organic Semiconductors with Helical Structure Based on Oligothiophene derivatives Exhibiting Chiral Nematic Phase

We have synthesized 2-[5-(4-[(S)-2-methylbutoxy]phenyl)-3-methyl]-2″′-propyl-2:5′-2′:5″-2″:5″′-quaterthiophene ( 10 ), a liquid crystalline semiconductor that exhibits a chiral nematic phase, and used the time-of-flight technique to determine its carrier mobility behavior. In the chiral nematic phase, the positive carrier mobility was 2 × 10−4 cm2/Vs at 120°C and the negative carrier mobility was 1 × 10−4 cm2/Vs, both of which we attribute to electronic processes. For the negative carrier, in addition to the fast electronic process, we also observed a slow carrier transport, having mobility on the order of 10−5 cm2/Vs, which we attribute to ionic conduction. Chiral dimers containing binaphthyl group exhibited stable glassy cholesteric phase and cholesteric thin films with selective reflection band in visible light area could be fabricated. They emitted circularly polarized light with dichroic parameter of 1.3 when UV light was illuminated.