Masahiro Misaki

Highly polarized polymer light-emitting diodes utilizing friction-transferred poly(9,9-dioctylfluorene) thin films

Polarized polymer light-emitting diodes (PLEDs) have been constructed utilizing friction-transferred poly(9,9-dioctylfluorene) (PFO) thin films. The friction transfer technique allows oriented PFO to be deposited directly onto an indium tin oxide anode without an alignment layer such as polyimide. Polarized absorption and photoluminescence spectra revealed that the polymer backbones are highly aligned in the friction direction. We fabricated PLEDs consisting of friction-transferred PFO as an emissive layer, vacuum-deposited bathocuproine as an electron transport and hole-blocking layer, and a vacuum-deposited LiF∕Al cathode. Highly polarized blue emission with an integrated polarization ratio of 31 and a luminance of up to 300cd∕m2 was observed from the PLEDs.

Molecular orientation and anisotropic carrier mobility in poorly soluble polythiophene thin films

Oriented thin films of a poorly soluble polythiophene, poly[benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl(3′,4,4″,4″′-tetradodecyl[2,2′:5′,2″:5″,2″′-quaterthiophene]-5,5″′-diyl)], were fabricated using the friction-transfer technique. The detail of the film structure was investigated using polarized UV-vis spectroscopy, transmission electron microscopy, and grazing-incidence x-ray diffraction observations. The friction-transfer technique enables control of the anisotropic carrier transport according to the face-on and/or edge-on molecular orientations in films with one-directionally aligned polymer backbones.

Device Performance of an n-Channel Organic Thin-Film Transistor with LiF/Al Bilayer Source and Drain Electrodes

We investigated the device properties of a copper (II) perfluorophthalocyanine (F16CuPc)-based n-channel organic thin-film transistor (TFT) with source and drain electrodes consisting of an LiF/Al bilayer. We clarified that the LiF thin film intercalated between the F16CuPc and Al layers played a key role in the observed field-effect transistor characteristics of the TFT. We concluded that the contact resistance was effectively reduced by the LiF layer, providing protection against an unfavorable chemical reaction with Al and possible formation of n-doped regions in the vicinity of the LiF/Al electrodes of the F16CuPc layer.

Fabrication of Phase-Separated Benzoporphycene/[6,6]-Phenyl-C61-Butyric Acid Methyl Ester Films for Use in Organic Photovoltaic Cells

The crystallization behavior of benzoporphycene (BPc) converted from a thermoconvertible BPc precursor (BPc-pre) in the presence of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was investigated. Upon annealing spin-coated films of BPc-pre and PCBM, BPc and PCBM crystallized individually to form a phase-separated structure. Repeated spin-coating of BPc-pre and PCBM in dichloromethane solution and subsequent annealing allowed the thickness of BPc films to be increased. BPc crystallized homoepitaxially on the underlying crystalline BPc layer after each deposition step. The thermal conversion conditions affected the size of BPc and PCBM crystals and the photovoltaic properties of the corresponding cells. An organic photovoltaic cell containing a repeatedly stacked BPc/PCBM film prepared by annealing at 250 °C for 1 min showed good rectification and a power conversion efficiency of 0.16%.

Thermal Conversion Behavior and Morphology Control of Benzoporphycene from a Novel Soluble Precursor

The thermal conversion behavior of a benzoporphycene precursor (BPc-pre) and the film formation of benzoporphycene (BPc) in the presence of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were reported. As-spun BPc-pre/PCBM films were prepared from a dichloromethane solution, and BPc-pre was converted into BPc by annealing. Both BPc and PCBM crystallized independently and PCBM crystals dramatically inhibited the aggregation of BPc, resulting in a phase-separated structure.

Crystal growth of rubrene in ionic liquids by vacuum vapor deposition

The crystal growth and unique morphological changes of organic semiconductor rubrene, fabricated by ionic liquid (IL)-assisted vacuum vapor deposition, were investigated. The texture and structure of rubrene films strongly depended on the thickness of IL films on substrates, namely, three-dimensional dendrites in 3D-ILs, two-dimensional microfibrils in 2D-ILs, and two-dimensional spherulites in 0D-ILs. The growth of two-dimensional spherulites would be promoted by the supersaturation of ILs, minutely controlling the rate and total amount of rubrene deposition. The growth mechanisms of rubrene fabricated by IL-assisted vacuum vapor deposition were different from that of the conventional one in terms of the nucleation and growth modes.

Oriented Polyfluorene Films Dye-Doped for Whitening of Polarized Electroluminescent Devices

Whitening of the emission of electroluminescent (EL) devices was performed by doping α-sexithiophene (6T), which is an orange emitting dye, into blue emitting oriented poly(9,9-dioctylfluorene) (PFO) polymer films. These oriented PFO films were produced by means of the friction transfer method and the doping of 6T was performed by means of the vapor transportation method which is a solvent-free process. The excellent orientation of PFO films and of the doped 6T led to very good polarization of the light emitted by these EL devices.

LiF/Al bilayer source and drain electrodes for n-channel organic field-effect transistors

We investigated electronic effects of an ultrathin LiF interlayer on device properties of copper(II) perfluorophthalocyanine (F 16 CuPc)-based field effect transistors (FETs) with Al source and drain electrodes. We observed FET operation only for the device with LiF/Al bilayer electrodes. We concluded that an effective electronic contact was formed between Al and F 16 CuPc layer by the presence of the LiF layer, which prevented the F 16 CuPc layer from reacting with Al and n-doped with dissociated Li atoms in the vicinity of the source and drain electrodes.

Photovoltaic properties of organic solar cell with octafluorophthalocyanine as electron acceptors

Organic photovoltaic (OPV) cells were fabricated using copper-2,3,9,10,16,17,23,24-octafluorophthalocyanine (F8CuPc) as an electron acceptor. Normal and inverted OPV cells using F8CuPc were showed rectification in dark and photovoltaic characteristics under illumination. The inverted-type cell had good durability. The power conversion efficiency (η) of the inverted-type cell was 0.11%, approximately 6.5 times higher than that of the normal-type cell. The cell using F8CuPc was more stable than that using fullerene in air. These results indicate the possibility of using F8CuPc as an electron acceptor for air-stable OPV cells.

Enhanced thermoelectric power of single-wall carbon nanotube film blended with ionic liquid

We have investigated the thermoelectric power of single-wall carbon nanotubes (SWCNTs) with an ionic liquid (IL). The SWCNT/IL films showed simultaneous increase in electrical conductivity and the Seebeck coefficient compared with the pristine SWCNT. No thermoelectric power was observed for the IL. The X-ray diffraction pattern and impedance diagram showed a unique behavior with the concentration of IL, which implies that the interaction between the SWCNTs and IL enhances the thermoelectric power of the SWCNTs. As a result of the simultaneous increase in these parameters, the power factor exhibited a 10-fold increase.