Keran Zhang

Degradation and failure of organic light-emitting devices

The degradation and failure of organic light-emitting device are observed via optical microscopy. The “degraded area” has been identified to be made up of three regions: (1) a dark spot at the center, (2) a nonemitting area forming the core, and (3) a weakly emitting area surrounding the core. It is found that due to metal migration, as evidenced from the secondary ion mass spectrometry profiles, the indium tin oxide/polymer interface roughens during operation. The intense local current at sharp points degrades the polymer causing the formation of the dark center. Further current stress caused the central core to carbonize which may lead to short and/or open circuits accompanied by fluctuations in the device current.

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.

Stabilization of electrode migration in polymer electroluminescent devices

A thin 3-nm-thick parylene layer is deposited by chemical vapor deposition at room temperature on the indium tin oxide (ITO) coated glass substrate to form a bilayer anode of an organic light emitting diode. The parylene layer forms a conformal film to cover the spikes present in the ITO film. This parylene film presents a smoother surface to the subsequent organic layers. The parylene film not only reduces the occurrence of dark spots, acting as a barrier for oxygen diffusion from either the ITO or from the atmosphere and stabilizing the migration of the electrodes during electrical stress, but also improves the injection of holes from the anode. By inserting another parylene layer in between the organic and cathode layers, the probability of formation of nonemissive areas is further reduced.

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.

Organic light emitting devices performance improvement by inserting thin parylene layer

Abstract An organic light emitting device (OLED) structure with a thin parylene layer deposited by low-temperature chemical vapour deposition (CVD) at the anode–organic interface was fabricated. Such a structure gives off higher efficiency, a smaller number and smaller size dark non-emissive areas, slower growth rate of the dark areas and a longer device lifetime compared to one without the parylene layer. The parylene modified indium tin oxide (ITO) surface shows an increased work-function and a reduced surface roughness compared to that of the bare ITO surface. The interface optimisation contributes to the device performance improvement.

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.

Bubble formation due to electrical stress in organic light emitting devices

The degradation in electroluminescence of poly(p-phenylene vinylene)-based organic light-emitting devices is studied using optical microscopy, scanning electron microscopy, and secondary ion mass spectroscopy. “Bubbles” are formed at the polymer and indium tin oxide interface or in the polymer layer within the nonemissive area. This formation, which occurs during device electrical stress, is accompanied by a fluctuation of the device current. The bubbles are formed by the degraded polymer and/or the gas released from disintegration of the polymer. High local current density flowing near the dark spot center and the resultant heating, decomposes the polymer layer. The resultant carbonized area causes either local short circuit and/or open circuit leading to the final light-emitting device failure.

Secondary ion mass spectroscopy study of failure mechanism in organic light emitting devices

Abstract Secondary ion mass spectroscopy is used to examine the dark, non-emissive defects on the organic light emitting device. Boundary movements are originated from electrode imperfection. Due to flexibility and movability of polymer layer, distribution variations and a more severe indium and calcium overlapping are detected in dark spot defect area. Boundary movements are not in good agreement between different layers. Interfaces became undulate. The closeness and proximity between the In sharp spikes and cathode metal protrusion leads to the initial point of dark spot. We demonstrate that the presence of cathode imperfection and interface roughness of different layers correlated to the device dark spot formation.

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.

Secondary Ion Mass Spectroscopy Study of Failure Mechanism in Organic Light Emitting Devices

Secondary ion mass spectroscopy is used to examine the dark, non-emissive defects on the organic light-emitting device. Boundary movements are originated from electrode imperfection. Due to flexibility and movability of polymer layer, distribution variations and a more severe Indium and Calcium overlapping are detected in dark spot defect area. Boundary movements are not in good agreement between different layers. Interfaces became undulate. The closeness and proximity between the In sharp spikes and cathode metal protrusion leads to the initial point of dark spot. We demonstrate that the presence of cathode imperfection and interface roughness of different layers correlated to the device dark spot formation.