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Fukuoka-shi, Japan

Komino T.,Kyushu University | Nomura H.,Kyushu University | Koyanagi T.,Life Center Kyushu | Adachi C.,Kyushu University | Adachi C.,Life Center Kyushu
Chemistry of Materials | Year: 2013

The efficiency roll-off characteristics in highly efficient thermally activated delayed fluorescence (TADF) based organic light-emitting diodes (OLEDs) were effectively suppressed by controlling the molecular orientation of a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host matrix. The hole mobility in the light emitting layer was found to govern the magnitude of this suppression. Three-dimensional finite-difference time-domain calculations and photoluminescence quantum yield measurements revealed that the optical characteristics of the fabricated devices and photophysical properties of the TADF emitter did not affect efficiency roll-off. CBP molecules adopted random orientations when films were fabricated at high temperature (350 K), resulting in low hole mobility, and shifting the recombination zone away from the interface of the emitting layer with the electron transporting layer. When CBP was randomly orientated, efficiency roll-off was suppressed by 30% at a current density of 100 mA cm-2. This result indicates that control of the molecular orientation of the host can allow us to indirectly tune the carrier balance in OLEDs. © 2013 American Chemical Society. Source

Nishio T.,Life Center Kyushu | Nishio T.,Lintec Co. | Kashiwagi M.,Life Center Kyushu | Kashiwagi M.,Kyushu Institute of Technology | And 7 more authors.
Applied Physics Express | Year: 2010

Nanoporous polymer film with the smallest pore size of 80 nm was obtained from 0.2 wt % polystyrene/dichloromethane solution at 4°C by the dip-coating method in conjunction with an evaporative humidifier. Fabrication parameters affecting the pore size such as wet coating thickness, solution temperature, and nozzle-substrate gap were varied. The distance between pores depends on the nozzle-substrate gap. For fabrication of the nanosized pores, an appropriate drying time prevents the water droplets from coalescing. Formation of the nanoporous surface structure in the films depends on the solution concentration and solution temperature preventing too rapid drying. © 2010 The Japan Society of Applied Physics. Source

Kashiwagi M.,Life Center Kyushu | Kashiwagi M.,Kyushu Institute of Technology | Hirata S.,Kyushu University | Harada K.,Life Center Kyushu | And 10 more authors.
Applied Physics Letters | Year: 2011

A porous thin film of Bi0.4Te3Sb1.6 with an enhanced figure of merit of 1.8 at room temperature was fabricated by flash evaporation on an alumina substrate containing hexagonally arranged nanopores with an average diameter of 20 nm, separated by an average distance of 50 nm. The thermal conductivity was significantly reduced compared with standard Bi0.4Te3Sb1.6 films to 0.25 W/(m·K) with no major decrease in either the electrical conductivity (398 S/cm) or the Seebeck coefficient (198 μV/K). The reduction in thermal conductivity was rationalized using a model for the full distribution of the phonon mean free path in the film. © 2011 American Institute of Physics. Source

Nakata M.,Life Center Kyushu | Nakata M.,Panasonic | Kawano K.,Life Center Kyushu | Kawano K.,Panasonic | And 8 more authors.
Thin Solid Films | Year: 2012

Organic nanostructures can enhance the device performance of organic photovoltaic (OPV) cells, because a significantly large area of a donor/acceptor heterointerface can be prepared. In this study, we improved the performance of OPV cells composed of cupper phthalocyanine (CuPc)/fullerene (C 60) by introducing a pentacene nanometer-sized grain layer into an anode/CuPc layer. The smallest featured grain sizes are as small as 100 nm, resulting in the formation of ragged heterojunctions. We show that power conversion efficiency is enhanced up to 20% compared with that of conventional planar heterojunctions. © 2012 Elsevier B.V. Source

Harada K.,Life Center Kyushu | Harada K.,Kyushu University | Sumino M.,Life Center Kyushu | Sumino M.,Kyushu University | And 5 more authors.
Applied Physics Letters | Year: 2010

We demonstrate an improved thermoelectric performance of small molecular thin films fabricated by thermal deposition of pentacene as a p -type conduction layer. To enhance the performance, a bilayer structure composed of an intrinsic pentacene layer and an acceptor 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane layer is utilized as the prototype thermoelectric element. With the bilayer structure, the electrical conductivity reaches 0.43 S/cm, exhibiting a positive Seebeck coefficient of about 200 μV/K. We thus obtain a high power factor of 2.0 μW/ mK2 with an optimized layer thickness. © 2010 American Institute of Physics. Source

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