Properties Of Organic Light-emitting Diodes Research Articles

알루미늄 도핑된 산화아연 양극을 적용한 고효율 유기발광다이오드

Properties of organic light-emitting diodes (OLEDs) with aluminum-doped zinc oxide (ZnO:Al) anodes showed different behaviors from OLEDs with indium tin oxide (ITO) anodes according to driving conditions. OLEDs with ITO anodes gave higher current density and luminance in lower voltage region and better EL and power efficiency under lower current density conditions, However, OLEDs with ZnO:Al anodes gave higher current density and luminance in higher voltage region over about 8V and better EL and power efficiency under higher current density over . These seemed to be due to the differences in conduction properties of semiconducting ZnO:Al and metallic ITO. OLEDs with ZnO:Al anodes showed nearly saturated efficiency under high current driving conditions compared with those of OLEDs with ITO anodes. This meant better charge balance in OLEDs with ZnO:Al anodes. These properties of OLEDs with ZnO:Al anodes are useful in making bright display devices with efficiency.

Improvement of highly efficient organic light-emitting diodes using Mg-doped ZnO buffer layers

In this paper, we report on the improvement of the anode buffer layer of Mg-doped ZnO (MZO) on the electrical and optical properties of organic light-emitting diodes (OLEDs). The MZO layers with different thickness were prepared by thermal-evaporating the MZO powders and then treated by ultraviolet (UV) ozone exposure. The turn-on voltage of OLEDs decreased from 4 V (4.2 cd/m 2) to 3 V (4.7 cd/m 2), the maximum luminance value increased from 16780 cd/m 2 to 35400 cd/m 2 and the power efficiency increased from 2.74 lm/W to 5.77 lm/W as the 1-nm-thick MZO layer was inserted between indium tin oxide (ITO) anodes and α-naphthylphenylbiphenyl diamine (NPB) hole-transporting layers. The surface of the MZO/ITO became smoother with the UV-ozone treatment. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) studies were performed to show that the formation of the MZO layer and the work function increased as the MZO/ITO layer was treated by UV-ozone for 20 min. Thus, the hole-injection energy barrier was lowered by inserting MZO buffer layer, consequently resulting in the decrease of the turn-on voltage and the increase of the power efficiency in OLEDs.

Electrical properties of organic light-emitting diodes by indium tin oxide chemical-mechanical polishing process

In this study, the optimum process parameters and the influences of their process parameters were investigated for indium tin oxide chemical-mechanical polishing (ITO-CMP) with a sufficient removal amount and good planarity. Next, the organic light-emitting display (OLED) device with the structure of glass∕ITO∕poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene]∕Al using a polished ITO surface as a bottom electrode (anode) was fabricated. The electrical characteristics, such as sheet resistance and current-voltage (I-V) relationship, are discussed in order to evaluate the possibility of the CMP application for an OLED device using an ITO film. The surface morphology and I-V characteristics of ITO thin film were improved after the CMP process using optimized process parameters compared to that of the as-deposited thin film before the CMP process.

Dielectric properties depending on frequency in organic light-emitting diodes

Dielectric properties of organic light-emitting diodes were investigated using ITO/Alq 3/Al device. In spite of various advantages in organic light-emitting diodes, a fundamental study on physical properties is not yet sufficient. Dielectric properties are used for studying fundamental physical properties of materials through a frequency-dependent response. We have investigated magnitude and phase of impedance, electrical conductivity, and the dielectric loss depending on a bias-voltage variation using ITO/Alq 3(60 nm)/Al device. The device shows a frequency-dependent response such that a major contribution is resistive below time constant and capacitive above time constant. Also, the device shows a voltage-dependent electrical conductivity in low-frequency region. A bulk resistance rapidly decreases as the frequency increases above 1 MHz. The dielectric loss shows that there appears an interfacial polarization in low-frequency region, and an orientational polarization in high-frequency region.

Retraction notice to: “Properties of organic light-emitting diodes by aluminum cathodes modification using Ar+ ion beam” [Org. Electron. 6 (4) (2005) 149–160

Retraction notice to: “Properties of organic light-emitting diodes by aluminum cathodes modification using Ar+ ion beam” [Org. Electron. 6 (4) (2005) 149–160

Improved Hole-Injection and Power Efficiency of Organic Light-Emitting Diodes Using an Ultrathin Li-Doped ZnO Buffer Layer

We report on the advantages of an anode buffer layer of Li-doped ZnO (LZO) on the electro-optical properties of organic light-emitting diodes (OLEDs). LZO layers with different thicknesses were prepared by thermally evaporating the LZO powders and then treating them with ultraviolet (UV) ozone exposure. The turn-on voltage of OLEDs decreased from , the maximum luminance value increased from and the power efficiency increased from when a thick LZO layer was inserted between indium-tin oxide (ITO) anodes and β-naphthylphenylbiphenyl diamine hole-transporting layers. X-ray and ultraviolet photoelectron spectroscopy were performed to show that the formation of the LZO layer and the work function increased by when the LZO/ITO layer was treated by UV-ozone for . The surface of the LZO/ITO film became smoother after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an LZO buffer layer, resulting in the decrease of the turn-on voltage and the increase of the power efficiency in OLEDs.

Effect of laser-etched indium tin oxide on optoelectrical properties of organic light-emitting diodes

This study investigated how laser and wet etching methods of ITO substrates affect the optoelectrical properties of OLEDs. Experimental results indicated that the OLED with a laser-etched ITO substrate has a lower driving voltage than that with a wet-etched ITO substrate. According to scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements, the ITO etching methods yielded different surface morphologies of ITO pattern edges. The lower luminance of the OLED with a laser-etched ITO substrate is attributable to the fringe at the edge of ITO pattern, which causes a high local electric field resulting in the recrystallization of organic materials.

Synthesis and blue electroluminescent properties of zinc (II) [2-(2-hydroxyphenyl)benzoxazole

This study reports on the properties of organic light-emitting diodes (OLEDs) with zinc (II) [2-(2-hydroxyphenyl)benzoxazole] as a hole-blocking layer. OLEDs devices are prepared in a conventional OLEDs structure (i.e., anode/HTL/EL/HBL/cathode and anode/HTL/HBL/EL/cathode). The luminescence efficiencies and the turn-on voltage are significantly affected by the existence of the hole-blocking layer. This is attributed to an excellent hole-blocking property, which is in turn due to the high HOMO energy level (6.5 eV). The device showed luminous efficiency 2.46 lm/W at 5 V. The maximum luminance of about 10,000 cd/m 2 is obtained, and the turn-on voltage (2.5 V) is affected by the existence of the hole-blocking layer.

Highly efficient organic light-emitting diodes fabricated utilizing nickel-oxide buffer layers between the anodes and the hole transport layers

The electrical and the optical properties of organic light-emitting diodes (OLEDs) fabricated utilizing nickel-oxide (NiO) buffer layers between the anodes and the hole transport layers were investigated. The NiO layer was formed by using a thermally evaporated nickel thin film and a subsequent oxidation process. The tunneling holes in the OLED were increased due to the existence of the NiO layer between the anode and the hole transport layer, resulting in enhanced efficiency for the OLED. These results indicate that OLEDs with NiO buffer layers hold promise for potential applications in highly-efficient flat-panel displays.

Spin injection into organic light-emitting diodes with a ferromagnetic cathode and observation of the luminescence properties

Electroluminescence (EL) properties of organic light-emitting diodes (LEDs) under an external magnetic field were investigated for observation of spin-polarized luminescence. When an external magnetic field was applied to organic LEDs with an Fe cathode, circularly polarized light was observed, while devices with an Al cathode did not show such behavior. In order to confirm whether the circularly polarized light was certainly due to spin injection or not, photoluminescence (PL) property of Alq 3 ( tris-(8-hydroxyquinolinato)-aluminum) film on Fe and that of Alq 3 film on Al under an external magnetic field were investigated. In the case of Alq 3 on Fe film, the degree of circular polarization from EL was much higher than that from PL. This suggested that circular polarization on EL was mainly due to spin injection.

Organic light-emitting diodes and organic light-emitting electrochemical cells based on silole–fluorene derivatives

Silole groups are known to present a high electron affinity. Initially, copolymerization of siloles with fluorene was aimed at improving electron injection into the polymer layer and so improving the electroluminescent properties of organic light-emitting diodes (OLED's) made from fluorene. But it also provides the ability to turn the light emission colour to the green part of the spectrum and to stop the well-known spectral shift degradation occurring in fluorene-based materials. In this paper we report the synthesis and the characterisation of 1,1-dimethyl-2,5-bis(fluoren-2-yl)-3,4-diphenylsilole 4, and of two soluble conjugated random copolymers derived from 9,9-ditetradecylfluorene and 1,1-dialkyl-2,5-diphenylsilole, where the alkyl group is either methyl 11a or n-hexyl 11b. Silole 4 crystallizes in the triclinic P-1 space group with a = 9.8771(8), b = 10.6240(10), c = 16.585(2) Å, α = 95.775(8), β = 97.025(7), and γ = 111.738(8)°. The results obtained with this molecule, operating in a single-layer OLED (luminance ≈450 Cd/m 2 at 12 V; η max = 0.2 Cd/A), give evidences for the complementarity of the silole and the fluorenyl moieties in the improvement of the charge injection processes when compared with 1,1-dimethyl-2,3,4,5-tetraphenylsilole. The results obtained from organic light-emitting electrochemical cells (LEC's) made from silole–fluorene copolymers 11a, 11b and molten salts show an improvement of both the device lifetime and the spectral stability when compared with polyfluorene. To explain devices performances electrical characterisation data and atomic force microscope (AFM) imaging were combined.

Electrical properties of organic light-emitting diodes using PECCP Langmuir–Blodgett films

Organic light-emitting diodes were fabricated using Langmuir–Blodgett (LB) technique with new emissive organic material. This new emissive organic material was synthesized and called PECCP [poly(3,6- N-2-ethylhexyl carbazolyl cyanoterephthalidene)], which has strong electron donor group and electron acceptor group in main chain repeated unit. This material has good solubility in common organic solvent and has a good stability in air. We made PECCP LB films and investigated current–voltage ( I– V) characteristics in ITO/PECCP LB/Al and ITO/PECCP LB/Alq 3/Al structures, and the Alq 3 was used as an electron transport layer. We have observed that the peak in electroluminescent spectrum of ITO/PECCP LB/Alq 3/Al device shifts to lower wavelength as the number of LB layer increases. The electroluminescent peaks are at about 507 nm, 505 nm, 503 nm, and 491 nm when the number of LB layers are 5, 7, 9, and 11 layers, respectively. From the I– V characteristics, the barrier height was obtained using Fowler–Nordheim analysis and it is in the range of 0.18 eV–0.26 eV.

유기 발광 소자의 바이어스 전압에 따른 유전 특성

We have investigated dielectric properties depending on bias voltage in organic light-emitting diodes using 8-hydroxyquinoline aluminum as an electron transport and emissive material. We analyzed the dielectric properties of organic light-emitting diodes using impedance of characteristics. Impedance characteristics was measured complex impedance Z and phase in the frequency range of 40 Hz to Hz. We obtained complex electrical conductivity, dielectric constant, and loss tangent of the device at room temperature. From these analyses, we are able to interpret a conduction mechanism and dielectric properties contributed by an interfacial and orientational polarization.

Highly efficient organic light-emitting diodes with hole injection layer of transition metal oxides

We report on the advantage of interlayers using transition-metal oxides, such as iridium oxide (IrOx) and ruthenium oxide (RuOx), between indium tin oxide (ITO) anodes and 4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl (α-NPD) hole transport layers on the electrical and optical properties of organic light-emitting diodes (OLEDs). The operation voltage at a current density of 100mA∕cm2 decreased from 17to11V for OLEDs with 3-nm-thick IrOx interlayers and from 17to14V for OLEDs with 2-nm-thick RuOx ones. The maximum luminance value increased about 50% in OLED using IrOx and 108% in OLED using RuOx. Synchrotron radiation photoelectron spectroscopy results revealed that core levels of Ru 3d and Ir 4f shifted to high binding energies and that the valence band was splitting from metallic Fermi level as the surface of the transition metal was treated with O2 plasma. This provides evidence that the transition-metal surface transformed to a transition-metal oxide. The surface of the transition metal became smoother with the O2 plasma treatment. The thickness was calculated to be 0.4nm for IrOx and 0.6nm for RuOx using x-ray reflectivity measurements. Secondary electron emission spectra showed that the work function increased by 0.6eV for IrOx and by 0.4eV for RuOx. Thus, the transition-metal oxides lowered the potential barrier for hole injection from ITO to α-NPD, reducing the turn-on voltage of OLEDs and increasing the quantum efficiency.

Thermal annealing of blended-layer organic light-emitting diodes

The effects of thermal annealing on the electro-optical and morphological properties of organic light-emitting diodes (OLEDs) containing a blend of poly[2-(2-ethylhexyloxy)-5-methoxy-1, 4-phenylenevinylene] and an electron-transporting material, 2,7-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]-9,9-dihexylfluorene (DFD) are reported. Improvements in the efficiency were found to depend on the composition of the blend film. Devices with a higher concentration of the electron-transport material showed a smaller increase in efficiency following the annealing step. The thermal treatment is thought to result in the formation of an interfacial layer between the organic film and the aluminum cathode, improving the injection of electrons from this electrode. Blended-layer OLEDs containing a high DFD concentration (⩾70%) retained their emission color following thermal processing.

RETRACTED: Properties of organic light-emitting diodes by aluminum cathodes modification using Ar+ ion beam

This article has been retracted at the request of the Editor-in-Chief and Author. Please see http://www.elsevier.com/locate/withdrawalpolicy . Reason:This article is retracted at the request of the editor as it is a duplication of a paper that has already appeared in at least nine other publications. As such this article represents a severe abuse of the scientific publishing system. It has wasted the time of many reviewers and also wastes costly journal space which is explicitly reserved for publishing original, not self-plagiarized work. Organic Electronics takes a very strong view on this matter. The editors want to inform our readership and authors, that those who publish multiple copies of their papers in Organic Electronics and elsewhere will lose their rights to further publication in this journal.

Temperature-dependent electrical properties of organic light-emitting diodes depending on cathodes

Abstract We have studied temperature-dependent electrical properties of organic light-emitting diodes with a variation of cathode materials; Al, LiAl, and LiF/Al. The organic light-emitting diodes emit a light by a recombination of injected charge carriers such as holes and electrons. Thus, the charge transport is affected by the injection barrier at the interface. By varying the cathode materials, the electron injection at the interface could be controlled because of the work-function change at the cathode. Temperature-dependent current–voltage luminance characteristics of the organic light-emitting diodes were measured in the temperature range from 10 to 300 K. The current-voltage characteristics were analyzed in terms of Fowler–Nordheim tunneling model, and the energy-barrier height was obtained. A measured lifetime of device with LiF/Al cathode is relatively longer than the other cathodes at room temperature: 4.5 h for Al cathode, 12.4 h for LiAl, and 29.6 h for LiF/Al. The device with LiAl and LiF/Al cathode, in the aspect of lifetime and luminous efficiency, is superior to one of other cathodes.

Impedance Spectroscopy of p-Conjugated Organic Materials

AC electrical properties of organic light-emitting diodes with poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV), poly[2,5-bia(dimethyloctylsilyl)-1,4-phenylenevinylene] (BDMOS-PPV), and tris-(8-hydroxyquinolate)aluminum (AlQ3) as light-emitting materials are studied. The frequency-dependent real and imaginary parts of impedance were fitted using an equivalent circuit. We found that the conduction mechanism is a space-charge limited current with exponential trap distribution.

Morphological, electrical and optical properties of organic light-emitting diodes with a LiF/Al cathode and an Al-hydroxyquinoline/diamine junction

We report the results of the morphological, electrical and optical characterisation of double-layer Alq 3-based organic emitting diodes with a lithium fluoride (LiF)/Al cathode. A detailed electron microscopy investigation of their cross-section shows the presence of LiF isolated grains underneath the Al film. Due to the introduction of the LiF layer, luminance was larger than 30,000 cd/m 2 at a bias voltage V B=25 V with a maximum external luminous efficiency as large as 46 lm/W at V B=20 V. Performing on/off V B cycles at a very low frequency, each time a recovery of the initial electric and luminous performance was observed. The non-exponential decay of both electric current and luminance during each on-cycle has been interpreted as the signature of dispersive transport controlled by multiple trapping on localised states in an amorphous system.

Electroluminescence Properties of Organic Light-Emitting Diodes with a Red Dye Doped into Alq3 : Rubrene Mixed Host

We have studied the electroluminescence (EL) properties of devices with a red fluorescent dye, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), doped into a mixed matrix of tris-(8-hydroxyquinoline)aluminum (Alq3) and rubrene. The devices doped with DCJTB in the Alq3:rubrene mixed host show an efficient red emission from DCJTB with negligible EL emission from both Alq3 and rubrene. The QE increases with increasing rubrene concentration and reaches a maximum of about 3.6 % for a DCJTB doping concentration of about 5 % in the Alq3:rubrene mixed (50:50 % ratio by weight) host, and then decreases at higher rubrene concentration, due to the concentration quenching effect. At a given bias voltage, the current density increases with increasing rubrene doping concentration, but it decreases with increasing DCJTB doping concentration. The results imply that the injected electrons and holes can transport via hopping through the energy levels of rubrene molecules while DCJTB acts as traps.