Week 10 - Organic Electronics: Foundations to Applications

Organic ElectronicsStephen R. Forrest

Week 10Light emitters 3

Outcoupling StrategiesReliability

Chapter 6.6.2-6.7

1


Substrate Mode Outcoupling: ~2X Improvement

ηext~40%

Microlens arrays: Polymer hemispheres much smaller than pixel

Möller, S. & Forrest, S. R. 2001. J. Appl. Phys., 91, 3324.

Fabrication sequence Spectrum angle independent

⇐ Scattering and surface roughness also can reduce substrate modes

Reidel, et al., Opt. Express 18 A631 (2010) Chang, et al., J. Appl. Phys., 113 204502 (2013) 2


3

Metal electrode pixel

Organics

Low-index grid

ITO

Glass substrate

Ray BRay A

Ray

C

Side view

wLIG

worg

Waveguide Mode Outcoupling:Embedded Low Index Grid

ηext~60% (incl. substrate modes)

Sun, Y. & Forrest, S. R. 2008. Nature Photon., 2, 483.


4

2µm

Low Index Grid Images

• OLED >> Grid size >> Wavelength• Embedded into OLED structure• May partially decouple waveguide mode from SPPs


5

400 600 8000.0

0.5

1.0

Nor

m. E

L (a

.u.)

Wavelength (nm)

Dev. 1 Dev. 4

10-6 1x10-5 1x10-4 10-3 10-2 10-1

0

10

20

30

Pow

er E

ffici

ency

(lm

/W)

Exte

rnal

Qua

ntum

Effi

cien

cy (%

)

Current Density (mA/cm2)

Dev. 4 Dev. 3 Dev. 2 Dev. 1

0

10

20

30

40

50

60

70

80

Dev. 1 x 1.32

Dev.1x1.68

Dev. 1 x 2.3

10-3 10-2 10-1 100 101 10210-3 10-2 10-1 100 101 102

Hybrid WOLED Performance Using Embedded Grids + Microlens Arrays

Device 1: ConventionalDevice 2: LIG onlyDevice 3: Microlenses onlyDevice 4: LIG + Microlenses

Method is WavelengthIndependent

Sun, Y. & Forrest, S. R. 2008. Nature Photon., 2, 483.


A better approach: Sub-Anode Grid

qA multi-wavelength scale dielectric grid between glass and transparent anode (sub-anode grid)

qThe grid is removed from the OLED active region

qWaveguided light is scattered into substrate and air modes

6

Cathode

nglass=1.5

nhost

norg=1.7, nITO=1.8ngridngrid

waveguided power + dissipation

Collect substrate mode power

Qu,et al., Nature Photonics (2015), 9, 758


Emission field calculations

7

WITH GRID

WITHOUT GRID

VERTICAL DIPOLE

HORIZONTAL DIPOLE

Mostly in-plane(Waveguided)

Mostly out-of-plane(Glass + air)

Qu,et al., Nature Photonics (2015), 9, 758


Optical Power Distribution

Thick-ETL organic structure:

340nm grid/70nm ITO/2nm MoO3/40nm TcTa/15nm CBP: Ir(ppy)3/10nm TPBi/230nm Bphen:Li/Al

2nm MoO3/40nm CBP/15nm CBP:Ir(ppy)3/xnm TPBi/1nm LiF/Al

Qu,et al., Nature Photonics (2015), 9, 758 8


Al OrganicITO

SEMLA

High IndexSpacer layer

Getting All the Light Out: Sub-Electrode Microlens Array (SEMLA)

Qu, Y., et al. 2018. ACS Photonics, 5, 2453. 9


0 20 40 60 80 100 120 1400

20

40

60

80

100

Frac

tion

of P

ower

(%)

ETL thickness (nm)

SPPLoss

WV

SEMLA

0 20 40 60 80 100 120 1400

20

40

60

80

100

Frac

tion

of P

ower

(%)

ETL thickness (nm)

SPP

WV

SubAir

Loss

SEMLAs Change the Outcoupling Landscape

Qu, Y., et al. 2018. ACS Photonics, 5, 2453.10


0 5 1010-7

10-3

101

Con SEM LA

J (

mA

/cm2 )

Vo ltage (V )

102 103 1040

20

40

60

80 Con_air Con_IMF SEMLA_air SEMLA_MLA SEMLA_IMF Sap_IMF

h EQE (%

)

Brightness (cd/m2)

-90

-60

-300

30

60

90 L am b. C on S E M L A M LA S E M LA H S

SEMLA

SEMLA

+MLA

SEMLA

+IMF

Sap1.0

1.5

2.0

2.5

3.0

3.541±3 %45 ±4%

2 7±3 %2 0±2 % 60 ±4%

65 ±5%

47 ±4%

E F

30±3 %

SEMLA Performance

Qu, Y., et al. 2018. ACS Photonics, 5, 2453. 11


Diffuse Reflectors: Low Cost & Simple

Teflon is the best diffuse dielectric reflector

Diffuse (Green) Mirror (Green) Diffuse (White) Mirror (White)

0.01 0.1 1 100

10

20

30

40

50

h EQE

(%)


PHOLED Active Area

Kim, J., et al. (2018), ACS Photonics, 5, 3315. 12


Pt(dbq)(acac)

Isotropic Orientation

HorizontalOrientation

Outcoupling Enhancements by Molecular Orientation

Ir(ppy)3

Prevents coupling to SPPs and waveguide modes

Planar molecules (e.g. Pt-complexes) more likely to align than octahedral (tris-Ir complexes)13


Example results

0 10 20 30 40 50 60 70 80 900.0

0.5

1.0

1.5

2.0

2.5

Detector Angle [Degree]N

orm

aliz

ed In

tens

ity

Isotropic (qhor= 66.7%) Horizontal (qhor= 100%) Fit (qhor= 45.9%) CBP:Pt(dbq)(dpm) 8wt%

Pt(dbq)(dpm)

0 10 20 30 40 50 60 70 80 900.0

0.5

1.0

1.5

2.0

Detector Angle [Degree]

Nor

mal

ized

Inte

nsity

Isotropic (qhor= 66.7%) Horizontal (qhor= 100%) Fit (qhor= 65.9%) CBP:Irppy3 8wt%

Ir(ppy)3

Approach challenges• Added constraints on molecular design• Added constraints on process (growth) conditions: may not align as expected• Added constraints on device architecture• Alignment is never “perfect”: only modest improvements

Θ =

TM!

TE⊥ +TM⊥ +TM!

Ratio of light emitting by vertical to horizontal dipoles:

14


• Waveguide thickness varies due to the corrugation.

• As the thickness changes, the mode distribution changes.

• When the waveguided power travels from thin to thick areas, the k vector needs to change direction to keep “being trapped”. Otherwise, the light is extracted.

0.8 0.85 0.9 0.95 10

0.5

1

1.5

2

2.5

3

3.5 x 10-3

k||/k

powe

r int

ensit

y

10nm30nm50nm70nm90nm

TM0

TE0

ETL thickness

Waveguide

nsub/norg=0.88

W. H. Koo, et al, Nat. Photonics 2010, 4, 222.

A possible approach: Surface buckling?FFT

Al on resin

Substrate Corrugations Can Outcouple Waveguide Modes

15


Reliability Testing Methodologies• Need to set clear metrics for failure

ØExample: Operating time for initial luminance (L0) to decrease 10% from its initial value (called T90, or LT90)

ØEmploy a population of equivalent devices and monitor their performance parameter (e.g. luminance) under normal operating conditions

ØIf degradation slow, then an empirical degradation relationship is determined to extrapolate time to failure• Example: Stretched exponential function:

• If degradation too slow, need to accelerate via increased T or L0.• Accelerated conditions must not introduce new failure modes• Need empirical relations to normalize lifetime to standard operating

conditions (called acceleration factors)

𝐿 𝑡 = 𝐿!𝑒𝑥𝑝 −𝑡/𝜏 b t, b = empirical constants

LTx L0( ) = LTx L0tst( ) i L0tst

L0

⎡

⎣⎢

⎤

⎦⎥

n

n = empirical acceleration factor16

Organic ElectronicsStephen R. Forrest1/kBTEL

1/(τ

2 jα )

T

Yoshioka, et al.. 2014,SID Digest Tech. Papers, 45, 642.

Meas

1τ 2

= ′′K jα exp −ΔEA0 kBT( )

Accelerated Degradation Methodologies

Sum of lifetimes alternative empirical relation):

DEA0=thermal activation of degradationa = current acceleration factor

Measuring populations of identical devices

L t( ) L0 = λ exp −t / τ1( ) + 1− λ( )exp −t / τ 2( )Burn-in Long term decay

Example data: Green PHOLED

17


18

Intrinsic Lifetime Limits of OLEDs

Giebink, et al., J. Appl. Phys., 103, 044509 (2008).

HTL ETLEML[mCP/ 9wt% FIr6]

drec

x2x3 xx1

FnE

FvE

HOMOhost

LUMOhost

tE

Guest

DefectJq

Jq

holes electrons

Energy ScaleRed light: ~ 2 eV

Green light: ~2.3 eVBlue light: ~ 2.9 eV

Organic ElectronicsStephen R. ForrestExciton-Polaron Annihilation

1 1/S T

0S

* */n nS T

energytransfer

Exciton-Exciton Annihilation

12

R

1 1/S T

0S

E

q

0D

*nD

energytransfer

2

0S

1 1/S TR

Degradation Routes

• Energetically Driven- Lifetime: R>G>B

• Two particle interactions lead toluminance loss

-Exciton on phosphor, polaronon host

- Exciton-exciton also possible

Triplet energy (~2.8 eV) + polaron (~3.3 eV) = hot polaron (≥ 6 eV)Bond cleavage

Broken bonds? à 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

19


0.0

0.2

0.4

0.6

0.8

1.0

Lum

inan

ce (n

orm

)

2010-1 100 101 102 103

0.0

0.2

0.4

0.6

0.8

1.0

t' (hrs)

Lum

inan

ce (n

orm

)

0.0

0.2

0.4

0.6

0.8

1.0

Lum

inan

ce (n

orm

)L0 = 1000cd/m2

drec = 12nm

L0 = 4000cd/m2

drec = 5nm

L0 = 1000cd/m2

L0 = 4000cd/m2

L0 = 1000cd/m2

L0 = 4000cd/m2

ExcitonLocalization

Exciton-ExcitonAnnihilation

Exciton-PolaronAnnihilation

Luminance Decay vs Time

Defect Generation Rates

•Blue PHOLED•Prepared and packaged using industry std.


21

8

9

10

11

12

13

14

Volta

ge

10-1 100 101 102 1038

9

10

11

12

13

14

Volta

ge

t' (hrs)

8

9

10

11

12

13

14

Volta

ge

L0 = 4000cd/m2

drec = 5nm

L0 = 1000cd/m2

L0 = 4000cd/m2

L0 = 1000cd/m2, drec = 12nm

L0 = 1000cd/m2

ExcitonLocalization

Exciton-ExcitonAnnihilation

Exciton-PolaronAnnihilation

L0 = 4000cd/m2

Drive Voltage Drift with Aging

•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

Conclusions

Giebink, et al., J. Appl. Phys., 103, 044509 (2008).

Organic ElectronicsStephen R. Forrest200 400 600 800 10001200

0.00.20.4

1.40.00.20.4

1.4

In

tens

ity (a

.u.)

m/z

Unaged

T40

H1-3Alq3

mCBP Ir(dmp)3

E4 D4-7

E3

Ir(dmp)3m/z = 1198

H1m/z = 155

mCBPm/z = 484

H2m/z = 168

H4m/z = 319

D2m/z = 526

H3m/z = 243

D1m/z = 334

D4m/z = 862

D3m/z = 668

Evidence for Defect Formation: Molecular Fragmentation

Jeong, et al. Org. Electron., 64, 15 2019

Laser Desorption Ionization-Time of Flight Mass Spectroscopy(LDI-TOF-MS)

-Molecular species identification

22


D12.1

0.6

D2-11.6

-12.5

D4-5.6

-8.2

CationAnion Radical Neutral

D6

-3.2

-4.8-4.5 (HOMOIr(dmp) )

-1.15 (LUMOmCBP)

D7

-0.9

-4.6

H1

-0.7

-6.1

H1-3.9

-4.8

H1-12.7

H14.3

H2

-0.7

-5.5

H2

-0.9

-5.4H2-7.9

-9.6

H2

3.6

-0.4

H3

-0.7

-5.3

H3

-2.3

-5.4

H4

-1.0

-5.3H3-7.9

-9.6

H32.1

0.7

H4

-2.2

-5.4H4-7.9

-8.4

H41.3

0.7

Non-trappingElectron traps(electrically active)

Triplet Energy (ET)

0 eV

2.8 eV (ET, Ir(dmp) )

D6<0.6

H1<1.4

H3<2.4

H4<2.8

H13.0

H23.2

H33.2

H43.1

D72.1

3

3

2.9 eV (ET, mCBP )

Identification of Defect Energies

Jeong, et al. Org. Electron., 64, 15 2019

Non-trapping

23


dopant

host

Con

cent

ratio

n

Position

Conventional Graded

Exciton density

LUMO

HOMO

Emission+

-

+

-

Y. Zhang, et al., Nature Comm. 5 5008 (2014)

Reducing Exciton Density to Increase Lifetime

ETL

HTL


mCBP mCBP mCBP mCBP

Alq3 Alq3 Alq3 Alq3

Li:Alq3

mCBP

Alq3

HATCN HATCN HATCN

HATCN

HATCN

NPD

mCBPAlq3

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

Spreading the recombination zone: Dopant/Host Grading3 Different test device structures

25


5.0eV

6.0eVh

eIr (dmp)3

mCBP

e

h

0 10 20 30 40 50 600.00

0.01

0.02

0.03

0.04

0.05 EML of D1

Exci

ton

dens

ity (a

rb. u

nits

)

Distance to anode (nm)

D1 D2 D3 D1S D3S

EMLs of D2, D3 and D4

Excitons in the EML

Exciton Sensing• Red Phosphor (PQIr)• 1.5 nm thick• Placed at 5 nm

intervals in EML• Measure red

emission intensity

Dopant conducts holes

Host conducts electrons

Y. Zhang, et al., Nature Comm. 5 5008 (2014)ETL

HTL

sensing layer positions

26


10 X Lifetime Improvement Over Conventional

Y. Zhang, et al., Nature Comm. 5 5008 (2014)

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 ,model

,

,

Luminan

ce,(no

rm.)

,

,

Luminan

ce,(no

rm.)

,

,

ΔV,(V)

T ime ,(hrs )

,

,

ΔV,(V)

T ime ,(hrs )

10X$

1000 cd/m2

Stacked/Graded

Graded

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 ,model

,

,

Luminan

ce,(no

rm.)

,

,

Luminan

ce,(no

rm.)

,

,

ΔV,(V)

T ime ,(hrs ),

,

ΔV,(V)

T ime ,(hrs )

10X$

P.Levermore et al, SID Digest, 2011.

Stacking is essential!

Panel15 cm x 15 cm 82% fill factor

2 Unit WSOLED

Luminance [cd/m2] 3,000

Efficacy [lm/W] 48

CRI 86

Luminous Emittance

[lm/m2]7,740

1931 CIE (0.454, 0.426)

LT70 [hrs] 13,000

27


• Luminance to achieve sRGB color gamut for G is 10X that for B

• ⇒B sub-pixel L0=100 cd/m2 (c.f. G with L0>1,000 cd/m2)

• ⇒B lifetime to T50=70,000 hr.

• Adopting Degradation acceleration factor: n =1.55 with

• ⇒B PHOLED lifetime to T50 = 1.3×105 hr.

• Commercial G PHOLED lifetime = 106 hours at L0 = 1000 cd/m2.

T50(100cd/m2 ) = T50(1000cd/m2 )× 1000cd/m2

100cd/m2

⎡

⎣⎢

⎤

⎦⎥

n

Not blue enough, T95 is required

Dopant Grading: Is it Good Enough?using acceleration factors to predict lifetime

28


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 brightness: ~4X

• Lifetime of blue lighting using grading: 50,000 hr

This is almost good enough

29


30

Photo illustrating good color rendering of the SWOLEDs in this report. The luminaire comprises 36 pixels (2 mm2) operated at 50-100k nits

Putting Grading Excited State Management to Work: Long lived all phosphor stacked WOLEDs

• Max Luminance > 200,000 nits • 50 lm/W max• CCT = 2780K• CRI=89

Coburn et al., ACS Photonics 5, 630 (2017)


All Phosphor SWOLED Performance

T70 SWOLED

ΔCCT -360 K

ΔCRI -0.8

ΔCIE (0.03,0)

400 500 600 7000

1

2

3

4 Stack example spectral shift after aging

Inte

nsity

(a.u

.)Wavelength (nm)

Spectralshi-sa-eraging

0

10

20

3040

5060

708090

3 Stack 4 Stack 5 Stack Cos(θ)

Lambertian

SWOLEDs

With outcoupling

T70 1000 nit(x103 hr)

T70 3000 nit (x103 hr)

ΔV/V0 (T70)(%)

SWOLED 80±40 14±5 ~+10%

Lifetime primarily limited by R/G sections

10-1 100 101 1020

20

40

60

LPE

(lm/W

)


w. outcoupling

w/o outcouplingSWOLED

ArchitectureBlue

degradation @ WOLED T70:

Conv T28

Grad-Managed T48

Coburn et al., ACS Photonics 5, 630 (2017) 31


What we learned about OLEDs• Chromaticity and the perception of color is quantified based

on eye response (photometic quantities)• OLEDs reach highest efficiency when both singlets and triplets

are harvested (heavy metal complexes and TADF molecules)• Optimized OLEDs have many layers serving purposes ranging

from charge conduction, contacting to electrodes, to light emission• Outcoupling methods essential to view substrate and

waveguide modes while limiting surface plasmons• Degradation of OLEDs particularly severe for blue due to

bimolecular annihilation• Lighting requires broad spectral emission using multilayer

devices or excimer emission• OLEDs provide uniform, area lighting vs. specular LED lighting 32

Week 10 - Organic Electronics: Foundations to Applications

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Transcript of Week 10 - Organic Electronics: Foundations to Applications