Mechanical Performance of Thin Films in Flexible Displays
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Mechanical Performance of Thin Films in Flexible Displays Jay Lewis, Sonia Grego, Erik Vick, Babu Chalamala, and Dorota Temple MCNC Research & Development Institute Research Triangle Park, NC 27709, U.S.A. ABSTRACT The use of brittle materials in flexible displays requires the understanding of the mechanical limitations of the materials and the various display architectures. We discuss various approaches to mechanical bend tests for components of flexible displays and describe a new test method based on a three-axis motion. We discuss the mechanical limitations of indium tin oxide (ITO) as a transparent conductor, and present results for a more mechanically robust multilayer transparent conductor made of an ITO-metal-ITO (IMI) stack. The IMI structures showed dramatically improved mechanical properties when subjected to bending both as a function of radius of curvature as well as number of cycles to a fixed radius. Organic light emitting devices fabricated on IMI anodes showed improved performance compared with those made on ITO anodes at current densities greater than 1 mA/cm2 due to the improved conductivity of the anode. We discuss the difficulties in analysis of the mechanical failure of transparent thin film permeation barriers. We present a novel approach for etching barrier-coated polymer substrates such that film cracking is readily visible. We report on the bend test results for sputterdeposited SiOxNy films. INTRODUCTION Rapid progress in the performance of organic light emitting devices (OLEDs) has positioned the technology as a potential replacement for many liquid crystal display applications. One advantage of OLEDs, because of the efficient, emissive, solid-state nature of the devices and the absence of viewing angle effects, is the ability to fabricate them at low temperature on a flexible substrate [1, 2]. Fabrication of active matrix transistor backplanes on polymer substrates is a challenge, but several groups have demonstrated both passive-matrix and active-matrix displays on polymer substrates [3-7]. However a significant hurdle for the introduction of flexible OLEDs into commercial applications is the limitation in flexibility imposed by the brittle films used as electrodes and permeation barriers. In this paper we discuss bend test techniques for components of flexible displays, which require increased standardization within the display community. We then discuss various issues regarding transparent conductors and thin film permeation barriers and present the status of these technologies with regard to mechanical limitations.
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BEND TESTING The transition of flexible display technology from the research lab to commercial applications will require the development of a consistent set of methods for bend testing materials and components that comprise the display. Standard methods and equipment exist for highly flexible materials such as flex circuits and textiles, as well as for moderately rigid materials such as smart cards. However the methods and techniques available for testing
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