Microsoft PowerPoint - MRS Fall 2006

MRS, November 16, 2006Peter Peumans ([email protected])

Efficient Organic Light-Emitting Diode Using Tapered-

Periodic and Aperiodic Dielectric Mirrors

Mukul Agrawal1, Yiru Sun2, Stephen. R. Forrest3 and Peter Peumans1*

1Electrical Engineering, Stanford University2Electrical Engineering, Princeton University

3Electrical Engineering and Computer Science, University of Michigan

MRS Fall Meeting, Nov 28, 2006

*[email protected]

Sponsors: SRC and UDC


Q r out PLη χη η φ=

• φPL: Photoluminescent quantum efficiency ~100%• ηr: charge recombination efficiency ~100%• ηout: outcoupling efficiency ~25%• χ: fraction of molecular excitations that are luminescent ~100%

ηQ= the number of photons emitted per

electron injected

OLED Quantum Efficiency


ITOglass

organiccathode

TIR

ESC

WG

TE, T

M m

odes

Hyb

rid m

odes

escape cone

Ray PictureModes confined by TIR, metal reflection

Wave PictureThree types of guided modes

Surface Plasmon ModesWaveguided modes (TE, TM or Hybrid)Substrate mode

OLED Outcoupling

SP m

odes


Angular Distribution Spectral Distribution

A randomly polarized dipole in standard OLED structure

10-0.8 10-0.6 10-0.4 10-0.2 100 100.2

100

101

Normalized kxy

Nor

mal

ized

Fra

ctio

n Po

wer

Air Modes

Sub Modes W/G Modes

SP Modes

Omni-DirectionalBroadBandResonance

Omni-DirectionalBroadBandAnti-Resonance

Angular and Spectral Power Distribution in A Typical OLED

Custom Tailored Microcavity?Can we design an optical structure that has omni-directional

AND broadband resonances AND anti-resonancessimultaneously?

G. W. Ford and W. H. Weber, Phys. Rep. 113, 195 (1984).R. R. Chance, A. Prock and R. Silbey, Adv. Chem. Phys. 37, 1-65

(1978)

600 700

0.4

0.8

1.6

1.8x 10-14

400 5000

1.2

Wavelength (nm)

~ 70nm

SP Modes (~19%)

W/G Modes(~35%)

Air Modes (~ 25%)N

orm

aliz

ed F

ract

ion

Pow

er

Broadband Emitter

Wide Angle CouplingAl

LiF (0.8 nm)BCP (40 nm)

Irppy3:TCTA (25 nm)NPD (40 nm)ITO (100 nm)

Glass

Sub Modes(~21%)

λ=550nmλ=520nmλ=490nm

0O 90O

kz

kxy

kAngle of mode in organic medium


How to Solve Outcoupling Problem –Existing Schemes

Periodic Bragg Reflectors (DBRs)Narrow-band emitters, narrow angle applications

Periodic Bragg GratingsMostly scatters out one of the modes – SPP mode

Surface PatterningUseful for extracting light from substrate modes

Microlens ArraysUseful for extracting light from substrate modes

Lee et. al., Appl. Phys. Lett. 82, 3779 (2003)

Lupton et. al., Appl. Phys. Lett. 77, 3340 (2000)

Carefully designed non-periodic gratings or mirrors?


Efficient OLEDs Amenable to Large Scale Integration

Scheme: 1D non-periodic photonic structures

Glass

ITOHTL

ETL

Al

EL Region R1

R2

Emissive layer

Exciton-optical vacuum mode interaction controls both

emission rate

mode distribution

Vacuum mode profile is controlled by the phase and amplitude of R1 and R2

A method to tune local photon density of states (LPDOS)

Non-PeriodicDielectric Mirror

Substrate mirror

Cathode mirror


Optimization Over a Very Complex Landscape

Extremely large number of local optima

Example problem – only two layersComplexity increasesexponentially with more layers

How do you optimize such a structure?

For passive optical filterdesigns, algorithms exist

Needle OptimizationAdapt it for active

OLED optimization


What Exactly Do We Want to Optimize?

Display ApplicationsAngular brightness

uniformity is crucial

Color saturation and angular uniformity is important

Brightness – viewing angle trade-off >100% effectivequantum efficiency is possible

Lighting ApplicationsPrimary concern: power conversion efficiencyAngular color and brightness uniformity: desirable

but may not be crucial. External focusing elements are common in these applications.


Experimental: TEM Micrograph

50nm Al95nm organic

150nm ITO

glass

SiNxSiO2

129

…SiNx

Glass/Stack/150nm ITO/30nm NPD/25nm CBP: Ir(piq)3 (7%)40nm BPhen0.8nm LiF/50nm Al


1 2 3 4 5 6 7 8 90

20

40

60

80

100

120

Layer Number

Thic

knes

s (n

m)

Periodic ThicknessesCathode

Organic layersTransparent Anode

Substrate

Dielectric Stack

0

30

60

90

Control (Measured)

Periodic (Theory)Tapered periodic (Measured)

Lambertian (Theory)

x1.6

•OLED with aperiodic stack appears 1.6 times as bright•Is Lambertian within viewing cone

Experimental: Red phosphorescent OLED on 9 layer SiNx/SiO2 on glass

41% of photons emit within ±30o cone

Lambertian: 25% of photons emit within ±30o

cone


550 600 650 700 750 550 600 650 700 750

Wavelength (nm)

Emitt

ed P

ower

Den

sity

[arb

s.]

0°10°20°30°

0°10°20°30°

0°

10°

20°

30°

Non-periodic SiO2/SiNx

Control

Periodic SiO2/SiNx

Aperiodic vs. Periodic


450 500 550 600 650 700 750 800 850 900

0.0

0.2

0.4

0.6

0.8

1.0

Inte

grat

ed E

L (a

.u.)

Wavelength (nm)

Control Device: Theory With Aperiodic Stack: Theory Control Device: Measured With Aperiodic Stack: Measured

Integrated EL Spectrum•Angularly integrated EL spectrum: theory and experiment match•Dielectric stack also “purifies” the spectrum


Overall (Full Half Space) Efficiency Results

1E-3 0.01 0.1 1

0123456789

10

EQ

E (%

)

J (A/cm2)

Full Half Space With Aperiodic StacksControl Device

1E-4 1E-3 0.01 0.1 1

0

1

2

3

4

5

6

7

8

J (A/cm2)

PE

(lm

/W)

Full Half Space With Aperiodic Stacks Control Device

Measurement:EQE improvement in full cone: Unchanged PE improvement in full cone: 13.2%

Theory:EQE improvement in full cone: 5% PE improvement in full cone: 15%


1E-3 0.01 0.1 1-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

EQE

(%)

J (A/cm2)

Within +-30o cone With Aperiodic Stacks Control Device

1E-3 0.01 0.1 1

0

1

2

3

4

J (A/cm2)

PE

(lm

/W)

Within +-30o Cone With Aperiodic Stacks Control Device

Measurement:EQE improvement in ±30o cone: (58±3)% PE improvement in ±30o cone: (81±3)%

Theory:EQE improvement in ±30o cone: 61% PE improvement in ±30 cone: 82%

Efficiency Improvement in ±30 Degree Viewing Cone

Even higher with higher index contrast dielectrics


Display Trade Off – Emit Where RequiredLambertian emission profile

within specified coneEffectively OLED appears

to be 2.5 times brighterDark outside cone

30

6090

60

0 0normalized power per unit solid angle

Optimized OLED+Stack: 63% effective outcoupling

Standard OLED: 25% outcouplingLambertian

100nm Al100nm organicEmissive

layer100nm ITO

glass

SnS2MgF2

12

14…

MgF2

30


Achievable Efficiencies With Display Quality Brightness Uniformity

a

b

cd

e

db

SnS2/MgF2stacks

0 10 20 30 40 50 60 70 80 900.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8Overall outcoupling efficiencyApparent outcoupling efficiency

Effi

cien

cy

Half Angle Lambertian Emission Cone

Trade-offHigh brightness vs. large viewing angle

Standard OLED


ConclusionsNovel OLED Photon Coupling Scheme• Non-periodic dielectric structure optimally tune the

LPDOS • Possible to guide light where required • Easy to integrate with existing OLED structures• No new technology required• Can complement other schemes• n2/n1↑ high performance (e.g. 2.5x for ±30deg)

Future work• Simultaneous optimization for CIE coordinate • Extension to 2D and 3D schemes• Metallodielectrics

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