the blue death, or how long can pholeds last?
TRANSCRIPT
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
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
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).
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
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
)
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
- +
- +
- +
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
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
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
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
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
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
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
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
Thanks!
Yifan Zhang
Jaesang Lee
The Evolution of the Blue PHOLED