Liquid Crystalline Semiconductors Research Articles

Solution-processed planarly oriented thin films of cholesteric liquid-crystalline semiconductors

We have successfully fabricated cholesteric liquid-crystalline (CLC) semiconductor thin films from the mixture of dimeric and monomeric compounds based on oligo(p-phenylenevinylene) units by spin-coating method. This is the first example of solution-processed planarly orientated thin film in CLC semiconductors. The mixture exhibited a CLC phase at lower temperatures than the compound alone, allowing the fabrication of spin-coated CLC thin films at room temperature. Furthermore, the dimeric compound has trifluoromethyl groups that exhibited high solubility in organic solvents such as chloroform due to its improved hydrophobicity. In addition, the high hydrophobicity of the trifluoromethyl groups lowers the surface tension of the liquid–crystal material, resulting in planar orientation.

High-speed blade-coating using liquid crystalline organic semiconductor Ph-BTBT-10

We have investigated the fabrication of crystalline thin films of a liquid crystalline organic semiconductor (2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene: Ph-BTBT-10) by high-speed blade-coating at 140 mm s−1. Uniform crystalline Ph-BTBT-10 films were fabricated at temperatures over 50 °C, which is the temperature of the liquid crystal phase, without inhomogeneous recrystallization, despite the high-speed blade-coating. Transistors fabricated using these films showed a high average carrier mobility of 4.8 cm2 Vs−1 for 26 devices, with a coefficient of variation of only 7.3%. We concluded that liquid crystalline organic semiconductors are useful materials for high-speed meniscus-guided-coating for practical use.

Accessing the electronic structure of liquid crystalline semiconductors with bottom-up electronic coarse-graining

A bottom-up electronic coarse-graining model predicts morphology-dependent electronic structure and charge transport properties for liquid-crystal semiconductors, without back-mapping or ad nauseam quantum chemistry calculations.

Solution-Processable liquid crystalline organic semiconductors based on 6-(Thiophen-2-yl)benzo[b]thiophene for organic thin-film transistors

Liquid crystalline organic semiconductors have garnered significant attention in the realm of organic thin-film transistors (OTFTs) due to their inherent controllability of phase and electrical properties with temperature variations. Typically, liquid crystalline semiconductors possess distinct structures comprising rigid anisotropic cores and flexible chains. Here, we sought to identify liquid crystalline semiconductor materials by appending a thiophene ring and octyl chain to the anisotropic 6-(thiophen-2-yl)benzo[b]thiophene (TBT) backbone. Among the investigated TBT derivatives, the compound featuring additional thiophene ring and octyl chains on both sides exhibited pronounced liquid crystalline characteristics, specifically the SmE phase. The optical, thermal, and electrical characteristics as well as thin film properties of the TBT derivatives were investigated. The TBT compound was employed through solution-shearing as an active layer for top-contact/bottom-gate OTFTs. The liquid-crystalline TBT (2,6-bis(5-octylthiophen-2-yl)benzo[b]thiophene) thin film annealed at low temperature (30 °C) showed hole mobility as high as 0.025 cm2/Vs and current on/off ratio of > 106. This results in a significant enhancement by an order of magnitude compared to the sample annealed at 70 °C in the SmE phase, and nearly a two-fold improvement compared to the performance observed in the sample annealed at 130 °C in the transition phase. The utilization of the TBT derivative as a p-channel material represents a significant step toward the development of novel liquid crystalline semiconductors.

Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor: Proficient hole transporting material for optoelectronic devices

Hole transport materials (HTMs) have a significant impact on the effectiveness of organic electronic devices; therefore, we present a molecular architecture of pyrazino[2,3-g]quinoxaline (PQ10)-based room-temperature organic liquid crystalline semiconductor (OLCS) as an alternative HTM. The PQ10 compound exhibits three different rectangular columnar (Colr) phases offering an impressive hole mobility of 8.8 × 10−3 cm2V−1s−1 which is found to be dexterous than most of existing polymeric hole transport materials. The charge transport mechanism is governed by the hole polarons hopping through H-aggregates of the PQ10 molecules and the hole mobility remains nearly constant throughout the mesophase range, but it decreases with increasing applied electric field. The current-voltage characteristics of the PQ10 have also been investigated in all three Colr phases and explained via the Poole-Frenkel conduction mechanism. The dielectric spectroscopy has been eventually carried out to understand the nature of dielectric permittivity and conductivity as a function of temperature and a correlation is established between the molecular architecture of the Colr phases and aforementioned physical properties. Solar cell simulation has been additionally performed to demonstrate that the PQ10 material can be a better choice as HTM for organic electronics and photovoltaic applications.

High-performance organic field effect transistor of liquid crystalline organic semiconductor using silver electrodes with bottom-gate top-contact configuration

We have investigated organic field effect transistors (OFETs) fabricated with liquid crystalline organic semiconductor, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), and silver (Ag) source-drain electrodes. Ph-BTBT-10 OFETs using Ag source-drain electrodes in bottom-gate top-contact configuration exhibit high mobility of 8.2 cm2 Vs−1, which is similar to those of gold (Au) electrodes. Contact resistance of Ph-BTBT-10 OFETs using Ag electrodes achieved 0.49 kΩcm by transfer length method, which is lower than those of Au electrodes, 2.7 kΩcm. Ph-BTBT-10 OFET using Ag source-drain electrodes in bottom-gate top-contact configuration is a promising candidate for high-performance and low-cost OFETs.

Differences between one- and two-photon excited circularly polarized light emission processes in cholesteric liquid-crystalline semiconductors

Recently, cholesteric liquid-crystalline (CLC)-semiconductors have been studied for the purpose of realizing circularly polarized light (CPL) emission by electrical pumping. In this study, we investigated the one- and two-photon excited CPL emission properties of glassy CLC film of oligo (p-phenylenevinylene) derivatives. In the case of one-photon excitation, the CPL emission spectra were significantly different between the forward and backward configurations of the detector to the excitation light. This is because spontaneous emission occurred near the sample surface due to the large absorption coefficient for the excitation light. In contrast, in the case of two-photon excitation, the CPL emission spectra in the forward and backward configurations were almost identical. This is because the absorption coefficient for two-photon excitation was smaller than that for one-photon excitation, and spontaneous emission occurred over the entire sample. This result indicates that two-photon excitation is a useful optical pumping method for spectral analysis in CLC-semiconductors.

Conformational Change of Alkyl Chains at Phase Transitions in Thin Films of an Asymmetric Benzothienothiophene Derivative.

Among many promising organic semiconducting materials, 2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) shows outstanding device performances for organic field-effect transistors. This compound has a highly ordered liquid crystalline state, i.e., the smectic E (SmE) phase. Although the transition from the crystalline state to the SmE phase is believed to accompany melting of the alkyl chains, no spectroscopic evidence has been found so far. In this study, the conformational change of the decyl chains in Ph-BTBT-C10 films across the phase transition is analyzed by temperature-dependent measurements in situ using infrared spectroscopy. The spectral analysis reveals that the polycrystalline film has latent conformational disorder (the gauche conformer), the rate of which becomes more pronounced with the heat treatment. As expected, melting of the decyl chains is observed above the transition temperature to the SmE phase. This study also highlights the discovery of some key bands sensitive to the phase transitions in liquid crystalline organic semiconductors.

1]Benzothieno[3,2-b][1]benzothiophene-based liquid crystalline organic semiconductor for solution-processed organic thin film transistors

Liquid crystalline organic semiconductors have drawn tremendous interest due to potentials of applications in organic thin-film transistors (OTFTs) with their high electrical performance and solution-processability. However, alkylated liquid crystals often encounter poor thermal durability and mobility drop at smectic E (SmE) phase. Here, we introduce new solution-processable [1]benzothieno[3,2-b][1]benzothiophene-based small molecules with different types of alkyl side chains for OTFTs. All BTBT derivatives showed smectic phase; dialkylated 2-octyl-7-(5′-octyl-[2,2′-bithiophen]-5-yl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (O-2Th-BTBT) and 2-(5′-(2-ethylhexyl)-[2,2′-bithiophen]-5-yl)-7-octylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (E-2Th-BTBT) suffered mobility degradation at SmE phase. However, monoalkylated 2-([2,2′-bithiophen]-5-yl)-7-octylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (2Th-BTBT) did not exhibit SmE phase over temperature range, resulting in high carrier mobility up to 0.31 cm2/Vs through molecular bilayer transition by thermal annealing. Physicochemical and liquid crystal properties were analyzed, and thin film characterization by atomic force microscopy and X-ray diffraction was discussed.

Chiral Liquid Crystalline Electronic Systems

Liquid crystals bearing extended π-conjugated units function as organic semiconductors and liquid crystalline semiconductors have been studied for their applications in light-emitting diodes, field-effect transistors, and solar cells. However, studies on electronic functionalities in chiral liquid crystal phases have been limited so far. Electronic charge carrier transport has been confirmed in chiral nematic and chiral smectic C phases. In the chiral nematic phase, consisting of molecules bearing extended π-conjugated units, circularly polarized photoluminescence has been observed within the wavelength range of reflection band. Recently, circularly polarized electroluminescence has been confirmed from devices based on active layers of chiral conjugated polymers with twisted structures induced by the molecular chirality. The chiral smectic C phase of oligothiophene derivatives is ferroelectric and indicates a bulk photovoltaic effect, which is driven by spontaneous polarization. This bulk photovoltaic effect has also been observed in achiral polar liquid crystal phases in which extended π-conjugated units are properly assembled. In this manuscript, optical and electronic functions of these chiral π-conjugated liquid crystalline semiconductors are reviewed.

Novel heterocyclic liquid crystalline semiconductors with polymorphism

ABSTRACT Novel heterocyclic liquid crystalline semiconductors incorporating fused thiazolo[5,4-d]thiazole (N, S), thiophene (S), pyridine (N), and/or alkoxy (O) units have been synthesised via simple and high efficient reaction routes and exhibit polymorphic transitions. The correlation between the molecular structures, especially the presence of strong polar and electronegative heteroatoms, and mesomorphic behaviour and transition temperatures was investigated. The energy levels and band gap for heterocyclic liquid crystalline 6 and 10 have also been measured to further evaluate their potentials for use as promising organic semiconductors.

Bulk photovoltaic effect in organic binary systems consisting of a ferroelectric liquid crystalline semiconductor and fullerene derivatives

The bulk photovoltaic effect of fluorophenylterthiophene derivative 1 bearing a chiral alkyl chain doped with fullerene derivatives 2 and 3 was studied for the construction of organic bulk photovoltaic systems with bulk heterojunction structures. Electrical polarization could be immobilized in the chiral ordered smectic phase of the mixtures of compound 1 with compound 2 or compound 3 by a DC bias application during the phase transition from the chiral smectic C (SmC*) phase to the chiral ordered smectic phases. The spectral sensitivity is expanded to the illumination of visible light in the smectic phases of the FLC 1 doped with the fullerene derivatives while FLC 1 had a spectral sensitivity below 480 nm. Because of segregation of fullerene derivatives in the smectic phases of FLC 1 doped with fullerene derivatives 2 and 3 , electron transport characteristics were enhanced in the smectic phases and the electron mobilities reached 1 × 10 −2 cm 2 V −1 s −1 . As a result, polarity inversion of this bulk photovoltaic effect was observed in FLC 1 doped with the fullerene derivatives. The open circuit voltage was also increased to 0.5 V in the ordered smectic phase of fullerene-doped FLC 1 in spite of the symmetrical structure of the cells. • Binary mixtures consisting of a p- and n-type components for an organic bulk photovoltaic system. • High electron mobilities because of nanosegregation of fullerene derivatives in the LC phases. • Reversible polarity inversion of the bulk photovoltaic effect. • Bulk photovoltaic effect for visible light illumination.

Scalable Ultrahigh-Speed Fabrication of Uniform Polycrystalline Thin Films for Organic Transistors.

The fabrication of organic semiconductor thin films by printing technologies is expected to enable the low-cost production of devices such as flexible display drivers, RF-ID tags, and various chemical/biological sensors. However, large-scale high-speed fabrication of uniform semiconductor thin films with adequate electrical properties for these devices remains a big challenge. Herein, we demonstrate an ultrafast and scalable fabrication of uniform polycrystalline thin films with 100% surface coverage using liquid crystalline semiconductors such as 2-phenyl-7-decyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) and 2.7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8), at a rate of 3 orders of magnitude higher than before, i.e., 40 mm/s (2.4 m/min) or more by dip-coating in the drainage regime. Organic transistors fabricated with polycrystalline thin films of Ph-BTBT-10 show average mobilities of 4.13 ± 0.75 cm2/(V s) in the bottom-gate-bottom-contact configuration and 10.90 ± 2.40 cm2/(V s) in the bottom-gate-top-contact configuration comparable to those of the devices prepared with single-crystalline thin films. More importantly, these films almost maintain the FET performance when the substrate size is extended up to 4 square inch. The present findings are available for other liquid crystalline semiconductors and bring us one step closer to the realization of printed electronics.

Linearly polarized electroluminescence device in which the polarized plane can be rotated electrically using a chiral liquid crystalline semiconductor

The polarized plane of linearly polarized electroluminescence is rotated by DC bias application during a phase transition of a chiral liquid crystal phase.

13 C NMR investigations of molecular order of rod-like, bent-core, and thiophene mesogens.

In this review, methods to obtain the orientational order of topologically variant molecular mesogens using by one- and two-dimensional (2D) solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy are described. Besides 13 C chemical shifts, the 13 C─1 H dipolar couplings measured from 2D-separated local field (SLF) technique are used for computing the order parameters of a variety of mesogens. The investigated molecules are composed of a variable number of rings in the core, that is, core ranging from simply one ring to five rings. Among the mesogens investigated, a special focus has been placed on mesogens with thiophene rings, which are gaining popularity as liquid crystalline organic semiconductors. The replacement of a phenyl ring by thiophene in the core has a dramatic influence on molecular topology, as observed from the measured order parameters. The review highlights the advantages of the 2D SLF method for understanding the local dynamics and for mapping the topology of mesogens through the measured order parameters. SLF NMR studies of as many as 24 molecular mesogens that vary in terms of the molecular structure as well as topology are covered in the review. Order parameters of the rings have been estimated from the 13 C─1 H dipolar couplings in the nematic, smectic A, smectic C, and tilted hexatic phases as well as in B1 and B2 mesophases of various mesogens. It is anticipated that, in the years to come, the 2D SLF method would provide advanced molecular information on structurally complex mesogens that are emerging in liquid crystal science through the incessant efforts of synthetic chemists. The mini review covers the orientational order of topologically variant molecular mesogens determined by 1D and 2D solid-state 13 C NMR spectroscopy. Accordingly, rod-like, bent-core, and thiophene mesogens were subjected to 2D SLF measurements to get the order parameters from which the topology was established. The replacement of phenyl ring by thiophene and its influence on order parameters as well as on molecular topology is also discussed.

2-Phenylbenzothiophene-based liquid crystalline semiconductors

We report herein the design and synthesis of some novel liquid crystalline semiconductors constructed from a biologically and pharmacologically active building block molecule, namely 6-methoxy-2-(4-methoxyphenyl)benzo[b]thiophene, presenting efficient luminescence and medium charge mobility rate. A first series of mesogenic 2-phenylbenzothiophene derivatives (nPBT) was simply and rapidly obtained in good yields by successive demethylation/alkylation reactions of the available methoxy precursor. The further stepwise oxidations moreover resulted in two new sets of the corresponding sulfoxide (nPBTO) and sulfone (nPBTO2) derivatives, respectively, that were also mesomorphic. The liquid crystalline behaviour was comprehensively characterized by DSC, POM and SAXS: all compounds exhibit smectic-like behaviour in agreement with their calamitic shape. More specifically, mesogens nPBT showed a SmA phase, with in addition, a higher ordered smectic phase at lower temperature. As for the oxidized mesogens, they displayed a SmA phase only, and over particularly large temperature ranges for the nPBTO with longer chain lengths (n ≥ 8). The photo-physical properties have also been studied both in solution and thin films, and the molecules were found to display strong absorption in the UV/vis domain and intense luminescence in the range of 400–650 nm (yellowish green light) in high quantum yields (up to 62%). Both absorption and luminescence were also found to be affected by the oxidation of the benzothiophene moiety. Finally, the semi-conducting behaviour of three PBT compounds in the various mesophases was investigated by photocurrent TOF technique. Hole mobility rates of ca. 4 × 10−3 cm2 V−1 s−1 were measured in the lower temperature ordered mesophase for all of them, with the best performances (temperature range and mobility values) however obtained for the shortest homolog (n = 6). With such highly reasonable mesomorphic, light-emitting and semiconducting functional features, as well as being cheap and easy to synthesize and to process, these materials become very attractive and may be incorporated into various kinds of electronic devices (OFET and OLED).

Observation of cavity polaritons in a metal-mirror Fabry-Pérot microcavity containing liquid-crystalline semiconductor based on perylene bisimide units.

We investigate the optical properties of a metal-mirror microcavity containing a liquid-crystalline (LC) perylene tetracarboxylic bisimide (PTCBI) derivative. Measurements of the transmission's incidence angle dependence show that the peaks are split in a complex way and shift as the angle changes. Further, measurements of the photoluminescence spectrum's emission angle dependence show that the peak also shifts with the angle, as in the transmission experiment. We also carry out a theoretical analysis; the theoretical and experimental results are in very good agreement, and we estimate the vacuum Rabi splitting energies to be about 212, 180, and 240 meV. In addition, the peak photoluminescence energy coincides with the lower polariton branch obtained by transmission experiment. Finally, in a time-resolved photoluminescence experiment, we observe a fast relaxation component that is not seen in the bare LC PTCBI film. We believe this is due to cavity effects increasing the spontaneous emission transition rate, indicating that the emissions are due to cavity polaritons.

Nonlinear Nonacenes with a Dithienothiophene Substructure: Multifunctional Compounds that Act as Columnar Mesogens, Luminophores, π Gelators, and p-Type Semiconductors.

Board-like liquid-crystalline semiconductors based on the dithieno[3,2-b;2',3'-d]thiophene (DTT) substructure were synthesized and their thermal, self-assembly, optical, and semiconducting properties investigated. These sanidic compounds, bearing eight peripheral chains, are mesomorphic and spontaneously self-assemble into columnar hexagonal mesophases (Colhex ) over broad temperature ranges, as confirmed by polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering. Strong blue photoluminescence with absolute quantum yields up to 33 % were measured. These compounds also form blue-light emitting gels in various organic solvents. The fibrillar-like morphology of these gels, reminiscent of the columnar structure, and stabilized by efficient intermolecular interactions, was characterized by scanning electron microscopy. The charge carrier mobility of these π-extended mesogens was measured by time-of-flight transient photocurrent technique in the Colhex mesophase, and showed good hole-transport performance with charge carrier mobility of 10-3 cm2 V-1 s-1 .

Novel calamitic liquid crystalline organic semiconductors based on electron-deficient dibenzo[c,h][2,6]naphthyridine: synthesis, mesophase, and charge transport properties by the time-of-flight technique

Mobility shows Poole–Frenkel like behaviour in the low order mesophase of liquid crystalline organic semiconductors based on an electron-deficient dibenzo[c,h][2,6]naphthyridine (DBN) derivative.

Molecular p-doping in organic liquid crystalline semiconductors: influence of the charge transfer complex on the properties of mesophase and bulk charge transport.

We explore the molecular nature of doping in organic semiconductors (OSCs) by employing a liquid crystalline organic semiconductor based on phenyl naphthalene as a model. The mesophase nature of composites that include a charge transfer complex (CTC) between the OSC (8-PNP-O12) and an electron acceptor (F4TCNQ) has been investigated by means of differential scanning calorimetry, polarized optical microscopy and X-ray scattering. Optical and vibrational spectroscopies allow us to explore the characteristics and the amount of charge transfer in the CTC and expose some properties that appear only in the complexed state. We have found this system to exhibit partial charge transfer, which manifests itself in all the phase states of the host 8-PNP-O12, as well as in solution. Due to the lowering of molecular symmetry as a result of the charge transfer, one of the previously IR-only vibrational bands of the nitrile group is found to be now active in the Raman spectrum. We have also made an attempt to further investigate the influence of dopant introduction on the bulk hole mobility of 8-PNP-O12. It is found that the presence of the CTC promotes the hole transport in the Smectic B mesophase, however it seems to have a somewhat negative influence in the less ordered smectic A mesophase. This work aims to establish the link between the inevitable change of molecular geometry that occurs on charge transfer with the results obtained by spectroscopic techniques and electronic charge carrier mobility measurements.