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