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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]
2U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
3U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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”
4U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
5U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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 SEMLA on a U of M Logo
6U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
7U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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||
8U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
9U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
10U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
11U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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=
12U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
13U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
14U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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 Green OLED on Diffuse Reflector Substrate
15U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
Metal reflectors are lossy
• Dielectric diffusive reflector No SPP Small absorption (Reflectance ~98%) No angle dependence Reduced micro-cavity effect
16U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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∞
=
= + − ⋅ + − ⋅ + ⋅⋅⋅ = − ⋅ =∑
17U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
18U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
19U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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
20U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
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)
21U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
• 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
22U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
Acknowledgements
• Optoelectronic Components and Materials Group (UM)– Xiaheng Huang– Jongchan Kim– Yue Qu– Caleb Coburn
• Department of Energy• Universal Display Corp.
23U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY
Thank You
University of MichiganStephen R. Forrest, Professor / Principal InvestigatorTel: (734) 647-1147 email: [email protected]