High Performance Encapsulants for Ultra High-Brightness LEDs

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1 High Performance Encapsulants for Ultra High-Brightness LEDs David W. Mosley, Kathy Auld, David Conner, Jay Gregory, Xian-Qian Liu, Angelo Pedicini, David Thorsen, Morris Wills, Garo Khanarian, Ethan S. Simon Rohm and Haas Company, Spring House, PA ABSTRACT The performance requirements for packaging ultra high-brightness LEDs are extremely demanding. These include high continuous use temperatures and light fluxes. Polymer encapsulants play a critical role in the extraction of light, as a host medium for the phosphors to convert blue to white light, and as capping lenses. Encapsulants must also meet critical requirements for viscosity, modulus, and optical transmission under varying environmental conditions, while having a high refractive index to maximize device efficiency. We describe the development of robust one-pot encapsulants having refractive indices of 1.57 and greater than 1.6. These encapsulants are stable at 2 o C for time periods greater than 4 weeks. Keywords: LED, encapsulant, silicone, refractive index, heat stable, light stable, siloxane I. INTRODUCTION Solid-state light-emitting diodes (LEDs) have found a wide range of uses from indicator lights to street lights to display applications. Initial applications have utilized LEDs for the intrinsic light they generate red LEDs for stoplights and display faces (alarm clocks), green LEDs for stoplights, etc. Applications utilizing white LEDs, produced by downconverting blue light to white light using phosphors, are now emerging. There are current applications for white LEDs in store display lighting, backlighting indicators, and as backlights in electronic displays. A large emerging market is in back light units (BLU) for LCDs and also automotive headlamps. The largest market in the future will be in the replacement of general lighting. Solid-state lighting technology based on high-brightness light-emitting diodes (HBLEDs) promises to become a major form of general lighting worldwide as the technical capabilities of devices improve. The technology is predicted to replace the classic light bulb. By 21, sales of LEDs are projected to be $9-1 billion[1]. The market projections in 21 do not include significant replacement of general lighting, so there is considerable upside in the future. There are still technical hurdles for HBLED development as general lighting replacements, since the critical lumens per dollar metric is still too high for this application (~$1-2/kilolumen). According to DOE estimates, LED costs would need to be in the range of $1-3 per kilolumen in order for HBLEDs to widely penetrate the general lighting market[2]. HBLEDs are under intensive development and every aspect of the devices is being optimized in order to meet the needs for general lighting. LED manufacturers focus on the internal quantum efficiencies of the devices, the efficiency and quality of conversion into white light, the light extraction out of the devices, and the directionality of light emitted from their packages. They design the device to run at a certain optimal temperature/current for either A) maximum light output per device, or B) maximum efficiency in terms of lumens per watt. Increasing temperature reduces the efficiency of phosphor conversion, reduces the device quantum efficiency, and places stringent demands on the packaging materials. However, many manufacturers find high operating temperatures offer the best performance for their devices in terms of the lumens output per device (i.e. per unit cost), and they accept lower efficiencies in exchange. Any gains that can be made in light extraction or in the temperature dependence of device efficiency will be important for the HBLED industry. Currently the LED encapsulant is critical in extracting light from the LED device, and so can have a direct impact on device efficiency. The refractive index (RI) of encapsulants - or whatever material is adjacent to the device - directly affects the light extraction, regardless of whether the device is flat, roughened, or patterned. There is demand for Light-Emitting Diodes: Research, Manufacturing, and Applications XII, edited by Klaus P. Streubel, Heonsu Jeon, Proc. of SPIE Vol. 691, 69117, (28) X/8/$18 doi: / SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol

2 the highest possible refractive index in the encapsulants refractive indices of would be the most useful in extracting light from GaN based HBLEDs. Since HBLEDs typically operate at high temperatures of 12-2 o C and at light fluxes of >1mW/mm 2, polymer thermal and photochemical stability must be exceptional. Polymers should exhibit no change in optical characteristics and very little change in mechanical characteristics under these conditions for projected lifetimes of 5, hours. Optical transmission should be greater than 9% at 45 nm; the majority of HBLED manufacturers are producing LEDs emitting at around 45 nm and downconverting with phosphors to produce white light. However, a number of manufacturers are also pursuing devices using 45 nm UV LEDs, and so an ideal encapsulant would be transparent and stable at 4 nm as well. The encapsulant should have no adverse interactions with the phosphors. Finally, the HBLED industry uses liquid prepolymers and vulcanizes them in place over the devices. So the polymer encapsulant should be shipped as a solvent-free prepolymer. Overall, the physical properties needed are challenging. Epoxies have been the dominant polymer encapsulant used for the last 2 years. However, epoxies show considerable yellowing at the higher operating temperatures of today s HBLEDs. Silicones can have excellent transparency and good thermal and photochemical heat stability. Room temperature vulcanizable silicones have become the replacement of choice. However, producing high RI silicones which meet all the necessary requirements continues to be a challenge. We have developed a new class of silicones that provides refractive indices of 1.6 at 589 nm, and ~1.63 at 45 nm. Furthermore, the material is shipped as a one-component thermoset with a variable viscosity, which is advantageous for manufacturing. II. ENCAPSULANT REFRACTIVE INDEX, LIGHT EXTRACTION, AND LED OPTICAL PATH Encapsulant refractive index plays a role in the efficiency of light extraction from LEDs due to Snell s Law and total internal reflection. LEDs are solid state devices, having refractive indices of 2 or higher. Light generated by an LED must somehow pass from a region of high refractive index (RI) to air, which has a refractive index of 1. From Snell s Law, there is a critical angle φ ca when light is passing from a high index to a low index material, where the light no longer passes the interface and is instead internally reflected. In LEDs, light reflected into the device can be reabsorbed and lost. Most HBLEDs are made using GaN, which has an RI of 2.5, and so the critical angle with air is 23.6 degrees. That critical angle can be changed by placing a higher refractive index material in contact with the GaN. Assuming light is emitted from the LED isotropically, one can calculate the amount of light which can escape the device for different refractive indices[3], as shown in Figure 1. interface P normal n sin(θ ) = n sin(θ ) θ n 1 v 1 O n 2 v 2 index velocity θ 2 % Power Out P P Light Extraction from GaN vs Encapsulant RI escape source 1 = 2 ( 1 cosφ ) ca Q RI of encapsulant Figure 1. Snell s Law and the integrated power out of an LED as the encapsulant RI varies. The power out can be a function of many factors, and the above calculation only considers light coming out the top (and bottom) of a flat LED chip based on total internal reflection considerations. Real world LEDs have far more complex optical paths than just emission through one interface of the device. Light is emitted out the sides, which is significant for 1 mm dies; light is reflected off of metallic mirrors or Bragg Proc. of SPIE Vol

3 reflectors; light must interact with phosphors, which causes scattering; LED surfaces may be roughened to enhance light extraction by a surface scattering phenomena; LED surfaces may be patterned with photonic slabs to enhance extraction and change directionality; LEDs may be mounted on transparent substrates which may affect light output; LEDs may be shaped as pyramidal structures to enhance extraction; and finally lenses are still commonly used to shape the light output and prevent further losses due to Snell s Law. For greater details, the reader may consult the book by Schubert[3], or the review by Zukauskas[4]. The complexity of the photon optical path and differing design options for HBLEDs makes it difficult to predict the amount of additional light which can be produced by using a higher refractive index encapsulant. In many cases the lens refractive index must be increased as well to see the full benefit. Surface roughening and photonic slabs may somewhat decrease the advantages of higher refractive index encapsulants or lenses. However, several researchers have concluded that, when surfaces are roughened or patterned, higher RI encapsulants will still contribute to higher light output at roughly the same order of magnitude as on smooth surfaces[5, 6]. Thus, one can project that changing encapsulant refractive index from 1.5 to 1.6 (589 nm values) will result in roughly 5-15 % more light out of a LED if the GaN surfaces are in direct contact with encapsulant. Figure 2 illustrates the trade-offs that must be made to produce higher refractive index optical materials. The general trend is that as the refractive index increases, the absorption cut-off for optical use also increases. Figure 2 shows organic examples, but the same corresponding relationship exists for inorganic solids as well. In the inorganic case, the refractive index is shifted higher. For instance, a solid such as TiO2 has a band edge at ~4 nm, but has an RI of Unfortunately, inorganic solids lack the processing advantages of organic polymers. Due to the relationship of optical absorption, polarizability, and refractive index, it is extremely challenging to produce higher refractive index encapsulants with sufficient transparency in the 4 nm region. For high index encapsulants, the challenge is to find outliers from the line in Figure 2. Considering the additional constraints of high heat and photochemical stability, the materials issues are difficult to overcome ><.O5- Polymers PES Kaplan Liquids copolymer PI-PDMS cdpalymer BIueLED wavelenglh I CO 4 SOC Cut off Wavelength Figure 2: Cut-off wavelengths of optical polymers (2 mm path length) versus refractive index. III. NOVEL HIGH REFRACTIVE INDEX SILICONES We have developed new high refractive index silicones with refractive indices ranging from 1.53 to 1.6 at 589 nm which exhibit excellent heat resistance and good optical characteristics. The general properties are summarized in Table 1. Thermosetting phenyl silicones have traditionally been limited to a maximum refractive index of 1.57, but through proprietary chemistry we have achieved a refractive index of 1.6. The Rohm and Haas (ROH) silicones have potential as encapsulants or injection moldable lens materials. We have also developed a catalyst for one-pot Proc. of SPIE Vol

4 formulations which gives room temperature stability of the viscosity for greater than 6 months. The one-pot system eliminates the need to pre-mix components before use, easing the manufacturing requirements. Viscosities of the one-pot systems are predicted to show little increase over 6 months storage at room temperature. Encapsulant Property Impact on HBLED performance ROH 1.6 ROH 1.57 ROH 1.53 ROH 1.41 Refractive Index More light extracted from GaN with higher RI 633 nm nm 633 nm 633 nm 633 nm Optical Transmission More light transmitted through encapsulant and phosphor >98%T at 4-7 nm 1 mm path >98%T at 4-7 nm 1 mm path >98%T at 4-7 nm 1 mm path >98%T at 4-7 nm 1 mm path Formulation at 25 Lower cost, better quality control One-pot system, One-pot system, One-pot system, ºC with one-pot systems stable > 6 months stable > 6 months stable > 6 months Two-pot system Ease of dispensing, adjustable 1,-1, cp at 25 1,-3, cp at 25 Viscosity at 25 ºC for applications ºC ºC 5-2, cp at 25 ºC <1 cp at 25 ºC Heat Stability at 2 ºC Can operate HBLED at higher junction temperature > 1 month > 1 month > 3 month > 6 month Hardness Combine encapsulation and molding of lens 45-8 Shore D 6-9 Shore A 1-2 Shore A 1-4 Shore A Table 1. General properties of ROH silicone encapsulants. The optical properties of these silicones have been characterized by UV-vis, refractive index measurements, and CIE color measurements. Figure 3 shows the optical spectra of ROH 1.6, ROH 1.57, ROH 1.53, and ROH 1.41 silicones. The spectra have been normalized to a 1 mm path length, and Fresnel reflections have been removed. Typical LED encapsulation needs involve material approximately 1 mm in depth. Note that the 1.41 silicone has a wider transparency window. All of the encapsulants have transparencies above 98% in the visible spectrum through a 1 mm thickness. Transmission UV-Vis Transmission of ROH Silicones ROH nm ROH nm ROH nm ROH nm Wavelength (nm) Refractive Index 1.66 Refractive Indices of ROH Silicones 45 nm nm ROH ROH Wavelength (Microns) Figure 3. Left: Optical spectra of ROH silicone encapsulants through a 1 mm pathlength. Fresnel reflections have been subtracted out of the data. Right: Dispersion curves of ROH 1.6 and 1.57 (at 589 nm) encapsulants, calculated on cured films using a Metricon Corporation prism coupler. Figure 3 shows dispersion curves of the ROH refractive index 1.6 and 1.57 materials. At the critical 45 nm wavelength, the refractive indices are 1.63 and 1.6 at room temperature. The lower index ROH silicones have refractive indices of 1.56 and 1.44 at 45 nm. Refractive index has been shown to be dependent on temperature for silicones, and so at HBLED operating temperatures of 15 o C the effective refractive index at 45 nm would be decreased by ~.2-.4 RI units[6]. We have followed the optical features of the encapsulants during heat aging at 2 o C. Aging at 2 o C is an accelerated aging test, reflecting an extended lifetime at LED operating temperatures of 15 o C. During aging, the UV- Proc. of SPIE Vol

5 vis band edge of most polymers red-shifts. However, we have found that the absorption shift does not necessarily correlate with the appearance of yellowing in the polymers, which is the real feature of interest for white lighting applications. In many cases the UV-vis absorption shifts but no yellowness is observed for many weeks. So we monitor encapsulant polymer aging by measuring the yellowness on the Commission Internationale de l'eclairage (CIE) 1976 L*a*b* scale. The polymer is measured as a 1 mm coating on a glass slide, using a calibrated white background. Figure 4 illustrates the merits of using CIE b values to track optical polymer aging. Figure 4 shows the UV-vis of a colored and uncolored 1.53 RI silicone formulation that has been aged at 2 o C for several weeks, along with the corresponding CIE b-values over time. When using the L*ab coordinates, the b value closely correlates with yellowness under our conditions. In a variety of cases, the UV-vis absorption region can change during heat aging while producing no change in color characteristics in the visible region; or as in this case, one sample can be noticeably yellow while having a virtually identical UV-vis spectrum to a non-yellow sample. Using CIE measurements as a tool, the ROH silicone encapsulants all have excellent color stability during 2 o C heat aging, as shown in Figure 4. Monitoring Aging by UV-vis Vs. CIE Measurements 16 3 CIE b Values of ROH Silicones During 2 o C Aging Absorbance CIE b Values water white sample yellowing sample CIE b Value 2 1 Visibly yellow polymer when b value >2 ROH 1.53 ROH 1.57 ROH 1.6 Days Aging Wavelength (nm) Days at 2 o C Figure 4: Left: UV-vis absorption spectrum of 1.53 RI silicone samples that aged at 2 o C for 7 days. The legend refers to both the large graph and the inset graph. Note that at 7 days the yellow sample was already quite yellow by CIE measurements, while its UVvis spectra was not significantly different from the clear sample. Samples under the test conditions are noticeably yellow when their CIE b values exceed 2. Right: CIE b values of ROH silicones during 2 o C aging. The ROH encapsulants, with the exception of the 1.41 RI silicone, are single-pot thermoset polymers. Cure times and temperatures range from o C, for times of 2-8 hrs. Initial gel times are typically one hour or less. The silicones cure equally well in the presence of phosphors. Weight loss upon cure is <1% in bulk, some of which is attributed to water absorption. The LED industry currently uses two-part silicone systems. However, one-part silicone systems have many advantages including ease of use, consistency and quality control, and very long pot-lives. In the past, one-part silicone systems have not been robust enough for the needs of many manufacturers. We set about developing one-pots which would be compatible with LED manufacturing processes, and would be stable at room temperature for extended times. The stability of the one-pot formulations to premature gelling has been evaluated by accelerated testing using an Arrhenius model, as well as by longer term testing. For Arrhenius calculations, the gel time versus temperature was monitored using a variable temperature viscometer at three or more temperatures between 5-13 o C. The method is demonstrated for ROH 1.57 in Figure 5. Since the viscosity build in a crosslinking system doesn t increase dramatically until near the gel point, we assume that the time to gel point approximates the useful shelf-life of a material. For the room temperature (25 o C) case of the Rohm and Haas encapsulants, all 3 of the one-pot formulations (1.53, 1.57, and Proc. of SPIE Vol

6 1.6 RI) are predicted to last well over 6 months. Figure 6 shows ongoing long-term viscosity aging data of the RI 1.57 and 1.6 formulations under recommended storage conditions. Figure 5. Arrhenius plot for the ROH 1.57 formulation, along with extrapolated and measured gel times. 5, Viscosity Tracking of One-Pot Formulations Under Recommended Storage Conditions 4, Viscosity (cp) 3, 2, 1, ROH 1.57 at 25 C ROH 1.57 at 4 C ROH 1.6 at 4 C Time (weeks) Figure 6. Long-term viscosity aging data for ROH one-pot silicone formulations under recommended storage conditions (ongoing). The ROH silicones have viscosities which can be varied from 1, to 1, cp at 25 o C for encapsulation needs. ROH 1.6 can have a substantially higher viscosity as well, which may be useful for injection molding applications. This ability to customize viscosities is useful as each manufacturer uses different processes which may require different viscosities. Furthermore, ROH silicones have very low metal impurities, as shown in Table 2, and are suitable for electronics applications. Proc. of SPIE Vol

1 ppb < 1 ppb Manganese < 1 ppb < 1 ppb Nickel < 1 ppb < 1 ppb Zinc < 1 ppb < 1 ppb Table 2. Metal impurities in ROH Silicones, in parts per billion. Metals were analyzed by ICP-MS.

7 Element ROH 1.57 ROH 1.6 Sodium < 1 ppb < 1 ppb Copper < 1 ppb < 1 ppb Potassium < 1 ppb < 1 ppb Calcium < 1 ppb 25 ppb Magnesium < 1 ppb < 1 ppb Iron < 1 ppb < 1 ppb Aluminum < 1 ppb 27 ppb Lead < 1 ppb < 1 ppb Chromium < 1 ppb < 1 ppb Manganese < 1 ppb < 1 ppb Nickel < 1 ppb < 1 ppb Zinc < 1 ppb < 1 ppb Table 2. Metal impurities in ROH Silicones, in parts per billion. Metals were analyzed by ICP-MS. We have tested the ROH silicone encapsulants in a variety of application specific ways. We employ 2 o C temperature aging extensively as a development tool. At these elevated temperatures, polymer degradation mechanisms become more numerous. It is essential to understand the real temperatures that polymers may see, because accelerated testing at higher temperatures can bring new mechanisms into play that do not affect the polymers under normal operating conditions. Currently, high performance LED manufacturers claim to be operating their devices in the range of 7-15 C. For that temperature range, we believe that 2 C aging is an excellent accelerated aging test. As shown in Figure 4, the ROH silicones do not yellow as determined by CIE measurements for at least 4 weeks at 2 o C. All of the silicones hold up to humidity cycling protocols similar to JESD22-A1C. We have cycled the ROH 1.6 encapsulant under a current bias of 1 amp using a Luxeon K2 die (Philips Lumileds Lighting Co.) and observed little change in light output characteristics over 3 weeks time. The custom-built LED testing apparatus is shown in Figure 7A. The encapsulants hold up well under constant humidity conditions either with or without bias, similar to JESD22- A11B. The encapsulants also withstand simple temperature cycling, -4 C to 125 C, JESD-A14G. It is important to ensure the encapsulants are fully cured before performing the tests. I-- S -4 A) B) Figure 7. A) A custom-built automated LED tester apparatus that can cycle temperature and humidity of operating 1 amp LEDs while taking real-time measurements of light output. Typically LUXEON K2 dies are used for testing. B) A laser aging apparatus utilizing an argon laser, with heating and humidity control capabilities, which is used for accelerated aging of encapsulants. It is a continuing challenge to approximate real operating conditions for each manufacturer s device. Different emission wavelengths, different phosphors, different configurations, and different encapsulant dimensions all need to be Proc. of SPIE Vol

8 considered for the highest performance HBLED devices. We have laser aged the ROH silicone encapsulants with a thickness of 1 mm at 2 o C and with a light flux of 4 mw/mm 2 at 488 nm for 3 weeks with no signs of damage. We have performed heat and laser aging both with and without phosphor present, with no discernible damage. However, manufacturers may soon surpass the light flux and power density of our argon laser, making this type of testing quite difficult in the future! IV. CONCLUSION We have developed new high refractive index silicones with refractive indices ranging from 1.53 to 1.6 at 589 nm which exhibit excellent heat resistance and good optical characteristics. These silicones have potential as LED encapsulants or injection moldable lens materials. We have also developed a catalyst for one-pot formulations which gives room temperature stability for greater than 6 months. The one-pot system eliminates the need to pre-mix components before use, easing the manufacturing requirements. The silicones have been tested by thermal aging, humidity and temperature cycling, and combined laser/heat aging, and maintain their excellent optical properties. REFERENCES [1] J. Rebello, "Solid State Lighting: New Lighting Applications Will Drive Market Growth," presented at Intertech LEDs 27, San Diego, CA, 27. [2] M. Kendall and M. Scholand, "Energy Savings Potential of Solid State Lighting in General Lighting Applications; U.S. Department of Energy Office of Building Technology," National Technical Information Service (NTIS), 21. [3] E. F. Schubert, Light-Emitting Diodes. Cambridge, United Kingdom: Cambridge University Press, 23. [4] A. Zukauskas, M. S. Shur, and R. Gaska, "Light-Emitting Diodes: Progress in Solid-State Lighting," Materials Research Bulletin, pp. 764, 21. [5] R. Windisch, C. Rooman, S. Meinlschmidt, P. Kiesel, D. Zipperer, G. H. Dohler, B. Dutta, M. Kuijk, G. Borghs, and P. Heremans, "Impact of texture-enhanced transmission on high-efficiency surface-textured lightemitting diodes," App. Phys. Lett., vol. 79, pp , 21. [6] Lighting Research Center, "Extracting More Light from LEDs," Rensselaer Polytechnic Institute LRC Project Sheet, 27. Proc. of SPIE Vol

White Paper: Pixelligent LED Encapsulation

White Paper: Pixelligent LED Encapsulation Maneesh Bahadur, Ph.D. Pixelligent Technologies LLC, 6411 Beckley Street, Baltimore, Maryland 21224 Email: mbahadur@pixelligent.com September 2014 Introduction