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In traditional lighting technologies, such as discharge and incandescent lamps, polymers were unsuitable materials because of the high temperatures and/or ultraviolet light. In solid state lighting (SSL), however, polymers and polymer-based composites are ubiquitous. They are used in housing materials, adhesives, encapsulants, lenses, flexible substrates, light-weight heat sinks, and in various other applications. Virtually all SSL products have some form of polymer present in their make-up. In this presentation, we will discuss the use of polymers in LED-based lighting, highlighting the challenges and outlook. The primary focus will be on their applications as encapsulants and adhesives. The most abundantly used technology in producing white light with LEDs is based on phosphor conversion. It has been estimated that >75% of the white LED market relies on phosphor conversion.[1] Whether the phosphor is present as a powder, polycrystalline ceramic, single crystal, or thin-film, polymers are often used to either glue the conversion layer to the LED chip, encapsulate the phosphor particles, or both. For these applications, optical silicones are the usual choice. Even though silicones have many desirable properties, there are three main challenges that limit their performance as adhesives or encapsulants: (i) low thermal conductivity, (ii) low refractive index, and (iii) long term photo-thermal stability. The Department of Energy (DOE) SSL R&D Plan lists LED encapsulation as a key issue and R&D priority.[2] In this talk, we will briefly present the history of polymer encapsulants for LEDs, leading up to the use of state-of-the-art optical silicones. We will discuss the associated limitations of these materials in terms of losses and how LED performance could improve with advancements in polymer and polymer-composite technology. References: [1] Company estimates based on McKinsey & Company, Lighting the Way: Perspectives on the Global Lighting Market, 1st ed. (2011). [2] DOE SSL Program, "R&D Plan," prepared by Bardsley Consulting, SB Consulting, SSLS, Inc., LED Lighting Advisors, and Navigant Consulting, Inc., May 2015; http://energy.gov/eere/ssl/downloads/solid-state-lighting-rd-plan. |
