Barrier Layer Technologies Drew Forman MS&E 542, Cornell University, Spring 2006 Abstract: The realization of flexible electronic devices will open the door to new, low cost commercial products. There are many technological hurdles that must be overcome to commercialize flexible electronic devices and even more challenges to achieve roll-to-roll processing. Significant progress has been made in many areas. However, due to the stringent requirements on the barrier layer, no clear technological leader has emerged. This document will review the requirements for a successful barrier layer and examine the progress made in each material currently being researched. Introduction One of the main difficulties in commercializing electronic organic molecules is lifetime. Electronic organic molecules and polymers can degrade in a few hours under normal environmental conditions. Devices are therefore fabricated in an inert environment and encapsulated to protect them from oxygen, water and ultraviolet light. In this manner lifetimes on the order of ten thousand hours or higher are possible.1 Organic light-emitting devices (OLEDs) are being investigated as an alternative to liquid-crystal displays (LCDs). While LCDs require heavy and brittle glass panels, OLEDs can be deposited on flexible plastic rolls. This makes OLEDs an attractive technology for low cost, light-weight, flexible displays.1All electronic organic molecules have different technical issues. Even among OLEDs each color has its own strengths and weaknesses. Blue OLEDs have particularly poor lifetimes. This makes full color display lifetimes a particularly large hurdle that must be overcome to make flexible electronics a reality.2 OLEDs have two degradation modes. The first is intrinsic loss of efficiency over long periods of time. The second mode is the growth of dark spots. It is the later that is responsible for the rapid degradation of OLEDs. The mechanism for dark spot growth is due to the presence of water and oxygen. Rapid OLED degradation can be prevented through encapsulation.3,4,5 Flexible barrier layers are required for flexible electronic devices. Flexible displays have the added burden of requiring transparency. In order to achieve sufficient lifetimes the barrier layer must transmit less than 10-6 g/m2/day of water vapor and less than 10-5 mL/m2/day of oxygen.6 Different applications require different degrees of flexibility. An OLED display for a soldier’s visor or a pilot’s cockpit only needs to be flexible during fabrication.7 Once fabricated, the device can be inserted between two shaped, inflexible panels that have sufficient barrier properties. Other applications will require the device to be flexible during operation. These will require transparent, flexible barrier layers. Polymers6, glass8 and composites9,10 are being investigated, as are stable organic semiconductors.1,11 No barrier layer technology has shown commercial viability at the time of this writing.Evaluating Barrier Layers Transmittances less than 10-6 g/m2/day of water vapor and less than 10-5 mL/m2/day of oxygen are two small to be measured with conventional techniques. A testing technique has been developed in which a thin film of calcium is deposited on the sample. As water permeates through the sample the calcium reacts to form a calcium salt. Optical transmission can measure the presence of the calcium salt. Measurements can be taken over the course of several days. In this manner the cumulative water uptake can be plotted versus time. The slope of a best fit line will reveal the rate at which water is transmitted through the barrier layer. This measurement technique is known as the “calcium test.”12 The effectiveness of a barrier layer is often determined by defects. This can lead to localized degradation, which is worse for a product than uniform degradation. An advantage of the calcium technique is that it allows for the evaluation of localized transmission rates allowing one to study the effects of defects in the barrier layer.13 In addition to directly measuring the transmittance of water through the barrier it can be useful to make in-situ measurements on the device as it ages. Electrical characteristics can be measured during thermal annealing using a high resolution in-situ electrical measurement technique. Light output is measured as a function of time, giving an Arrhenius relation and the corresponding activation energy of failure.14 FT-IR and UV-Vis spectroscopy experiments can also be performed in-situ while the device is heated. This enables the study of reaction mechanisms and kinetics. Furthermore,observing the degradation of characteristic peaks with time can result in an Arrhenius plot, revealing the activation energy for a mechanism.14 Finally, Photothermal Deflection Spectroscopy is a useful technique for measuring oxygen content. Oxygen will react with organic semiconductors forming defects. These defects can be measured by modulating the intensity of an incident light beam resulting in periodic heating. Index of refraction is temperature dependant and can be reliably measured.14 Metal Foil Barriers Flexible electronic applications that do not require transparency can utilize metal foil barrier layers. Metal foil barriers have superior mechanical properties and are easier to process than glass. Additionally metal foil barriers have excellent barrier properties. Metal foil barrier layers have low cost, excellent thermal conductivity properties, deform predictably and have high operating temperatures. Unlike other barrier materials, metal foil has a rough surface. Furthermore the coefficient of thermal expansion of a metal foil substrate is mismatched with the coefficient of thermal expansion of organic electronic components. Most importantly metals are opaque, making them unsuitable for flexible displays except as a backplane.15 Polymer Barriers Polymer films are highly desirable materials for flexible electronics. Polymers can be low cost, are relatively smooth and are transparent. Furthermore mechanical properties can be fine tuned for desired flexing and the coefficient of thermal expansion is a good match for organic electronicdevices. However, plastic substrates have limited operating temperatures and poor thermal conductivity. They also have very poor barrier properties.15 Two polymers have been heavily investigated as barrier materials: polyethylene