environmental barrier coatings

                        example: moisture barriers

MATERIALS / APPLICATION AREA::  In contrast to intermediate-temperature and high-temperature barriers (see "application area" listed herein) that utilize purely inorganic refractory materials, ambient-temperature diffusion barriers are a very different materials and problem set, meeting a different class of barrier requirements.  Thin film barriers for applications in normal human environments are typically referred to as "environmental barrier coatings," "moisture barriers," or similar nomenclature, depending on the subset of applications, where barrier properties are suited for blocking egress of molecular gas/vapor in more-or-less ambient conditions.  Apart from its more technically-trivial forms (such as the ubiquitous metallic coatings used in disposable food -preservation containers) these barriers can be specified with quite demanding requirements in applications such as flexible electronics, envirinmentally-sensitive compound semiconductors, transparent flexible display interfaces, medical applications, various thin film solar panels, and other applications where a simple, ductile metal coating is incompatible with the application.   Helicon's basis in multiple applications for multilayer diffusion barrier technologies has been ongoing since 1990, when it's founder was selecting and reactively depositing high-temperature, multi-layer, diffusion barrier designs for zone-melt re-crystallization (ZMR) of lithium niobate for processing at >1200 Celsius. Since then, Helicon has developed barrier coating technology for many applications, ranging from high-temperature solid oxide fuel cells (patents) and corrosive reactor surfaces, to room-temperature barriers for flexible solar panels and organic light emitting diodes (OLED, AMOLED, etc) display technologies.  APPLICATION:  Helicon's work in moisture barriers has been recognized internationally as seminal work in the development of recent high-performance generations of this technology.   Some of our work in this area is cited in many of the most recent and authoritative textbooks on the technology of thin film environmental barriers (see cit. 2,3,5,6)

FIGURE 1:  flexible active-matrix displays, organic solar cells, and other organic opto-electronic devices rely on organic semiconductors, which are inherently susceptible to moisture diffusion and contamination.  While metal-electrode layers are easily utilized as an effective moisture barrier on optically non-transmissive faces of a device, more enabling, flexible transparent moisture barriers for the optically transmissive face are an enduring technical challenge.  Flexible transparent moisture barriers are widely understood to be the single greatest technical obstacle preventing widespread commercial use of such devices.  These transparent moisture barriers will typically comprise alternating pairs of inorganic and organic layers.  Our research discovered new mechanisms for symbiotic improvement of barrier properties in these structures.

The primary barrier to the introduction of ultra-thin, flexible displays (such as a flexible, roll-up, thin televisions; e.g., photo below right), rather than as the relatively heavy, glass-encapsulated AMOLED monitors now available commercially, is squarely centered in the difficulty of preventing egress of moisture into the easily contaminated organic light-emitting diodes (OLED), where a flexible polymer film is used to replace the heavier, rigid glass as the structural support.  Since polymers alone are not physically capable of sufficiently low moisture diffusion rates, transitioning to manufacturable flexible displays on thin, flexible, polymer sheet requires exceedingly thin, reliable, flexible, transparent, moisture barriers that can reliably incorporate the barrier capability of inorganic dielectric thin films.

Beginning over a decade ago, Helicon Thin Film Systems was approached for our expertise in surface science by prominent physicist in the field of room-temperature diffusion barriers, Dr. John A. Affinito, so as to explore barrier materials and insights for this high-profile area of thin-film-based diffusion barriers.  Through our work and discussion on this problem, we came upon several fundamental discoveries, leading to patent applications and the co-authoring of a now broadly-cited paper, presented by Helicon at the 2004 SVC conference.   Until our work in this area, all published approaches to this problem involved attempting to produce perfectly pin-hole-free films.  Our published review thoroughly covers the various pre-existing physical models that were existing in the field.

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In our work, we explored various ranges in surface energies, micro/nano-scaled microstructures, and the physical chemistry of many monomeric compounds, thus determining combinations of surface activation, surface morphology, and monomeric chain properties that could result in a nearly instantaneous infiltration of polymeric chains into even the most tortuous intergranular spaces that would be present in standard dielectric deposition processes; processes that could be used in encapsulation suitable for OLED displays.      Our exploration into ultra-fast infiltration properties in thin film architectures allowed us to develop a new paradigm in multilayer organic/inorganic barrier function, as well as simultaneously allow the throughput required for commercially viable manufacturing.  These behaviors were also modelled via Maxwell-Garnett theory, in addition to other analytical approaches, for establishing the viability of the preferred acrylate monomers were further established as being very well suited for providing this instantaneous wicking behavior.   We also modelled time constants for complete positioning of the adsorbing/wicking of monomer chains into nano-scale intergranular spaces within freshly deposited dielectrics layers, thus enhancing toruosity and lowering egress of moisture by several orders of magnitude.  In this work, numerous monomeric chain molecules were found to provide the desired behavior. Our findings have been subsequently corrobrated and referenced by multiiple peer-reviewed studies (some of these studies are supporting citations in referenced textbooks below)(list papers/groups).  Therefore, we both successfully introduced this model into the field for the first time, as well as successfully predicted with good accuracy, the precise manner in which these ultra-barrier infiltration phenomena were then repeatedly observed to take place; in particular, with more sophisticated characterization techniques later made available. The work performed at Helicon in this area is now considered fundamental, providing a primary model for successful barrier performance in the various applications addressed. These lamellar, polymer/inorganic composite coatings are a continued area of interest for us, as some applications are possibly of greater societal impact than flexible displays.   In an invited paper from Helicon Thin Film Systems, presented in a thorough treatment at SVC 2004, Dallas, a methodical explanation of these results is published.           . More generally, Helicon has performed numerous roles in this area of environmental barriers as applied to organic semiconductors, both in theoretical development and in practical R&D.  In the late 1990's we were contracted to configure tooling for evaporative deposition of OLED structures by the Optical Sciences Center at the University of Arizona, where Helicon also configured and baselined custom R&D system having 10-source, robotically serviced, load-locked metal/organic cluster evaporation system that was developed for deposition of experimental OLED compositions and electrode structures.  Below are a few of the recently published textbooks that discuss our well-known work  (Affinito and Hilliard) in environmental barriers.  Helicon's thin film work in this OLED/barrier area has also been published and widely cited in numerous other textbooks, peer-reviewed papers, and patent applications, in addition to the references below.

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1. Affinito, J., and Hilliard, D., (2004) A New Class of Ultra-Barrier Materials,  Proceedings of the 47th Annual Technical Conference of the Society of Vacuum Coaters, pp. 563–593. 2. Arza Seidel (Ed.), Processing and Finishing of Polymeric Materials, 2 Volume Set,  John Wiley & Sons, 2012  3. Mittal, Vikas,   Encapsulation Nanotechnologies , John Wiley & Sons, 2013  4. Frederik C. Krebs , Stability and Degradation of Organic and Polymer Solar Cells  John Wiley & Sons, 2012  5. Otto G. Piringer, A. L. Baner (Ed.) Plastic Packaging: Interactions with Food and Pharmaceuticals , John Wiley & Sons, 2008  6. Herbert Weber  (Ed.) ,  Nanotechnologie in der Lebensmittelindustrie: zum Kenntnisstand nanoskaliger Wirkstoffcarrier in Lebensmitteln und Verpackungsmaterialien , Behr's Verlag DE, 2010

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