Showing papers by "Chihaya Adachi published in 2011"
TL;DR: In this article, a material possessing a very small energy gap between its singlet and triplet excited states, ΔE1−3, which allows efficient up-conversion of triplet excitons into a singlet state and leads to efficient thermally activated delayed fluorescence (TADF), is reported.
Abstract: A material possessing a very small energy gap between its singlet and triplet excited states, ΔE1−3, which allows efficient up-conversion of triplet excitons into a singlet state and leads to efficient thermally activated delayed fluorescence (TADF), is reported. The compound, 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine, breaks the restriction of a large energy gap, with a ΔE1−3 of just 0.11 eV, while maintaining a high fluorescent radiative decay rate (kr∼107). The intense TADF provides a pathway for highly efficient electroluminescence.
906 citations
TL;DR: In this article, the authors highlight the potential of employing dye-doped emission layers with emitting molecules having horizontally oriented transition dipole moments and demonstrate that a mainly horizontal dipole orientation enhances light outcoupling by around 45%.
211 citations
TL;DR: In this paper, it was shown that the intermolecular C-H···N hydrogen bonds can be formed even in vacuum-deposited organic films having flat interfaces.
Abstract: Simple bottom-up fabrication processes for molecular self-assembly have been developed for the construction of higher-order structures using organic materials, and have contributed to maximization of the potential of organic materials in chemical and bioengineering. However, their application to organic thin-film devices such as organic light-emitting diodes have not been widely considered because simple fabrication of a solid film containing an internal self-assembly structure has been regarded as difficult. Here it is shown that the intermolecular C–H···N hydrogen bonds can be simply formed even in vacuum-deposited organic films having flat interfaces. By designing the molecules containing pyridine rings properly for the intermolecular interaction, one can control the molecular stacking induced by the intermolecular hydrogen bonds. It is also demonstrated that the molecular stacking contributes to the high carrier mobility of the film. These findings provide new guidelines to improve the performance of organic optoelectronic devices and open up the possibilities for further development of organic devices with higher-order structures.
142 citations
TL;DR: In this article, 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.
Abstract: 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.
100 citations
TL;DR: In this paper, a tris[4-(5-phenyl thiophen-2-yl)phenyl]amine layer between the donor and the PEDOT:PSS layer was introduced to prevent exciton quenching.
Abstract: We demonstrate enhanced power conversion efficiency (ηPCE) for small molecular-based organic photovoltaic cells with an exciton blocking layer (ExBL) at the anode/donor interface. Although poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) films are widely used as anodic buffer layers, they also act as exciton quenchers. To prevent exciton quenching, we introduced a tris[4-(5-phenyl thiophen-2-yl)phenyl]amine layer between the donor and the PEDOT:PSS layer and clarified its effect. By a combination of dual ExBLs at both the anode and cathode sides, we achieved significantly enhanced short circuit current and ηPCE values; the highest ηPCE = 5.24% was obtained by optimizing the device parameters.
100 citations
TL;DR: A unique OLED containing a neat liquid semiconductor is developed, i.e., a liquid OLED that can be used for lighting and display applications and can be replaced by a fl ow of fresh organic liquid emitters, removing the problem of OLED degradation resulting from the decomposition of organic materials.
Abstract: Research on organic light-emitting diodes (OLEDs) containing solid state organic semiconductors has advanced signifi cantly in the past twenty years, resulting in indispensible technology for fl at panel displays [ 1 ] and lighting applications. [ 2 ] This is because OLEDs exhibit not only high-performance light-emitting characteristics [ 3 , 4 ] but also provide unique benefi ts such as light weight and fl exibility, [ 5 , 6 ] which are diffi cult to realize using inorganic LEDs. Taking into account the advantages of organic materials, we developed a unique OLED containing a neat liquid semiconductor, i.e., a liquid OLED that can be used for lighting and display applications. [ 7 ] It is expected that liquid OLEDs will allow the realization of truly fl exible displays because detachment between the liquid emitting layer and electrodes does not occur even when the devices are bent signifi cantly. Furthermore, liquid emitters degraded by long-term use in an OLED can be replaced by a fl ow of fresh organic liquid emitters, removing the problem of OLED degradation resulting from the decomposition of organic materials (see Figure S1 of Supporting Information). Figure 1 demonstrates electroluminescence (EL) from a liquid OLED. Here, a liquid emitter was passed from the top to the bottom of the device by capillary action between two patterned indium tin oxide (ITO) glass substrates separated by a gap of 5.0 ± 0.5 μ m. After injection of the liquid emitter at the top, it penetrates into the gap between the substrates by capillary action and fl ows downward (Figure 1(i)) . The location of the liquid emitter can be detected by photoluminescence as shown in Figure 1(i)–(iv) . EL was not observed before the liquid emitter reaches the area where the two ITO electrodes overlap, as shown in Figure 1(v) . In contrast, intense EL was observed when the liquid emitter reached the area between the electrodes (Figure 1(vi)) . The area exhibiting EL then expanded gradually
94 citations
TL;DR: In this paper, the performance of n-type organic thin-film thermoelectric elements utilizing a bilayer structure composed of C60 and Cs2CO3 was demonstrated.
Abstract: The high performance of n-type organic thin-film thermoelectric elements utilizing a bilayer structure composed of C60 and Cs2CO3 was demonstrated. By employing an underlying layer, the electrical conductivity and the power factor of the n-type thermoelectric elements were significantly improved, and a maximum power factor of 20.5 μW m−1 K−2 at room temperature was demonstrated. In addition, an organic p-n prototype thermovoltaic device was demonstrated.
88 citations
TL;DR: A new method is demonstrated that applies the ambipolar light-emitting transistors to directly observe the recombination zone, and unambiguously indicates that the light emission comes from the electric-field-induced p-i-n homojunction in ambipolar LETs.
Abstract: A new method for investigating light-emitting property in organic devices is demonstrated. We apply the ambipolar light-emitting transistors (LETS) to directly observe the recombination zone, and find a strong link between the transistor performance and the zone size. This finding unambiguously indicates that the light emission comes from the electric-field-induced p-i-n homojunction in ambipolar LETs.
79 citations
TL;DR: It is demonstrated that periodic grating structures with almost µm-scale can be used to extract SPs as well as waveguide modes and therefore enhance the outcoupling efficiency in light-emitting thin film structures.
Abstract: Organic light-emitting diodes (OLEDs) usually exhibit a low light outcoupling efficiency because a large fraction of power is lost to surface plasmons (SPs) and waveguide modes. In this paper it is demonstrated that periodic grating structures with almost µm-scale can be used to extract SPs as well as waveguide modes and therefore enhance the outcoupling efficiency in light-emitting thin film structures. The gratings are fabricated by nanoimprint lithography using a commercially available diffraction grating as a mold which is pressed into a polymer resist. The outcoupling of SPs and waveguide modes is detected in fluorescent organic films adjacent to a thin metal layer in angular dependent photoluminescence measurements. Scattering up to 5th-order is observed and the extracted modes are identified by comparison to the SP and waveguide dispersion obtained from optical simulations. In order to demonstrate the low-cost, high quality and large area applicability of grating structures in optoelectronic devices, we also present SP extraction using a grating structure fabricated by a common DVD stamp.
64 citations
TL;DR: This study grew cupper phthalocyanine (CuPc) crystalline nanopillar arrays by conventional thermal gradient sublimation technique using a few-nanometer-sized trigger seeds composed of a CuPc and 3,4,9,10-perylene-tetracarboxylic-dianhydride stacked layer and optimized the pillar density by tuning crystal growth condition in order to apply it to OPV cells.
Abstract: To enhance the performance of organic photovoltaic (OPV) cells, preparation of organic nanometer-sized pillar arrays is fascinating because a significantly large area of a donor/acceptor heterointerface having continuous conduction path to both anode and cathode electrodes can be realized. In this study, we grew cupper phthalocyanine (CuPc) crystalline nanopillar arrays by conventional thermal gradient sublimation technique using a few-nanometer-sized trigger seeds composed of a CuPc and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) stacked layer. We optimized the pillar density by tuning crystal growth condition in order to apply it to OPV cells.
49 citations
TL;DR: In this article, the synthesis and characterization of two DPP-based organic molecules functionalized with phenanthrene and ethynylphenanthrene was reported and their application as the donor materials of solution-processed OPVs.
TL;DR: The dependence of the amplified spontaneous emission threshold (Eth) on molecular orientation in vacuum deposited thin films fabricated from diphenyldiamine- and carbazole-substituted spirofluorene derivatives was investigated in this article.
Abstract: The dependence of the amplified spontaneous emission threshold (Eth) on molecular orientation in vacuum deposited thin films fabricated from diphenyldiamine- and carbazole-substituted spirofluorene derivatives was investigated. The molecular orientations were modified by changing the substrate temperature during deposition and were almost parallel to the substrate at around 300 K in all of the films. The orientation became random in the film containing diphenyldiamine compound at around 380 K, resulting in decreased optical anisotropy. However, for the carbazole compound, the molecular orientation scarcely changed from 300 to 380 K, while the root-mean-square roughness increased from 0.28 to 0.94 nm. Eth approximately doubled for both the films when the optical anisotropy decreased because of the randomization of molecular orientation or when the surface roughness increased.
TL;DR: In this paper, three suitable techniques for extracting surface plasmons (SPs) from organic light-emitting diodes (OLEDs) are discussed from a theoretical point of view and investigated experimentally in simplified OLED-like structures.
Abstract: Organic light-emitting diodes (OLEDs) usually exhibit a low light-outcoupling efficiency of only 20%. Typically, more than 30% of the available power is lost to surface plasmons (SPs). Consequently, the overall efficiency could be strongly enhanced by recovering SP losses. Therefore, three suitable techniques for extracting SPs-index coupling, prism coupling, and grating coupling-are discussed from a theoretical point of view and investigated experimentally in simplified OLED-like structures. The basic physical processes are clarified by systematic variations of the involved layer thicknesses and by excited state lifetime measurements. In addition, the analysis of the results is supported by optical simulations based on a dipole model. Finally, the advantages and disadvantages of each method, their potential efficiency for recovering SP losses, as well as the applicability in OLEDs are compared.
TL;DR: A linear-shaped phosphorescent platinum(II) complex that adopts a horizontal orientation in amorphous films was synthesized in this article, and the complex possesses a photoluminescence quantum efficiency of ΦPL = 50±3% in thin films with the host of m-biscarbazolylbenzene and 4,4'-bis(N-carbazyll)biphenyl.
Abstract: A linear-shaped phosphorescent platinum(II) complex that adopts a horizontal orientation in amorphous films was synthesized. The complex possesses a photoluminescence quantum efficiency of ΦPL = 50±3% in thin films with the host of m-biscarbazolylbenzene and 4,4'-bis(N-carbazolyl)biphenyl. Polarized emission from the edge of these thin films confirmed the horizontal orientation of the complex within them. An organic light-emitting diode containing the complex possessed a maximum external quantum efficiency of ηEQE = 15.8±3%. The light out-coupling efficiency of the device was estimated to be at least Φout = 32±3%, which was ascribed to the horizontal orientation of the complex.
Patent•
29 Aug 2011TL;DR: A delayed fluorescence material containing a mixture of an acceptor compound and a donor compound that satisfies the following formulae (1) to (4):.............. T 1 A −S 1 > 0.2 eV
Abstract: A delayed fluorescence material containing a mixture of an acceptor compound and a donor compound that satisfies the following formulae (1) to (4):
T 1 A −S 1 >0.2 eV (1)
T 1 D −S 1 ≧0.2 eV (2)
|LUMO A |>2.0 eV (3)
|HOMO D |≦5.3 eV (4)
wherein T 1 A represents the excited triplet energy of the acceptor compound; T 1 D represents the excited triplet energy defined of the donor compound; S 1 represents the excited singlet energy of the exciplex; LUMO A represents the energy level of LUMO of the acceptor compound; and HOMO D represents the energy level of HOMO of the donor compound.
TL;DR: In this article, the authors synthesized spirobifluorene derivatives; hexylbiphenyl-spiro-brifluororene (HBP-Spiro) and triphenylamine-SPiro-drone (TPA-TPA) and demonstrated the operation of a distributed-feedback polymer waveguide blue laser, achieving an operation lifetime of 27,000 shots for a pump-pulse duration of 1.2
TL;DR: The fluorescence detection system was successfully used in a flow-based enzyme-linked immunosorbent assay on a PDMS microfluidic device for the rapid determination of immunoglobulin A (IgA), a marker for human stress.
Abstract: A fluorescence detection system for a microfluidic device using an organic light-emitting diode (OLED) as the excitation light source and a charge-coupled device (CCD) as the photo detector was developed. The OLED was fabricated on a glass plate by photolithography and a vacuum deposition technique. The OLED produced a green luminescence with a peak emission at 512 nm and a half bandwidth of 55 nm. The maximum external quantum efficiency of the OLED was 7.2%. The emission intensity of the OLED at 10 mA/cm(2) was 13 μW (1.7 mW/cm(2)). The fluorescence detection system consisted of the OLED device, two band-pass filters, a five microchannel poly(dimethylsiloxane) (PDMS) microfluidic device and a linear CCD. The fluorescence detection system was successfully used in a flow-based enzyme-linked immunosorbent assay on a PDMS microfluidic device for the rapid determination of immunoglobulin A (IgA), a marker for human stress. The detection limit (S/N=3) for IgA was 16.5 ng/mL, and the sensitivity was sufficient for evaluating stress. Compared with the conventional 96-well microtiter plate assay, the analysis time and the amounts of reagent and sample solutions could all be reduced.
TL;DR: The formation of interfacial hole carriers between a rubrene single crystal and a 2 nm-thick molybdenum oxide layer resulted in the formation of a highly conductive interface with a high electrical conductivity of 0.16 S/cm.
TL;DR: In this article, the authors revealed that the cause of low carrier mobility is due to the presence of local upheaval regions generated by the volume change through the polymerization process of diacetylene monomers, in order to suppress the occurrence of the upheaval regions, they found that electron beam irradiation is effective, resulted in the highest carrier mobility of µmax = 3.8 cm2 V-1 s-1.
Abstract: The highest carrier mobility of polydiacetylene (PDA) thin films in field-effect transistors has been limited to less than 0.8 cm2 V-1 s-1, although the main chain conduction should show higher carrier mobility potentially. We revealed that the cause of the low carrier mobility is due to the presence of local upheaval regions generated by the volume change through the polymerization process of diacetylene monomers. In order to suppress the occurrence of the upheaval regions, we found that electron beam (EB) irradiation is effective, resulted in the highest carrier mobility of µmax = 3.8 cm2 V-1 s-1.
TL;DR: In this article, the photophysical properties of BCT and BCT-t-Bu were investigated both in solution and films, and it was shown that BCT was significantly medium-dependent.
Patent•
01 Mar 2011TL;DR: In this article, a source and a drain electrode containing the first and second organic compound layers of a tetrathiafulvalene derivative expressed by the following general formula I and a layer of an electron accepting compound are laminated.
Abstract: To provide an organic semiconductor element, containing: a source electrode containing a first organic compound layer and a second organic compound layer, at least one of the layers having an organic semiconductor active region; and a drain electrode containing the first organic compound layer and the second organic compound layer, as well as providing an organic electrode, containing: a laminated film, in which a layer of a tetrathiafulvalene derivative expressed by the following general formula I and a layer of an electron-accepting compound are laminated:
TL;DR: In this paper, the synthesis and properties of a series of fluoro-substituted phenyleneethynylenes are presented, and the properties of these are discussed.
Abstract: Synthesis and properties of a series of fluoro-substituted phenyleneethynylenes are presented.
TL;DR: A double layer of DATTF and fullerene (C60) on an n++-Si wafer pretreated with n-octyltrichlorosilane exhibited a high electrical conductivity of σ= 0.12 S/cm and was used as source-drain electrodes in organic field effect transistors as discussed by the authors.
Abstract: A double layer of dianthratetrathiafulvalene (DATTF) and fullerene (C60) on an n++-Si wafer pretreated with n-octyltrichlorosilane exhibited a high electrical conductivity of σ= 0.12 S/cm and was used as source–drain electrodes in organic field effect transistors (OFETs). A simplified OFET device architecture composed of an organic semiconducting active layer and an organic electrode layer was easily fabricated by successive vacuum deposition of organic donor and acceptor layers. It was confirmed that this device configuration is applicable for both p- and n-type FET operation.
TL;DR: In this paper, it was shown that horizontally oriented organic semiconducting molecules in amorphous films can be oriented uniaxially by mechanical brushing, and the absorption of polarized light in the direction parallel to the brushing direction was higher than the absorption in the perpendicular direction.
Abstract: We demonstrate that horizontally oriented organic semiconducting molecules in amorphous films can be oriented uniaxially by mechanical brushing. The absorption of polarized light in the direction parallel to the brushing direction was higher than the absorption in the perpendicular direction. Thus, simultaneous uniaxial and in-plane molecular orientation in amorphous films was realized by brushing. In addition, we demonstrate a polarized electroluminescence from the films treated by mechanical brushing.