Ramadas Senthil Kumar

Low moisture permeation measurement through polymer substrates for organic light emitting devices

Low permeation measurements through polymer substrates is very critical and there is no definite commercial method for measuring a water vapor transmission rate of 10 gym yday at 39 8C and 95% RH at atmospheric pressure.A novel moisture y 62 permeation measurement method and model was developed qualitatively and quantitatively with the ‘calcium degradation test’ method.It is well known that organic light emitting device structures and other active display materials degrade in the presence of oxygen and moisture.This method would be highly suitable for assessing ultra high barrier properties of polymer substrates for organic light emitting device applications. 2002 Elsevier Science B.V. All rights reserved.

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.

Low-frequency noise measurement and analysis in organic light-emitting diodes

Low-frequency noise characteristics of organic light-emitting diodes are investigated. Two noise components were found in experimental low-frequency noise records, namely: 1) 1/f Gaussian noise from device bulk materials and 2) an excessive frequency-related part of noise related to device interfaces or defects and traps. 1/f noise is said to be related to carrier mobility. Degradation, especially photo-oxidation of the electroluminescence polymer, is a possible reason that affects carrier mobility. The excessive part of noise is believed to be related to the carrier numbers and could come from the interface deterioration, defects and traps generation and furnish. The excessive part of noise increases much faster during device stress. This shows that the degradation related interface defects and traps is much faster.

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.

Au-ITO anode for efficient polymer light-emitting device operation

A thick gold layer is deposited on indium tin oxide (ITO) to improve the interface quality between the ITO anode and the organic layer in organic light-emitting diodes (LEDs). With this improvement, the device with structure ITO/Au/hole-transport-layer(HTL)/poly(p-phenylenevinylene)/Ca/Ag, achieved a lower turn-on voltage from 4 V to about 1.6 V and an increase in luminescence intensity by more than a factor of two at the same voltage. The work function of the Au facilitates the formation of an ohmic contact and good mechanical adhesion to the HTL. The experimental results suggest that the ITO contact limits the supply of current for radiative recombination. The improvement of the device performance is due to the smoother Au surface and the matching of the Au work function with the highest occupied molecular orbital level of adjacent HTL layer.

Building blocks for ultrathin flexible organic electroluminescent devices

Displays based on organic electroluminescent (EL) materials have entered the marketplace already and demonstrated remarkable contrast, high brightness and crisp colors. However, one of the key advantages of this new technology has not been commercially exploited yet: Fabrication of a display that is still fully functional even when it is bent or flexed. This is possible since organic EL devices comprising only thin, amorphous solid state films and optical properties have no critical dependence on the film thickness. In this paper we address the important elements that are required to produce a flexible organic EL display. Most crucial is the selection of a flexible substrate. Here we present results obtained with ultra-thin inorganic glass materials as well as polymeric foils. For the glass substrates we determined the ultimate mechanical properties for different device configurations. In the case of polymeric substrates permeation of water and oxygen molecules through the substrate is the governing factor. We compare the performance of different barrier systems. In summary we demonstrate that OLED devices with certain flexibility can be reliably built on ultra-thin glass substrates. For polymeric substrates a lot of progress has been achieved in terms of the required barrier properties and other necessary ingredients and this might result later into a commercial organic EL product.

Indium-tin-oxide-free organic light-emitting device

Sapphire substrates coated with a gold (Au) layer in place of indium-tin-oxide (ITO) on glass substrates are used as hole-injecting anodes in organic light-emitting devices (OLEDs). Due to the unique quality of the sapphire/Au interface and the match of the Au work function with the highest occupied molecular orbital level of the adjacent hole transport layer (HTL) and the smoothness of the interface, the ITO-free OLED, with the structure sapphire/Au/HTL/poly(p-phenylenevinylene)/Ca/Ag, achieved an increase in current efficiency by more than a factor of three. In addition, the flawless sapphire substrate and anode/polymer interface make dark nonemissive areas decrease in number and area. The diodes show substantially slower degradation, and the lifetime in air increases by a factor of two or more.

Correlation of Current Noise Behavior and Dark Spot Formation in Organic Light-Emitting Diodes

A correlation between current 1/f noise and dark spot formation is reported. Our results show that the dark spot is primarily correlated to current 1/f noise slope; the higher the slope, the poorer the interface, and the more abnormal dark spot growth rate and the shorter lifetime. Besides, there is a correlation between current 1/f noise magnitude and the dark spot initial size. A higher 1/f noise magnitude generally indicates a larger dark spot initial size. A seemingly identical current-voltage curve does not render the same characteristics of dark spot formation, which can be clearly distinguished from the subtle difference in 1/f noise behavior. The noise measurement can be used to predicate device lifetime and degradation behavior.

Blocking Impurities in Organic Light Emitting Device by Inserting Parylene Interlayer

Secondary-ion mass spectrometry is used to study ion diffusion from a substrate into an organic film, which is considered as one of the reasons for organic-light-emitting-device degradation and instability. Results show that a 1 µm-thick parylene layer inserted between an indium–tin–oxide (ITO) anode and a soda-lime glass substrate effectively controls the diffusion of sodium, potassium, silicon and sulphur ions from the substrate to the device. The effect is the same as that in the case of using a plastic substrate which is sodium- and potassium-free. Also a 3 nm-thick parylene layer grown in between an ITO anode and a hole transport layer (HTL) not only shows improvement in device performance, but also is capable of blocking impurities such as sodium, potassium, silicon and sulphur ions. This study shows that the use of a parylene layer is effective for controlling contamination coming from the substrate.

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.