Oleg Shchekin Device Architecture LUMILEDS, San …...©2015 Lumileds Holding B.V. | February 8,...

LED technology considerations for high luminance sources

Oleg ShchekinDevice ArchitectureLUMILEDS, San Jose CA, USA

DOE SSL R&D WorkshopFebruary 2, 2017

©2015 Lumileds Holding B.V. | 2February 8, 2017

Tremendous progress in LED efficiency


Dominant high efficiency architectures

Mid-Power LEDs

High-Power LEDs

Die

Phosphor fill - High extraction efficiency die

- Low operating power densities allow for high

epi IQE

- Large highly reflective cup reduces optical

losses

- Large volume of phosphor relative to die area

reduces irradiance levels: lower

photoquenching and high package efficiency

- High extraction Thin-Film or Flip-Chip die

- Highly reflective and thermally conductive

submount

- Flip-Chip die allows for reduction in photo-

thermal quenching of phosphors

- Die footprint 2mm2 or larger to keep epi IQE

high

- Large silicone dome to aid photon extraction

Phosphor

n-GaNp-GaN

Sapphire

Tile or interposer-- +


LEDs for low-drive vs. high-drive applications

100

120

140

160

180

200

220

240

0 10 20 30 40 50 60 70

Eff

icacy (

lm/W

)

Current density (A/cm2)

MP production 4000K/80

MP lab 4000K/80

HP production 4000K/70

HP lab 4000K/70

Typical operating

range for low-

drive applications

(e.g. indoor linear)

Typical operating range

for high-drive applications

(e.g. streetlight)

At 25 °C junction

temperature


Etendue and brightness limitations of dominant high efficiency architectures

Etendue G = n2∙A∙Ω, Where A is the area of the emitting source

Ω is the emission solid angle

n is the index of refraction

Etendue can only increase in an optical system

(optical equivalent of Entropy)

n

LED

I(θ)

2θ1/2

ALED

Alens

ΩLED Ωlens

Die

Phosphor fill

Phosphor

n-GaNp-GaN

Sapphire


n~ 1.5

- Large source area

- increases etendue

- Input power limited by:

- die design

- die attach

- Common die footprint 1-2mm2 or larger

- increases etendue

- Silicone dome

- increases etendue

- Multi-side emitters: larger solid angle

- increases etendue


Versatility of low etendue, high brightness sources

0

20000

40000

60000

80000

100000

-20 -15 -10 -5 0 5 10 15 20

cd/k

lm

degrees

0.5mm21mm22mm24mm28mm2

8° FWHM TIR optic

for a 2 mm2 die 10.2 mm

8.0 mm

Beam profiles for various die areas using the same optic

Smaller source size will reduce FWHM and increase punch

20 mm

8° beam example Die area (mm2)

0.5 1 2 4 8

FWHM (°) 4.0 5.7 8.3 12.3 17.1

factor from 2mm2 0.5 0.7 1.0 1.5 2.1

Punch (cd/lm) 105.1 58.4 30.8 15.8 8.0

factor from 2mm2 3.4 1.9 1.0 0.5 0.3

Still using an 8° FWHM TIR

optic, going to a 0.5 mm2 die,

reduces x, y and z by a factor

of 2: 8x system volume

reduction

5.1 mm

4 mm

10 mm

-We illustrate system level impact of source

etendue by varying source area

-Lower source etendue allows greater

freedom to optimize for light utilization and

system size

Light

utilization

System size


High-luminance LED architectures

• Key features of low-etendue, high-luminance architectures:

– Small source size

– High current density die

– Low thermal resistance

– Proximity “on-chip” phosphor

– Single-sided emitter (side-coated phosphor and die as needed)

– No dome

• Challenges:

– Lower optical Package Efficiency due to absence of dome and the addition of side-coat

– With current densities above 35A/cm2 need to consider:

- impact of EPI IQE droop

- impact phosphor photo-thermal quenching

Phosphor

GaN

Sapphire

Tile or interposer

-- +

Chip-Scale Package (CSP)

Phosphor

AlSi or Si or Ge

GaN

Tile or interposer

+--

Vertical Thin-Film (VTF)

Phosphor

GaN

Tile or interposer

-- +

Thin-Film Flip-chip (TFFC)


Package efficiency penalty for high luminance architectures

• Reduced extraction with removal of domes

• Reduced extraction of photons with side-coat

• Due to finite thickness of converters need side-coat even with Thin-Film architectures

• Reduced phosphor heat dissipation and higher photo-quenching compared to multi-side emitters

Key focus area: optical absorption in the pump chip

Phosphor

n-GaNp-GaN

Sapphire



Phosphor droop in LEDs

• For this example of a typical warm white pcLED,

– the PCE relative drop is ~4% for each doubling of blue light power density

– This drop accounts for 20-25% of the total white LED efficiency droop with drive

0%

5%

10%

15%

20%

25%

30%

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0

Frac

tio

n o

f th

e t

ota

l dro

op

du

e t

o C

E

No

rmal

ize

d E

QE

Current (A/mm2)

85C pulsed

Warm White Thin-Film

pcLED

Underlying

Blue Thin-Film

Pump LED


Photo-thermal quenching of phosphors in LEDs: impact on conversion efficiency• Photo-quenching in Eu2+ red nitrides shows

strong dependence on temperature

• Cerium-doped aluminum garnets show little dependence of photo-quenching on temperature, but, depending on composition, may exhibit considerable thermal quenching

• Photo-thermal quenching of phosphors readily translates into Conversion Efficiency (CE) quenching in pcLEDs

• At typical operating conditions of a HP LED Tphosphor>100C, irradiance 0.7 – 1.5 W/mm2

Temperature

irradiance

WW High-Power LED example

25C

85C

125C

(Ba,Sr)2Si5N8: Eu

A Ce-doped

Aluminum Garnet

25C

85C

150C

25C85C

150C


Dependence of phosphor droop on temperature: Photo-Thermal Quenching

(Ba,Sr)2Si5N8: Eu powder phosphor doped with 2.5% Eu2+ in a film of silicone

• Photo-quenching in Eu2+ red nitrides shows strong dependence on temperature

• Thermal quenching is rather low at low excitation where QE measurements are usually done and quoted

• Thermal and photo effects on QE need to be considered in LEDs20406080100120140160

0

0.5

1

1.5

2

70

75

80

85

90

95

100

Temp (degC)Fluence (W/mm2)

QE

(%

)Q

uantu

m E

ffic

iency (

%)

(Ba,Sr)2Si5N8:Eu


0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10 100

Qu

antu

m E

ffic

ien

cy (

%)

blue light irradiance (W/mm2)

Quantum Efficiency vs excitation for 23% vol (Ba,Sr)2Si5N8: Eu in silicone film with 3.2% Eu2+ concentration

150C

85C

25C

25C fit

85C fit

150C fit

Example of phosphor QE in different applications

MP

LEDsHP

LEDsLasers


Droop dependence on activator concentration

• Photo-quenching in Eu2+ red nitrides shows strong dependence on activator concentration

• Red nitride phosphors with higher Eu2+

concentration are used for high CRI applications

• Conversion efficiency droop due to PTQ is most pronounced for warm white, high Ra emitters

75

80

85

90

95

100

0.0 0.5 1.0 1.5

Qu

antu

m E

ffic

ien

cy (

%)

blue light irradiance (W/mm^2)

Integral Quantum Efficiency vs excitation for 16% vol (Ba,Sr)2Si5N8: Eu in silicone film with

varying Eu 2+ concentrations

3.2% Eu

2.5% Eu

1% Eu

85 C

Adapted from: Oleg B. Shchekin

et al, Phys. Status Solidi RRL, 1–

5 (2016) / DOI

10.1002/pssr.201600006


Converter materials for high-power density operation

• Keep activator concentration in the phosphor low to minimize PTQ

• Maximize heat conductivity through the converting layer and out

• Low activator concentration in powder phosphors leads to increased phosphor layer thickness or loading to achieve a target color point. This results in:

– Efficiency penalty due to excessive scattering

– Efficiency penalty due to poor thermals of the thicker layer

• In a ceramic phosphor, scattering can be tightly controlled to allow for thicker layers enabling lower activator concentration

• High thermal conductivity of ceramic allows for flexibility in phosphor thickness

• Ceramic phosphors are well suited for high power density applications

Lumileds Lumiramic

(ceramic phosphor)

technology examples in high

power density applications

LUXEON F PC Amber

LUXEON Altilon H1K


• Other than QDs, don’t yet have a phosphor material allowing full freedom in spectral engineering

Gains in conversion efficiency from phosphor emission

linewidth

CE vs phosphor FWHM with optimum

peak wavelength; 3000K, 80 CRI

800

1300

1800

2300

600 620 640 660 680FW

HM

[cm

-1]

peak emission [nm]

red nitrides in

the market

SLA

KSiF


– Don’t have materials fulfilling all the requirements; fundamental materials development needed

– Focus needed on PTQ and QE at operating conditions in addition to spectral characteristics

Status of red phosphors for high luminance

applications

FWHM WL

QE low

drive

QE high

drive

Eu2+ 258

and SCASN

nitrides

SLA

KSIF

QDs


Summary

• High luminance LEDs can enable significant value add from system form factor, weight and cost reductions.

• Developing efficient high-luminance LEDs requires improvement in

– epi droop

– die design for high power densities

– Die and package technologies for high photon extraction

– Low droop phosphors for WW and high Ra

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