Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited states.

The design of organic compounds with nearly no gap between the first excited singlet (S1) and triplet (T1) states has been demonstrated to result in an efficient spin-flip transition from the T1 to S1 state, that is, reverse intersystem crossing (RISC), and facilitate light emission as thermally activated delayed fluorescence (TADF). However, many TADF molecules have shown that a relatively appreciable energy difference between the S1 and T1 states (~0.2 eV) could also result in a high RISC rate. We revealed from a comprehensive study of optical properties of TADF molecules that the formation of delocalized states is the key to efficient RISC and identified a chemical template for these materials. In addition, simple structural confinement further enhances RISC by suppressing structural relaxation in the triplet states. Our findings aid in designing advanced organic molecules with a high rate of RISC and, thus, achieving the maximum theoretical electroluminescence efficiency in organic light-emitting diodes.

Influence of vacuum chamber impurities on the lifetime of organic light-emitting diodes.

We evaluated the influence of impurities in the vacuum chamber used for the fabrication of organic light-emitting diodes on the lifetime of the fabricated devices and found a correlation between lifetime and the device fabrication time. The contact angle of the ITO substrates stored the chamber under vacuum were used to evaluate chamber cleanliness. Liquid chromatography-mass spectrometry was performed on Si wafers stored in the vacuum chamber before device fabrication to examine the impurities in the chamber. Surprisingly, despite the chamber and evaporation sources being at room temperature, a variety of materials were detected, including previously deposited materials and plasticizers from the vacuum chamber components. We show that the impurities, and not differences in water content, in the chamber were the source of lifetime variations even when the duration of exposure to impurities only varied before and after deposition of the emitter layer. These results suggest that the impurities floating in the vacuum chamber significantly impact lifetime values and reproducibility.

Solution-Processed Organic-Inorganic Perovskite Field-Effect Transistors with High Hole Mobilities.

A very high hole mobility of 15 cm2 V-1 s-1 along with negligible hysteresis are demonstrated in transistors with an organic-inorganic perovskite semiconductor. This high mobility results from the well-developed perovskite crystallites, improved conversion to perovskite, reduced hole trap density, and improved hole injection by employing a top-contact/top-gate structure with surface treatment and MoOx hole-injection layers.

Fluorometric flow-immunoassay for alkylphenol polyethoxylates on a microchip containing a fluorescence detector comprised of an organic light emitting diode and an organic photodiode.

A compact fluorescence detector was constructed on a microchip from an organic light emitting diode (OLED) as the light source and an organic photodiode (OPD) as the photo-detector and was used in an immunoassay for alkylphenol polyethoxylates (APE). The OLED based on a terbium complex emitted a sharp light at the main wavelength of 546 nm with a full width at half maximum of 9 nm. The incident photo-to-current conversion efficiency (IPCE) of the OPD fabricated with Fullerene 70 (C70) and tris[4-(5-phenylthiopen-2-yl)phenyl]-amine (TPTPA) was approximately 44% for light at a wavelength of 586 nm. The performance of the fluorescence detector was evaluated for the determination of resorufin (λ(em)=586 nm) and the photocurrent of the OPD due to the fluorescence of resorufin was proportional to the concentration of resorufin in the range from 0 to 18 µM with a detection limit (S/N=3) of 0.6 µM. The fluorescence detector was successfully utilized in a competitive enzyme-linked immunosorbent assay for APE, where an anti-APE antibody was immobilized on the surface of the channel of the Polydimethylsiloxane (PDMS) microchip or on the surface of magnetic microbeads. After an immunoreaction with a sample solution of APE containing a horse radish peroxidase (HRP)-labeled APE, the fluorescence of resorufin generated just after introduction of a mixed solution of Amplex Red and H2O2 was measured using the fluorescence detector. The calibration curve for the photocurrent signals of the OPD due to the fluorescence of resorufin against the logarithmic concentration of APE was sigmoidal in shape. The detection limits defined as IC80 were ca. 1 ppb and ca. 2 ppb, respectively, for the methods using the anti-APE antibody immobilized on the surface of the microchannel and in the case where the antibody was immobilized on the surface of magnetic microbeads.

Photometric flow injection determination of phosphate on a PDMS microchip using an optical detection system assembled with an organic light emitting diode and an organic photodiode.

A compact photometric detector was constructed from an organic light emitting diode (OLED) based on a europium complex, europium(diben-zoylmethanato)3(bathophenanthroline) (Eu(DBM)3bath), as the light source and an organic photodiode (OPD) fabricated from a hetero-junction of two layers of copper phthalocyanine (CuPc)/fullerene (C60) as the photo-detector on a microchip prepared from poly(dimethylsiloxan) (PDMS) and was applied to the determination of phosphate. The OLED and the OPD were fabricated by a vapor deposition method on an indium tin oxide (ITO) coated glass substrate with the following layered structure; Glass (0.7 mm)/ITO (110 nm)/4,4'-bis[N-(1-naphthyl)-N-phenyl amino]-biphenyl (α-NPD) (30 nm)/4,4'-di(N-carbazolyl)biphenyl (CBP): Eu(3+) (8 wt%, 30 nm)/bathocuproine (BCP) (30 nm)/aluminum tris(8-hydroxyquinoline) (Alq3) (25 nm)/magnesium and silver (MgAg) (100 nm)/Ag (10nm) and Glass (0.7 mm)/ITO (110 nm)/CuPc (35 nm)/C60 (50 nm)/BCP (10 nm)/Ag (50 nm), respectively. The OLED based on the europium complex emitted a sharp light at the wavelength of 612 nm with a full width at half maximum (FWHM) of 8 nm. The performance of the photometric detector assembled was evaluated based on measurements of the absorbance of different concentrations of malachite green (MG) solutions for a batch system with 1cm long path length. The molar absorptive coefficient of the MG solution, calculated from the photocurrent of the OPD, was in good agreement with the value reported in the literature. A microchip with two inlets and one outlet U-shaped channel was prepared by a conventional photolithograph method. The OLED and the OPD were configured so as to face each other through the PDMS microchip in parallel in order to align the light axis of the OLED and the OPD with the flow cell (optical path length of 5mm), which was located at the end of outlet. For the determination of phosphate, an ion-association reaction between MG and a molybdenum-phosphate complex was utilized and a good linear relationship between the concentration and absorbance was observed in the concentration range 0-0.2 ppm, with a detection limit (S/N=3) of 0.02 ppm. The assembled photometric detector was also applied to the determination of phosphate by the flow injection of river water samples using the reagent solution containing MG and molybdenum ammonium in sulfuric acid. A good recovery (97-99%) for the river water samples, which had been spiked with the standard 0.08 ppm, with an RSD of ca 5% (n=5) was obtained using the constructed system.

An organic thin film photodiode as a portable photodetector for the detection of alkylphenol polyethoxylates by a flow fluorescence-immunoassay on magnetic microbeads in a microchannel.

An organic thin film photodiode (OPD) was successfully employed as a portable photodetector in a competitive enzyme-linked immunosorbent assay (ELISA) of a class of nonionic surfactants, namely alkylphenol polyethoxylates (APnEOs) which are an environmental pollutant. Microbeads that were chemically immobilized with an anti-APnEOs antibody were used in the assay. The OPD consisted of a layer of copper phthalocyanine (CuPc), C60 and a second layer of bathocuproine (BCP) with a bulk heterojunction composed of CuPc and C60 prepared by a vapor deposition method on an indium-tin oxide coated glass substrate. The OPD showed an incident photon-current efficiency (IPCE) of approximately 19% for light at a wavelength of 585 nm. This relatively high IPCE at 585 nm makes it suitable for detecting the fluorescence of resorufin (λem=585 nm), the product of the competitive ELISA, produced through the enzymatic reaction of Amplex Red with horseradish peroxidase (HRP) and H2O2. A fluorometric detector was assembled on a microchip by combining the fabricated OPD and a commercial LED as a photodetector and a light source, respectively. The photocurrent of the OPD due to the fluorescence of resorufin was proportional to the concentration of resorufin in the concentration range from 0 to 8 µM. When the fabricated OPD was used as a portable photodetector, the competitive ELISA of APnEOs using HRP labeled APnEOs (HRP-APnEOs) was performed on magnetic microbeads on which surface an anti-APnEOs antibody had been immobilized. A typical sigmoidal calibration curve was obtained and the data were in good agreement with a numerical simulation, where the photocurrent of the OPD was plotted against the concentration of APnEOs, determined via the competitive ELISA. The detection limit of the immunoassay for APnEOs was approximately 2 and 4 ppb in batch and flow system, respectively.

Droplet manipulation by an external electric field for crystalline film growth.

Combining droplet manipulation by the application of an electric field with inkjet printing is proposed as a unique technique to control the surface wettability of substrates for solution-processed organic field-effect transistors (FETs). With the use of this technique, uniform thin films of 2,7-dioctyl[1]benzothieno[2,3,-b][1]benzothiopene (C8-BTBT) could be fabricated on the channels of FET substrates without self-assembled monolayer treatment. High-speed camera observation revealed that the crystals formed at the solid/liquid interface. The coverage of the crystals on the channels depended on the ac frequency of the external electric field applied during film formation, leading to a wide variation in the carrier transport of the films. The highest hole mobility of 0.03 cm(2) V(-1) s(-1) was obtained when the coverage was maximized with an ac frequency of 1 kHz.

Performance of an organic photodiode as an optical detector and its application to fluorometric flow-immunoassay for IgA.

The performance of an organic thin film photodiode (OPD), fabricated from a hetero-junction comprised of two layers of C(60) and a phthalocyanine-Cu(II) complex was evaluated by detecting the chemiluminescence generated from the reaction of luminol with horseradish peroxidase in the presence of H(2)O(2), and the fluorescence from resorufin, as an optical detector. The photocurrent of the OPD was linear with respect to the power of light from a commercial LED. The sensitivity of the OPD was sufficient for detecting chemiluminescence with a power 0.1µW/cm(2). The OPD was successfully used in a flow-immunoassay for IgA, a marker of human stress, in which a sandwich immunoassay was carried out on the microchip and the fluorescence from resorufin, produced by the enzymatic reaction, was detected. The detection limits for resorufin and IgA were 5.0µM and 16ng/mL, respectively. The photosensitivity of the OPD remained relatively constant for a minimum of one year.

An integrated enzyme-linked immunosorbent assay system with an organic light-emitting diode and a charge-coupled device for fluorescence detection.

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.

Formation of organic crystalline nanopillar arrays and their application to organic photovoltaic cells.

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.

High current density in light-emitting transistors of organic single crystals.

We measured the external electroluminescence quantum efficiency (eta(ext)) in light-emitting field-effect transistors (LETs) made of organic single crystals and found that, in the ambipolar transport region, eta(ext) is not degraded up to several hundreds A/cm(2) current-density range, which is 2 orders of magnitude larger than that achieved in conventional organic light-emitting diodes. The present result indicates the single-crystal organic LET is a promising device structure that is free from various kinds of nonradiative losses such as exciton dissociation near electrodes and exciton annihilations.

Electrical properties of 1,4-bis(4-(phenylethynyl)phenylethynyl)benzene and its application for organic light emitting diodes.

We found that a phenylene ethynylene derivative, 1,4-bis(4-(phenylethynyl)phenylethynyl)benzene (BPPB), provides very high photoluminescence efficiency both in solution (Phi(PL) = 95 +/- 3%) and thin films (Phi(PL) = 71 +/- 3%); further, we observed blue electroluminescence (EL) of lambda(EL(max)) approximately 470 and 510 nm with an external EL efficiency of eta(EL) approximately 0.53% and maximum luminance of approximately 70000 cd m(-2) at current density of approximately 2 A cm(-2) with BPPB as an emitter; also we identified that BPPB functions as a hole transport layer in organic light emitting diodes.