Luminescent Materials for High Brightness LEDs

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Marcia Willa Hudson
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1 Luminescent Materials for High Brightness LEDs September 14 th,

Evolution of Light Sources General Lighting Light generation occurs from 19

C, Os, W Hg, Na, Ne, Xe AlInGaP,AlInGaN 1,0 1,0 1,0 Emission Intesinty [a.u.

] 0,8 0,6 0,4 0,2 Normierte Intensität 0,8 0,6 0,4 0,2 0,0 300 400 500 600

2 Evolution of Light Sources General Lighting Light generation occurs from 19 th century 20 th century 21 st century solid state gas discharge solid state C, Os, W Hg, Na, Ne, Xe AlInGaP,AlInGaN 1,0 1,0 1,0 Emission Intesinty [a.u.] 0,8 0,6 0,4 0,2 Relative intensity [a.u.] 0,8 0,6 0,4 0,2 Normierte Intensität 0,8 0,6 0,4 0,2 0, Wavelength [nm] 0, Wavelength [nm] 0, Wellenlänge [nm] -2-

3 Evolution of Light Sources LEDs 1970 GaAsP < 0.1 W < 0.1 lm yellow, red 2004 AlInGaP, AlInGaN W lm all colors + UV-A -3-

4 Evolution of Light Sources - LED Lumen Package 1000 Flux/Package (lumens) Year LED Flux per package has doubled every months for last 30 years! 500 lm LED (30 W) demonstrated in lab! -4-

5 Evolution of Light Sources High Brightness LEDs Emission intensity [a.u.] 0,35 0,30 0,25 0,20 0,15 0,10 0,05 LED U(V) I(A) x y Blue Green Red , Wavelength [nm] AlInGaP nm Quenching ~0.7%/K AlInGaN nm Quenching ~0.1%/K Spectral shift with drive and temperature Input power Voltage Current Chip temperature 1 W (2000) 4.5 V 0.2 A 120 C 5 W (2002) 4.5 V 1.1 A 150 C -5-

6 Evolution of Light Sources Efficiency of LEDs Energy efficiency Eye response Luminous efficiency External Quantum Efficiency 60% 50% 40% 30% 20% 10% In x Ga 1-x N (1) (2) (Al x Ga 1-x ) 0.5 In 0.5 P V(λ) Luminous Performance (lumens/watt) T j =25 C 10 AlGaInN AlGaInP Eye Response Curve (CIE) High Pressure Sodium (1kW) Fluorescent (40W) Mercury Vapor (1kW) AlGaAs Halogen (30W) Tungsten (60W) Red-Filtered Tungsten (60W) 0% Peak Wavelength (nm) Yellow Efficiency Gap Peak Wavelength (nm) LEDs are already more efficient than (filtered) incandescent and halogen lamps -6-

7 Spectra of InGaN LEDs Normalised emission intensity 1,0 0,8 0,6 0,4 0,2 increasing x (In x Ga 1-x )N Increasing In concentration 0, Wavelength [nm] InGaN quantum well transition energy decreases: 370 nm 550 nm Broadening of emission band Decrease in quantum efficiency -7-

8 White Light Generation by LEDs UV LED Blue LED Green LED Red LED Phosphor blend Single phosphor or phosphor blend Additive color mixing by secondary optics White light by fluorescence White light by colour mixing -8-

White Light Generation by Fluorescence Fluorescent light sources comprise a luminescent screen, which convert absorbed energy into visible light by means of luminescent materials (phosphors) How does

9 White Light Generation by Fluorescence Fluorescent light sources comprise a luminescent screen, which convert absorbed energy into visible light by means of luminescent materials (phosphors) How does a phosphor work? Layer of YAG:Ce µ-particles Excitation Heat S Heat A Energy Transfer Emission Relative intensity 1,0 0,8 0,6 0,4 0,2 Emission spectrum Excitation spectrum Reflection spectrum x = y = LE = 395 lm/w QE 254 = 89% RQ 254 = 20% Emission 0, Sample YAGaG:Ce 800 Wavelength [nm] -9-

10 White Light Generation by pcleds nm nm nm nm lm/w lm/w CRI = CRI = lm/w lm/w CRI = CRI = low CRI at low T c high CRI at all T c low CRI at low T c high CRI at all T c UV light causes polymer degradation and requires safety measures 390 nm LED (3.2 ev) 570 nm (2.2 ev) Quantum deficit = 0.69 Leak through of blue light depends on optical pathway Colour point = f(viewing angle) 460 nm LED (2.7 ev) 570 nm (2.2 ev) Quantum deficit =

11 Phosphor Converted LEDs (pcleds) 1,0 Absorption 1,0 phosphor Intensity [a.u.] 0,8 0,6 0,4 0,8 0,6 0,4 InGaN die Blue LED chip: nm emitting InGaN Phosphor conversion layer: 1. Yellow T c > 4000 K Cool white 2. Yellow + red T c < 4000 K Warm white 3. Green + red 2000 K < T c < 8000 K 4. Red Magenta colors Ag-Mirror Silicone 0,2 Light Source Emission of phosphor converter 0,0 0, Wavelength [nm] -11-0,2

12 Selection Criteria for pcled Phosphors In General strong absorption at LED chip emission wavelength spin and parity allowed transition, e.g. 4f n 4f n-1 5d 1 quantum efficiency higher than 90% stability towards O 2, CO 2, and H 2 O stability under high photoexcitation density Blue + Yellow Concept broad emission band at around nm Ce 3+ phosphors (ground state depletion 2 F 5/2 + 2 F 7/2 ) Blue + Green/Yellow + Red Concept green/yellow phosphor Eu 2+ or Ce 3+ red phosphor Eu

13 Spectra of Ce 3+ and Eu 2+ Phosphors E 5d Free ion gaseous Nephelauxetic effect Crystal field Stokes Shift ε c ε cfs 4f Free ion: Eu 2+ Ce 3+ 4f n-1 5d 1 level: cm cm -1 ε c : centroid shift due to nephelauxetic effect ε cfs : crystal field splitting Both parameters depend on type of host lattice, i.e. composition -13-

14 Normalised emission intensity Emission Spectra of Ln 3 Me 5 O 12 :Ce Emission spectra 1,0 YAG:Ce1% YAG:Ce2% (Gd,Y)AG:Ce2% 0,8 (Lu,Y)AG:Ce1% LuAG:Ce1% 0,6 0,4 0,2 Color points 0, Wavelength [nm] Garnet structure Ln 3 Me 5 O 12 Ln = Y, Ce, Gd, Lu dodecahedral Me = Al, Ga tetrahedral(3), octahedral(2) Enhance conc. of Ce nm 565 nm (Red-Shift) Replace Y 3+ by Gd 3+ or Tb nm 580 nm (Red-Shift) Replace Al 3+ by Ga 3+ or Y 3+ by Lu nm 520 nm (Blue-Shift) -14-

15 Emission intensity White pcleds based on (Y,Gd) 3 Al 5 O 12 :Ce Spectra of cool white pcleds Tc = 5270 K: CRI = 82 Tc = 4490 K: CRI = 79 Tc = 4110 K: CRI = 76 Tc = 3860 K: CRI = 73 Tc = 3540 K: CRI = Wavelength [nm] Color rendition CRI = Cool white light emission Wall plug efficiency: high brightness 30 lm/w low brightness 50 lm/w Lifetime 50000h, 90% at 12000h Lack of red radiation and colour rendering is f(t c )! (Y,Gd) 3 Al 5 O 12 :Ce is very efficient and (photo)chemically stable, but high color rendering at low color temperature requires additional red emitter! -15-

16 White pcleds with a high Color Rendition 1. Blue LED + (Y,Gd)AG:Ce CRI > 75 only for T c > 4000 K 2. Blue LED + YAG:Ce + Red CRI > 85 for T c < 4000 K red phosphor yellow phosphor Intensity [a.u.] 1,0 0,8 0,6 0,4 0,2 Blue LED Yellow Phosphor Red Phosphor InGaN die Silicone 0, Wavelength [nm] -16-

17 Red Phosphors Shortcomings of Ce 3+ Phosphors Rather narrow absorption band Rather broad emission band No known red-emitting Ce 3+ phosphor with a high thermal quenching temperature at the same time Alternatives Mn 2+ activated: Sensitisation required, saturation Eu 3+ activated: Y 2 O 2 S:Eu, CT band < 360 nm Eu 2+ activated: Strong covalency and CFS required Best choice are Eu 2+ activated luminescent materials -17-

18 Color of Eu 2+ Phosphors Chemical composition λ max CaS:Eu 655 nm Sr 2 Si 5 N 8 :Eu 615 nm SrS:Eu 610 nm Ba 2 Si 5 N 8 :Eu 580 nm Sr 2 SiO 4 :Eu 575 nm SrGa 2 S 4 :Eu 535 nm SrAl 2 O 4 :Eu 520 nm Ba 2 SiO 4 :Eu 505 nm Sr 4 Al 14 O 25 :Eu 490 nm SrSiAl 2 O 3 N:Eu 480 nm BaMgAl 10 O 17 :Eu 450 nm Sr 2 P 2 O 7 :Eu 420 nm SrB 4 O 7 :Eu 368 nm Increase fo covalency or crystal-field strength Nitrides + Sulfides Oxynitrides + Oxides -18-

Spectra of (Ca 1-x Sr x )S:Eu Substitution of Sr by Ca yields a red-shift and improves stability 1,0 0,8 SrS:Eu stability Crystal field strength Centroid shift CaS:Eu Relative intensity 0,6 0,4 0,2

19 Spectra of (Ca 1-x Sr x )S:Eu Substitution of Sr by Ca yields a red-shift and improves stability 1,0 0,8 SrS:Eu stability Crystal field strength Centroid shift CaS:Eu Relative intensity 0,6 0,4 0,2 Emission spectrum Excitation spectrum Reflection spectrum 0, Sample PS1-38 Wavelength [nm] Composition QE [%] Abs. [%] LE [lm/w] x y SrS:Eu > 95 > CaS:Eu > 95 >

20 Warm White pcleds LUXEON-warm white -the components blue YAG:Ce CaS: Eu JAZZ 3300K BB 3300K nm nm 750 Red emitter is added to the plain (cold) white of blue +YAG:Ce. We chose CaS:Eu 2+ (Philips + Lumileds IP) More red: lower CCT, more blue: higher CCT. black body 3600 K fluorescent, CCT=3600 K CCT range: 2700 to 5500 K with excellent CRI > nm -20-

21 Warm White pcleds Warm white pcleds comprising YAG:Ce and CaS:Eu CRI > 90 for T c between 2700 and 5500 K Colour cannot be better but efficiency... Standard White ((Y,Gd)AG:Ce) 5500 K, CRI ~ lm/w opt. ~ 30 lm/w el. Warm White (YAG:Ce + CaS:Eu) 3200 K, CRI ~ lm/w opt. ~ lm/w el. -21-

22 Trichromatic RGB White pcleds 1. Blue LED + (Y,Gd)AG:Ce CRI > 75 only for T c > 4000 K 2. Blue LED + YAG:Ce + Red CRI > 90 for T c < 4000 K 3. Blue LED + Green + Red CRI > 85 for T c = K red phosphor green phosphor Intensity [a.u.] 1,0 0,8 0,6 0,4 0,2 Blue LED Green Phosphor Red Phosphor InGaN die Silicone Alternative concept: Application of RGB LEDs, but... 0, Wavelength [nm] -22-

23 Trichromatic RGB White pcleds Blue LED + SrGa 2 S 4 :Eu + SrS:Eu Emission intensity [a.u.] 0,14 Tc2700K: CRI = 89 Tc2900K: CRI = 92 0,12 Tc4000K: CRI = 93 Tc5000K: CRI = 93 0,10 Tc6300K: CRI = 91 Tc8000K: CRI = 89 0,08 Tc8600K: CRI = 89 0,06 0,04 0,02 0, Wavelength [nm] Color rendition is > 89 for 2700 K < CCT < 8000 K (R. Mueller-Mach, G.O. Mueller, Proc. SPIE 3938, 2000, 29) Problem: Stability of phosphor blend (sulfides) -23-

Advantages over oxides and sulfides highly condensed anionic networks high density high chemical stability high hardness high quenching temperature Nitride Phosphors higher charge density between

24 Advantages over oxides and sulfides highly condensed anionic networks high density high chemical stability high hardness high quenching temperature Nitride Phosphors higher charge density between activator and nitride ligands: oxides < oxynitrides < nitrides < nitridocarbides large red-shift of emission band Si X = O 2- X = N 3- X = C 4- r [pm] Electronegativity χ Ionic bonding Si-X [%]

25 Nitride Phosphors Several efficient phosphors have been recently developed for LED purposes (Ca,Sr,Ba) 2 Si 5 N 8 :Eu 2+ W.S. Schnick et al. λ em = nm YSiO 2 N:Tb 3+ λ em = 545 nm Y 2 Si 3 O 3 N 4 :Tb 3+ Gd 2 Si 3 O 3 N 4 :Tb 3+ B. Hintzen et al. but low absorption at 450 nm Green Nitride:Eu 2+ P.J. Schmidt et al. SrSiAl 2 O 3 N:Eu 2+ Osram λ em = nm λ max = 480 nm Nichia introduced first white LED comprising a nitride phosphor into the market, but due to low T-stability only low-power versions are sold (< 1 W) -25-

26 Optical Properties of (Ca,Sr,Ba) 2 Si 5 N 8 :Eu Reflection Spectra Emission Spectra Reflection [%] Sr 2 SiO 4 :Eu Intensity [counts] Ba-Eu solid solution: Ba 2-x Eu x Si 5 N 8 Ex-slit 3.0 nm; Em-slit 1.5 nm colour point Excitation by 523 nm x y Sample 29; x = 0, Sample 49; x = 0, Sample 48; x = 0, Sample 36; x = 0, Sample 47; x = 0, Sample 56; x = 0, Sample 40; x = 0, Sample 41; x = 1, Sample 43; x = 1, Sample 45; x = 1, Ba258-2 (x ~ 0.02) Wavelength [nm] Sr 2 Si 5 N 8 :Eu Wavelength [nm] Composition Body color Emission band Stability Sr 2 SiO 4 :Eu yellow 575 nm decomposes in H 2 O Ba 2 Si 5 N 8 :Eu orange 580 nm decomposes in conc. acids Sr 2 Si 5 N 8 :Eu orange-red 615 nm decomposes in conc. acids -26-

27 Thermal Quenching of Sr 2 Si 5 N 8 :Eu Emission spectra Thermal quenching Emission intensity [a.u.] T25 T75 T100 T150 T200 T250 T Wavelength [nm] Normalised emission intensity 1,0 0,8 0,6 0,4 0,2 0,0 Model: Boltzmann-Sigmoidal Chi^2/DoF = R^2 = A ± A2 0 ±0 x ± dx ± Peak intensity Integral y = A2 + (A1-A2)/(1 + exp((x-x0)/dx)) Temperature [ C] High absorption strength between 200 and 500 nm Quantum efficiency > 90% under 450 nm excitation Thermal quenching TQ 50% (450 nm excitation) Sr 2 Si 5 N 8 :Eu 340 C SrS:Eu 300 C -27-

28 Application of pcleds - Illumination Presently achieved performance of white LEDs 1. Standard white LEDs: (Y,Gd)AG:Ce T c = 5500 K CRI ~ lm/w opt. 30 lm/w el. 2. Warm white LEDs: YAG:Ce + CaS:Eu T c = 3200 K CRI > lm/w opt lm/w el. 3. Trichromatic white LEDs: Green nitride + (Ca,Sr,Ba) 2 Si 5 N 8 :Eu T c = K CRI ~ lm/w opt lm/w el. to address main concerns of LEDs lumen package lifetime price per lumen colour homogeneity (has been solved) -28-

29 Application of pcleds - Illumination Flash lights Light tiles Head lamps Spot lighting Architectural lighting Contour lighting Desktop lamps Automotive lighting Backlighting (General lighting) (Street lighting) Advantages long lifetime easily dimmable slim profile little T-dependence fast switching speed low voltage arbitrary color (temperature) robustness.. pcleds provide new opportunities for many illumination applications -29-

Application of pcleds - Color on Demand Blue LED (420 480 nm) + single phosphor conversion layer Examples Magenta: Blue LED + red phosphor Cyan: Blue LED +

30 Application of pcleds - Color on Demand Blue LED ( nm) + single phosphor conversion layer Examples Magenta: Blue LED + red phosphor Cyan: Blue LED + green phosphor Application in corporate logos signals mood lighting advertisement lighting LEDs can provide almost all colors on demand without colour filters -30-

31 Conclusions Further increase in LED energy efficiency ( 100 lm/w) and CRI ( 90-95) expected this decade Next generation high color quality white LEDs will use Eu 2+ activated luminescent materials Stable red-emitting phosphor has been found, e.g. CaS:Eu or Sr 2 Si 5 N 8 :Eu Further increase in power and lumen package per LED 0.1 W (1970) 1 W (2000) 5 W (2002) 20 W (2007) Novel phosphors with a high stability and strong absorption are urgently required green and yellow nitrides/oxynitrides Besides warm white, red and amber highly wanted for new applications (automotive, colour on demand, displays) Wide range of new nitride/oxynitride based color converters will be developed in the near future Phosphor converted LEDs will widely penetrate the lighting market! -31-

32 Acknowledgement PFL Aachen IPM Maarheeze Lumileds San Jose Volker Bachmann Jan Broere Mike Krames Wolfgang Busselt Rene Hochstenbach Gerd Mueller Petra Huppertz Jan Migchels Regina Mueller-Mach Walter Mayr Wouter Schrama Jörg Meyer Dick vd Voort LMU Munich Cees Ronda Henning Höppe Peter J. Schmidt Wolfgang Schnick Dominik Uhlich Florian Stadler Dieter Wädow Detlef U. Wiechert and support from many others

Anorganische Leuchtstoffe

Anorganische Leuchtstoffe - Schlüsselmaterialien für hocheffiziente Lichtquellen - tj@fh-muenster.de thomas.juestel@philips.com 15. März 2005-1- Inorganic luminescent materials Application areas Performance