Organic Light-emitting Diode Structure Research Articles

Reduction of driving voltage in organic light-emitting diodes with molybdenum trioxide in CuPc/NPB interface

A novel structure of organic light-emitting diode was fabricated by inserting a molybdenum trioxide (MoO 3) layer into the interface of hole injection layer copper phthalocyanine (CuPc) and hole transport layer N,N′-diphenyl-N,N′-bis(1-napthyl–phenyl)-1,1′-biphenyl-4,4′-diamine (NPB). It has the configuration of ITO/CuPc(10 nm)/MoO 3(3 nm)/NPB(30 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq 3)(60 nm)/LiF(0.5 nm)/Al. The current density –voltage –luminance ( J–V–L) performances show that this structure is beneficial to the reduction of driving voltage and the enhancement of luminance. The highest luminance increased by more than 40% compared to the device without hole injection layer. And the driving voltage was decreased obviously. The improvement is ascribed to the step barrier theory, which comes from the tunnel theory. The power efficiency was also enhanced with this novel device structure. Finally, “hole-only” devices were fabricated to verify the enhancement of hole injection and transport properties of this structure.

Novel Amorphous Red Electroluminescence Material Based on Pyranylidene Indene-1,3-Dione Derivative

The organic light emitting diode (OLED) is a promising device for future technologies, like flat panel displays and novel light sources. So far the OLED structures have mostly been made by thermal evaporation in vacuum. An alternative approach is to use small molecules which form solid state with glassy structure from solutions. Such compounds can be used in the ink-jet printing technologies and result in reducing the OLED prices.

Análisis de la característica corriente-voltaje de diodos orgánicos emisores de luz (OLEDs) basados en polímeros depositados por spin coating.

<p>Polymer-based organic light-emitting diodes (OLEDs) with the structure ITO / PEDOT:PSS / MDMO-PPV / Metal were prepared by spin coating. It is known that electroluminescence of these devices is strongly dependent on the material used as cathode and on the deposition parameters of the polymer electroluminescent layer MDMO-PPV. <strong>Objective.</strong> In this work the effect of i) the frequency of the spin coater (1000-8000 rpm), ii) the concentration of the MDMO-PPV: Toluene solution, and iii) the material used as cathode (Aluminium or Silver) on the electrical response of the devices, was evaluated through current-voltage (I-V) measurements. <strong>Materials and methods</strong>. PEDOT:PPS and MDMO-PPV organic layers were deposited by spin coating on ITO substrates, and the OLED structure was completed with cathodes of aluminium and silver. The electric response of the devices was evaluated based on the I-V characteristics. <strong>Results.</strong> Diodes prepared with thinner organic films allow higher currents at lower voltages; this can be achieved either by increasing the frequency of the spin coater or by using concentrations of MDMO-PPV: Toluene lower than 2% weight. A fit of the experimental data showed that the diodes have two contributions to the current. The first one is attributed to parasitic currents between anode and cathode, and the other one is a parallel current through the organic layer, in which the carrier injection mechanism is mediated by thermionic emission. <strong>Conclusions.</strong> The results fitting and the energy level alignment through the whole structure show that PPV-based OLEDs are unipolar devices, with current mainly attributed to hole transport.</p> <p><strong>Key words:</strong> organic semiconductors, OLEDs, electroluminescent polymers, MDMO-PPV, PEDOT:PSS, Spin coating, HOMO, LUMO, carrier injection, thermionic emission.</p><br />

Optimization of 2-Stack WOLED Structure With Consideration on Color Gamut and Power Consumption

We fabricated 2-stack white organic light-emitting diode (WOLED) structure with luminance efficiency of 27.4 cd/A and color point of (0.32, 0.29) where one unit device emitted blue color and the other emitted red and green colors. Comparing with other configurations possible for 2-stack WOLED, it was found that our structure had merits of higher color gamut and lower power consumption, in spite of low luminance efficiency. In our 2-stack structure, we found that the blue efficiency which determines the power consumption of panel was strongly dependent on the position of the blue emitting layer. Based on an optical simulation and a viewing-angle analysis, we proved that the change of the luminance of blue color came from the microcavity effect depending on the distances between anode and cathode and between blue emitting layer and cathode.

Anode‐voltage‐control circuit for compensation of luminance deterioration

Abstract— An external driving circuit that has realized long lifetime, power‐consumption control, and peak luminance for organic light‐emitting diode (OLED) displays have been developed. This circuit realizes an effective method for constant‐anode‐voltage (CV) driving refered to as clamped inverter (CI) driving. The feature of CV driving is to achieve low‐power consumption compared with constant‐anode‐current (CC) driving and to control the power consumption and peak luminance according to the image because display luminance can be easily changed by controlling the anode voltage. On the other hand, CV driving has the problem that luminance deterioration appears to be serious compared with that of CC driving because the current of the OLED element decreases according to usage time. To cope with this, a lifetime compensation circuit that has increased the anode voltage so that it compensates for the luminance deterioration has been developed. This circuit can compensate not only the decrease in current but also the decrease in luminance at a constant current that CC driving cannot. However, increasing the anode voltage causes an increase in stress on the OLED element. The influence of stress on OLED lifetime was verified. As a result, it was confirmed that this circuit can extend the lifetime by 32% even if the anode voltage is increased, causing stress on the OLED structure.

Energy level alignment at the interfaces in a multilayer organic light-emitting diode structure

Received 2 March 2009; revised manuscript received 4 May 2009; published 11 June 2009We use photoelectron spectroscopy to study the electronic structure and energy level alignment throughoutan organic light-emitting diode. The structure under investigation is a state-of-the-art long-living red phospho-rescent device composed of doped charge-injection layers, charge-blocking layers, and an emission layer. Byconsecutively building up the whole device, the key parameters of every interface are measured. Our resultsshow that the doped layers have a significant influence on the device energetics, especially in controlling thebuilt-in potential, and that there are mostly only small dipoles present at the interfaces of the intrinsic organiclayers.DOI: 10.1103/PhysRevB.79.245308 PACS number s : 79.60.Fr, 85.60.Jb

Interface electronic structures of organic light-emitting diodes with WO 3 interlayer: A study by photoelectron spectroscopy

The energy level alignment and chemical reaction at the interface between the hole injection and transport layers in an organic light-emitting diode (OLED) structure has been studied using in-situ X-ray and ultraviolet photoelectron spectroscopy. The hole injection barrier measured by the positions of the highest occupied molecular orbital (HOMO) for N,N′-bis(1-naphthyl)- N,N′-diphenyl-1,1 ′-biphenyl-4,4 ′-diamine (NPB)/indium tin oxide (ITO) was estimated 1.32 eV, while that with a thin WO 3 layer inserted between the NPB and ITO was significantly lowered to 0.46 eV. This barrier height reduction is followed by a large work function change which is likely due to the formation of new interface dipole. Upon annealing the WO 3 interlayer at 350 °C, the reduction of hole injection barrier height largely disappears. This is attributed to a chemical modification occurring in the WO 3 such as oxygen vacancy formation.

Efficiency analysis of organic light-emitting diodes based on optical simulation

In spite of huge progress in improving the internal quantum efficiency of organic light-emitting diodes (OLEDs), these devices still suffer from poor light out-coupling. Loss mechanisms are for example waveguiding in the organic layers and the substrate as well as the excitation of surface plasmons at metallic electrodes. Their relative strength and the mutual dependence on the OLED structure have been studied both experimentally and by numerical simulation. Here, we consider the impact of the radiative quantum efficiency of the emitter material on predictions of light extraction from OLEDs. Competing processes resulting in non-radiative recombination of charge carriers usually reduce the emitter quantum efficiency in a real device. We show that optical simulation leads to erroneous conclusions when neglecting these competing processes. Furthermore, we demonstrate a method, which allows determining both the radiative quantum efficiency and the charge recombination factor via simulation based analysis of experimental data. This analysis of device efficiency is applied on a set of red-emitting electrophosphorescent devices.

Single Isolation Structure for Organic Light-Emitting Diodes

We have developed a single isolation structure for small molecule organic light-emitting diodes (OLEDs), where the insulator and cathode separators are formed by a single layer of image reversal photoresist to shorten the fabrication process and to increase the aperture ratio. The single isolation structure and its process parameters are described in this paper. Because the single isolation structure simultaneously forms the insulator and cathode separators with a single layer, the number of process steps is reduced by 29% compared with that of a conventional bilayered isolation structure, where the insulator and cathode separators are formed by a positive photoresist and a negative photoresist, respectively. Moreover, since the adhesion problem at the interface between the insulator and cathode separators is eliminated, their width is reduced to increase the aperture ratio. In addition, to verify its applicability and reliability, 1.17'' 96 ×RGB ×96 passive matrix OLEDs are fabricated using the single isolation structure.

2121 有機EL素子の薄膜構造最適化による射出光スペクトルの設計(OS21.計算力学と最適化(5),オーガナイズドセッション)

Organic light emitting diode (OLED) is next generation emission device because it has sliminess, high brightness, and high response. The emission spectrum depends on its film thickness and refractive index of each layer, so structural optimization of OLED is effective to improve of the light extraction efficiency and achieve the target emission spectrum. We developed emission spectrum analysis and optimization programs. In this paper, emission spectrum analysis by changing refractive index and structural optimization of OLED are reported. Results for some examples show that the peak wavelength is able to control by optimization of thicknesses and indexes of films in OLED structure.

HTL:EML(DPVBi:NPB)층의 조성비 변화에 따른 청색 유기 발광 소자 개발

The structure of organic light-emitting diodes(OLEDs) with typical heterostructure consists of anode, hole injection layer, hole transport layer, light-emitting layer, electron transport layer, electron injection layer, and cathode. 4,4bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl(NPB) used as a hole transport layer and 4'4-bis(2,2'-diphenyl vinyl)-1,1'-biphenyl(DPVBi) used as a blue light emitting layer were graded-mixed at selected ratio. Interface at heterojunction between the hole transport layer and the elecrtron transport layer restricts carrier's transfer. Mixing of the hole transport layer and the emitting layer reduces abrupt interface between the hole transport layer and the electron transport layer. The operating voltage of OLED devices with graded mixed-layer structure is 2.8 V at 1 <TEX>$cd/m^2$</TEX> which is significantly lower than that of OLED device with typical heterostructure. The luminance of OLED devices with graded mixed-layer structure is 21,000 <TEX>$cd/m^2$</TEX> , which is much higher than that of OLED device with typical heterostructure. This indicates that the graded mixed-layer enhances the movement of carriers by reducing the discontinuity of highest occupied molecular orbital(HOMO) of the interface between hole transport layer and emitting layer.

Molecular nanocrystals in polyaniline-based light-emitting diode structures

It is found that one of the types of molecular organic nanocrystals, the so-called J-aggregates, is able to modify optoelectronic properties of electroactive polymers. It is intrinsic that the polyaniline interpolymeric complex, known as p-type semiconductor, turns into electroluminescent material in the presence of J-aggregates of 3,3-di(gamma-sulfopropyl)-5,5-dichlorotiamonomethincyanine triethylammonium salt. During the studying of new organic light-emitting diode structures, in which the polyaniline/J-aggregates polymeric composite forms both recombination and emitting layers, a polyaniline intrinsic electroluminescence spectrum is obtained. The electroluminescence band completely coincides with the polyaniline photoluminescence spectrum; it has maximum at 400 nm. The electroluminescence mechanism in the studied nanocomposites is discussed.

The magnetic field effect on the transport and efficiency of group III tris(8-hydroxyquinoline) organic light emitting diodes

Magnetoresistance and efficiency measurements of organic light emitting diode structures based on the group III hydroxyquinolates (Mq3) have been made as a function of magnetic field and Mq3 thickness, where M=Al, Ga, and In. For all quinolates, independent of thickness, we observed very similar behavior for the efficiency of the devices, with large increases in efficiency occurring at low values of applied field, which rapidly saturate as the field is increased. The current through these devices is found to be a strong function of both the device thickness and the metal ion. For Alq3 based devices, the current changes appear to correlate strongly with the triplet population in the devices. For Gaq3 and Inq3 devices, the magnetoresistance is found not to correlate with the triplet concentration and this may be evidence that there is little energetic barrier for carrier trapping in these materials. For all materials, a further dependence of the magnetoresistance on applied field was observed, which needs closer investigation.

Source Driver Channel Reduction Schemes Employing Corresponding Pixel Alignments for Current Programming Active-Matrix Organic Light-Emitting Diode Displays

We propose two types of novel scheme for reducing the number of output channels of driver-integrated circuit (D-IC) for the current programming compensation pixel structures of active-matrix organic light-emitting diodes (AMOLEDs). One is a 2:1 data demultiplexing technique that can reduce the number of output channels of D-IC by half. The proposed second scheme is a vertically aligned red (R), green (G), and blue (B) subpixel scheme instead of a horizontally aligned R–G–B subpixel one, which is regarded as the conventional pixel alignment scheme. We have also successfully implemented these schemes in a 2.4-in.-sized QCIF + (176 × RGB × 220) AMOLED using p-type excimer laser annealing (ELA) low-temperature polycrystalline silicon (LTPS) technology and evaluated key performance characteristics.

Distribution of radiation from organic light-emitting diode structures with wavelength-scale gratings as a function of azimuth and polar angles

The angular distribution of radiation emitted from organic electroluminescent diodes fabricated on substrates with wavelength-scale gratings was measured using an optical Fourier transform instrument. A simple geometrical model is derived that specifies the polar angle of the exiting photon as a function of the azimuth angle, the grating pitch, the wavelength of light, and the effective index of the refraction of the light emitted by the fluorescing excitons. The radiation pattern of the extracted light is shown to fit that predicted by the model if one assumes that it comes from surface plasmon polaritons and bound TE waveguide modes.

Laminated active matrix organic light-emitting devices

Laminated active matrix organic light-emitting device (AMOLED) realizing top emission by using bottom-emitting organic light-emitting diode (OLED) structure was proposed. The multilayer structure of OLED deposited in the conventional sequence is not on the thin film transistor (TFT) backplane but on the OLED plane. The contact between the indium tin oxide (ITO) electrode of TFT backplane and metal cathode of OLED plane is implemented by using transfer electrode. The stringent pixel design for aperture ratio of the bottom-emitting AMOLED, as well as special technology for the top ITO electrode of top-emitting AMOLED, is unnecessary in the laminated AMOLED.

Magnetoresistance in organic light-emitting diode structures under illumination

We have investigated the effect of illumination on the organic magnetoresistance (OMR) in organic light-emitting diode (OLED) structures. The results show that it is possible to obtain OMR at voltages below ``turn-on,'' where no OMR was visible for devices operated in the dark. The photoinduced OMR has a field dependence that is identical to that obtained for OLEDs containing very thin layers, where triplet dissociation at the cathode was a major component of the OMR. At voltages around the open circuit voltage, where the current through the device is very small, very large OMRs of $\ensuremath{\sim}300%$ can be observed. The results support our proposed model for organic magnetoresistance as being caused in part by the interaction of free carriers with triplet excitons within the device. The results suggest that the introduction of a low field magnet could provide a simple means of improving the efficiency of organic photovoltaic cells.

The role of magnetic fields on the transport and efficiency of aluminum tris(8-hydroxyquinoline) based organic light emitting diodes

Magnetoresistance and efficiency measurements of aluminum tris(8-hydroxyquinoline) (Alq3) based organic light emitting diode structures have been made as a function of magnetic field and Alq3 thickness. Both positive and negative magnetoresistances can be observed depending on the thickness of the Alq3 layer, the drive voltage, and the applied field. In all devices, large increases in device efficiency are observed. We suggest that the increase in device efficiency is due to conversion of triplet states into singlets through a hyperfine scale interaction. The changes in the magnetoresistance are a result of the reduction in the triplet concentration and operate either through the reduced role of free carrier trapping at triplet states or through the reduction in triplet dissociation at the cathode interface depending on the Alq3 thickness.

Transparent organic light-emitting diodes using resonant tunneling double barrier structures

A semitransparent cathode of indium tin oxide (ITO)/Ag/ITO was developed as a resonant tunneling double barrier structure for transparent organic light-emitting diodes. A weak negative differential resistance was observed in devices using a 100nm thick ITO/Ag/ITO layer as a cathode in combination with a thin LiF∕Al layer. The current injection of devices was dominated by resonant tunneling, which induced no luminance at low voltage. This was achieved by employing an e-beam evaporated ITO/Ag/ITO cathode due to the double quantum barriers of ITO and the quantum well of Ag. The authors also applied the multilayer cathode to small molecule devices, which showed the same resonant tunneling currents.

Design of Multilayer Structure for UV Organic Light-Emitting Diodes Based on 2-(2-Naphthyl)-9,9'-spirobifluorene

With the goal of obtaining UV organic light-emitting diodes (UV OLEDs), the device architecture of OLEDs containing a novel UV-emitting material, 2-(2-naphthyl)-9,9'-spirobifluorene (2SBFN), has been studied. UV emission from the 2SBFN layer was attained by employing the multilayer structure where poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) was used as a hole-injection layer (HIL), while using conventional materials such as 4,4',4''-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (mMTDATA) and copper phthalocyanine (CuPc) as HIL resulted in the emission from a material other than 2SBFN. These results suggest that the appropriate choice of HIL material is of critical importance in obtaining UV OLEDs.