Patent Application
20150380615
The present disclosure relates generally to light emitting diodes (LEDs), high index of refraction materials, and polymer composites comprising nanoparticles.
Light emitting diodes (LEDs) consume less power than incandescent and halogen lights for the same amount of light produced and have longer service lifetimes. Unlike fluorescent lights, LEDs contain no mercury. LEDs are typically built on a semiconductor wafer that is diced apart to make individual chips and the LED can be as small as 1 mm2 or less. LEDs resemble basic p-n junction diodes however LEDs also emit light. LEDs are typically comprised of a semiconducting material doped with impurities to create a p-n junction. The wavelengths of light emitted by a LED depends on the material used to form the p-n junction and LEDs that emit near IR, visible, or near-UV light are all possible.
The material described and illustrated is provided to exemplify aspects and is not meant to limit scope. For simplicity and clarity of illustration, elements illustrated in the figures are not necessarily drawn to scale. Further, where appropriate, reference labels have been repeated among figures to indicate corresponding or analogous elements. In the figures:
In the following description, numerous specific details are set forth in order to provide an understanding of certain embodiments. Embodiments may be practiced without one or more of these specific details and frequently specific details of one embodiment may be practiced with other disclosed embodiments, as will be apparent to one of skill in the art. In other instances, well-known features are not described in detail in order to not obscure the description.
Most materials used to form light emitting diode chips (LEDs) have high indexes of refraction. The large difference in refractive index for the LED versus the air or polymer at the air-LED or polymer-LED interface causes a significant amount of light to be reflected back into the LED. The reflection of light back into the LED causes reduction in the efficiency of light emission. Generally, a flat-surfaced uncoated LED chip (a LED, or a LED semiconductor chip) will emit light perpendicularly to the semiconductor surface and a few degrees to the side in a conical shape, which is sometimes referred to as the light cone or the escape cone. The maximum angle of incidence is referred to as the critical angle. When the critical angle is exceeded, the photons of light are reflected back into the LED. Although some of these internal reflections can escape through other crystalline faces, in the typical device, a significant number are not emitted and are lost as heat in the device.
LEDs are typically packaged or encapsulated using a polymer (plastic) material. A polymer casing, shell, or package can protect the LED and its associated wiring from damage. The polymeric casing can also provide a cover or lens that can act as a diffusing lens that allows light to be emitted at a higher angle of incidence from the LED than the angle of incidence at which a bare LED chip would be able to emit light.
LEDs have dramatically improved in efficiency over the past 10 years due in part to improved process capabilities and materials; however, significant tight is still lost due to internal reflection. Optimization of the refractive index (RI) between the LED and the polymer used to package the LED can improve LED light emission efficiency. Typical polymers used to package LEDs include, for example, silicone or silicon/epoxy co-polymer 1,2. For example, 1,2-epoxy-4-vinyl-cyclohexane with 1,3,5,7-tetramethycyclotetrasiloxane is a transparent silicon epoxy co-polymer. A higher RI material can potentially improve the light intensity by 30% or more.
The efficiency of the light production in a LED can be reduced in a second significant way which is also related to differences in refractive indexes between materials. The efficiency of light emission from the LIED 100 is described by the equation:
ηex=ηescTSETEA
where ηex is the extraction efficiency, ηesc is escape efficiency (the efficiency of light transfer from the semiconductor 100 to the polymer cover 125), TSE is the transmission efficiency from the substrate to polymer or polymer composite layer, and TEA is the transmission efficiency from the polymer or polymer composite layer to the air. The escape efficiency ηesc, can be described by the equation:
TSE can be described by the equation:
and TEA can be described by the equation:
where ne is the refractive index for a polymer cover 125 and ns is refractive index for the LED 100 material. Table 1 demonstrates the dependence of the extraction efficiency on the refractive index of the polymer cover 125 material. In Table 1, the substrate refractive index is taken to be 2.45.
It can be seen from Table 1, that increasing the refractive index of the material directly on top of the LED substrate (the cover 125) is important to increasing the overall extraction efficiency for the LED. A cover 125 material having an index of refraction of 1.5 yields an overall extraction efficiency of 8.9% whereas a cover 125 material having an index of refraction of 2.45 yields an overall extraction efficiency of 39.4%.
Useful polymer materials for the covers 125, 235, and 310 that are adjacent to LED 100, 210, and 305 demonstrate photo and thermal stability and are transparent at desired LED emission wavelengths. in embodiments, polymers are stable up to at least 130° C., up to at least 140° C., or up to at least 150° C. Useful polymer materials include polymer composites comprising zirconium dioxide nanoparticles or hafnium dioxide nanoparticles. In embodiments, polymer composites comprise both nanoparticles of zirconium dioxide and nanoparticles of hafnium dioxide. In further embodiments, polymer composites comprise nanoparticles that comprise both zirconium and hafnium dioxide. In additional embodiments, polymer composites comprise titanium dioxide nanoparticles. In further additional embodiments, a polymer composite comprises titanium dioxide, zirconium dioxide, and/or hafnium dioxide nanoparticles. In additional further embodiments, individual nanoparticles of the polymer composite each comprise titanium, zirconium, and or hafnium dioxides. In embodiments, nanoparticles are between 1 and 5 nm in average diameter. Particles that are less than 5 nm in average diameter are smaller than the wavelengths of light emitted by typical LEDs, and allow light scattering by the nanoparticles to be minimized. Additionally, at these dimensions the nanoparticles can react with polymer monomers to create stable polymer matrices having properties of both the nanoparticles and the polymer base. In embodiments polymer composites have an inorganic content of between 35 weight % and 85 weight %. Inorganic materials are materials that essentially do not contain ca bon or hydrogen. Nanoparticles of zirconium dioxide, hafnium dioxide, and titanium dioxide are considered to be inorganic materials. One of skill in the art will recognize that it is unlikely that a material can be totally free from impurities or trace quantities of other substances, however, an inorganic material is considered to be without carbon or hydrogen even though it may contain trace, difficult to detect, or insignificant amounts of these elements.
In embodiments, useful polymers exhibit sulfur weight percentages of between 8 and 37 weight %. In embodiments, polymer composites exhibit refractive indexes of between 1.55 and 2.35, or between 1.60 and 2.35, or between 1.60 and 2.30, or between 1.65 and 2.30, or between 1.70 and 2.30, or between 1.70 and 2.25 at 500 nm. The refractive index of the polymer composite is in part dependent on the concentration of inorganic nanoparticles, such that higher concentrations of nanoparticles yield composites having higher indexes of refraction.
In general, a LED chip can comprise one light-emitting diode or an array of light-emitting diode regions. The LEDs of the array can be the same type of LED or the array can comprise different types of LEDs. LED chips can have a variety of sizes, wattages, color spectrums, and or number of LEDs. LED chips can also comprise additional electronics, such as, for example, LED drivers and dimmer circuits.
Substrates can include substrates that provide connections between and among electronic components, such as chips, and power supplies. Semiconductor and LED chips can be attached to one or both sides of the substrate. Substrates can be used to provide electrical connections between small-scale semiconductor chips and larger-scale power sources. Substrates can be, for example, wire boards or circuit boards.
Persons skilled in the relevant art appreciate that modifications and variations are possible throughout the disclosure as are substitutions for various components shown and described. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, but does not necessarily denote that they are present in every embodiment. Various additional layers and/or structures may be included and/or described features may be omitted in other embodiments.
