Furong Zhu

Lithium–fluoride-modified indium tin oxide anode for enhanced carrier injection in phenyl-substituted polymer electroluminescent devices

Phenyl-substituted polymer electroluminescent (EL) devices using an insulating lithium–fluoride (LiF) layer between indium tin oxide (ITO) and poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT) hole transporting layer have been fabricated. By comparing the devices made without this layer, the results demonstrate that the former has a higher EL brightness operated at the same current density. At a given constant current density of 20 mA/cm2, the luminance and efficiency for devices with 1.5 nm LiF-coated ITO were 1600 cd/m2 and 7 cd/A. These values were 1170 cd/m2 and 5.7 cd/A, respectively, for the same devices made with only an ITO anode. The ultrathin LiF layer between ITO and PEDOT modifies the hole injection properties. A more balanced charge carrier injection due to the anode modification by an ultrathin LiF layer is used to explain this enhancement.

Optimized indium tin oxide contact for organic light emitting diode applications

This work aimed to develop indium tin oxide (ITO) coatings with optimum optoelectronic properties required for applications in organic light emitting diode (OLED) devices on flexible thin glass substrates. This involved introducing hydrogen in gas mixture during ITO film preparation using radio frequency magnetron sputtering method. Uniform ITO films with resistivity of 2:7 £ 10; 24 Vcm and transparency of 85% over the visible wavelength region was achieved. The presence of hydrogen gas in the sputtering processes was shown to increase the number of charge carriers in the ITO films. The feasibility of using the ITO films with high carrier concentration was examined and tested in OLED devices composing a fluorene based polymeric hole transporting layer (HTL) and green-emitting polymer, benzothiadiadzolefluorene. The devices made with different ITO substrates had an identical configuration of ITO/HTL/emitter polymer/Ca (6 nm)/Ag (200 nm). The device showed that a maximum luminance of 4:36 £ 10 4 cd/m 2 and an efficiency of 4.14 cd/A were achieved when an optimal ITO layer was used. These results are comparable with that of devices made with the best commercially available ITO products operated at the same conditions. q 2000 Elsevier Science S.A. All rights reserved.

Improvement of hole injection in phenyl-substituted electroluminescent devices by reduction of oxygen deficiency near the indium tin oxide surface

We report the use of an in situ four-point probe method to investigate the relation between oxygen plasma treatment on indium tin oxide (ITO) and the variation in its sheet resistance. Analyses on the ITO surface composition made with time-of-flight secondary ion mass spectroscopy probe a dual-layer parallel resistor model for oxygen plasma-treated ITO anodes. We have shown that the increase in the ITO sheet resistance can be attributed to the reduction of oxygen deficiency near the surface. The improvement in carrier injection in phenyl-substituted poly(p-phenylenevinylene)-based light-emitting diodes correlates directly with a layer of low conductivity, several nanometers thick. This was induced on the ITO surface and serves as an efficient hole injecting anode.

Tris-(8-hydroxyquinoline)aluminum-modified indium tin oxide for enhancing the efficiency and reliability of organic light-emitting devices

Tris-(8-hydroxyquinoline)aluminum (Alq3), which is typically used as an electron transport material for organic light-emitting devices (OLEDs), was used in this study for OLED anode modification. The electronic structure at the indium tin oxide (ITO)/organic interface for improvement of carrier injection was studied using ultraviolet photoelectron spectroscopy. The interfacial analysis reveals that the barrier height at the ITO/organic interface can be varied from ∼0.6–1.08eV. It is demonstrated that the barrier for hole injection from an ITO anode to a hole transporting layer can be engineered by inserting an ultrathin interlayer of Alq3, a few nanometers thick. The presence of an Alq3 interlayer is shown to improve the current balance, leading to an enhancement in the electroluminecent efficiency and operational stability of OLEDs.

An in situ sheet resistance study of oxidative-treated indium tin oxide substrates for organic light emitting display applications

Indium tin oxide (ITO) substrates are subjected to oxygen plasma and UV ozone treatments. It is observed that the treatments produced a surface layer rich in oxygen, as investigated by X-ray photoelectron spectroscopy (XPS), and this is correlated with the sheet resistance of ITO measured by a four-point probe. A method has been devised to measure the change in the sheet resistance more prominently and this measurement is carried out under purified nitrogen atmosphere after the ITO is being treated. With an oxygen-rich ITO surface layer formed on ITO after the oxidative treatments, the resistance of ITO is considered to be that of a parallel-resistor combination and the thickness of the surface layer is being estimated based on this approach.

Effect of ITO Carrier Concentration on the Performance of Organic Light-Emitting Diodes

The indium tin oxide (ITO) anodes for organic light emitting diode (OLED) were made from an oxidised target with In2O3 and SnO2 in a weight proportion of 9:1 using the RF magnetron sputtering method. The comparable ITO anodes with different carrier concentrations were prepared by varying the hydrogen partial pressure during film deposition. The current-luminance-voltage characteristics of the devices indicated that a high carrier concentration in ITO plays a role in improving OLED performance. A maximum efficiency of 3.8 cd/A was achieved when an ITO anode with a higher carrier concentration of 9×10 cm was used in a fluorene based OLED. This efficiency is about 1.5 times higher than that of an identical device made with an ITO anode having a lower carrier concentration of 5×1020 cm-3. The increase in electroluminescent efficie ncy reflects an enhanced hole-injection in the device. We consider that enhanced hole injection is due to the reduced band bending in ITO when it has a high carrier concentration.

Aging effect on balance of carrier injection in polymer light-emitting diodes

We propose an approach to study the oxidation effect on the degradation process of phenyl-substituted polymer based light emitting diodes (LEDs) using acceleration aging of device in air. The poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT) is used as hole transporting layer. We have made conventional balanced electron-hole injection together with electron and hole injection dominant devices. The acceleration aging of these devices in air was performed at a fixed current density that produces the maximum electroluminescence (EL) efficiency of the corresponding LEDs. The results show that the fast decrease of electron current in these devices with regard to the aging in air was mainly responsible for reduction of EL efficiency. This implies that the degradation of the polymer LED is due to the deterioration of the cathode contact for carrier injection.