Organic Light Emission Research Articles

Silicon nitride PhC nanocavities as versatile platform for visible spectral range devices

We propose silicon nitride two-dimensional photonic crystal resonators as flexible platform to realize photonic devices based on spontaneous emission engineering of nanoemitters in the visible spectral range. The versatility of our approach is demonstrated by coupling the two dipole-like modes of a closed band gap H1 nanocavity with: (i) DNA strands marked with Cyanine 3 organic dyes, (ii) antibodies bounded to fluorescent proteins and (iii) colloidal semiconductor nanocrystals localized in the maximum of the resonant electric field. The experimental results are in good agreement with the numerical simulations, highlighting the good coupling of the nanocavities with both organic and inorganic light emitters.

A high performance inverted organic light emitting diode using an electron transporting material with low energy barrier for electron injection

A high performance inverted green emission organic light emitting diode with a maximum external quantum efficiency of 20% and a maximum power efficiency of 80 lm/W was realized by properly selecting an electron transporting material to have no energy barrier for electron injection between the n-doped electron transporting layer (n-ETL) and the ETL. Based on the energy levels and the current density–voltage characteristics of electron only devices, we demonstrate that the interface between an n-ETL and an ETL even in homo-junction is as important as the interface between the cathode and the n-ETL for efficient electron injection into an emitting layer.

Improving the Efficiency of Organic Light Emitting Diodes by Use of a Diluted Light-Emitting Layer

The use of a thin mixed layer consisting of an inert diluent material and a light emitting material between the hole-transport layer and electron-transport layer of organic lightemitting diodes leads to an increase in the external quantum efficiency. The efficiency improvement is highly dependent on the thickness of the diluted light-emitting layer and driving current. Significant improvement seen at low current densities is explained in terms of effective hole confinement by the mixed layer while a modest decreases in efficiency at higher current densities may be attributed to luminescence quenching at the hole-transport layer/inert diluents material interface. The phenomena are demonstrated with three different inert diluents materials. A maximum external quantum efficiency improvement of about 40% is found for a diluted light-emitting layer thickness between 40 A and 60 A.

Novel Field Emission Organic Light Emitting Diodes with Dynode

This work presents novel field emission organic light emitting diodes (FEOLEDs) with dynode, in which an organic EL light-emitting layer is used instead of an inorganic phosphor thin film in the field emission display (FED). The proposed FEOLEDs introduce field emission electrons into organic light emitting diodes (OLEDs), which exhibit a higher luminous efficiency than conventional OLED. The field emission electrons emitted from the carbon nanotubes (CNTs) cathode and to be amplified by impact the dynode in vacuum. These field emission electrons are injected into the multi-layer organic materials of OLED to increase the electron density. Additionally, the proposed FEOLED increase the luminance of OLED from 10 820 cd/m^2 to 24 782 cd/m^2 by raising the current density of OLED from an external electron source. The role of FEOLED is to add the quantity of electrons-holes pairs in OLED, which increase the exciton and further increase the luminous efficiency of OLED. Under the same operating current density, the FEOLED exhibits a higher luminous efficiency than that of OLED.

Novel Field Emission Organic Light Emitting Diodes

This work presents a novel field emission organic light emitting diode (FEOLED), in which an inorganic phosphor thin film is replaced by an organic EL light-emitting layer in the configuration of a field emission display (FED). The field emission electrons emitted from the carbon nanotubes (CNTs) cathode of the proposed FEOLED intensify the electron density in the multi-layer organic materials of the OLED; thus, resulting a higher luminous efficiency than that of a conventional OLED. Additionally, the luminance of the proposed FEOLED can be further increased from 10,820 cd/m 2 to 27,393 cd/m 2 by raising the current density of OLED through an external electron source. A balanced quantity of electrons and holes in the OLED, which is achieved by the proposed FEOLED increases the number of excitons and attributes the enhancement of luminous efficiency of the OLED. Under the same operating current density, the proposed FEOLED exhibits a higher luminous efficiency than that of a conventional OLED.

Both Luminous Efficiency and Lifetime Enhancement in Blue Fluorescence OLEDs by Modifying Molecular Structure of Hole Transporting Material

ABSTRACTBoth luminous efficiency and lifetime in blue fluorescence organic light emitting devices (OLEDs) have been improved by modified HTMs with higher LUMO energy levels. The LUMO energy levels of HTM were increased by modifying substituent in HTM molecules. Two HTMs containing ortho and meta biphenyl substituent and one HTM containing thiophene substituent were synthesized via palladium catalyzed amine coupling reactions to compare with a para biphenyl substituent HTM-1 as a standard molecule. According to TDDFT calculations, these three modified HTMs showed 0.05-0.15 eV higher LUMO energy levels compared to the para biphenyl substituent HTM-1. The luminous efficiency and the lifetime (LT90) of OLEDs using HTM-2 at 500 cd/m2 have been enhanced up to 20 % and 52 %, respectively, compared to the standard device using HTM-1.

Organic light emitters: Molecularly designed for better performance

Organic light emitters: Molecularly designed for better performance

Outcoupling efficiency of organic light emitting diodes and the effect of ITO thickness

Outcoupling efficiency of bottom emission organic light emitting diodes (OLEDs) have been analyzed using a model (Adv. Chem. Phys. 37 (1987) 1, [17]) based on classical electromagnetic theory with a full vectorial approach. The analyses presented in this study take into account many factors, including the thickness of the ITO layer. Maximum outcoupling efficiencies over 27% and 30% are achievable if thin ITO layers in the range of 50–70 nm and 50–100 nm are used for blue and green phosphorescent OLEDs, respectively. Even higher efficiencies of 29% for the blue and 31.6% for the green OLED can be obtained if we use thicker organic layers supporting two antinodes suggesting that phosphorescent OLEDs can have a much higher external quantum efficiency than 20%.

Electron-Bombardment-Induced Damage to Organic Light Emission Layer

Damage to an aluminum(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq) organic film by direct electron irradiation was investigated. An electron-bombardment system, which can supply a desired amount and energy of electrons, was set up. Changes in the photoluminescence (PL) intensity of the BAlq layer were used to evaluate the damage produced by electron bombardment. A marked reduction of the PL intensity was observed with the increase in the amount and energy of electrons irradiated to the BAlq films, although no changes in the Fourier transform infrared (FTIR) spectra of the BAlq layer due to electron bombardment were detected. A change in surface morphology, however, was observed by atomic force microscopy (AFM). These results suggest that electron bombardment induces significant damage to the organic layer and that a reduction of electron bombardment during the sputter deposition of electrode films is necessary to improve the performance of the organic layer.

A solution processed top emission OLED with transparent carbon nanotube electrodes

Top emission organic light emitting diodes (OLEDs) with carbon nanotubes (CNTs) as topelectrodes were fabricated and characterized. Devices were fabricated on glass substrateswith evaporated bottom Al/LiF cathodes, a spin coated organic emissive layer and aPEDOT-PSS hole injection layer. Transparent thin CNT films were deposited on top of theemission layer to form the anode by micro-contact printing with a polydimethylsiloxanestamp. A very good device performance was obtained, with a peak luminance of3588 cd m − 2 and a maximumcurrent efficiency of 1.24 cd A − 1. This work shows the possibility of using CNTs as transparent electrodes toreplace ITO in organic semiconductor devices. Furthermore, the top emissionnature of such devices offers a broader range of applications of CNTs on anytype of substrate. By combining with solution processed organic materials, it isanticipated that lower cost fabrication will be possible through roll-to-roll manufacture.

Highly Efficient Blue-Light-Emitting Diodes Based on Styrylamine Derivatives End-capped with a Diphenylvinyl Group

In this paper, we reported the synthesis and electroluminescent properties of blue fluorescent styrylamine derivatives end-capped with a diphenylvinyl group. A new series of styrylamine derivatives have been synthesized via the HornerWadsworth-Emmons reaction. To explore electroluminescent properties of these molecules, multilayer organic lightemitting devices with the configuration of ITO/NPB/1-5 doped in MADN/Bphen/Liq/Al were fabricated. All devices exhibited blue emissions with good EL performances. Among those reported herein, the device using dopant 5 exhibited a maximum luminance of 24,000 cd/m 2 at 11.0 V, a luminous efficiency of 12.5 cd/A at 20 mA/cm 2 , a power efficiency of 6.50 lm/W at 20 mA/cm 2 , and CIEx,y coordinates of (x = 0.173, y = 0.306) at 8.0 V, all of which demonstrate the superiority of these materials in blue OLEDs.

Effect of the electric field on the decay of excited states in poly-phenylenevinylene

To gain an insight into the effect of electric field on the electro-luminescence of organic light emission diode, the ultrafast pump-probe spectroscopy is used to investigate the transient phenomena of the excitons induced by the electric field. Under the excitation density of 230 μJ/cm2, the decay of the singlet excitons shows a fast and a slow component. The amplitude factor and the relaxation time of fast component are field-dependent. Compared with the zero biascase, the relaxation time of the fast component becomes faster at a bias of 6.4×105 V/cm, its amplitude factor is increased from 22% to 72%, about 50% initial excitons are dissociated by the electric field. The slow component is field-independent, its relaxation time-constant is about 890 ps. The longitudinal acoustic phonons with sound velocity of 17 /ps generated by excitation pulse are observed.

Materials, Devices, and Circuits of Transparent Amorphous-Oxide Semiconductor

This paper presents the following recent investigations of transparent amorphous-oxide semiconductors (TAOS) from materials to devices and circuits. 1) Composition of metals in TAOS are widely explored with the aim of seeking semiconductors suitable for the channel layers of thin-film transistors (TFTs) composing backplanes for flat-panel displays. It is found in combinatorial approaches to the materials exploration that indium-based ternary TAOS (In-X-O) and their TFTs show the properties and the performance as good as those of the most popular material of amorphous In-Ga-Zn-O (alpha-IGZO) when X = Zn or Ge. 2) Defects and impurities in TAOS are investigated by theoretical approaches. The first-principle calculation of the electron states reveals that charge-neutral oxygen vacancy or interstitial forms the density of states around mid-gap level and does not generate carriers directly, while hydrogen impurity raises the Fermi level beyond the conduction-band minimum and acts as a donor in TAOS. 3) Device structures of TAOS-TFTs are also investigated extensively for better performance and stability. It is found in channel-etch type TFTs with bottom-gate inverse-stagger structures that the TFT characteristics and stability are significantly improved by chemically removing the back-channel layer in a wet-etching process. Coplanar homojunction (CH) structure is proposed as a novel device structure where conductive alpha-IGZO regions work as the source and drain electrodes to the channel region of semiconductor alpha-IGZO. The CH TFTs show excellent characteristics and stability, with low series resistance without any difficulty in making good electrical contact between metals and TAOS. 4) Circuits using TAOS-TFTs are demonstrated. A ring oscillator composed of fifteen-stage inverters with a buffer circuit operates as designed by circuit simulation with a TFT model for hydrogenated amorphous Si TFTs. Pixel circuits composed of three TFTs and one transparent capacitor successfully drive organic light-emission diode cells without a planarization layer on a 2-in diagonal panel having 176 times144 times 3 pixels.

Application of High Work Function Anode for Organic Light Emitting Diode

We fabricated bottom emission organic light emitting diode (OLED) by using high work function material anode with sputtering method and investigated their electrical properties. Nickel oxide (NiO) was found to reach about 5.2 eV, much higher than those of other oxide metal and indium tin oxide (ITO). The bottom emission OLED using a thin oxidation of nickel oxide layer upon ITO as an anode and an Al layer as a cathode has been fabricated. Device performance was found to improve greatly due to an efficient hole injection from nickel oxide; moreover, the transmittance of NiO-ITO was nearly same as the transmittance of ITO.

Ultrafast field-induced decay of singlet excited-state in conjugated polymers

An ultrafast pump−probe spectroscopy was used to investigate the transient phenomena of the excitons induced by the electric field to gain an insight into the effect of electric field on the organic light emission diode. At high excitation density of 230 μJ/cm 2 the decay of the singlet excitons showed fast and slow components. The amplitude factor and the relaxation time of the fast component are field-dependent. The slow component is field-independent, and its relaxation time-constant is about 890 ps. We also observed the longitudinal acoustic phonons with sound velocity of 17 Å/ps generated by excitation pulse.

A hybrid control scheme for driving current sources of pm-oled panel

This paper proposes a new amplitude width modulation (AWM) scheme for PM-OLED (passive matrix organic light emission device) data driver IC. The data driver controls brightness of OLED by adjusting the amplitude and width of the current through OLED. There are two conventional modulation schemes; pulse amplitude modulation (PAM) and pulse width modulation (PWM). The PWM suffers from lower light emitting efficiency at lower luminance levels. The PAM gives us large chip area, due to DACs for each column. The proposed AWM accurately controls the current level using MSB data and increases luminance using LSB data that improves inefficiencies of the PAM and PWM. We fabricated a one channel data driver chip with AWM using a 0.18 um standard CMOS process. The proposed scheme reduces power consumption 50% compared with PWM.

Raman lasing near 650 nm from pure water microdroplets on a superhydrophobic surface

We demonstrate Raman lasing near 650 nm in pure water microdroplets located on a superhydrophobic surface. In the experiments, stationary, pure water microdroplets were prepared on a superhydrophobic surface and excited by a pulsed, frequency-doubled Nd:YAG laser at 532 nm. Intense laser emission was observed at frequencies corresponding to the whispering gallery mode resonances of the water microdroplets near 650 nm where Raman resonances due to OH-stretching bonds of water are located. On–off behavior was observed during lasing and the average temporal inter-burst separation was determined from the time-dependent intensity traces. Our results can find applications in the development of novel organic light emitters for short-haul communication systems, and in the spectroscopic characterization of water microdroplets on a surface.

Organic light emitters: Star quality

Organic light emitters: Star quality

Active-Matrix Organic Light Emission Diode Pixel Circuit for Suppressing and Compensating for the Threshold Voltage Degradation of Hydrogenated Amorphous Silicon Thin Film Transistors

A new hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) pixel circuit for active-matrix organic light emission diodes (AM-OLEDs), which significantly compensates the OLED current degradation by memorizing the threshold voltage of driving TFT and suppresses the threshold voltage shift of a-Si:H TFTs by negative bias annealing, is proposed and fabricated. During the first half of each frame, the driving TFT of the proposed pixel circuit supplies current to the OLED, which is determined by modified data voltage in the compensation scheme. The proposed pixel circuit was able to compensate the threshold voltage shift of the driving TFT as well as the OLED. During the remaining half of each frame, the proposed pixel circuit induces the recovery of the threshold voltage degradation of a-Si:H TFTs owing to the negative bias annealing. The experimental results show that the proposed pixel circuit was able to successfully compensate for the OLED current degradation and suppress the threshold voltage degradation of the driving TFT.

PEDOT as an alignment layer and electrode in AFLC devices

AbstractPEDOT [Poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate)] is a material typically used organic light emission displays (OLEDs) manufactured as a conductor, or rather, as a hole injector. In this paper, three different issues have been studied: the use of rubbed PEDOT as alignment layer, the resistivity of the applied PEDOT layers, i.e., its potential as a replacement of ITO electrodes, and the induction of asymmetry in the electro-optical response of liquid crystal cells by assembling the display with dissimilar coated aligning surfaces — PEDOT on one side and buffed nylon on the counter electrode