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)

2

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.

3

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.

4

Contents

Transparent electrode

Ag based transparent electrode

Ag based electrode for transparent OLED (TrOLED)

Micro-cavity simulation for high performance OLED

Summary

5

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

6

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

7

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.

8

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.

9

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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11

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