1. Thank you IESNYC Sponsors!

2. Further Improving White LED Performance : Thermal Analysis of LED Phosphor Layer Ukwatte L. Indika U. Perera Lighting Research Center Rensselaer Polytechnic Institute, Troy, NY New York City Section

3. Motivation For phosphor converted white LEDs to reach 226 lm/W by 2020 further improvements are needed. The down-conversion and LED encapsulant materials are key areas that require improvement in achieving this target. Improvement in encapsulant material identified as a key area of research only in the 2015 U.S. DoE R&D roadmap Heat generated in the LED package affects both lumen output and lumen degradation. 3 20152020Goal Cool-white efficacy (lm/W)185226250 Warm-white efficacy (lm/W)162220250 Source: U.S. DoE R&D roadmap, 2015

4. Motivation The heat generated in an LED package LED chip Phosphor layer – Increased optical power LEDs, the heat in the phosphor layer is becoming a significant problem. Thermal management LED: Studied extensively Phosphor layer: Studies are scarce Thesis motivation: To study phosphor layer heat transfer to improve LED system performance 4 In 2014 U.S. DoE SSL R&D Roundtable meeting identified thermal management of the down-conversion material layer as a key area for research LED optics phosphor layer LED heat sink LED heat dissipated via heat sink Phosphor heat

5. Background Heat is generated within a phosphor layer due to Phosphor conversion efficiency losses, Stokes shift losses, and trapped photons within the phosphor layer The increase in temperature affects LED performance Reduced light output – Phosphor quenching Reduced lifetime – LED and encapsulant degradation Chromaticity shift – At the chip – At the phosphor Barton et al. 1998; Tamura et al. 2000; Narendran 2005; Tsai et al. 2009; Setlur 2009; Chen et al. 2010; Wenzl et al. 2012, 2013; Huang et al. 2013 5 Source: Tamura et al. (2000) Source: Chen et al. (2010) (Y 2.93 Ce 0.07 ) Al 5 O 12 under 460 nm

6. Background … cont. Phosphor layer heat buildup depends on Distance between LED and phosphor Arik et al. 2003; Fan et al. 2007; Hwang et al. 2010; Yan et al. 2011; Hu et al. 2012; Dong et al. 2013 Thickness of the phosphor layer Yan et al. 2011; Wenzl et al. 2013; Kolodeznyi et al. 2014; Mou et al. 2014 Concentration of phosphor Arik et al. 2003; Yan et al. 2011; Hu et al. 2012; Huang et al. 2013; Luo and Hu 2014; Mou et al. 2014 Binding media used for encapsulating phosphor Arik et al. 2003; Wiesmann et al. 2012 LED package structure Arik et al. 2003; Narendran et al. 2004, 2005; Kim et al. 2005; Zhu 2006; Hoelen et al. 2008; Yan et al. 2011; Hu et al. 2012; Luo et al. 2013 Conversion efficiency of phosphor Mueller-Mach et al. 2002; Arik et al. 2003; Setlur et al. 2004, 2006; Wenzl et al. 2012; Luo et al. 2013; Luo and Hu 2014 6

7. Summary: literature review Methods to lower phosphor temperature Binding materials and or fillers to improve thermal conductivity – Limited experimental conductive binding material studies – Computer modeling with limited experimental validation Arik et al. 2003 ; Huang et al. 2004; Lin et al. 2010; Chung et al. 2012; Wiesmann et al. 2012; Luo et al. 2013 Use of modified drive current – Computer modeling with limited validation – Reduced light output and perceivable color shift Wenzl et al. 2013 Use of composite structures to dissipate heat – Limited studies quantifying performance of these methods – Reduced light output and limited analysis of the reasons – No studies looking into reliability of these type of composite heat sink structures Allen et al. 2010; Lowery et al. 2011; Tong et al. 2011; Perera and Narendran 2013 7

8. Problem statement and thesis goal Problem: Heat buildup in phosphor layer negatively affects phosphor-converted white LED performance – Short-term and long-term effects Thesis goal: To better understand the heat transfer in the phosphor layer to reduce phosphor operating temperature and improve system performance. – Heat induced lumen degradation in short and long term 8 Source: Narendran et al. 2004 Long-term effects Short-term effects Source: Lin et al. (2010)

9. Knowledge gap Limited studies on how heat generation in the phosphor layer affects The temperature and the temperature distribution The light output of the LED system (short- and long-term) Only a few studies have considered thermal management of the phosphor layer to improve system performance Most of these studies have not quantified or experimentally validated their findings. There are no models that predicts temperature of the phosphor layer and the resulting light output of the LED system for different system configurations. 9

10. Thesis objectives To analyze and quantify the contribution of heat transfer mechanisms in the thermal management solution used in my MS thesis study. To develop a theoretical model that can predict phosphor layer temperature profile and the light output for an LED system. To validate the model by comparing the model results with observed past experiment results. 10

11. Theoretical model Objective: To predict phosphor layer temperature and total light output 11 Light propagation model Thermal model

12. Literature survey 12 Thermal modeling of phosphor layer heat Predefined heating profile and FEM Heating profile from light propagation Ray-trace and FEMFD/FEM Arik et al. 2003; Fan et al. 2007; Hwang et al. 2010; Krivic et al. 2012; Dong et al. 2013; Luo et al. 2013; Kolodeznyi et al. 2014 Yan et al. 2011; Wiesmann et al. 2012; Wenzl et al. 2013 Kang et al. 2006; Hu and Luo 2012; Huang et al. 2013; Luo and Hu 2014

13. Phosphor layer analysis with FD/FEM Study author Theory/ Method Light propagation Heat propagation Notes Kang et al. 2006 Beer-Lambert (Analytical) YesNo*Doesn’t assume scattering of light * Analytical solution. Hu and Luo 2011 Kubelka-Monk (FD) YesNo Huang et al. 2013 Beer-Lambert and Energy Eq. (Analytical) Yes *Scattering included but eliminates some model parameters for analytical solution. *No temp. dependence. *Do not solve heat equation for temperature distribution Luo and Hu 2014 Kubelka-Monk and Energy Eq. (FD) Yes *No temp. dependence. *Do not solve heat equation for temperature distribution 13

14. Theoretical model development 14

15. Combined light propagation and heat model 15 Optical model parameters Thermal model parameters Geometrical model parameters Boundary conditions Model equations ** SPIE 2014 : Mathematical model to analyze phosphor layer heat transfer of an LED system One of the most downloaded papers from the SPIE Digital Library in February 2015.

16. Experiment validation of models The developed model was used to estimate the total light output and phosphor layer top surface temperature of the experimental setup. 16 dc power supply

17. Experiment validation of models Validated the model Illustrated the developed model’s improved ability of estimation compared to other models. Temperature dependence of light propagation Absorption of light in other directions to the direction of light propagation 17

18. Further validation of thesis model It was hypothesized that model developed in this thesis will predict light output and temperature results of past experiments Study A : Heat transfer mechanism contribution Study B : Study verifying light output Study C : Model verification for phosphor concentration and thickness of phosphor layer 18

19. Study A: Quantify contribution of mechanisms Objective: To compare the results from the thesis model to past experiment results. To determine the contribution of heat transfer mechanisms in dissipating phosphor layer heat 19 dc power supply for fan dc power supply for LEDs

20. Study A … contd. 20 Validation of the temperature profile and quantification of mechanisms predicted from model ** ITHERM 2014 : Understanding Heat Dissipation of a Remote Phosphor Layer in an LED System

21. Study B: Verify light output Objective: To check the ability of the developed thesis model to capture the reduction in light output as the interface area is increased in the phosphor layer heat sink 21

22. Study B … contd. The model captured the reduction in light output observed in experimental studies 22

23. Study C: Verify the model for concentration To predict the top surface temperature of remote phosphor plates of different thicknesses and phosphor concentrations 23 Source: Mou et al. (2014)

24. Study C … contd. The model predicted light output of the LED system and phosphor layer top surface temperature Phosphor layer thickness and phosphor weighting 24

25. Study on long-term effects of heat sinking Objective was to investigate the long-term effects of optical radiation on phosphor layer with heat sink Light output emitted from the phosphor layer was measured The surface temperature of the phosphor layer on the emitting side was measured 25

26. Long-term lumen depreciation study 26 The degradation of the remote phosphor layer was significantly lower due to the reduced phosphor layer temperature

27. Knowledge contribution – e.g., identification of key research tasks Material Thermal conductivity [W/(m.K)] Epoxy (binding material in the phosphor layer)0.42 Acrylic0.18 Glass1.4 Steel AISI 434044.5 Aluminum238 Copper400 27

28. Knowledge contribution – e.g., identification of key research tasks … cont’d Base case Enhanced encapsulant case High-efficiency down-converter case Phosphor layer heat sink material Acrylic [0.18 W/(m.K)] Phosphor layer composition Nitride phosphor in epoxy Nitride phosphor in enhanced encapsulant Hypothetical Nitride phosphor in epoxy Thermal conductivity of phosphor layer 0.42 W/(m.K)1 W/(m.K)0.42 W/(m.K) Quantum efficiency of phosphor 0.6 0.95 28

29. Knowledge contribution – e.g., design tool 29 ApplicationMR 16 LED power7 W Optical power1.75 W Phosphor layer maximum temperature 100°C Phosphor concentrationDesign variable Phosphor layer thicknessDesign variable 6 mm 50.8 mm 25.4 mm

30. Knowledge contribution – e.g., design tool … cont’d 30 Phosphor layer diameter constraint by reflector geometry Not possible to maintain the operating temperature with 10% by weight

31. Knowledge contribution – e.g., design tool … cont’d 31 Design constraint achieved with 6 mm thick phosphor layer with 2% by weight concentration

32. Conclusion of dissertation New knowledge contribution to the field: Development of a theoretical model for estimating radiant power and temperature profile of a LED phosphor layers – Useful for assessing factors that affect LED system performance. – Useful in identifying key research tasks and as a design tool 32

33. Acknowledgement Advisor: Nadarajah Narendran, Ph.D. Committee members: Mark Rea Ph.D., Russell Leslie AIA, FIES, Theodorian Borca-Tasciuc Ph.D. LRC faculty, staff and students Jean Paul Freyssinier, Yiting Zhu, Andrew Bierman, Martin Overington, Yi- wei Liu, John Bullough, Nick Skinner, Howard Ohlhous, Robert Wolsey, Jennifer Taylor, Lenda Lyman, Bonnie Westlake Funding ASSIST(# H11014), FAA, RPI/LRC (IRA), Henkel, The Besal Lighting Education Fund (Acuity Brands Lighting) My family and friends 33

34. Acknowledgement 34 Alliance for Solid-State Illumination Systems and Technologies Members

35. Thank You !