Effective Electron Injection Research Articles

29.3: Very Bright and Efficient Top-Emitting OLED with Ultra-Thin Yb as Effective Electron Injector

Very bright and efficient top-emitting organic light-emitting diodes (TOLEDs) using an ultra-thin ytterbium (Yb) layer capped with a semitransparent Ag layer as the effective electron-injection cathode were demonstrated. TOLED with Yb/Ag cathode exhibited lower operation voltage and higher power efficiency, as compared to the one with commonly used Ca/Ag cathode. With bis(2-phenylpyridine)iridium [(ppy)2Ir(acac)] as the phosphorescent dopant, the Yb/Ag based TOLED showed a current efficiency of 88 cd/A and a power efficiency of 67 lm/W, considerably greater than those (56 cd/A and 44 lm/W) obtained from the corresponding bottom-emitting one. The good performance of these TOLEDs is attributed to the efficient electrons injection from the Yb/Ag cathode as well as a micro-cavity effect.

Efficient electron injection from bilayer cathode with aluminum as cathode

A new bilayer cathode has been developed for varieties of RGB emitting polymers to form an effective electron injector. An effective electron injection from high work function metal, such as Al cathode can be realized by incorporation of a thin layer of amino-ended alkyl-substituted polyfluorene copolymer and corresponding ammonium salt-cationic polyelectrolyte between metal cathode and emitting layer. The performance of polymer light emitting-diodes (PLEDs) fabricated with such bilayer cathodes can be enhanced to the levels comparable or even higher than that obtained by using Ca and Ba as the cathode (for red QE = 2–3%, for green 5–8%, for blue 1–2% respectively). The new electron-injection layer can be processed from environment-friendly solvents such as alcohol or water in which most of light-emitting polymers are insoluble. The mechanism of lowering the barrier height for electron injection from Al is discussed based on I-V characteristics, build-in potential, and X-ray diffraction data.

An effective cathode structure for inverted top-emitting organic light-emitting devices

Inverted top-emitting organic light-emitting devices (OLEDs) combine technical merits of top-emitting OLEDs and inverted OLEDs for active-matrix OLED displays. One major challenge in inverted top-emitting OLEDs, however, is to prepare a reflective bottom cathode capable of effective electron injection. In this letter, we report an effective cathode structure for enhancing the electron-injection capability of the bottom cathode in inverted top-emitting OLEDs. Such an approach does not involve handling reactive metals during fabrication and permits use of highly reflective materials such as Al and Ag as the bottom cathodes. Efficient inverted top-emitting devices employing such a cathode scheme have been demonstrated.

23.1: Efficient Electron Injection from Bilayer Cathode with High Work Function Metal

AbstractA novel bilayer cathode has been developed to variety of RGB emitting polymers to form an effective electron injector. An effective electron injection from high work function metal such as Al can be realized by incorporation of a thin layer of water‐ or alcohol‐soluble copolymer between metal and emitting layer. The device performance of polymer LEDs fabricated with such bilayer cathodes can be enhanced to levels comparable to or even higher than that obtained by using Ca and Ba cathode.

Preparation and properties of dye-sensitized solar cell using chlorophyll derivative immobilized TiO 2 film electrode

A dye-sensitized solar cell using the visible light sensitization of chlorophyll-a derivative, chlorine-e 6 (Chl-e 6) immobilized on TiO 2 film was developed. From fluorescence spectrum of Chl-e 6 immobilized on TiO 2 film, the emission of Chl-e 6 was effectively quenched by TiO 2, indicating that the effective electron injection from the excited singlet state of Chl-e 6 into the conduction band of TiO 2 occurred. The short-circuit photocurrent density ( I SC), the open-circuit photovoltage ( V OC), and the fill factor (FF) of solar cell consisting of Chl-e 6 immobilized on TiO 2 film electrode and platinum-coated substrate electrode were estimated to be 1.47 mA cm −2, 425 mV, and 57.0%, respectively. IPCE values were reached a maximum around the wavelength of absorption maximum (11.0% at 400 nm, 4.7% at 541 nm and 7.9% at 661 nm), indicating that the dye-sensitized solar cell using visible light sensitization of TiO 2 film by Chl-e 6 was developed.

(AlN) 1/(GaN) n1 ] m/(AlN) n2 -based quantum wells for quantum-cascade-laser application

[(AlN) 1/(GaN) n1 ] m /(AlN) n2 -based quantum wells (QWs) constructed by periodically introducing several atomic layers of AlN into (AlN) 1/(GaN) n1 short-period superlattices have a great potential for the application to mid-infrared quantum-cascade lasers. Effective electron injection from the first to second subbands in the (AlN) 1/(GaN) n1 short-period superlattice is expected through the inserted (AlN) n2 layer which has a large polarization field. The [(AlN) 1/(GaN) n1 ] m /(AlN) n2 QWs were prepared by hot wall epitaxy, and the structure was characterized by transmission electron microscopy and X-ray diffraction. The quality of [(AlN) 1/(GaN) n1 ] m /(AlN) n2 QW depends significantly on the quality of GaN buffer layer, and high-quality QW structures were prepared on GaN films grown on Al 2O 3 ( 0 0 0 1 ).

Bio-photovoltaic conversion device using chlorine-e 6 derived from chlorophyll from Spirulina adsorbed on a nanocrystalline TiO 2 film electrode

A bio-photovoltaic conversion device based on dye-sensitised solar cell (DSSC) using the visible light sensitisation of chlorine-e 6 (Chl-e 6) derived from chlorophyll from Spirulina adsorbed on a nanocrystalline TiO 2 film was developed. Form fluorescence spectrum of Chl-e 6 adsorbed on a nanocrystalline TiO 2 film, the emission of Chl-e 6 was effectively quenched by TiO 2 nanocrystalline indicating that the effective electron injection from the excited singlet state of Chl-e 6 into the conduction band of TiO 2 particles occurred. The short-circuit photocurrent density ( I SC), the open-circuit photovoltage ( V OC), and the fill factor (FF) of solar cell using Chl-e 6 adsorbed on a nanocrystalline TiO 2 film electrode were estimated to be 0.305±0.012 mA cm −2, 426±10 mV, and 45.0%, respectively. IPCE values were reached a maximum around the wavelength of absorption maximum (7.40% at 400 nm; 1.44% at 514 nm and 2.91% at 670 nm), indicating that the DSSC using visible light sensitisation of nanocrystalline TiO 2 film by Chl-e 6 was developed.

Effects of oxygen and hydrogen on electron field emission from microwave plasma chemically vapor deposited microcrystalline diamond, nanocrystalline diamond, and glassy carbon coatings

Electron field emission from chemically vapor deposited polycrystalline diamond, nanocrystalline diamond, and glassy carbon coatings are reported. Effects of hydrogen-rich or oxygen-containing CVD precursors on electron field emission from CVD carbon coatings are presented. Surface conductivity and negative electron affinity resulting from hydrogen termination of crystalline diamond were found to promote electron field emission for discrete diamond particles and non-continuous diamond films but not for high quality and continuous diamond films as well as nanocrystalline diamond and glassy carbon coatings containing conductive graphitic carbon. Graphitic carbon in nanocrystalline diamond coatings and glassy carbon coatings provided additional conduction paths for electrons and allowed more effective injection of electrons into electron emitters. Nanocrystalline diamond coatings deposited by methanol/ethanol plasmas contained more graphitic carbon than that deposited by hydrogen/methane plasmas resulting in lower turn-on electric field and higher electron field emission current. Glassy carbon coatings deposited by both processes exhibited excellent electron emission characteristics. Low turn-on electric field, i.e., 1 V/μm, for electron field emission was achieved for diamond and glassy carbon coatings deposited under optimized conditions. Among these three classes of carbon coatings, glassy carbon deposited at elevated substrate temperatures were found to exhibit the best electron field emission properties.

Dye-sensitized nanocrystalline TiO2 solar cells based on novel coumarin dyes

Abstract We have developed dye-sensitized nanocrystalline TiO2 solar cells (DSSCs) based on novel coumarin-dye photosensitizers. The absorption spectra of these novel dyes are red-shifted remarkably in the visible region relative to the spectrum of C343, a conventional coumarin dye. Introduction of a methine unit (–CHCH–) connecting the cyano (–CN) and carboxyl (–COOH) groups into the coumarin framework expanded the π-conjugation in the dye and thus resulted in a wide absorption in the visible region. These novel dyes performed as efficient photosensitizers for DSSCs. A DSSC based on 2-cyano-5-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)-penta-2,4-dienoic acid (NKX-2311), produced a 6.0% solar energy-to-electricity conversion efficiency (η), the highest performance among DSSCs based on organic-dye photosensitizers, under AM 1.5 irradiation (100 mW cm–2) with a short-circuit current density (Jsc) of 14.0 mA cm–2, an open-circuit voltage (Voc) of 0.60 V, and a fill factor of 0.71. Our results suggests that the structure of NKX-2311 whose carboxyl group is directly connected to the –CHCH– unit, is advantageous for effective electron injection from the dye into the conduction band of TiO2. In addition, the cyano group, owing to its strong electron-withdrawing ability, might play an important role in electron injection in addition to a red shift in the absorption region. On a long-term stability test under continuous irradiation with white light (80 mW cm–2), stable performance was attained with a solar cell based on the NKX-2311 dye with a turnover number of 2.6×107 per one molecule.

Characterization of AlN/GaN Quantum‐Cascade Structures Prepared by Hot‐Wall Epitaxy

[(AlN)1/(GaN)n1]m/(AlN)n2 quantum-cascade (QC) structures were prepared by hot wall epitaxy for midinfrared laser application, periodically inserting several atomiclayers of AlN into (AlN)1/(GaN)n1 short period superlattices. The (AlN)1/(GaN)n1 short-period superlattice has a large first-subband broadening, and effective population inversion between first and second subbands is expected through the electron injection into the second subband. The AlN/GaN quantum-well system grown to [0001] direction shows a large piezo-electric effect, and the effective electron injection into the second subband is expected by the periodically inserted AlN layers. The QC structure was characterized by X-ray diffraction, transmission electron microscopy (TEM). The (0002) X-ray diffraction pattern showed good agreement with the theoretical one, and (10—14) reciprocal mapping of the cascade structure showed the QC structure was grown coherently on the GaN buffer layer.

Molecular Design of Coumarin Dyes for Efficient Dye-Sensitized Solar Cells

We have developed novel coumarin dyes for use in dye-sensitized nanocrystalline TiO2 solar cells (DSSCs). The absorption spectra of these novel coumarin dyes are red-shifted remarkably in the visible region relative to the spectrum of C343, a conventional coumarin dye. Introduction of a methine unit (−CHCH−) connecting both the cyano (−CN) and carboxyl (−COOH) groups into the coumarin framework expanded the π conjugation in the dye and thus resulted in a wide absorption in the visible region. These novel dyes performed as efficient photosensitizers for DSSCs. The monochromatic incident photon-to-current conversion efficiency (IPCE) from 420 to 600 nm for a DSSC based on NKX-2311 was over 70% with the maximum of 80% at 470 nm, which is almost equal to the efficiency obtained with the N3 dye system. The IPCE performance of DSSCs based on coumarin dyes depended remarkably on the LUMO levels of the dyes, which are estimated from the oxidation potential and 0−0 energy of the dye. The slow charge recombination, on the order of micro to milliseconds, between NKX-2311 cations and injected electrons in the conduction band of TiO2 (observed by transient absorption spectroscopy) resulted in efficient charge separation in this system. A HOMO−LUMO calculation indicated that the electron moves from the coumarin framework to the −CHCH− unit by photoexcitation of the dye (a π−π* transition). Our results strongly suggest that molecular design of the sensitizer is essential for the construction of highly efficient DSSCs. The structure of NKX-2311, whose carboxyl group is directly connected to the −CHCH− unit, is advantageous for effective electron injection from the dye into the conduction band of TiO2. In addition, the cyano group, owing to its strong electron-withdrawing ability, might play an important role in electron injection in addition to a red shift in the absorption region.

Si超微粒子を用いたELデバイスからの発光特性の改善

Electroluminescent device (ELD) using nanocrystalline silicon (nc-Si) was fabricated by annealing of co-sputtering of Si and silicon dioxide (SiO2) targets and subsequently hydrofluoric (HF) acid solution treatment. The HF treatment made effective electron injection into the nc-Si and the resultant decrease in resistance of the ELD. The HF treatment also made Pb-center, which act as a role of non-radiative recombination center, compensate with hydrogen atoms. From above effects of the HF treatment, the HF-treated ELD emits high efficiency red light with external quantum efficiency of 0.35% under low operating voltage of +4.5 V. Moreover, intensity of the red light emission from the HF-treated ELD is stable for continuous operation above 10 days (operating time of 15000 min).

Alkali metal acetates as effective electron injection layers for organic electroluminescent devices

The effect of alkali metal (Li +, Na +, K +, Rb +, and Cs +) acetates and fluorides at an aluminum/tris(8-hydroxyquinoline) aluminum (Alq 3) interface on performance of organic electroluminescent (EL) devices is described. We also studied the effect of acetates with different metals, such as Mg 2+ and Al 3+, on the performance of the EL devices. EL characteristics of the devices with Al/alkali metal acetates were compared with those of devices with Al/alkali metal fluorides or an Al cathode itself. We found that the bilayer cathodes with alkali metal acetates exhibited better device performance than the bilayer cathodes with alkali metal fluorides. The improvement can be attributed to lowering of the work function of the Al cathode and doping of Alq 3 with alkali metals that were formed by the higher reactivity of alkali metal acetates than alkali metal fluorides with hot Al atoms impinging on these salts during the vapor deposition. The enhanced electron injection resulted in more balanced charge-carrier injection and thus higher EL efficiencies.

Precision adjustment of an ECR plasma chamber relative to the magnetic confinement field using strong focused electron beams

Strong focused electron beams produced by the magnetic field of an ECR ion source and a sequence of thin aluminum foils located in the ECR plasma chamber were used for a precision adjustment between the geometric axis of the ECR plasma chamber and the magnetic field of the source. After adjustment the acting radial component of the magnetic field on the source axis was not more than a few 10 −4 T. The presented method allows it to optimize the ECR ion source construction and to create suppositions for an effective injection of additional electrons in ECR plasmas to increase the electron density inside the source plasma.

Design Criterion and Operation Mechanism for 4.5 kV Injection Enhanced Gate Transistor

This paper investigates the injection enhancement effect of a trench gate structure for a metal oxide semiconductor (MOS) controlled power transistor called an injection enhanced gate transistor (IEGT). By virtue of the enhancement in effective electron injection efficiency, 4.5 kV IEGTs attain a thyristor-like carrier profile during the on-state, and hence achieve the same low on-state voltage drop as that of thyristors. The operation mode of the IEGT was studied using a two-dimensional numerical simulation, and verified by device fabrication. It was confirmed that the proposed novel trench gate geometry acts as an injection enhancer by restricting the hole diffusion current which flows from the n-type high-resistance base layer to the cathode electrode. It is shown for the first time that the effective electron injection efficiency of the n-ch insulated gate bipolar transistor increases to nearly 1. It is also shown that the new trench gate structure effectively decreases the forward voltage drop without degradation of turn-off capability.

Study of 4.5 kV MOS-Power Device with Injection-Enhanced Trench Gate Structure

We propose two new MOS-power device structures, which realize lower on-state voltage than previously proposed injection-enhanced gate transistors (IEGTs). One is an improved IEGT (or IEGN), and the other is a MOS-controlled diode (or IEGD). Using 2-D numerical simulations, it has been confirmed that the IEGN and the IEGD realize a lower on-state voltage drop than the conventional IEGT, retaining MOS gate drivability. This means that the injection-enhanced gate structure improves the effective electron injection efficiency not only for transistor structures, but also for devices with a low-injection efficiency emitter. It has also been confirmed that the 4.5 kV IEGN and IEGD can operate under the same circuit conditions as a 4.5 kV GTO-thyristor.

Amorphous Hydrogenated Silicon Films Studied by Schottky Barrier Method

For several years authors have been making series of technological experiments with a-Si:H films on glass substrate by d.c. magnetron sputtering. The main intention of the research is to obtain large area solar cells and a matrix of thin film transistors for liquid crystal displays [1-3]. This study is a part of series of experiments in which the problem of the effective ohmic junction and the question of the barrier height in the thin film stuctures are investigated. Various types of sandwich structures were prepared, as shown in the Table. D.c. magnetron sputtering was used to obtain the stuctures and all the films. D.c. planar magnetron system was specially made to optimize the preparation conditions of the a-Si:H films as well as hydrogen partial pressure pH2, argon partial pressure p AS , substrate temperature Τs , input power P and substrate bias Vs. The a-Si:H films were obtained in the conditions earlier reported in [1, 2]. Schottky diodes were fabricated in a sandwich structure. Molybdenum or aluminum sputtered onto glass substrate, served as the back contact. An ohmic contact characterized by effective electron injection to a-Si:H, was only obtained if additional film of 0.3 μm polycrystalline silicon was sputtered in an 99% of argon and 1% of phosphin atmosphere. Then, the a-Si:H film and top Schottky electrode were deposited on the substrate. Active area of the junction was 0.005 cm 2 . Diode rectification ratios about 10 4 are observed at room temperature, as shown in Fig. 1. This ratio is ultimately limited by the series resistance of the quasi-neutral region which affects the forward current. The undoped hydrogenated silicon is highly resistive with p 109 Ωcm. The rectification ratio about 104 was

An amorphous SiC thin film visible light-emitting diode with a μc-SiC:H electron injector

We have prepared μc-SiC:H films with high dark conductivities (10 −3 -1 S/cm) and wide optical band gaps (2.1–2.4 eV) at a higher gas pressure (around 5mTorr) than that for conventional electron cyclotron resonance (ECR) plasma chemical vapor deposition (CVD), with suppression of hydrogen ions impinging on an underlying layer. We have also applied them to a-SiC:H based thin film (TF) light-emitting diodes (LEDs) as electron injectors and obtained the emission over the whole range of visible spectrum from 400 to 780 nm. For a-SiC TF-LEDs, it is the first time that the emission in shorter wavelengths (400–500 nm) has been observed, and it is considered that the improvement of the interface due to the reduction of the process-induced damage during the deposition of μc-SiC:H causes an effective injection of electrons.

Very low-threshold separate-confinement-heterostructure lasers prepared by liquid phase epitaxy

AlGaAs separate-confinement-heterostructure lasers with low threshold current density (lowest averaged Jth of 317 A cm−2) are reproducibly prepared by liquid phase epitaxy. All optical-cavity layers of the lasers are undoped and their compositions are not graded. The low threshold is attained by increased internal efficiency resulting from effective injection of electrons into the active layer in the thin cavity, as well as by low internal loss of the optical cavity.