the blue death, or how long can pholeds last?
TRANSCRIPT
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The Blue Death, Or How Long Can PHOLEDs Last?
Stephen Forrest
Departments of Physics, EECS and Materials Science and Engineering
University of Michigan
Ann Arbor, MI 48109
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Efficiency and Operational Lifetime of OLEDs & PHOLEDs
Phosphorescent dopants Fluorescent dopants
Color CIE LE (cd/A) t50 (hrs)
Red [0.64, 0.36] 30 900,000
Green [0.31, 0.63] 85 400,000
Blue [0.14, 0.12] High 10’s
Color CIE LE (cd/A) t50 (hrs)
Red [0.67, 0.33] 11 160,000
Green [0.31, 0.63] 37 200,000
Blue [0.14, 0.12] 9.9 11,000
Universal Display Corp. Idemitsu Kosan
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3
HTL ETL EML
[mCP/ 9wt% FIr6]
drec
x2 x3 x x1
FnE
FvE
HOMOhost
LUMOhost
tE
Guest
Defect J
qJ
q
holes electrons
Intrinsic Lifetime of OLEDs
N. Giebink, et al., J. Appl. Phys., 103, 044509 (2008).
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Exciton-Polaron Annihilation
1 1/S T
0S
* */n nS T
energy
transfer
Exciton-Exciton Annihilation
1 2
R
1 1/S T
0S
E
r
0D
*
nD
energy
transfer
2
0S
1 1/S TR
Degradation Routes
• Energetically Driven - Lifetime: R>G>B
• Two particle interactions lead to luminance loss
-Exciton on phosphor, polaron on host - Exciton-exciton also possible
Triplet energy (~2.8 eV) + polaron (~3.3 eV) = hot polaron (≥ 6 eV)
Bond cleavage
Broken bonds? Defects!
Bond BE(eV) Bond BE(eV)
C-C 3.64 N-N 1.69
C-H 4.28 N-O 2.08
C-O 3.71 N-H 4.05
C-N 3.04 O-O 1.51
C-F 5.03 H-H 4.52
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0.0
0.2
0.4
0.6
0.8
1.0
Lum
inance
(norm
)
5 10-1
100
101
102
103
0.0
0.2
0.4
0.6
0.8
1.0
t' (hrs)
Lum
inance (
norm
)
L0 = 1000cd/m2
L0 = 4000cd/m2
L0 = 1000cd/m2
L0 = 4000cd/m2
Exciton-Exciton Annihilation Exciton-Polaron Annihilation
Luminance Decay vs Time
•Blue PHOLEDs
•Prepared and packaged using industry std.
•Q~1018 cm-3 50% increase in quenching
•At 1000 cd/m2, formation rate = 1012cm-2s-1
-1 in 5 x108 E-P encounters leads to defect
-Increasing recombination zone width
extends lifetime
- Guest triplets/host polarons most active
10-1
100
101
102
103
0.0
0.2
0.4
0.6
0.8
1.0
t' (hrs)
Lu
min
an
ce
(n
orm
)
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WOLED vs. SOLED Lifetime Comparison
Panel 15 cm x 15 cm 82% fill factor
Single Unit
WOLED*
2 Unit
WSOLED
Luminance
[cd/m2] 3,000 3,000
Efficacy [lm/W] 49 48
CRI 83 86
Luminous
Emittance
[lm/m2]
7,740 7,740
Voltage [V] 4.3 7.4
1931 CIE (0.471,
0.413)
(0.454,
0.426)
Duv 0.000 0.006
CCT [K] 2,580 2,908
Temperature [oC] 27.2 26.2
LT70 [hrs] 4,000 13,000
P. Levermore et al, SID Digest, 2011
WOLED SOLED
SOLED : ~ 3x LT70 improvement vs. single unit
WOLED with similar color and power efficacy
charge generation layer
Io , Vo
Lo
cathode
ITO organics -
+
3xLo
Io , 3xVo
CGL
- +
- +
- +
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dopant
host
Conce
ntr
ation
Position
Conventional Graded
Exciton density
LUMO
HOMO
Emission
+
-
+
-
Y. Zhang, et al., Nature Comm. 5 5008 (2014)
Distributing Excitons to Increase Lifetime
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mCBP mCBP mCBP mCBP
Alq3 Alq3 Alq3 Alq3
Li:Alq3
mCBP
Alq3
HATCN HATCN HATCN
HATCN
HATCN
NPD
mCBP
Alq3
Li:Alq3
mCBP
Alq3
HATCN
HATCN
NPD
NPD
Host: mCBP Dopant: Ir(dmp)3
13 vol% uniform 8 to 18% vol% graded
D1 D2 D3 D1S D3S
a
b
Spreading the recombination zone: Dopant/Host Grading
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5.0eV
6.0eV
h
e Ir (dmp)3
mCBP
e
h
0 10 20 30 40 50 600.00
0.01
0.02
0.03
0.04
0.05EML of D1
Exciton d
ensity (
arb
. units)
Distance to anode (nm)
D1
D2
D3
D1S
D3S
EMLs of D2, D3 and D4
Grading Reduces Exciton Pile-UP
Exciton Sensing • Red Phosphor • 1.5 nm thick • Placed at 5 nm
intervals in EML • Measure red
emission intensity
Dopant conducts holes
Host conducts electrons
ETL
HTL
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Fig.2 (a) (b)
a
b
0 5 10 15 2010
-3
10-2
10-1
100
101
102
D1 D2 D3 D1S D3S
Voltage (V)
Curr
ent D
ensi
ty (
mA
/cm
2)
0
1000
2000
3000
4000
5000
Lum
inance
(cd/m
2)
0.1 1 10 1000
5
10
15
20
EQ
E (
%)
Current Density (mA/cm2)
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm) Inte
nsi
ty (
norm
.)
Fig.2 (a) (b)
a
b
0 5 10 15 2010
-3
10-2
10-1
100
101
102
D1 D2 D3 D1S D3S
Voltage (V)
Curr
ent D
ensi
ty (
mA
/cm
2)
0
1000
2000
3000
4000
5000
Lum
inance
(cd/m
2)
0.1 1 10 1000
5
10
15
20
EQ
E (
%)
Current Density (mA/cm2)
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
Inte
nsi
ty (
norm
.)
Performance Improves with Grading
D1= Conventional/Uniform doping D2= No HTL/Uniform doping D3= Graded doping DxS=Stacked
CIE=[0.15, 0.29]
EQE w. grading
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Y. Zhang, et al., Nature Communications, 5 5008 (2014)
10 X Lifetime Improvement Over Conventional
0.5
0.6
0.7
0.8
0.9
1.0
0.5
0.6
0.7
0.8
0.9
1.0
0 150 300 450 6000.0
0.5
1.0
1.5
2.0
0 150 300 450 6000.0
0.3
0.6
0.9
1.2
D 1
D 2
D 3
D 1S
D 3S
T P A model
E X P m odel
Lu
min
an
ce
(n
orm
.)
Lu
min
an
ce
(n
orm
.)
DV
(V
)
T im e (hrs )
DV
(V
)
T im e (hrs )
10X
1000 cd/m2
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Dopant Grading for Lighting: Is it OK?
• Current state of stacked WOLED: T70=13,000 hrs
• Mostly limited by blue lifetime
• Only light blue required
• Estimated increase in lifetime for stacked blue at
lighting brightnesses: ~4X
• Lifetime of blue lighting using grading: 50,000 hr
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What about TADF?
Uoyama, Adachi, et al., Nature 492, 234–238 2012
• Broad spectra • Long lived triplets: 2-20µs • Excitations maintained in triplet manifold • Identical degradation mechanism to long-lived
blue PHOLEDs Can benefit from same solutions
TADF sensitized fluorescence • Identical concept to phosphor sensitized
fluorescence (Baldo, Thompson, Forrest, 2000) • Narrow spectra • Long lived triplets: 1-20µs • Excitations maintained in triplet manifold • Need UV sensitizer to access blue fluorescence
dopant energies Degradation too rapid to be practical for
lighting applications
Nakanotani, Adachi, et al., Nature Comms. 2014
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Conclusions
Under practical modes of operation, there appears to be no fundamental reason why organics should be less (or more) reliable than inorganic semiconductors
• Excitons responsible for ~100% efficiency of phosphorescent OLEDs and for the photodetection in OPVs, but…
• Excitons responsible for molecular decomposition due to local energy dissipation
• YOU CAN HAVE IT BOTH WAYS: High efficiency AND long lifetime through exciton management.
• And it’s also about purity, purity, purity
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Thanks!
Yifan Zhang
Jaesang Lee
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The Evolution of the Blue PHOLED