silver-based ultrathin transparent top electrode for ... kwan hyun cho... · kwan hyun cho*, heui...
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Kwan Hyun Cho*, Heui Seok Kang, Kyung Tae Kang and Yong-Cheol Jeong
Silver-based Ultrathin Transparent Top Electrode for Organic Light Emitting Diodes
SESSION 25: EMERGING CAPABILITIES
THURSDAY, JUNE 22, 2017
Center for Advanced Printed Electronics (CAPE)Korea Institute of Industrial Technology (KITECH)
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Korea Institute of Industrial Technology (KITECH)
Convergence Technologies
(Micro process ,Robot,
Textile)Gyeonggi
AutomotiveComponents,Applied Optics &Energy relatedtechnology
Clean/smartMFG** System
(Green process& materials, etc.)
Chungcheong
Honam
RootTechnologies*(Molding,Weldign, etc.)
Incheon
ConvergenceComponents &
materialsDongnam
Mechatronics(Nano-level sensors
& actuators)
Daekyeong
Capital Region
Chungcheongregion
Daegu-Kyeongbukregion
Honamregion
Dongnamregion
• Root technologies : Foundry, Molding, Welding, Forming, Heat treatment, Surface treatment• MFG : Manufacturing
GangWon Center
NonferrousMetal Technology
GangWon
GangWon region
KITECH is a government supported research institute, which has 7 regional divisions.
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Center for Advanced Printed Electronics (CAPE)
More than 10 years research experience in printed electronics area. 10 Ph.D.’s in mechanical, electrical, material majors.
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Contents
Transparent electrode
Ag based transparent electrode
Ag based electrode for transparent OLED (TrOLED)
Micro-cavity simulation for high performance OLED
Summary
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Transparent electrode
Samsung Display @ SID2017
Flexible/Stretchable Electronics
Materials for transparent electrode
Demands for flexibility
Flexibility
High transparent
High conductivity
Low cost process
Large area deposition
Patterning
Damage free to underlying films
Device performance
Indium Tin Oxide
Nanowire
Metal mesh
Metal based thin film
Graphene
Carbon Nanotube
Conducting polymer
Hybrid
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Metal based thin film
Dielectric/Metal/Dielectric (DMD)
Oxide/Metal/Oxide (OMO)
Dielectric/Metal/Dielectric (DMD)
Organic/Metal/Organic
Hybrid
Metal
(Bottom) Dielectric
(Top) DielectricMetal
Dielectric
Dielectric
Top emission OLED Bottom emission OLED
Metal
Dielectric
Dielectric
Organic
Electrode
Organic
Electrode
Optical properties
(transmittance, reflection)
Electric properties
(conductivity, charge injection)
Damage free deposition
Optical properties
(transmittance, reflection)
Electric properties
(conductivity, charge injection)
Easy patternability
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Superiority of Ag film
Electrical conductance
Metal Resistivity (𝝁𝝁Ω 𝒄𝒄𝒄𝒄) Metal Resistivity (𝝁𝝁Ω 𝒄𝒄𝒄𝒄)Ag 1.6 In 8.0Cu 1.7 Pt 10.0Au 2.4 Pd 11.0Al 2.8 Sn 11.5Mg 4.6 Cr 12.6W 5.6 Ta 15.5Mo 5.7 Ti 39.0Zn 5.8 ITO 200~500Ni 7.8
Resistivity of metal :Handbook of Chemistry and Physics, CRC Press, (1997)/ Resistivity of ITO: NATURE PHOTONICS, VOL 6, (2012).
Ag is praised for their excellent electrical performance.
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Superiority of Ag film
Optical transmittance
Cr
Co
CuAu
Ir
MoNiPt
AgTi W
ITOSiO2
Cr
Co
CuAu
Ir
MoNiPt
AgTi W
ITOSiO2
Absorption Transmittance
10-nm-thick metal films at 550nm wavelength
Mg Mg
Product of n and k Absorption , transmittance
Ag is expected to be good candidate materials for the high transmittance.
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Technically challenging
the nucleation and evolution of discrete nanoscopic clusters (I,II); complete coalescence between relatively small and regular clusters (III), followed by incomplete coalescence between large and irregular clusters (IV); the formation of a nanotrough network at the percolation threshold (V); and the transition from a nanotrough network to a continuous film (VI–VIII) with increasing metal thickness.
Adv. Funct. Mater. 2017, 1606641
Coalescence mode: no electrical paths (electrical) localized surface plasmon resonance (optical)
Nanotrough network mode: establishing electrical paths (electrical) the penetration depth of the incident light (optical)
percolation threshold
3D growth mode of Ag film
island-like metal clusters
coalescence
nanotrough network
continuous film
A reduction in metal thickness of the percolation threshold (𝑇𝑇𝑝𝑝𝑝𝑝𝑝𝑝−𝑡𝑡𝑡𝑝𝑝𝑝𝑝𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡) is important.
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Well known approaches (1)
Bottom dielectric layer
Interfacial adhesion
𝑇𝑇𝑝𝑝𝑝𝑝𝑝𝑝−𝑡𝑡𝑡𝑝𝑝𝑝𝑝𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 on ZnO < 𝑇𝑇𝑝𝑝𝑝𝑝𝑝𝑝−𝑡𝑡𝑡𝑝𝑝𝑝𝑝𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡on TiO2
The Zn−O bonding in ZnO is one of the weakest amongthe oxide candidates, whereas the Ti−O bonding in TiO2, similar to SiO2, is among the strongest. Ag atoms adsorbed on ZnO can form a strong bond to the oxygens at the topmost surface of ZnO.
Adv. Funct. Mater. 2017, 1606641
Adv. Funct. Mater. 2015, 25, (2015) Surface energy𝑇𝑇𝑝𝑝𝑝𝑝𝑝𝑝−𝑡𝑡𝑡𝑝𝑝𝑝𝑝𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 on ZnS
𝑇𝑇𝑝𝑝𝑝𝑝𝑝𝑝−𝑡𝑡𝑡𝑝𝑝𝑝𝑝𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 on MoO3
𝑇𝑇𝑝𝑝𝑝𝑝𝑝𝑝−𝑡𝑡𝑡𝑝𝑝𝑝𝑝𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 on WO3
𝑇𝑇𝑝𝑝𝑝𝑝𝑝𝑝−𝑡𝑡𝑡𝑝𝑝𝑝𝑝𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 on Glass
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Well known approaches (2)
Change in the number density of Ag clusters as a function of thethickness of the Sn surfactant
Seed layer Metal seed layer
kinetic approachthe activation energy barrier for the surface diffusion of Ag metals is expected to increase on the metallic seed layer compared to pristine oxide substrates.
thermodynamic approachthe reduction in the driving force for the surface diffusion of Ag metals to lower the difference in the surface free energy between the Ag metals and the substrate.
Surf. Sci. 2008, 602, L49./ Adv. Energy Mater. 2013, 3, 438./ Adv. Funct. Mater. 2017, 1606641
1 nm seed layer/ Ag with a thickness of 7 nm.number density of Ag clusters
Polymer seed layerAdv. Mater. 2014, 26, 3618–3623/ Adv. Energy Mater. 2014, 4, 1400539
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Well known approaches (3)
Change in the number density of Ag clusters as a function of thethickness of the Sn surfactant
Co-depositionAdv. Mater. 2014, 26, 5696–5701/ Adv. Funct. Mater. 2017, 1606641
Ag:Al co-deposition
c) 9-nm pure Ag fi lm, d) 9-nm Al-doped Ag fi lm.
Ca:Ag co-deposition
Even without any seed layer, the Ca:Ag blend electrode shows a high mean transmittance of 79.5%
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Top electrode for TrOLED
Transmittance spectra
Al/Ag bilayer cathode transmission clearly exhibits an increased transmittance, and the shape of the spectrum is similar to those of the calculated theoretical transmission.
Ag-only cathode shows a substantial difference compared with the theoretical calculation.
Organic Electronics 33 (2016) 116-120
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Top electrode for TrOLED
SEM images
Al/Ag bilayer show the continuous bulk-like Ag film.
Ag-only cathodes show separately island-like Ag films, and the sample of thickness with 8 nm show starting to become continuous Ag film.
Organic Electronics 33 (2016) 116-120
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Top electrode for TrOLED
Sheet resistance & Figure of merit (FOM)
The FOM of the Al/Ag cathode based on the calculated values is high and similar to the ITO.
Maximum Tlum value of 86% was obtained for the Al/Ag bilayer cathode (@ 4 nm thickness).
Organic Electronics 33 (2016) 116-120
Ag (measurement) Al/Ag (measurement) Ag (calculation) Al/Ag (calculation)
Ag(measurement) Al/Ag(measurement) Ag(calculation)
Luminous transmittance Sheet resistance Figure of merit (FOM)
Luminous transmittance
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Top electrode for TrOLED
Alq3 (60nm)
Glass
ITOPEDOT:PSS (50nm)
LiF (1nm)Al (1nm)Ag (x nm)
NPB (60nm)Alq3 (40nm)
The maximum value was 72 % at 550nm wavelength with the 4 nm Ag thickness.
The transmittance of the TrOLED devices decreased as the Ag layer thickness increased.
Device structure Transmittance of TrOLED
Transmittance of the TrOLED
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Transmittance calculation of TrOLED
ITO based TrOLED: thickness variation from 0 to 200 nm (ITO), 0 to 200 nm (WO3).
WAW based TrOLED: thickness variation from 0 to 50 nm (Ag), 0 to 200 nm (WO3).
calculated using Setfos (Fluxim)Transmittance of TrOLEDs (ITO vs WAW)
Glass (incoherent)ITO(x)WO3(y)
NPB(50nm)Alq3(50nm)
LiF(1nm)/Al(1nm)/Ag(8nm)CPL(70nm)
Air(incoherent)Encap. glass(incoherent)
Al(1nm)/Ag(x)WO3(y)
NPB(50nm)Alq3(50nm)
LiF(1nm)/Al(1nm)/Ag(8nm)CPL(70nm)
Air(incoherent)Encap. glass(incoherent)
WO3WO3(50nm)
Glass(incoherent)
WAW based TrOLED ITO based TrOLED
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Transmittance calculation of TrOLEDcalculated using Setfos (Fluxim)
Transmittance spectra (ITO vs WAW)
400 500 600 700 800
20
40
60
80
Tr
ansm
ittan
ce (%
)
Wavelength (nm)
Transmittance of TrOLEDs WAW based TrOLED ITO based TrOLED
Max. transmittance of ITO based TrOLED: 74% (@550nm, 150nm and WO3 101nm).
Max. transmittance of WAW based TrOLED: 76.4% (@550nm, Ag 12nm and WO3 13nm).
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Micro-cavity simulationcalculated using Setfos (Fluxim)
Micro-cavity effect
Reflector(Al, Ag)
Semitransparent (ITO, Ag, Al)
Substrate
dZ0
𝑓𝑓𝐹𝐹𝑃𝑃 : Multiple beam interference (= Fabry-Perot effect)
𝐺𝐺𝑐𝑐𝑎𝑎𝑣𝑣 (λ) =𝑓𝑓𝐹𝐹𝑃𝑃 (λ)×𝑓𝑓𝑇𝑇𝐼𝐼 (λ)
𝑓𝑓𝑇𝑇𝐼𝐼 : Two beam interference
Micro-cavity effect in OLED
Iout (λ) = 𝐺𝐺𝑐𝑐𝑎𝑎𝑣𝑣 (λ) × IEML (λ)
Optical length(organic layer thickness, d) and semitransparent electrode thickness is important for the micro-cavity.
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Micro-cavity simulationcalculated using Setfos (Fluxim)
Device structure for micro-cavity simulation
Bottom emission
Top emission
Device 1-reflector: Al-semitransparent: ITO
Device 2-reflector: Al-semitransparent: : Ag
Device 3-reflector: Ag-semitransparent: Al
Device 4-reflector: Ag-semitransparent: Ag
ITO (x nm)
LiF (1 nm)
NPB (50 nm)Alq3 (50 nm)
Al (100 nm)
WO3 (y nm)
Glass
Ag (150 nm)
LiF (1 nm)
NPB (50 nm)Alq3 (50 nm)
Al (x nm)
WO3 (y nm)
Glass
CPL (70 nm)
Encap Glass
Ag (150 nm)
LiF (1 nm)
NPB (50 nm)Alq3 (50 nm)
Al/Ag (1/x nm)
WO3 (y nm)
Glass
CPL (70 nm)
Encap Glass
Ag (x nm)
LiF (1 nm)
NPB (50 nm)Alq3 (50 nm)
Al (100 nm)
WO3 (y nm)
Glass
WO3 (50 nm)
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Micro-cavity simulation_current efficiency
WO
3 Th
ickn
ess
WO
3 Th
ickn
ess
WO
3 Th
ickn
ess
WO
3 Th
ickn
ess
ITO Thickness Ag Thickness
Al Thickness Ag Thickness
Device 1 Device 2
Device 3 Device 4
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Micro-cavity simulation
Device1: (ITO: 1~11 nm, WO3: 96 nm)Device2: (Ag: 27 nm, WO3: 11 nm)Device3: (Al: 9 nm, WO3: 6 nm)Device4: (Ag: 25, WO3: 6 nm)
Max. emission and current efficiency @ the thickness of
Device1 Device2 Device3 Device40
10
20
30
40
50
Cur
rent
Effi
cien
cy (c
d/A
)
Max. current efficiency
Electrode material for micro-cavity effect: Ag > Al> ITO
400 500 600 7000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Emis
sion
(W*m
-2*n
m-1
*sr-
1)
Wavelength (nm)
Device 1 Device 2 Device 3 Device 4
Max. emission Max. current efficiency
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Poster Session Title: The hybrid blue organic light-emitting diodes and quantum dot color converter
for flexible white lighting
Glass
LiF/Al (100nm)
Outer WO3 (70nm)
NPB (70nm)
Bphen (30nm)
Al/Ag (16nm)Inner WO3 (x nm)
DPVBi (30nm)
Quantum Dot
LiF (100nm)
Glass
LiF/Al (100nm)
WO3 (x nm)
NPB (70nm)
Bphen (30 nm)
DPVBi (30nm)
Quantum Dot
ITO (150nm)
ITOW60ITOW80
ITOW100
ITOW120
WAW120
WAW100
WAW60
WAW80
The CIE 1931 color coordinate of the hybrid QD/OLED Cavity enhancement factor
Schematics of hybrid red QD/blue OLED
We achieved a wide variation of color coordinates, including blue, near-green, and near-white regions, with a simplearchitecture of a blue OLED and red QD.
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Summary
Demands for transparent electrode; damage free deposition, device performance and low cost process are important.
Ag film have high superiority of electrical conductance and optical transmittance.
We demonstrate the enhanced optical and electrical properties of an ultrathin silver (Ag) film by applying an aluminum (Al) seed layer.
The transparent OLED devices that employed the Al/Ag cathode showed a transmittance of 72% at a 550 nm wavelength.
In the micro-cavity simulation, OLED device having Ag based top and bottom electrode was obtained the maximized current efficiency.
25Center for Advanced Printed Electronics