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Encapsulation Approaches for Flexible Solar Panels, Displays, and Antennas


OBJECTIVE: Develop encapsulation approaches to protect flexible solar panels, displays, and/or conformal antennas from environmental threats with minimal impact on packaged weight. DESCRIPTION: Flexible electronic devices such as organic light emitting diode (OLED) displays, conformal antennas, and thin film solar panels have the potential to provide revolutionary new capabilities at significantly lower cost. However these materials share a common challenge in their vulnerability to environmental threats. In order to maximize the lifetime of traditional solar panels, the devices are typically encapsulated in glass or in plastics such as Teflon and/or Tedlar, which protect the solar cells and interconnects from weather, dust, and other environmental threats. Although this approach has enabled greater than twenty-year lifetimes for rigid solar cells such as those fabricated from single crystal silicon, this approach is less practical for flexible solar panels. Flexible solar cells, however, could potentially be integrated with personal electronics, clothing, windows, and curved structures. In these cases, encapsulation approaches are needed that maintain the inherent advantages of flexible solar technology the light weight, thin form factor, and flexibility. In addition, for military applications these encapsulation approaches must provide rugged protection during flexing and survive extreme temperatures. Flexible electronics and devices have numerous commercial and military applications, including providing auxiliary power to aircraft, air bases, and special operations forces with flexible solar panels. Current state of the art encapsulation approaches for flexible solar cells typically include plastics such as Tedlar or Teflon as well as layers that provide adhesion and handling support. In the packaged solar panel, the photovoltaic layer itself can make up as little as 1-10% of the total thickness and weight. For unmanned aircraft system (UAS) applications, any added weight would reduce the potential flight endurance gains associated with the solar panels. A similar tradeoff analysis would be required for conformal sensors or any aircraft application since weight reduction is a pervasive goal. Some preliminary progress has been made in reducing packaging weight by employing a co-curing scheme involving attaching the solar panels before the composite structural components are treated in an autoclave. While a co-curing approach has demonstrated limited reductions in packaging weight, a more universal design is needed that would allow for light-weight, durable encapsulation of flexible electronics on UAS or other structures. Encapsulation approaches could include thin polymer film sheets that are attached with adhesives or more innovative solutions such as protective layers deposited by spraying. An ideal encapsulation approach would be compatible with the full range of foreseeable applications (e.g., personal electronics, clothing, windows, and curved structures). For this SBIR project, the encapsulation approach will be demonstrated on a flexible solar panel. Any encapsulation scheme should hermetically seal the solar panels and provide protection from environmental threats such as rain, sand, and mild acids/bases for>3 years. The encapsulant should be>80% transparent in the visible-near IR and resistant to UV degradation and scratching. To enable compatibility with organic electronic devices, the encapsulation process should not exceed 130C, and the encapsulant should provide protection to>150C. The encapsulant should survive the flexing conditions to which these thin film devices are typically subjected (<1 cm radius of curvature). Evaluation methods should include at a minimum ASTM standards E117-09 (cyclic temperature and humidity), E1830-09 (mechanical integrity), and E1799-08 (visual inspection). PHASE I: Develop a hermetic encapsulation process for flexible devices that protects them from environmental threats (e.g., chemical, erosion, scratching) while maintaining solar performance characteristics. Demonstrate this process at a scale of at least 1 square inch. The added weight due to the encapsulant/packaging should be less than 200 grams per square meter. PHASE II: Scale-up the encapsulation process to at least 10"x 10"and demonstrate it on functioning commercially available flexible solar panels. Conduct lifetime testing to demonstrate viability of encapsulation approach. The added weight due to the encapsulant/packaging should be less than 125 grams per square meter. The initial prototype will be evaluated for use on small UAS to extend the flight time. PHASE III: Potential Air Force applications could include evaluation of the encapsulant on an emeriging solar-modified UAS system or on photovoltaic sheets for deployed airbases. Commericial applications could include protection for flexible displays and building-integrated photovoltaics. REFERENCES: 1. Jason Maung, K.; Hahn, H. T.; Ju, Y. S. Solar Energy 2010, 84, 450-458. 2. Ronald F, G. Composite Structures 2010, 92, 2793-2810.
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