Liquid Crystalline Organic Semiconductors Research Articles

The effect of thermal annealing on the layered structure of smectic liquid crystalline organic semiconductor on polyimide gate insulator and its OFET performance

The electrical performance of organic field-effect transistors (OFETs) depends on the quality of thin-film organic semiconductors, which is significantly affected by solution-processing conditions and additional processes. We investigated the effects of post-thermal annealing on the thin-film morphologies of liquid crystalline organic semiconductors on polyimide gate insulator surface and the FET performances of the films. The active material selected for the OFETs was 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10), which shows a highly ordered smectic E (SmE) mesophase and polycrystalline thin films feature very high mobility. We aimed to produce well defined molecular orientation and crystal structure in thin-film Ph-BTBT-C10 on polyimide gate insulator not typical SiO2 gate insulator via thermal annealing process. Uniform bilayer- or monolayer-structured polycrystalline thin films were obtained on polymer gate insulator after thermal annealing at a SmE (over 148°C) and SmA (over 213°C) liquid crystalline phase temperature, respectively. The OFET using bilayer-structured thin film showed high performance with a mobility of 2.27cm2/Vs.

Development of a scanning time of flight microscope and its application to the study of charge transport in phase separated structured organic semiconductors

We describe a tool for studying the two-dimensional spatial variation in electronic properties of organic semiconductors: the scanning time-of-flight microscope (STOFm). The STOFm simultaneously measures the transmittance of polarized light and time-of-flight current transients with a pixel size <30 μm, making it especially valuable for studies of the correlations of structure with charge generation and transport in liquid crystalline organic semiconductors (LC OSCs). Adapting a previously developed photopolymerization technique, we characterize the instrument using patterned samples of a LC OSC bounded by a non-semiconducting polymer matrix.

Photogeneration and enhanced charge transport in aligned smectic liquid crystalline organic semiconductor

Liquid crystalline organic semiconductors are emerging candidates for applications in electronic and photonic devices. One of the most attractive aspects of such materials is the potential, in principle, to easily control and manipulate the molecular alignment of the semiconductor over large length scales. Here, we explore the consequences of alignment in a model smectic liquid crystalline semiconductor, and find that the photogeneration efficiency is a strong function of incident polarization in aligned samples. A straightforward theory shows that such behavior is a general feature of aligned materials, regardless of the details of photophysics. Furthermore, we uncover tentative evidence that the mobility of aligned samples is substantially enhanced. Both of these phenomena are of significant technological importance.

Intrinsic Charge Carrier Mobilities at Insulator–Semiconductor Interfaces Probed by Microwave-based Techniques: Studies with Liquid Crystalline Organic Semiconductors

Abstract The intrinsic, local-scale hole and electron mobilities at the interface between perylenediimide (PDI)-based liquid crystalline organic semiconductors and insulating polymers were evaluated by field-induced time-resolved microwave conductivity (FI-TRMC). The PDI liquid crystals having alkyl and semifluoroalkyl tails showed ambipolar hole and electron mobilities of 0.2 and 0.3 cm2 V−1 s−1, respectively. In contrast, the PDI having triethylene glycol and semifluoroalkyl tails showed no significant conductivity, suggesting an important role of side chain structures on the charge transport properties. The FI-TRMC technique demonstrated here serves as a powerful tool to screen out the interfacial structure of liquid crystalline semiconductor materials.

電極/液晶性有機半導体界面の電子構造V：電極の仕事関数の変化の分析

電極/液晶性有機半導体界面の電子構造V：電極の仕事関数の変化の分析

Persistence of nematic liquid crystalline phase in a polyfluorene‐based organic semiconductor: A high pressure study

ABSTRACTThe role of high pressure on a low molecular weight nematic liquid crystalline organic semiconductor, ethyl‐hexyl substituted polyfluorene (PF2/6) is investigated using photoluminescence (PL), Raman scattering, and X‐ray scattering studies at pressures from 1 to 8 GPa. The PL and the Raman data under pressure are consistent with each other with no abrupt changes in the pressure coefficients of PL or Raman peaks. The PL energies redshift and broaden, consistent with both enhanced intra‐ and interchain interactions. The Raman peak positions yield pressure coefficients similar to other phenyl based π‐conjugated polymers. The broadening of a doublet peak in the 1135 cm−1 region indicates a more planar backbone conformation with increasing pressure. X‐ray scattering indicates that the torsion angle between adjacent repeats reduces with increasing pressure and reverts back with decompression. The intermolecular structure is weakly ordered (frozen nematic) and essentially maintained with increasing pressure, in contrast to a high molecular weight PF2/6. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1014–1023

Electronic Structure of Interface between Liquid-Crystalline Organic Semiconductor and Electrode IV: Ionization Energy in Liquid-Crystalline Phase

Electronic Structure of Interface between Liquid-Crystalline Organic Semiconductor and Electrode IV: Ionization Energy in Liquid-Crystalline Phase

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

Solvent‐Free Directed Patterning of a Highly Ordered Liquid Crystalline Organic Semiconductor via Template‐Assisted Self‐Assembly for Organic Transistors

Highly ordered organic semiconductor micropatterns of the liquid-crystalline small molecule 2,7-didecylbenzothienobenzothiophene (C10 -BTBT) are fabricated using a simple method based on template-assisted self-assembly (TASA). The liquid crystallinity of C10 -BTBT allows solvent-free fabrication of high-performance printed organic field-effect transistors (OFETs).

Microresonator-enhanced electroluminescence of an organic light emitting diode based on a columnar liquid crystal

The liquid crystalline organic semiconductor perylene-3,4,9,10-tetracarboxylic-tetraethylester is used to create resonant cavity enhanced organic light emitting diodes. The results indicate that the emitted intensity can be increased and the emission spectrum narrowed by embedding a suitable sequence of several organic layers, including the discotic liquid crystal, with appropriate thicknesses in a microresonator consisting of a highly reflecting metal electrode and a Bragg reflector. The experimental data are in good agreement with theoretical calculations. Resonant cavity enhancement revealed to be suitable for improving the performance of liquid crystal-based electroluminescent devices.

38.1: Highly Thermally‐Stable OFETs Fabricated with Liquid Crystalline Organic Semiconductors

AbstractWe have developed a new soluble FET material having a highly ordered smectic liquid crystal phase of a phenyl and alkyl‐substituted benzothienobenzothiophene derivative, Ph‐BTBT‐10. This BTBT derivative had high processability for solution processes and highly thermal stability thanks to the solid‐like nature of the highly ordered smectic phase. Polycrystalline thin films of ph‐BTBT‐10 spin‐coated at a liquid crystalline temperature of 110 °C were uniform in macroscale and molecularly flat in microscale, and FETs fabricated with the thin films showed high FET mobility of 5.7 cm2/Vs, kept high mobility of 3 cm2/Vs after the thermal stress for 5 minutes up to 150 °C, and still operated even after thermal stress at 170°C.

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.

Liquid crystalline organic semiconductors: nematic spiro[cyclopentyl-1,9′]fluorenes

A number of nematic spiro[cyclopentyl-1,9′]fluorene reactive mesogens with polymerisable oxetane or non-conjugated diene end-groups have been successfully synthesised. The melting and clearing points of these new spiro[cyclopentyl-1,9′]fluorenes are much higher than those of the corresponding 9,9-diethyl- or 9,9-dipropyl-substituted fluorenes, which are present in many light-emitting polymers currently used in polymer light-emitting diodes. The significant difference in transition temperature may be attributed to the lower intermolecular separation induced by the more rigid and less bulky cyclopentyl group, confirmed by molecular modelling. The presence of either these short but flexible aliphatic chains, or a rigid alicyclic group of similar size and shape, results in only small differences in the photoluminescence spectra and efficiency of these model liquid crystalline organic semiconductors, and in their improved thermal stability.

Synthesis of a novel smectic liquid crystalline glass and characterization of its charge carrier transport properties

A new organic semiconductor material with both a self-organized molecular alignment in smectic liquid crystals and a solid nature in amorphous materials, i.e., a glassy liquid crystalline organic semiconductor has been proposed, and its model compound, 1,3,5-benzenetricarboxylic acid, tris{12-[6′-(4′′-octylphenyl)-2′-naphthyloxy]-1-dodecyl ester} (LCG-triester 8-PNP-O12), was synthesized, and we characterized its phase transition behaviors in slow cooling and rapid cooling. It exhibited smectic A (SmA) and smectic B (SmBhex) phases in slow cooling, and a SmBhex glassy phase with a high Tg close to 60 °C when cooled rapidly from the SmA and SmBhex phases. Its charge carrier transport properties in the smectic mesophases, including the liquid crystal glassy phase, were first studied by the time-of-flight (TOF) technique, which revealed that the hole mobility in the SmBhex glassy phase was about 3.5 × 10−4 cm2 v−1s−1, comparable to that in the SmBhex liquid crystal phase. We concluded that the high charge transport properties of the smectic mesophase are successfully maintained in the solid glassy state.

Effect of ω-hydroxyl Group of Side Chains on Phase Transition Behavior and Charge Transport Properties in Terthiophene Liquid Crystals

We have synthesized liquid crystalline terthiophene (TTP) derivatives having hydroxyl groups in ω-position of side chains, i.e., 5-hexyl-5’’-(6-hydroxyhexyl) terthiophene (6-TTP-6ol) and 5, 5’’-bis (6-hydroxyhexyl) terthiophene (6-TTP-6diol), and characterized their liquid crystallinity by differential scanning calorimeter (DSC), polarized optical microscopic observation (POM) and X-ray diffraction (XRD) measurement. And charge carrier transport properties by time-of-flight (TOF) measurement. These hole mobilities determined by TOF had not been changed much despite relatively the large dipole of hydroxyl group of 1.8-2.2D, compared with a non-hydroxyl derivative and their values are 10−3-10−2cm2V−1s−1. This fact indicates that the charge carrier mobility is not affected by polar groups, if it is substituted at the far −end position from the TTP core responsible for carrier transport, indicating a possible molecular design of a functionalized liquid crystalline organic semiconductor with polar groups without decreasing the mobility.

Novel High-Performance Liquid-Crystalline Organic Semiconductors for Thin-Film Transistors

A novel class of liquid-crystalline organic semiconductors based on 2,5′-bis-[2-(4-pentylphenyl)vinyl]-thieno(3,2-b)thiophene and 2,5′-bis-[2-(4-pentylphenyl)vinyl]-(2,2′)bithiophene were synthesized through proper structural design. The materials exhibited high field-effect mobilities up to 0.15 cm2 V−1 s−1 and current on/off ratio of 1 × 106. The high performance is attributed to their abilities to form highly ordered structures through molecular packing, which is revealed by single-crystal and thin film X-ray diffraction (XRD). The structures were analyzed by polarized optical microscope and showed obvious changes in texture corresponding to the phase changes in DSC diagrams, indicating their abilities to form liquid-crystalline structures. High environmental stabilities have been demonstrated with these new compounds, which is consistent with their low HOMO levels and large band gaps. In addition, UV−vis spectra of the thin films formed from these compounds have showed that their absorption peaks main...

Reversible Switching between Molecular and Charge Transfer Phases in a Liquid Crystalline Organic Semiconductor

We report the first experimental example, to our knowledge, of reversible switching between a molecular and a charge transfer phase in an organic semiconductor. An oriented film of liquid crystal perylene diimide molecules reversibly switches between a red phase with narrow conduction and valence bands and a large bandwidth black phase as the π-stacked chromophores shift just 1.6 Å relative to their neighbors. This shift causes a substantial change in the intermolecular electronic overlap between molecules. The polarization of maximum absorbance rotates ∼90°, from an apparently molecule centered transition to an intermolecular charge transfer (CT) transition polarized along the π−π stacking axis. The experimental results are further explored via density functional theory calculations on a dimer model that demonstrate the variations in energy and oscillator strength of the molecular (Frenkel) and CT transitions as the longitudinal molecular offset is varied. These results demonstrate the exquisite sensitivity of the electrical properties of organic semiconductors to slight variations in molecular stacking.

Optical and Conductive Properties of Large-Area Liquid Crystalline Monodomains of Terthiophene Derivatives

The optical and conductive properties of the liquid-crystalline organic semiconductor 5,5‘ ‘-bis (5-hexyl-2-thienylethynyl)-2,2‘:5‘,2‘ ‘-terthiophene (TR5-C6) spin-coated on rubbed polyimide (PI) were studied. The absorption of light linearly polarized parallel to the rubbing direction largely exceeds that in the perpendicular direction. In contrast, a small anisotropy in the photoconductance was found from time-resolved microwave conductivity (TRMC) measurements. From analysis of the optical data, it is inferred that the terthiophene chains in the TR5-C6 molecules are oriented along the PI rubbing direction with a tilt angle of 53 degrees with respect to the plane of the substrate. The absence of a strong anisotropy in the mobility of charge carriers can be understood on the basis of calculated charge transfer integrals, which were found to be comparable for charge-transfer steps in different directions. This is due to arrangement of the molecules in a herringbone structure. Due to the relatively small values of the charge transfer integrals, it is likely that charge transport in TR5-C6 occurs via polaron hopping between localized states.

Electronic structure and charge injection at interface between electrode and liquid-crystalline semiconductor

The measurement and analysis of the current-voltage characteristics of a liquid-crystalline organic semiconductor 2-(4′-octyphenyl)-6-dodecyloxynaphthalene (8-PNP-O12) in contact with electrodes of Pt, Au, ITO, Cr, and Al (in the order of work function) have revealed that the injection of holes from the electrodes of Pt, Au, and indium tin oxide to 8-PNP-O12 takes place according to the Richardson-Schottky model and that an electric double layer is formed at the interface between each of these electrodes and 8-PNP-O12, making it difficult to inject holes from the former to the latter.

Analysis of Ohmic Current under Low Electric Field in Liquid-Crystalline Organic Semiconductor in Contact with Electrodes

Analysis of Ohmic Current under Low Electric Field in Liquid-Crystalline Organic Semiconductor in Contact with Electrodes