Akihiko Fujii

Nanoparticle-Stabilized Cholesteric Blue Phases

The authors report the expansion of the temperature range of cholesteric blue phases by doping nanoparticles. When spherical gold nanoparticles with a mean diameter of 3.7 nm were doped in a blue phase-exhibiting multi-component liquid crystal mixture, the temperature range of the cholesteric blue phase increased from 0.5 to 5 °C, while the clearing temperature decreased by approximately 13 °C. We believe that the mechanism stabilizing the cholesteric blue phase is similar to that of polymer-stabilized cholesteric blue phases: the nanoparticles accumulate in the lattice disclinations, stabilizing the overall cholesteric blue structure.

Solution Processable Organic Solar Cell Based on Bulk Heterojunction Utilizing Phthalocyanine Derivative

Organic thin-film solar cells based on a bulk heterojunction utilizing the phthalocyanine derivative 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2) have been studied. C6PcH2 is soluble in common organic solvents such as chloroform, and the blend uniform thin film with the fullerene derivative 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM) could be fabricated by a spin-coating method. Solar cells with an indium–tin-oxide/polymer hole transport layer/C6PcH2:PCBM/Al structure, the active layer of which was prepared by a wet process using a low-weighted molecular system, have demonstrated a high external quantum efficiency of more than 70% in the Q-band absorption region of C6PcH2 and a high energy conversion efficiency of 3.1%.

Nanoparticle-Dispersed Liquid Crystals Fabricated by Sputter Doping

2010 WILEY-VCH Verlag Gm Nanoparticle (NP) dispersions in liquid crystals (LCs) is an exciting interdisciplinary research topic attracting interest from both academia and industry, as the resultant suspensions exhibit functions not observed in their pure state: improved response speed and lowering of the Frederiks transition voltage has been demonstrated in nematic LCs either doped with metal, ferroelectric, or dielectric NPs, and dispersions of core–shell semiconductor NPs have been predicted to allow for tunable refraction including negative and zero refractive index. While some of these suspensions can be fabricated by simply mixing the LC with the NPs, the production of metal NPs dispersed in LCs usually require their surface to be modified chemically, in order to stabilize them in the anisotropic host without causing agglomeration. However, the chemical approach is not time and cost efficient, since specific reactions must be performed for different materials, and the byproducts of the reaction must be removed to ensure the performance of the resultant suspension. Here, we demonstrate a robust, single-step technique to fabricate metal NP–LCdispersions without having to prepare the NPs chemically. We show that highly dispersed gold NP suspensions are produced simply by sputter depositing the target material on the host LC. We provide direct proof of the dispersed NPs through electron microscope observation, and show that an improved electro-optic property is obtained in the resultant suspension. This technique is in principle applicable to any kind of material and LC phase, and thus provides a versatile route to prepare stable NPs dispersed in LCs. To date, four main kinds of materials have been dispersed in LCs as NPs: metals, insulators, ferroelectrics, and carbon nanomaterials (fullerenes and nanotubes). In all suspensions, novel functionalities are elicited depending on the optical and electrical properties of the NPs and the interaction of the NPs with the orientational field of the host LC. Reduction of the Frederiks threshold voltage has been observed in nematic LCs doped with insulating or semiconducting NPs, while a memory effect in which a residual transmittance remains after the electric field is switched off has been observed in aerosol-filled nematic LCs, attributed to the stabilization of the LC orientation by the aerosols. A frequency dependent electro-optic response has been reported in metal NP-doped nematic LCs as a result of the dielectric property of the metal NPs coupling with the host LC, and was explained in the framework of the Maxwell–Wagner theory. On the other hand, upon doping ferroelectric NPs, the coupling of the spontaneous polarization with the LC director caused an increase in the order parameter and the LC temperature range. Carbon nanotubes dispersed in nematic LCs were found to align along the LC director and cause a large increase in the conductivity, while fullerenes also caused a decrease in the Frederiks threshold voltage. Depending on the nanomaterial to be doped, different approaches have been adopted to ensure dispersion. The dispersion of dielectric materials and larger particles has been achieved by simply mixing the NPs with the host and sonicating the mixture; however, to date, in all cases where metallic NPs were dispersed in LCs, the NPs were treated chemically with capping agents to attain sufficient solubility in the host. Since this requires the NPs to be prepared beforehand, treated by tedious chemical reactions for functionalization, and then treated again for the removal of unwanted byproducts, the purity and throughput becomes limited and the production process is not efficient. Herein, we propose a method that can drastically simplify the procedure to fabricate metal NP–LC suspensions: we claim that such suspensions can be fabricated by sputter depositing the target material on the host LC. It has long been known that liquid substrates used in place of conventional solid substrates are capable of holding NPs of the target material deposited by vacuum evaporation. However, because deposition techniques require a reduced-pressure environment, only a limited variety of liquids, such as oils or ionic liquids, have been used so far. Here, we exploit the fact that certain LC molecules also possess a vapor pressures smaller than 1Pa at room temperature, which is sufficient for them to undergo the sputter deposition process, which is performed at a pressure of a few to several tens Pa. Figure 1a shows the photographs of 4-pentyl-40-cyanobiphenyl (5CB), a compound showing a nematic phase between 24 and 35 8C, before and after sputter deposition of gold. The vapor pressure of 5CB is reported to be less than 0.23Pa at room temperature, so the gold can be deposited without any noticeable evaporation of the host. The pure 5CB appears white at room temperature

Organic solar cells using few-walled carbon nanotubes electrode controlled by the balance between sheet resistance and the transparency

Organic photovoltaic devices (OPD) using high conductive transparent few-walled carbon nanotubes (FWNT) film prepared by spraying was fabricated as a selective hole collection. Photovoltaic response with different sheet resistance (Rs) and the transparency (T) of FWNT film was investigated. Maximum efficiency of OPD up to 0.61% with the structure of FWNT (T=70%, Rs=86 Ω/◻)/poly(3-hexylthiophene): [6-6]phenyl-C61-butyric acid methyl ester/Al demonstrates a promising alternative of ITO (0.68%) with almost identical operation. The performance improvement results from the optimal balance between sheet resistance and transparency with three-dimensional network interface between nanotubes and polymers.

Efficiency enhancement in mesogenic-phthalocyanine-based solar cells with processing additives

Bulk heterojunction (BHJ) solar cells, fabricated by spin casting processes, with phase-separated small molecules including a phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2), and a fullerene derivative, 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM), have been demonstrated to have a power conversion efficiency exceeding 4.1%. The C6PcH2:PCBM BHJ surface morphology and the phase separation have been controlled by utilizing processing additives in various solvents. The effects of the processing additives on the crystallization of the discotic C6PcH2 columns in hexagonal structures have been discussed.

Pentacene:Fullerene Multilayer-Heterojunction Organic Photovoltaic Cells Fabricated by Alternating Evaporation Method

Novel small-molecule organic photovoltaic cell structures with pentacene and fullerene multilayers are reported. Pentacene is an attractive material for photovoltaic cells, but it tends to crystallize with molecular orientation during vacuum deposition. Therefore, thus far, it has been difficult to fabricate a bulk-heterojunction active layer with fullerenes by coevaporation, despite the demand for pentacene/fullerene bilayer heterojunction cells owing to their good photovoltaic performance. Here, instead of preparing an active layer by coevaporation, we prepare a pentacene and fullerene multilayer-heterojunction active layer by alternating evaporation. It was found that multilayer cells show superior photovoltaic performance to bilayer cells owing to the improvements in their exciton dissociation efficiency and charge carrier transport properties.

Electric Field Dependence of Lasing Wavelength in Cholesteric Liquid Crystal with an In-Plane Helix Alignment

The authors report continue tuning of the lasing wavelength in cholesteric liquid crystals (ChLCs) by an applied electrical fields. Although the pitch length and the corresponding lasing wavelength is sensible to external fields, only a discrete shift in the pitch is observed in the conventional cell configuration where the helix axis is lying the cell-normal direction, since the molecules are anchored to a single direction at the substrate interfaces. In this study, continuous wavelength tuning is achieved by laying the helix in the cell-plane direction, mitigating the effects of anchoring: this alignment is attained simply by applying an electric field while cooling the sample from the isotropic state, in a planarly rubbed sandwich cell. We also show that the tuning characteristics as that predicted based on calculations using the elastic theory.

Alkyl Substituent Length Dependence of Octaalkylphthalocyanine Bulk Heterojunction Solar Cells

Alkyl substituent length dependences of photovoltaic performance of bulk-heterojunction (BHJ) organic solar cells (OSCs) utilizing 1,4,8,11,15,18,22,25-octaalkylphthalocyanine (CnPcH2) mixed in [6,6]-phenyl C71 butyric acid methyl ester have been studied. By shortening the alkyl substituents, stacking of the discotic CnPcH2 columns is probably changed from 2D rectangular lattices to pseudohexagonal structures, and Davydov splitting at the Q-band of CnPcH2 absorbance spectra decreases, which results in the higher hole mobility and the deeper highest occupied molecular orbital energy levels. As a result, the power conversion efficiencies of CnPcH2-based BHJ OSCs are improved from 0.3 to 3.7% by changing the alky substituent length.

Improved Lasing Threshold of Cholesteric Liquid Crystal Lasers with In-Plane Helix Alignment

In this paper, we report the improved lasing threshold in cholesteric liquid crystal (ChLC) lasers using an in-plane helix alignment, where the helix axis lies in the in-plane direction of the cell. The in-plane helix alignment of ChLCs was obtained by applying an in-plane electric field in a cell with homeotropic surface treatment while cooling the sample from the isotropic phase. The energy threshold of this device was 0.3 mJ/(cm2pulse), which was found to be less than 1/35 that of conventional planarly aligned ChLC lasers.

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.