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.     

Effect of the plasma treatment of anode electrode of the organic light-emitting diodes on the growth of hole-injection layer

In this paper, we focused on the effects of the plasma treatment of the anode electrode of organic light emitting diodes (OLED) on the growth of hole-injection layer (HIL). The CF₄ plasma (CF₄-P) treatment, which is known for efficient method to enhance the performance of OLED, was not effective on the OLED with the HIL material copper phthalocyanine (CuPc). The CF₄-P treated OLED showed remarkably reduced electroluminescence (EL) characteristics while the O₂ plasma treated OLED showed improved EL efficiency. The dependence of the CuPc growth on the polarity of substrate induced the morphological difference of the HIL, and finally resulted in the different device characteristics.     

Characteristics of organic light emitting diodes with copper iodide as injection layer

We have studied the use of a thin copper iodide (CuI) film as an efficient injection layer of holes from indium tin oxide (ITO) anode in a light-emitting diode structure based on tris-8-hydroxyquinoline aluminium (Alq3). The results of impedance analysis of two types of diode structures, ITO/CuI/Alq3/poly(ethylene glycol) dimethyl ether/Al and ITO/Alq3/poly(ethylene glycol) dimethyl ether/Al, are presented. Comparative analysis of their current density-voltage, luminance-voltage and impedance characteristics shows that presence of CuI layer facilitates injection of holes from ITO anode into the light-emitting layer Alq3 and increases electroluminescence efficiency of the organic light emitting diodes.     

Correlation of recombination zone and device performances of phosphorescent white organic light-emitting diodes

We demonstrate a general method for tuning the color performance of white organic light-emitting diodes (WOLEDs) by inserting 0.5 nm thick red emitting layer in different location of blue phosphorescent emitting layer. The Commission Internationale de L'Eclairage (CIE) coordinates of WOLEDs were dependent on the position of red emitting layer and they were correlated with recombination zone of the blue phosphorescent emitting layer. Red shift of white CIE was observed as the location of red emitting layer get close to recombination zone of blue emitting layer. In addition, CIE of WOLEDs was kept stable between 100 cd/m² and 10,000 cd/m².     

t-Butyl group-substituted triphenylamine-containing orange-red fluorescent emitters for organic light-emitting diodes

Efficient orange-red fluorescent compounds, 4-(dicyanomethylene)-2-adamantyl-6-(4-(N-(4-tert-butylphenyl)-N-(3,5-di-tert-butylphenyl)amino)benzene)vinyl-4H-pyran (DCATP) and 2,6-bis[4-(N-(4-tert-butylphenyl)-N-(3,5-di-tert-butylphenyl)amino)benzene]vinyl-4-(dicyanomethylene)-4H-pyran (BDCTP) containing the tert-butylated triphenylamine in donor moieties, were synthesized and characterized. In these red emitters, bulky groups, such as t-butyl group and adamantane were introduced to increase the steric hindrance between the red emitters. In particular, an efficient orange-red device containing the emitter DCATP as a dopant showed a luminous and power efficiency of 6.87 cd/A and 2.70 lm/W, respectively, at 20 mA/cm² with the CIE coordinates of (0.48, 0.50) at 7.0 V. In addition, an efficient red organic light-emitting diode using BDCTP as a dopant exhibited a luminous and power efficiency of 2.30 cd/A and 1.31 lm/W, respectively, at 20 mA/cm² and CIE coordinates of (0.61, 0.39).     

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.     

Study on the antimony tin oxide as a hole injection layer for polymer light emitting diodes

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) is commonly used as a hole transfer layer in polymer light emitting diodes (PLEDs). However, Indium tin oxide transparent electrodes are corroded by poly(styrenesulfonate) and the erupted indium diffuses into the active layer, which in turn decreases the brightness, efficiency and lifetime of the device. In this study, therefore, antimony tin oxide (ATO) was introduced as a hole injection layer (HIL) in PLEDs. The work function and pH of ATO were − 5.1 eV and ~ 7.5, respectively. When annealed at 200 °C, high conductivity (~ 0.18 S/cm) was observed, which represents good HIL characteristics. Here, the maximum luminance (26,114 cd/m²) and maximum efficiency (1.55 cd/A) of the PLEDs were increased by 33% and 20% respectively. Their stability improved as well.     

Influence of ambient air exposure on surface chemistry and electronic properties of thin copper phthalocyanine sensing layers

In this paper we present photoemission studies of the influence of 12-hour exposure to the ambient air on the chemical and electronic properties of thin 16-nm copper phthalocyanine (CuPc) sensing layers deposited on n- and p-type silicon Si(111) substrates covered with the native oxide. The surface chemistry and electronic parameters of organic thin film including surface band bending, work function, electron affinity and their variations upon the exposure have been monitored with X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy techniques. We found that after the exposure, the surface chemistry of CuPc remained unaffected, however the work function and surface band bending increased by 0.55 eV and 0.45 eV for the layers on n-Si and by 0.25 eV and 0.30 eV for those on p-Si. Additionally, we detected a slight surface dipole at CuPc on n-Si manifested by a small shift in electron affinity of 0.10 eV. In order to explain these changes we developed a model basing on the interaction of ionic species with the phthalocyanine surface.     

Influence of oxygen deficiency in indium tin oxide on the performance of polymer light-emitting diodes

We investigated effects of oxygen deficiency in the indium tin oxide (ITO) on the performance of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)-based polymer light-emitting diodes, in which the ITO anode was deposited by radio frequency magnetron sputtering at different oxygen flow rates. We found that the degree of oxygen deficiency in the ITO films can affect the device performance significantly and is a source of current leakage. At the optimal oxygen flow rate, the leakage current of devices can be reduced and the balance between hole and electron fluxes can be promoted in the MEH-PPV layer to improve device efficiency.     

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.     

Structural transformation and crystallization of amorphous copper phthalocyanine nanostructures

Vapor condensation was performed to prepare nanostructures of copper phthalocyanine. With a lower pressure and longer evaporation duration, a fine and dense network of thin and uniform nanowires was obtained. Nanorods with tube-like branches were formed at higher working pressures. The absorption coefficients and optical band gaps of the nanostructures were derived from the UV-vis absorption spectra. After heat treatment, the amorphous nanowires were transformed into crystalline straight β-phase nanostructures with a smooth surface. Reduced intensity of the absorption spectrum and a red shift of the optical band gap were also observed.     

Efficiency optimization of green phosphorescent organic light-emitting device

Using a narrow band gap host of bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp₂) and green phosphorescent Ir(ppy)₃ [fac-tris(2-phenylpyridine) iridium III] guest concentration as low as 2%, high efficiency phosphorescent organic light-emitting diode (PHOLED) is realized. Current and power efficiencies of 62.5 cd/A (max.), 51.0 lm/W (max.), and external quantum efficiency (max.) of 19.8% are reported in this green PHOLED. A low current efficiency roll-off value of 10% over the brightness of 10,000 cd/m² is noticed in this Bepp₂ single host device. Such a high efficiency is obtained by the optimization of the doping concentration with the knowledge of the hole trapping and the emission zone situations in this host-guest system. It is suggested that the reported device performance is suitable for applications in high brightness displays and lighting.     

Influence of ITO patterning on reliability of organic light emitting devices

Indium tin oxide (ITO) films are widely used for a transparent electrode of organic light emitting devices (OLEDs) because of its excellent conductivity and transparency. Two types of ITO substrates with different surface roughness were selected to use as anode of OLEDs. In addition, two types of etching process of ITO substrate, particularly the etching time, were also carried out. It was found that the surface roughness and/or the etching process of ITO substrate strongly influenced on an edge of ITO surface, further affected the operating characteristics and reliability of devices.     

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.     

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.