Showing papers by "Chihaya Adachi published in 2007"

Correlation of hole mobility, exciton diffusion length, and solar cell characteristics in phthalocyanine/fullerene organic solar cells

Abstract: The authors investigated heterojunction organic solar cells composed of different metal phthalocyanines (MPcs, M=Fe, Co, Ni, Cu, and H2)/fullerene (C60) and compared the solar cell characteristics with the field-effect hole mobilities (μh) and exciton diffusion length (Lex) of the different MPcs. They observed that the short circuit current (JSC) is linearly dependent on the μh of the MPcs, except for ZnPc. They also estimated the Lex of the MPcs by creating a line of best fit using the action spectra of the external quantum efficiency in the solar cells and found that JSC is closely correlated with the Lex of the MPcs.

Highly efficient and stable red phosphorescent organic light-emitting device using bis[2-(2-benzothiazoyl)phenolato]zinc(II) as host material

Abstract: The red-emitting phosphorescent organic light-emitting device employing Bis[2-(2-benzothiazoyl)phenolato]zinc(II) (Zn(BTP)2) as a host material has been demonstrated. The device generates saturated red emission with Commission Internationale de l’Eclairage coordinates of (0.67, 0.33), characterized by a low driving voltage of 3.2V and high external quantum efficiency of 10.3% at 10mA∕cm2. Moreover, the efficiency and lifetime are improved by using 30% NPB-doped Zn(BTP)2 host in the emitting layer, where NPB is 4,4′-bis(N-phenyl-1-naphthyl-amino)biphenyl. The maximum external quantum efficiency of the device reaches 12.6% at 1.7mA∕cm2. The device has exhibited excellent stability. The half-luminance lifetime is 240h at 80mA∕cm2 (initial luminance of 6070cd∕m2). The relationship between the employed host material and device performance is discussed.

Material design of hole transport materials capable of thick-film formation in organic light emitting diodes

Abstract: In this study, the authors show an empirical guideline for designing hole transport materials (HTMs) that suppress rises in driving voltage even with a few hundred nanometer thick film in the organic light emitting diodes (OLEDs). In a device structure of indium tin oxide (110nm)/hole transport layer (HTL) (Xnm)∕4,4′-N,N′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (10nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (50nm)∕MgAg (100nm)∕Ag (10nm), the authors compared electroluminescence characteristics of the OLEDs having a thin-film HTL (X=50nm) and a thick-film HTL (X=300nm) using 13 kinds of HTMs. They observed a closed correlation between suppression of the driving voltage and the HTMs’ thermal characteristics. Highly thermally stable HTMs resulted in a small increase in the driving voltage.

Extremely low-threshold amplified spontaneous emission of 9,9′-spirobifluorene derivatives and electroluminescence from field-effect transistor structure

Abstract: By doping 2,7-bis[4-(N-carbazole)phenylvinyl]-9,9′-spirobifluorene (spiro-SBCz) into a wide energy gap 4,4′-bis(9-carbazole)-2,2′-biphenyl (CBP) host, we demonstrate an extremely low ASE threshold of Eth = (0.11 ± 0.05) μJ cm–2 (220 W cm–2) which is the lowest ASE threshold ever reported. In addition, we confirmed that the spiro-SBCz thin film functions as an active light emitting layer in organic light-emitting diode (OLED) and a field-effect transistor (FET). In particular, we succeeded to obtain linear electroluminescence in the FET structure which will be useful for future organic laser diodes.

Estimation of electron traps in carbon-60 field-effect transistors by a thermally stimulated current technique

Abstract: The authors investigated the influence of O2 and H2O molecules absorbed in carbon-60 (C60) films on their electron trap and n-type field-effect transistor (FET) characteristics. Electron traps in the C60 films were directly measured using a thermally stimulated current (TSC) technique. The TSC results demonstrate that the absorption of O2 and H2O molecules in the C60 films induced an increase in the electron trap concentration, which degrades C60 FET characteristics. By annealing the C60 films at 100°C for 8h, the electron trap concentrations were markedly lowered, enhancing the C60 FET characteristics. An electron mobility of 0.017cm2∕Vs and a current on/off ratio of 106 were observed from the degassed C60 FETs.

Ambipolar field-effect transistor based on organic-inorganic hybrid structure

Abstract: The authors developed an ambipolar field-effect transistor (FET) based on an organic-inorganic hybrid structure that consisted of an indium zinc oxide and pentacene double layer fabricated on a SiO2∕n++-Si substrate. Although the FETs based on an indium zinc oxide or pentacene single layer only showed unipolar FET characteristics, the hybrid FET showed definite ambipolar FET characteristics. The authors obtained a highly saturated field-effect hole and electron mobilities of 0.14 and 13.8cm2∕Vs. Furthermore, the authors demonstrated electroluminescence from hybrid FETs using tetracene as an emitting layer. The authors’ success shows that the hybridization of organic and inorganic materials opens up a new field in electronics.

Ambipolar light-emitting organic field-effect transistors using a wide-band-gap blue-emitting small molecule

Abstract: The authors applied a wide-band-gap (2.9eV) molecule of 4,-4′-bis(styryl)biphenyl (BSBP) as an active layer in light-emitting organic field-effect transistors. They found that BSBP provided both relatively high field-effect hole mobility of 0.01cm2∕Vs and photoluminescence efficiency of 20% in thin film. They achieved ambipolar operation by without breaking vacuum through devices’ preparation and measurements, applying aluminum contacts, and inserting a hydroxyl-free poly(methylmethacrylate) layer, and light emission was observed when the device was operated in the ambipolar mode. The results presented here will open the way to fabricating efficient light-emitting transistors with high mobility.

Optical properties of oligo(9,9-diarylfluorene) derivatives in thin films and their application for organic light-emitting field-effect transistors

Abstract: We demonstrated that oligo(9,9-diarylfluorene) derivatives have high potential for optoelectronic applications such as organic lasers and light-emitting organic field-effect transistors (LE-OFETs) The oligo(9,9-diarylfluorene) derivatives have high photoluminescence quantum efficiencies up to ΦPL ≈ 70−75% and very low amplified spontaneous emission thresholds (Eth) down to 05 μJ/cm2 in their vacuum-deposited neat films In particular, the trimer derivatives (B3 and T3) show higher ΦPL and lower Eth than those of the dimers (B2 and T2) Efficient deep-blue LE-OFETs with the electroluminescence (EL) peaking at λpeak = 429 nm were demonstrated using the ter(9,9-diarylfluorene) as the active layer Rather high luminance up to L ≈ 150 cd/m2 and EL quantum efficiency up to ηext ≈ 060% were achieved with the optimum source-drain channel length, indicating bipolar carrier injection in the terfluorene layer under the FET operation

Spectrally narrow emission from organic films under continuous-wave excitation

Abstract: The authors observed a narrow emission from a 500-nm-thick codeposited organic thin film, 6wt% 4,4′-bis[(N-carbazole)styryl]-biphenyl (BSB-Cz) doped into a 4,4′-bis(N-carbazole)-biphenyl host, under continuous-wave excitation. Although they observed no clear threshold characteristics on the spectral narrowing, they found that the emission centered at a wavelength of λASE=462nm, corresponding to the zero to one transition of BSB-Cz, has a transverse electric mode with a narrow full width at half maximum of λFWHM=3.4nm, indicating the occurrence of amplified spontaneous emission. No excited state absorptions of singlet and triplet excited states of BSB-Cz are confirmed around λASE, leading to large net gain for the light amplification.

Top Light-Harvesting Organic Solar Cell Using Ultrathin Ag/MgAg Layer as Anode

Abstract: We fabricate a top light-harvesting organic solar cell (TL-OSC) with a Si substrate/SiO2 layer/cathode/organic layer/semitransparent metal anode structure. We obtain power conversion efficiency (ηp) comparable to that of conventional bottom-type OSCs. To harvest light from a top anode, we use a 1 nm Ag layer/4 nm MgAg layer having a transmittance of 55–80% in the visible and near-infrared regions. We also insert an ultrathin molybdenum oxide (MoO3) layer at the cathode/organic layer interface to increase hole collection efficiency from the organic layer into the cathode.

Ambipolar field-effect transistor of high photoluminescent material tetraphenylpyrene (TPPy) single crystal

Abstract: An ambipolar field-effect transistor (FET) based on a 1,3,6,8-tetraphenylpyrene (TPPy) single crystal, a highly photoluminescent material, has been successfully fabricated. Several kinds of metal electrodes have been employed to investigate the charge injection characteristics into the single-crystal FET. Hole and electron mobilities of 0.34 and 7.7×10-2 cm2/(Vs) were achieved using Au and Ca electrodes, respectively. The ambipolar characteristic of this device gives a prospect for further development in light-emitting FET operation.

Semiconductor, semiconductor device, complementary transistor circuit device

Abstract: A semiconductor device including a semiconductor 1, a first electrode 2, an insulating layer 3 interposed between the semiconductor 1 and the first electrode 2, the second electrode 4 which is in contact with the semiconductor 1 and is detached from the first electrode 2, and the third electrode 5 which is in contact with the semiconductor 1 and is detached from the first electrode 2 and the second electrode 4, wherein the semiconductor 1 is provided with the organic semiconductor layer 10 and the oxide semiconductor layer 11.

High-current Injection and Transport on Order of kA/cm2 in Organic Light-emitting Diodes Having Mixed Organic/Organic Heterojunction Interfaces

Abstract: We manufactured multilayer organic light-emitting diodes (OLEDs) with mixed organic/organic heterojunction interfaces composed of alpha-sexithiophene and phenyldipyrenylphosphine oxide carrier-transporting layers. In the mixed-interface OLEDs, we achieved a low driving voltage of 3.6 V at a current flow of 100 mA/cm2, which was caused by efficient carrier injection at the mixed interfaces. We investigated how much current can flow through these OLEDs with the aim of fabricating electrically pumped organic laser diodes. We found that an OLED of this type with a small active area of 625 µm2 on a high-thermal-conductivity sapphire substrate can sustain high current densities of 1.1 kA/cm2 and emits bright electroluminescence of 7.9 Mcd/m2 under direct current.

Blue Organic Electrophosphorescence Diodes using Diarylamino-substituted Heterocyclic Compounds as Host Material

Abstract: Diarylamino-substituted heterocyclic compounds (DHCs), 2,6-dicarbazolo-1,5-pyridine (PYD2) and 2,4,6-tricarbazolo-1,3,5-pyrimidine (PYM3), possess a high triplet energy level (T1) of T1= 2.93 eV (PYD2) and T1= 2.89 eV (PYM3), respectively, in their solid state films. 6wt%-Iridium(III)bis[(4,6-di-fluorophenyl)-pyridinato-N,C2]picolinate (FIrpic):PYD2- and 6wt%- FIrpic:PYM3-doped films showed high photoluminescence (PL) efficiencies of ΦPL= 88±2 % and ΦPL= 80±1 %, indicating near perfect confinement of FIrpic triplet excitons. By increasing the FIrpic concentration in DHC hosts, lower driving voltage and higher external electroluminescence (EL) quantum efficiencies (ηEL) were observed, indicating enhancement of electron injection and transport (electron current) in these emitting layers

Observation of Extremely High Current Densities on Order of MA/cm2 in Copper Phthalocyanine Thin-Film Devices with Submicron Active Areas

Abstract: Using contact photolithography and electron-beam lithography techniques, we manufactured copper phthalocyanine thin-film devices with active areas ranging from 1,000,000 to 0.04 µm2 to investigate how much current can flow through these devices with the aim of fabricating electrically pumped organic laser diodes. From the results of our current density–voltage (J–V) measurements, we found that the device with the smallest active area of 0.04 µm2 on a silicon substrate exhibits an extremely high current density of 6,350,000 A/cm2 due to improved thermal management. The J–V characteristics of the devices are controlled by shallow-trap space-charge-limited current (SCLC), trap-free SCLC, and two-carrier injection current mechanisms over a wide range of current densities between nA/cm2 and MA/cm2.

Alternating copolyfluorenevinyles with polynuclear aromatic moieties: Synthesis, photophysics, and electroluminescence

Abstract: Three new copolymers, namely poly(fluorenevinylene-alt-naphthalenevinylene) (N), poly(fluorenevinylene-alt-anthracenevinylene) (A) and poly(fluorenevinylene-alt-pyrenevinylene) (P), were synthesized by Heck coupling of 9,9-dihexyl-2, 7-divinylfluorene with a polynuclear aromatic dibromide. The 9,10-disubstituted anthracene was obtained exclusively for A while N and P were obtained as a mixture of two isomers with predominant the 1,4-disubstituted naphthalene and 1,8-disubstituted pyrene, respectively. The polymers were soluble in common organic solvents and decomposed above 370 °C. Their glass transition temperature increased from 58 to 110 °C by increasing the number of the phenyl rings of the polynuclear aromatic moiety. Rather high-efficiency blue and blue-greenish photoluminescence (PL) of these copolymers in solution was largely decreased in their films, indicating the presence of concentration quenching in the solid state. The OLED using these polymers demonstrated green EL in the case of copolymers N and A, and red EL in the P derivative with ηEL = 0.26–0.31%. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4661–4670, 2007

Analysis of Carrier Traps in Continuously Operated 4,4'-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl/tris(8-hydroxyquinoline)aluminum-Based Organic Light-Emitting Diodes by Thermally Stimulated Current Measurement

Abstract: We measured the reliability of two different organic light-emitting diodes (OLEDs) composed of indium tin oxide (ITO)/4,4'-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD)/tris(8-hydroxyquinoline)aluminum (Alq3)/MgAg (two layer) or ITO/α-NPD/α-NPD:Alq3 (mixed layer)/Alq3/MgAg. Changes in the carrier trap distribution were estimated by thermally stimulated current (TSC) measurements. Improved device reliability was observed in the mixed-layer device. In the case of the two-layer device, the TSC peaks gradually changed depending on the duration of continuous operation, while the mixed device showed no change in its TSC spectra. These spectral differences indicate that the mixing of α-NPD and Alq3 occurs during the continuous operation in the two-layer device.

Spectrally Narrow Emission at Cutoff Wavelength from Edge of Electrically Pumped Organic Light-Emitting Diodes

Abstract: We demonstrate spectrally narrow emissions from the edge of electrically pumped organic light-emitting diodes having a fine "line-shaped" waveguide structure with a silver cathode. A 4,4'-bis[(N-carbazole)styryl]-biphenyl (BSB-Cz) layer, which has a very low amplified spontaneous emission threshold, was used as an emitting layer. We observed extremely narrow emissions having polarized transverse electric (TE) mode. The full width at half maximum became narrower down to 6.5 nm as the peak wavelength of the emission approached the wavelength of the 0–1 transition of the BSB-Cz layer. The edge emission intensity showed a superlinear relationship with the current density, and the spectral shapes changed appreciably depending on the current density, indicating the occurrence of amplification of the emitted light. The peak wavelengths of the edge emissions were attributed to the leaky mode very close to the cutoff wavelength of the waveguide structure having the metal cladding layer.

Organic electroluminescence element and organic laser diode

Abstract: PROBLEM TO BE SOLVED: To provide an organic EL element which can extract luminescence with extremely narrow spectrum width even under excitation of low energy density (current density). SOLUTION: An organic electroluminescence element comprises a substrate, a positive electrode layer, an organic layer including at least one organic material and a negative electrode layer. The main light extracting direction is in the parallel direction against the surface of the substrate. One of the cut-off wavelengths of the transverse electric modes of an optical waveguide that has an organic layer constituted by the element as a part of its core is included in the wavelength band of full width at half maximum of the fluorescence spectrum of one of the organic materials included in the organic layer. COPYRIGHT: (C)2009,JPO&INPIT

Semiconductor device, its manufacturing method and display

Abstract: PROBLEM TO BE SOLVED: To contribute to reduction in manufacturing cost by using an organic semiconductor, and to enhance manufacturing efficiency by appropriately determining the balance of carriers to be injected SOLUTION: The semiconductor device comprises a light-emitting portion 1 composed of an organic semiconductor layer 10 and an oxide semiconductor layer 11, a first electrode 2, an insulator layer 3 formed between the light-emitting portion 1 and the first electrode 2, a second electrode 4 touching the light-emitting portion 1 and spaced apart from the first electrode 2, and a third electrode 5 touching the light-emitting portion 1 and spaced apart from the first and second electrodes 2 and 4 wherein the light-emitting portion 1 emits light by the recombination of a hole and an electron COPYRIGHT: (C)2009,JPO&INPIT

Features of Conductivity and Electroluminescence of New Poly (9,9-Dioctylfluorenyl-2,7-Diyl) – End Capped With Polyhedral Oligomeric Silsesquioxanes

Abstract: The emission and conductivity of a new polymer poly (9,9-dioctylfluorenyl-2,7-diyl) end capped with polyhedral oligomeric silsesquioxanes (PFO-POSS) have been investigated by the differential approach. Thin films of PFO-POSS showed pure blue photoluminescence (PL) with structured spectrum and a maximum at λPL = 423 nm. However, the OLED structures formed on the PFO-POSS base emitted green light with broadened spectrum and a maximum at λEL = 510 ∼ 529 nm. The electroconductivity and electroluminescence have shown that the greatest influence on the current voltage and luminance current characteristics is rendered by introducing the electron transport layer to the multilayer OLED structure under investigation.

Organic light emitting devices from OLED to organic laser diode

Abstract: We mention some significant progresses on organic light emitting diode (OLED), organic laser diode (OLD) and organic field effect transistor (OFET). We discuss prospect of OLD based on recent developments of organic light emitting devices.

Frontier of organic light emitting devices

Abstract: We report recent progresses on organic light emitting devices such as organic light emitting diode (OLED), light-emitting organic field effect transistor (LE-OFET) and organic laser diode (OLD) We demonstrate very low driving voltage of OLEDs using a p-i-n structure which also provides high current carrier injection over kA/cm2 Further we demonstrate novel organic devices such as light emitting FET We also discuss future prospect of OLD based on our recent materials' development and analysis of operation mechanism

Lowering the Driving Voltage of Organic Light Emitting Diodes by Chemical Doping

Abstract: Although a very high electroluminescence (EL) efficiency has been demonstrated with organic light emitting diodes (OLEDs) using electrophosphorescence, enhancement of energy conversion efficiency, that is decreasing driving voltage, has been required from the standpoint of practical device applications. In this paper, we report on a decrease in operation voltage by doping carrier transport layers and discuss the possible mechanisms. In particular, we demonstrate an OLED with very low driving voltage (2.9 V at J = 100 mA/cm2) with a p-i-n structure.