eliminating plasmon losses in high efficiency white ... · – trapped at the metal cathode...

1U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY

Eliminating Plasmon Losses in High Efficiency White Organic Light Emitting Devices for Light Applications

University of MichiganStephen R. Forrest, Professor / Principal InvestigatorTel: (734) 647-1147; [email protected]


Contents

I. Motivation• Where does the Light Go?• Approach Principles

II. Sub-Electrode MicroLens Array• Design Scheme• Optical Design• Device Performance & Analysis

III. Sub-Electrode Diffuse Reflector• Design Scheme• Optical Design• Device Performance & Analysis

IV. Conclusion OLED Lighting

Reference. Luflex, LG Display


Where does the Light Go?

• Internal quantum efficiencies (ηIQE) ~100%• But extraction efficiency (ηout) is a major limit• ηEQE = ηIQE × ηout ≈ 20% (TIR and other losses)

• Refractive index change at interfaces lead to trapped light – At glass / air interface, “Glass modes”– At high-index ITO / organic layers, “Waveguide modes”– Trapped at the metal cathode interface, “Surface plasmons”


Approach Principles

Approaches:Acceptable solutions must have the following properties Low cost Viewing angle and wavelength independence Non-invasive of the OLED structure

- Solutions that outcouple > 70% of the emitted light- Demonstrate scalability of the methods investigated


Contents




IV. Conclusion SEMLA on a U of M Logo


Al OrganicITO

SEMLA

Spacer layer

• Micron-scale lens array between the bottom electrode and the glass substrate

• Flat spacer layer

• High refractive index

• Microlens array embedded into glass

Design Scheme

T Komoda et al, Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp. 2012, 43 (1), 610− 613


As nsub/norg goes up, more waveguided light is squeezed out

Optical Design – Organic wg SEMLA

n=1.6 n=1.7 n=1.8nsub/norg nsub/norg

ITOSubstrate

organicAl

θk=2πn/λ

k||


Optical Design – SEMLA Glass Substrate

• As nSEMLA/nsub goes up, both transmission and escaping angle decrease

• nSEMLA/nsub = 1.8/1.5 = 1.2

nSEMLA/nsub1.11.21.31.41.51.8

1.11.21.31.41.51.8


Optical Design – High Refractive Index Spacer

NOA 170, Norland Products Inc.

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

0 20 40 60 80 100 120 1400

20

40

60

80

100

Frac

tion

of P

ower

(%)

ETL thickness (nm)

SPPLoss

WV

SEMLAFr

actio

n of

Pow

er (%

)

ETL thickness (nm)

Conventional

SEMLA


Device Performance – Measurement

0 5 1010-7

10-3

101

Con. glass Sub1

J (m

A/c

m2 )

Voltage (V)

• The SEMLA• Conventional glass (n=1.45)• Sapphire (n=1.77)

GlassSEMLA

• External microlens arrays (MLA)• Index matching fluid (IMF)

Internal Extraction

External Extraction


S1 S2 S3Sap

1.0

1.5

2.0

2.5

3.0

3.541±3%45±4%

27±3%20±2% 60±4%

65±5%

47±4%

E F

30±3%

GreenWhite

Device Performance – Efficiency

• Control Conventional– Peak EQE ~ 25% (Green)– Peak EQE ~ 17% (White)

• Enhancement factors (EF) with EQEs

EF= η/ ηglass

• Independent on emissive wavelengths

ηEQE=


Device Analysis – Angle Dependence

• SEMLA with MLA and large Hemisphere lens (HS)

• no blue shift at large angles (at 60 degree)

500 600 700

0o 30o 60o

Inte

nsity

(a.u

.)

Wavelength (nm)

SEMLA HS

SEMLA MLA

Con


Device Analysis – Resolution Impact

• No visible impact on the image resolution

• Patterning of the SEMLA can only be seen under the microscope

100 μm100 μm


Contents




IV. Conclusion Green OLED on Diffuse Reflector Substrate


Metal reflectors are lossy

• Dielectric diffusive reflector No SPP Small absorption (Reflectance ~98%) No angle dependence Reduced micro-cavity effect


Device Scheme

out TA D Sη η η η= +ηTA : Light power fraction to top surfaceηD : Light extracted / Light fraction into planarization layerηS : Light power fraction into planarization layer

𝜂𝜂TA𝜂𝜂D 𝜂𝜂S𝜂𝜂D 𝜂𝜂S

𝜂𝜂S

2

0(1 ) (1 ) (1 ) 1n

D S S S S S S Sn

R R R R R R Rη∞

=

= + − ⋅ + − ⋅ + ⋅⋅⋅ = − ⋅ =∑


Optical Design – ηD and ηS

• Outcoupled light from planarization layer (ηD)– Thickness, absorption↓ ηD ↑– Major loss channel Planarization layer

absorption

Input/output power of Planarization Layer

• Coupled light into the planarization layer (ηS)– Increase with nP nP = 1.8 wg mode vanish


Device Performance – Efficiency

• External Quantum Efficiency (EQE)– Mirror 15 ± 2%– Diffuser 37 ± 4% (×2.5)

• Identical J-V– No influence on device structure

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

0.01 0.1 1 100

10

20

30

40

50

η EQ

E (%

)

Current Density (mA/cm2)

DiffuseMirror

0 4 8 12 16 2010-4

10-3

10-2

10-1

100

101

102

Cur

rent

Den

sity

(mA/

cm2 )

Voltage (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Luminance (cd m

-2)

x104


Device Performance – White Spectrum

• White OLED– No spectral shift

• Weak cavity (high index Planarization layer)– Lambertian output pattern

• Scattering via diffuse reflection

0.5

1.0

1.5

400 500 600 700 8000.0

0.5

1.0

1.5Diffuse

0 Degree 30 Degree 60 Degree

Nor

mal

ized

Inte

nsity

Wavelength (nm)

Mirror

90

60

300

30

60

90 Diffuse Mirror Lambertian

Device Area


Device Analysis – Peripheral Emission

• Light emitted outside the defined active area : Peripheral Emission• Device area ↑ peripheral emission ↓• Planar. layer thickness ↓ peripheral emission ↓, EQE ↑ (68%, ×3.4 @50um)


• Outcoupling methods of following features were demonstrated Highly efficient Low-cost Scalable Wavelength/viewing angle-independent No spectrum shift Non-intrusive into the device structure

• Same enhancement factors for white and monochromatic devices• No impact on image sharpness (SEMLA)• No need of external outcoupling, Simple fabrication (Diffuser)

Conclusion

Y. Qu, J. Kim, C. Coburn and S. R. Forrest, ACS Photonics 5, 6, 2453–2458 (2018)J. Kim, Y. Qu, C. Coburn and S. R. Forrest, ACS Photonics 5, 8, 3315–3321 (2018)

Reference


Acknowledgements

• Optoelectronic Components and Materials Group (UM)– Xiaheng Huang– Jongchan Kim– Yue Qu– Caleb Coburn

• Department of Energy• Universal Display Corp.


Thank You

University of MichiganStephen R. Forrest, Professor / Principal InvestigatorTel: (734) 647-1147 email: [email protected]

eliminating plasmon losses in high efficiency white ... · – trapped at the metal cathode...

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