Week 2-4 - Organic Electronics: Foundations to Applications

Organic ElectronicsStephen R. Forrest


Week 2-4Light emitters 4

Stacked WOLEDsOutcoupling

Chapter 6.5.4-6.6

1



2

White Phosphorescent SOLEDs

Io , Vo

Lo

3xLo

Io , 3xVo

Conventional OLED Stacked OLED (SOLED)

cathode

ITOorganic layers-

+

charge generation layer

-+

-+

-+

Lifetime and efficiency of OLEDs can be increased by vertically stacking multiple OLED units in series

Requires less current for same luminance as a single unit device• Longer lifetime at same luminance• Less current for a given luminance = reduced resistive power losses and heating


SOLED Operation Principles

Blue=FlzIr Green=Ir(ppy)3Red=PQIr

Archetype stacked WOLED

CGLEBL

Two types of CGLS: Recombination vs. tunnel junctions

Kanno et al., Adv. Mater., 18, 339 (2006) 3


White SOLED Panel: Efficacy vs. Luminous Emittance

Panel Area = 15 cm x 15 cm

• Efficacy approaching 50 lm/W at 10,000 lm/m2

• CRI = 83• LT70 = 4,000 hrs

1,000 cd/m2

2,000 cd/m2

ITOHIL

HTL

R:G EML

BL

ETL

Cathode

Blue EML

BL

ETL

CGLHILHTL

4


White PHOLED Panel

Panel15 cm x 15 cm15 mm thick

At 1,000 cd/m2

At 3,000 cd/m2

Efficacy [lm/W] 58 49

CRI 82 83

Luminous Emittance[lm/m2] 2,580 7,740

Voltage [V] 3.8 4.3

1931 CIE (0.466, 0.413) (0.471, 0.413)

Duv 0.001 0.000

CCT [K] 2,640 2,580

EfficacyEnhancement 1.75x 1.75x

Temperature Rise [oC] 0.7 7.2

LT70 [hrs] 30,000 4,000

Cathode

AnodeHIL

Red-Green EML

Blue EML

ETL/EIL

Glass

BL

HTL

LT70 ~ 30K hrs at 1000 cd/m2

Warm White with CCT 2640K

t = 60 min3000 cd/m2

28.7oC

27.5oC

290 300 310 320 330 340 35010

100

1000

10000

Life

time

to L

T80

[hrs

]

Temperature [K]

10K Lower Temperature ~ 1.65x Longer Lifetime

Lower current at constant L⇒lower temperature⇒longer lifetime

5


WOLED vs. SOLED Panel ComparisonPanel

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, 72.2, p.1060, 2011.

WOLED

SOLED

SOLED architecture: ~ 3x LT70 improvement vs. single unit WOLED with similar color and power efficacy

ITOHILHTL

R:G EML

BLETL

Cathode

Blue EML

ITOHILHTL

R:G EML

BL

ETL

Cathode

Blue EML

BL

ETL

CGLHILHTL

6


OLEDs: Not All Light Goes to the Viewer

• Optical paths outcoupled with hemispherical lens

7


• Good solutions§ Inexpensive§ Viewing angle independent§ Independent of OLED structure

• Among those things that have been tried§ Optical gratings or photonic crystals1

§ Corrugations or grids embedded in OLED2

§ Nano-scale scattering centers3

§ Dipole orientation management

8

1Y .R. Do, et al, Adv. Mater. 15, 1214 (2003).2Y. Sun and S.R. Forrest, Nat Phot. 2, 483 (2008).3Chang, H.-W. et al. J. Appl. Phys. 113, - (2013).

Getting all the photons out



k !

k Θ

z

dmedium 1

medium 2

Molecules are radiating dipoles in inhomogeneous media

9


Cathode metal (mirror)

Organics

ITO

Glass substrate

k||

kθ

• Air modes: EQE first increases, then decreases with ETL thickness

• Waveguide modes: Only one waveguide mode TE0 due to thin ETL (<30nm). TM0 appears when >50nm.

• Surface plasmon polariton modes: Reduced with ETL thickness• Both waveguide and SPP modes are quantized• Total energy is the integral of Power Intensity x cos(θ), so SPP

not as small as it looks

Where do all the photons go?

0 0.2 0.4 0.6 0.8 1 1.20

0.5

1

1.5

2

2.5

3

3.5 x 10-3

k||/k

powe

r int

ensit

y

10nm30nm50nm70nm90nm

Air modes

Substr

atemodeswav

eguid

e

SPP

ETL thickness

(20%) (20%)

(15%) (45%)

10


WOLED Outcoupling Can Yield True Color

Color balance achieved at two ETL thicknesses

ETL Thickness Controls Coupling to lossy surface plasmons

11


Surface Plasmon Polariton (SPP) Modes: Major Loss Channel

ηext > 80% (incl. substrate + waveguide modes)

• Waveguided light excites lossy SPPs in metal cathode• Major loss channel partially eliminated by rapid outcoupling of waveguide modes• Most difficult to eliminate cost-effectively without impacting device structure

12


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) 13


14

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.


15

2µm

Low Index Grid Images

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


16

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

17

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

18

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 19


Substrate Au Ag Mirror on Grid Surface

SiO2IZOOrganicAnti-reflectionlayer

Eliminating SPPs Using Sub-Anode Grid + MirrorTop Emitting OLED

3 Lift-off

Substrate Fabrication

Qu, et al. ACS Photonics, 4, 363 2017

Mirror remote from EML eliminates SPP excitation

20


a. Ag

IZO/MoO3 80nmETL 60 nm

Substrate

EML 20 nmHTL 40 nm

IZO/MoO3 80nm

SiO2 65 nm

Ag

IZO/MoO3 80nmETL 60 nm

Substrate

EML 20 nmHTL 40 nm

IZO/MoO3 80nm

SiO2 245 nm

a. Ag

ETL 35 nm

Substrate

EML 15 nmHTL 30 nmAg 15nm

Sub-electrode grid modelingVariable Waveguide Widths Above Mirror Prevent Mode Propagation via Scattering

Qu, et al. ACS Photonics, 4, 363 2017

raised mirror section depressed mirror section no waveguide

SPP at k‖>1

21


0 2 4 6 8 10 12 1410-5

10-4

10-3

10-2

10-1

100

101

102

Voltage (V)

Curre

nt De

nsity

(mA/

cm2)

1

10

100

1000

10000

Lum

inanc

e (Cd

/m2)

0

20

40

6080100

120

140

160

180

Con Grid

0.1 1 10 1000

5

10

15

20

25

30

hEQE (

%)

Current Density (mA/cm 2)

Performance with and without grid+mirror

Grid

No Grid

Qu, et al. ACS Photonics, 4, 363 2017 22


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. 23


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.

SPPs not completely eliminated

24


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. 25


Gratings Provide Efficient Waveguide Mode Outcoupling

Process for fabricating 1D and 2D gratings using optical interference resist exposure

rotate and re-expose for 2D grating

AdvantagesCan be very efficient

DisadvantagesRequires high resolution photolith over large areasVery wavelength and angle dependent

Bocksrocker et al., Opt. Express, 20, A932 (2012) 26


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

(%)

Current Density (mA/cm2)

PHOLED Active Area

Kim, J., et al. (2018), ACS Photonics, 5, 3315. 27


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)28


Outcoupling Improvements via Alignment

Calculated dipole field

Kim et al., Adv. Funct. Mater., 23, 3896 (2013)

29


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:

30


Alignment is Driven by Energy and Deposition Dynamics

Alignment can be “forced” by seeding the substrate with a thin molecular template

Molecular functionalization can drive orientation: Aliphatic groups avoid substrate

Example: Ir(ppy)2acac

aliphatic acacLampe, et al. Chem. Mater. 28, 712 (2016)

Kim, et al. Adv. Mater. 31, 1900921 (2016) 31


• 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

32

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Week 2-4 - Organic Electronics: Foundations to Applications