Single-layer small molecular organic light emitting diodes without hole transport layer

Organic light emitting devices based on tris-8-hydroxyquinoline aluminum with an ultra-thin insulating film instead of a hole-transporting layer were investigated. Three kinds of single-layer devices were fabricated with silicon dioxide (SiO₂), alumina (Al₂O₃) or polytetrafluoroethylene (Teflon™) as the insulating films, and they showed better performance than devices without insulating films. The hole injection of these devices was also investigated using hole-only devices. The holes injection enhancement by tunneling effect and electrons blocking of the ultra-thin insulating layers were experimentally proved to have yielded high performance of single-layer devices.     

Tuning the colour of white polymer light emitting diodes

Colour tuning of white polymer light emitting diode (LED) light sources can be attained by various methods at various stages in the production process of the lamps and/or by the design of the active material incorporated in the LEDs. In this contribution we will describe the methods and discuss the physical background of colour tuning. Furthermore, the material design has led to polymers which are more stable during electrical stress, so that colour shift during lifetime can be excluded for white polymer LEDs.     

Micropatterning of emitting layers by microcontact printing and application to organic light-emitting diodes

The microcontact printing (μCP) technique, which is a simple and low damage fabrication technique for thin films, was successfully applied to fabricate patterned emitting layers such as polyfluorene (PF). We fabricated micropatterns by transferring dried and uniform thin films, and observed strong electroluminescence (EL) from the fabricated organic light-emitting diodes (OLEDs) with the patterned emitting layers. The performance of the fabricated device was superior to that of a conventionally fabricated device. This demonstrates the well-controlled interfaces achieved by μCP. Furthermore, we succeeded in fabricating OLEDs with multiple emitting layers. These results show that this technique is promising for application to cost-effective, high luminance and multicolored OLED displays.     

Dielectric properties depending on frequency in organic light-emitting diodes

Dielectric properties of organic light-emitting diodes were investigated using ITO/Alq₃/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/Alq₃(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.     

Surface conditioning of indium-tin oxide anodes for organic light-emitting diodes

Oxygen-plasma treatment of indium-tin oxide (ITO) anodes is now widely used as one of the most effective ways to improve the device performance of organic light-emitting diodes (LEDs). However, the role of oxygen-plasma treatment has not been clearly understood. We have performed detailed studies of the surface and bulk of the ITO thin films exposed to oxygen-plasma. We employed a multitude of experimental techniques, including X-ray and ultraviolet photoelectron spectroscopies, atomic force microscopy, dynamic contact angle measurement, four-point probe and Hall measurements to investigate the changes induced by the plasma. We have also analyzed the device characteristics of polymer LEDs fabricated with these anodes. We found significant modifications of the physico-chemical, morphological, transport and optical properties of the oxygen-plasma treated ITO. Although oxygen-plasma does not show any measurable etching effect, it induces considerable changes leading to an increase in work function, electron carrier concentration and conductivity. It also increases the surface energy and polarity. We relate these modifications to enhancement of the device performance, such as electroluminescence efficiency and lifetime, through their effects on hole injection, and interface structure and stability. Finally, we show that even in the presence of a hole-transport layer such as a poly(styrene sulphonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) inserted between the anode and the emissive polymer layer, oxygen-plasma treatment of the ITO anodes is still beneficial for the devices.     

Efficient top-emitting organic light-emitting diodes with Sm/Ag bilayer cathode

A bilayer is used as a semitransparent cathode for top-emitting organic light emitting devices (top-emitting OLEDs). The bilayer cathode consists of samarium (Sm) and silver (Ag). Top-emitting OLEDs with the bilayer cathode showed enhanced current injection and high electroluminescence efficiency as compared with a Sm cathode. The maximum current efficiency of the top-emitting OLEDs with a Sm/Ag cathode is 9.9 cd/A, much greater than 4.9 cd/A obtained from the top-emitting OLEDs with a Sm cathode. The improved performance can be attributed to the balance between optical transparency and electrical conductivity of the Sm/Ag cathode.     

Radiation damage and transmission enhancement in surface-emitting organic light-emitting diodes

A systematic study has been carried out on organic light-emitting diodes (OLEDs) with a transparent MgAg/ITO cathode. The most dominant imperfections in surface-emitting OLEDs were electrical shorts and attributed to radiation damage incurring in ITO deposition due to the presence of pinholes in the MgAg protective layer. When adding an ITO surface layer on a semitransparent MgAg thin film, an increase in luminance efficiency was observed and exhibited a strong dependence on ITO thickness. By carefully selecting processing conditions and device layer structures we fabricated surface-emitting OLEDs with excellent luminance efficiency and good operational stability.     

Low-cost and reliable thin film encapsulation for organic light emitting diodes using magnesium fluoride and zinc sulfide

We report highly efficient gas diffusion barriers for organic light emitting diodes (OLEDs) with an encapsulation structure composed of alternating magnesium fluoride (MgF₂) and zinc sulfide (ZnS) layers grown by vacuum thermal deposition. The half lifetime of yellow OLEDs under an initial luminance of 2000 cd/m² with rubrene as an emitter reached 245 h using three pairs of MgF₂/ZnS layers. The device lifetime was obviously improved using MgF₂ and ZnS as passivation layers before UV-cured epoxy seal without desiccant with the lifetime for the initial luminance dropping to 56% being over 500 h. This simple and inexpensive encapsulation method can potentially be applied to top-emitting OLEDs due to good light transmission characteristic of the passivation film.     

Optical and electroluminescent properties of samarium complex-based organic light-emitting diodes

The optical and electroluminescent (EL) properties of newly synthesized tris(hexafluoroacethylacetonato)(phenanthroline)samarium(III)mono-methanol [Sm(hfa)₃(phen)₂MeOH]-based organic light-emitting diodes (OLEDs) were investigated. The as-prepared photoluminescence (PL) spectrum of Sm(hfa)₃(phen)₂MeOH-doped PMMA film exhibits the peaks at the wavelength around 564, 598, 645 and 710 nm which correspond to the ⁴G_5/2 → ⁷H5_/2, ⁴G_7/2 → ⁷H_7/2, ⁴G_5/2 → ⁷H_9/2 and ⁴G_5/2 → ⁷H_11/2 transitions of the Sm³⁺ ion, respectively. The best organic light-emitting device performance is obtained for a device using 8 wt.% Sm(hfa)₃(phen)₂MeOH and 40 wt.% 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4,-oxadiazole doped in poly(N-vinylcarbazole) as a emitting layer. The optimal device exhibits maximum luminance of 135 candela (cd)/m² at the current density of 0.16 A/cm², with current efficiency of 0.1 cd/A at the current density of 0.08 A/cm². The EL spectrum from optimal device has the Commission Internationale De L'Eclairage (CIE) coordinate of (0.60, 0.36).     

Improved performance of the single-layer and double-layer organic light emitting diodes by nickel oxide coated indium tin oxide anode

The electrical and optical properties of the NiO films deposited under various conditions were first characterized. An ultra-thin layer of nickel oxide (NiO) was then deposited on the indium-tin oxide (ITO) anode to enhance the hole injection in the organic light-emitting diode (OLED) devices. A very low turn-on voltage (3 V) was actually observed for the device with the ITO/NiO anode in the conventional double layer heterojunction OLEDs. The enhancement of hole injection by the ITO/NiO anode was further verified by the hole-only device and by the device with a patterned NiO layer on the ITO anode. The luminance and the current density of the single-layer OLED device were also significantly improved by using the ITO/NiO anode to enhance the hole injection. Although the luminescence efficiency was low, the reasons of low efficiency were studied and the improvement method was proposed. Our results suggest that the NiO/ITO anode is an excellent choice to enhance the hole injection in OLED devices.     

高效稳定性有机黄光电致发光器件

主要通过红绿磷光材料R-4B和GIr1掺杂的方法,制备了黄光OLED器件,器件结构为ITO/MoO3(X)/NPB(40nm)/TCTA(10nm)/CBP:GIr1 14%:R-4B2%(30nm)/BCP(10nm)/Alq3(40nm)/LiF(1nm)/Al(100nm),TCTA和BCP分别为电子和空穴阻挡材料,同时结合TCTA和BCP对载流子的高效阻挡作用,研究了MoO3对器件效率和稳定性的影响。发现当增加MoO3的厚度为90nm时,在较大的电压范围内,器件都具有较高的效率和色坐标稳定性。在电流密度为7.13mA/cm2时,器件达到最高电流效率29.2cd/A,亮度为2081cd/m2;电流密度为151.7mA/cm2时,获得最高亮度为24430cd/m2,电流效率为16.0cd/A;器件色坐标稳定性较好,当电压为5、10、15V时,色坐标分别为(0.5020,0.4812)、(0.4862,0.4962)、(0.4786,0.5027)。器件性能的改善主要归因于载流子注入与传输的平衡以及阻挡层对发光区域的有效限定。     

Pure red organic electroluminescent devices using a novel europium(III) complex as emitting layer

Stable vacuum deposition of a new europium(III) complex, Eu(DBM)₃(L) {DBM=dibenzoylmethanato, L=3-ethyl-2-(4′-dimethylaminophenyl)imidazo[4,5-f]1,10-phenanthroline}, were verified by ultraviolet–visible and infrared spectroscopy. By using the vacuum deposited film of the Eu(III) complex as the emitting layer, aluminum tris(8-hydroxyquinolinate) (AlQ) as electron-transporting layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as hole-blocking layer, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD) as hole-transporting layer, a four-layer electroluminescent device of (+)indium–tin oxide/TPD(40 nm)/Eu(DBM)₃(L)(40 nm)/BCP(10 nm)/AlQ(40 nm)/Mg:Ag(110 nm)/Ag(60 nm)(−) gave high efficient and pure red light emission with a luminance of 230 cd/m². A comparison of the electroluminescence properties of the four-layer device with those of a two-layer and a three-layer device was made.     

High performance inkjet printed phosphorescent organic light emitting diodes based on small molecules commonly used in vacuum processes

High efficiency phosphorescent organic light emitting diodes (OLEDs) are realized by inkjet printing based on small molecules commonly used in vacuum processes in spite of the limitation of the limited solubility. The OLEDs used the inkjet printed 5 wt.% tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃) doped in 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) as the light emitting layer on various small molecule based hole transporting layers, which are widely used in the fabrication of OLEDs by vacuum processes. The OLEDs resulted in the high power and the external quantum efficiencies of 29.9 lm/W and 11.7%, respectively, by inkjet printing the CBP:Ir(ppy)₃ on a 40 nm thick 4,4′,4″-tris(carbazol-9-yl)triphenylamine layer. The performance was very close to a vacuum deposited device with a similar structure.     

Effect of Ca and buffer layers on the performance of organic light-emitting diodes based on tris-(8-hydroxyquinoline) aluminum

The effects of buffer layers, including LiF, LiCl, NaF, NaCl, NaI, KI, RbF, RbCl, CsF, CsCl, MgF₂, CaF₂, BaF₂, and BaCl₂ on electron injection and device performance in organic light-emitting diodes based on tris-(8-hydroxyquinoline) aluminum, were investigated systematically. The insertion of the buffer layers at the organic/cathode interface not only reduced the operating voltage, but also enhanced the luminance and efficiency, which is attributed to the improvement of electron injection efficiency. It was found that the efficiency of the electron injection was closely related to the inherent properties of the buffer layer, such as its melting point (MP) and dielectric constant (ε), as well as with the buffer layer's interface with the metallic electrode through the effective work function (WF). Low MP, low ε and low WF values result in an effective improvement in the injection of the electrons, and thus to the device performance. The electroluminescent performance was further improved by the introduction of calcium between the buffer layer and the aluminum electrode.     

Printed organic diode backplane for matrix addressing an electrophoretic display

An organic matrix diode backplane for driving an electrophoretic display with an unpatterned front plane electrode is reported. Using thin film diodes instead of transistors as the active components in the pixels enable the backplane to be manufactured using roll-to-roll compatible printing processes. The backplane functionality is demonstrated in an electrophoretic display using 16 copper pixel electrodes, each of which is connected to two printed organic diodes. The printed diodes have sputtered copper cathodes and printed silver anodes. The diodes and pixels are arranged in a lateral 4 by 4 matrix structure with a dielectric layer separating the row and column electrodes. The dielectric, semiconductor and silver layers are all printed using a laboratory-scale rotary gravure press. The display module is finalized by laminating the front plane material onto the backplane.     

Photo- and electroluminescent properties of bithiophene disubstituted 1,3,4-thiadiazoles and their application as active components in organic light emitting diodes

Photo- and electroluminescence of five bithiophene disubstituted 1,3,4-thiadiazoles, constituting a new class of solution processable materials for organic opto-electronics, were studied. It was found that the introduction of alkyl solubilizing substituents bathochromically shifted the photo- and electroluminescence bands. The most pronounced effect was observed for the substitution at the C_α position which changed the emitting light color from bluish to green. All five derivatives were tested in host/guest type organic light emitting diodes (OLEDs) with either poly(N-vinylcarbazole) (PVK) or poly(N-vinylcarbazole) + 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK + PBD) matrices. The latter matrix turned out especially well suited for these guest molecules yielding devices of varying color coordinates. The best luminance (750 cd/m²) was measured for 2,5-bis(5′-octyl-2,2′-bithiophene-5-yl)-1,3,4-thiadiazole with the luminous efficiency exceeding 0.4 cd/A.     

The optical linewidth of InGaN light emitting diodes

A comparative study of the optical linewidths of high-quality InGaN epilayers and commercial single quantum well light emitting diode structures was undertaken using photo- and electroluminescence. Optical linewidths show a linear increase with increasing indium concentration in both cases. We assess the contribution of three mechanisms to the luminescence linewidth: alloy fluctuations, well width fluctuations and strain effects. It is found that the broadening of the emission line is an intrinsic property of InGaN alloys. The piezoelectric effect in wurtzite semiconductors is proposed as a novel line-broadening mechanism.     

A review on the light extraction techniques in organic electroluminescent devices

Organic electroluminescent devices are becoming increasingly important because of their potential applications for large area flat-panel displays and general lighting. The internal quantum efficiency of these devices have been achieved near 100% using electro-phosphorescent materials with proper management of singlet and triplet excitons, however, the external quantum efficiency of conventional devices remains near 20% because of losses due to wave-guiding effect. Recently, there has been great progress to enhance the light out-coupling efficiency of organic electroluminescent devices by means of various internal and external device modification techniques. In this review we report recent advances in light out-coupling techniques, such as, substrate modification methods, use of scattering medium, micro-lens arrays, micro-cavity effect, photonic crystals and nano-cavity, nano-particles, nano-structures and surface plasmon-enhanced techniques that have been implemented to enhance the external extraction efficiency of organic electro-luminescent devices.     

Synthesis and red electrophosphorescence of a novel cyclometalated iridium complex in polymer light-emitting diodes

The preparation and characterization of a new cyclometalated iridium complex with 2-(1-naphthalene) pyridine ligand were reported. An electrophosphorescent device was fabricated by using this new iridium complex as guest and poly-(cyano-paraphenylene) as host. Red electrophosphorescence was observed with an emission peak at approximately 600 nm. An external quantum efficiency of 1.3% was achieved in this electrophosphorescent polymer light-emitting devices.     

Anode material properties of Ga-doped ZnO thin films by pulsed DC magnetron sputtering method for organic light emitting diodes

This study examined the anode material properties of Ga-doped zinc oxide (GZO) thin films deposited by pulsed DC magnetron sputtering along with the device performance of organic light emitting diodes (OLEDs) using GZO as the anode. The structure and electrical properties of the deposited films were examined as a function of the substrate temperature. The electrical properties of the GZO film deposited at 200 °C showed the best properties, such as a low resistivity, high mobility and high work function of 5.3 × 10^− 4Ω cm, 9.9 cm²/Vs and 4.37 eV, respectively. The OLED characteristics with the GZO film deposited under the optimum conditions showed good brightness > 10,000 cd/m². These results suggest that GZO films can be used as the anode in OLEDs, and a lower deposition temperature of 200 °C is suitable for flexible devices.