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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
Sponsors: SRC and UDC
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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?
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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.
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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%
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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
MRS, November 16, 2006Peter Peumans ([email protected])
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