Brief Review of

OLED Technology

Cheah Kok Wai (謝國偉謝國偉謝國偉謝國偉)Department of Physics and

Centre for Advanced Luminescence Materials(先進發光材料中心先進發光材料中心先進發光材料中心先進發光材料中心)

Hong Kong Baptist University (香港浸會大學香港浸會大學香港浸會大學香港浸會大學)Kowloon Tong (九龍塘九龍塘九龍塘九龍塘), Hong Kong

Cheah Kok Wai (Cheah Kok Wai (謝國偉謝國偉謝國偉謝國偉謝國偉謝國偉謝國偉謝國偉))Department of Physics andDepartment of Physics and

Centre for Advanced Luminescence MaterialsCentre for Advanced Luminescence Materials

((先進發光材料中心先進發光材料中心先進發光材料中心先進發光材料中心先進發光材料中心先進發光材料中心先進發光材料中心先進發光材料中心))Hong Kong Baptist UniversityHong Kong Baptist University ((香港浸會大學香港浸會大學香港浸會大學香港浸會大學香港浸會大學香港浸會大學香港浸會大學香港浸會大學))

Kowloon TongKowloon Tong ((九龍塘九龍塘九龍塘九龍塘九龍塘九龍塘九龍塘九龍塘), ), Hong KongHong Kong

Content

1. Historical background

2. Basic Principle of OLED

3. OLED Technology and Market Trend

4. OLED Display

5. OLED Lighting

6. The Future

Introduction

• In this presentation, a review on OLED technology and the market trend will be made.

• The presentation consists of both my personnel view as well as statements made by various organisation internationally.

• The aim is to give a broad and concise view on OLED technology within the limit of the time period.

Rochester University

Dr. Fred CH ChenHKBU/NJTU

OLED History

Green Materials Wanted

Display Light Source

OLED LCD

View Angle >1600 Limited

Response Time µs ms

Light Emission Active Passive ( Backlighting)

Temperature Range ~ -500C to ~800C ~ -00C to ~1000C

Mfg. Processes ~6 steps ~9 steps

Contrast high Low

Basic Principle of OLED

How OLED Works

Traditional Semiconductor Diode Organic LED

OLED Device Structure

• The early device structure is quite simple; ITOglass/Hole-layer/Emission-layer/buffer/Aluminum.

• The emission layer is acting as electron transport layer (ETL).

• But not all emission material is good ETL and stability of single material emission layer is good.

• Hence materials for different functions are introduced e.g. Hole transport, electron transport, dopant etc.

• Dye dopants are used to stabilised OLED emission and colour tuning.

Electron Transport Materials

• Probably most prominent OLED material in OLED is Alq3.

• Not only it is a emissive material, it is also an electron transporter.

• More recent electron transport material is ADN.

N

NO

OAl

O

N

Hole Transport Materials

N N

• There are several established hole transport materials.

• The more mature materials are, NPD and TPD.

• Both are used extensively in the current OLED design.N N

CH3

CH3

NPD

TPD

Dyes

• These are several dye examples.

• They are for two main purposes:

• For colour tuning.

• For colour stabilisation.

• Rubrene is used to dope Alq3 for yellow emission.

• DCM II and DCJTB are used to dope Alq3 for red emission.

Rubrene

OCH3

NC CN

N

DCM II

N

NC CN

tBu

DCJTB

Dyes – colour stabilisation

• C545 is used for green colour stabilisation.

• Perlene is used for blue colour stabilisation.

N

S

O ON

Doping in OLED

Polymer based LED

• Polymers have bigger molecules and therefore cannot be thermally deposited.

• Instead, thin films are spin coated onto the substrate.

• Although it has advantage of fabricating device in ambient atmosphere, it has solvent problem when depositing multi-layer thin films.

• An alternate approach is being considered instead of spin coating.

Solution Process

• Cambridge Display Technology (UK) and Toppan Printing (Japan) have an ink-jet based relief printing that can be used in roll to roll production for polymer LED.

• In parallel, DuPont (USA) has also developed a printing process for small molecule based AMOLED with a claim lifetime of better than 10,000 hours.

• DuPont also claims that their method can lead to 30% less in production cost than LCD.

• Another advantage is that printing on flexible substrate is possible.

Inkject Process

PixDro, a subsidiary of OTB-Group, has released a new desktop inkjet printer for the development of printed electronics applications.

OLED for Display

• Small molecule based full colour display is mature enough for commercial production.

• While Korea and Japan (e.g. Samsung and Sony) concentrate on larger display unit like TV, Taiwan and others are working hard on smaller display for handheld device application.

• 2007 was the lowest point for OLED and this year we see some up-turn this year.

• Some of the reasons are listed here:

2008 – the return of OLED?

• Samsung SDI continues to boost her AMOLED panels monthly production from 1.5 million in 2007 second half to 4.5 million in 2008 third quarter.

• Both Mitsubishi Heavy Industries, Toshiba and Sumitomo have declared their intention to expand their OLED activities.

• Nokia decided that they will use OLED display in their mobile phones (N85).

• Sony sells the first world OLED TV; 11” XEL-1.

Some Key Issues in OLED Full Colour Display

• There are several possible device configuration for full colour display in OLED.

• Standard RGB, RGBW, colour by blue and colour by white.

• In all the configurations above one of the key colour needed is deep blue.

• The reason is as follow:

0.00 0.05 0.10 0.15 0.200.30

0.35

0.40

0.45

0.50

0.55

Power (W)

Blue-y

CIE Color Coordinate vs Power Consumption

in Full Color OLEDs

Power ratio

Blue-y

Major Stable Blue Host Materials Used in OLEDs

1. Kodak’s Blue Host J. Shi, C. W. Tang, Appl. Phys. Lett. 80, 3201 (2002)

2. Idemitsu’s Blue Host

ADN

DPVBi350 400 450 500 550 600 650

Wavelength (nm)

429 nm

Solution PL

350 400 450 500 550 600 650

Wavelength (nm)

427 nm

Solution PL

C. Hosokawa, H. Higashi, H, Kakamura, and T, Kusumoto, Appl. Phys. Lett. 67, 3853 (1995)

Major Stable Blue Host Materials Used in OLEDs

3. Kodak’s Blue Host Meng- Ting Lee, Hsian- Hung Chen et al, Appl. Phys. Lett. 85, 3301 (2004)

MADN

CH3

350 400 450 500 550 600 650

Wavelength (nm)

452 nm

PL

OLED Fabrication

• The backplane of OLED panel has many similarities with that of LCD.

• Broadly speaking the fabrication can be divided by three parts:

– Substrates preparation

– Device deposition

– Encapsulation

OLED Process Flow

2 ITO/Cr Pattern

1 Substrate Preparation

3 Base / Pillar Formation

4 Plasma Pretreatment

5 Organic Evaporation

6 Cathode Evaporation

7 Encapsulation

8 Test & Assembly

Passive OLED Display

The Basic Device Cross-section Structure of aPassive Dot MatrixDisplay

Passive Matrix OLED

Cathodes

Anodes

Substrate

Organic layers

Active OLED Display

The Basic Device

Poly-silicon glass backplane similar to the TFT-LCD

Active Matrix OLED Layout

OLED Feature Advantages

All Angle

Ultra-thinHigh Contrast

Motion

Power Efficient

Early Applications of OLEDs:Importance of Display Attributes

H = High; M = Medium; L = Low

Thin light Low power Clear Image Fast Broad color Wide view Long life gamut

Smart cards H H H L L L M

Head-mounted H H H M M M M

Car radios M L M L L M L

Voice phones H H M L M L L

P D A H H H L M M L

Camera view ers H H H L/M H M L

Mass Production of OLED

• Small molecule based OLED devices are fabricated from thermal deposition.

• Complete end to end production line is available from many suppliers from Japan and Korea.

• Examples are Tokki, Ulvac and Sunic etc.

• However from experience, all systems required customised tuning to suit individual manufacturers’ production conditions.

Pilot Production System

Tokki Pilot Production System

Glass substrate: 200 x 200 mm, with an effective deposition area of 160 x 160 mmTwo Deposition Chambers

Mass Production System

• Tokki Mass Production system•A fully automated system with two thin film deposition clusters, one encapsulation cluster.• Glass substrate 370 x 470mm.

Commercial Products

• OLED display has began to make appreciable in-road into the commercial market.

• Initially OLED display is being used in handheld devices; mobile phone, MP3 and MP4.

• When 3G is a common place and 4G is being realised, then OLED display will have an edge because of the faster time response, contrast and better colour saturation.

• Larger display for TV, notebook (?) and picture frame are being actively investigated.

Double sided AMOLED by AUO (10.12.04)

• 143 ppi• 200 nits• Independent image• 1.8 mm• Single module design• New desiccant• Perfect for mobile phone with 2” main and 1.5” subdisplay

Diesel watch; blue and green colour

Palm size MP3 with radio

Hanheld Devices

Mobile Phones and Laptop

Nokia N85

Motorola U9

OLED TV – Large Display

SONY XEL-1, 11” TV with luminance of

600 cd/m2 and contrast 1,000,000:1.

Resolution: 960x540

Lifetime: 30,000 hours.

Cost: US$2,295.00 - US$2,499.99

Samsung develops 21-in a-Si TFT AM-OLED (1.4.2005)

• 1000 nits brightness

• 6.22 million pixels resolution

• 5000:1 contrast ratio capability

• 1000 x response time over LCD

• Reduced production cost using

LCD G4 line

• Market $830 US in 2005

• 2.2 Billion in 2008

Bigger Samsung – 40” TV Demo

OLED Research in Hong Kong

• Research in OLED in Hong Kong started in @1997 with small group of chemists and physicists.

• Since then there are major research groups in City U, HKBU, HKUST and HKU.

• Many of the projects are ITC funded in partnership with industry.

CALM – HKBU Research Centre

• CALM (Centre for Advanced Luminescence Materials) was formed 4 years ago.

• Its mission is to research in advanced organic electronic materials e.g. OLED, OPV and OTFT and introduce the relevant technology/knowledge to HK industry and beyond.

• CALM has been working with industry in OLED projects supported by ITC since 2004.

• A short summary of the results are shown in the following slides.

Stacked OLED

Current density Yield (cd/A) Luminance Voltage

DSOLED 20 5.2 1040 14.07

100 5.713 5713 16.763

Standard OLED 20 3.513 702 6.12

100 3.69 3691 8.35

Double Stacked OLED Summary

ITO

OLED 1

Inter connection

LiF/Al

OLED 2

Emitting Layer

NPB

BPhen

2TNATA:F4TCNQ

Robust OLEDs Material Robust OLEDs Material

Materials

Tg (℃℃℃℃) Device structureVoltage

(V)Yield (cd/A)

Half-life (hrs)

Others

BUHI-3 126

ITO\BUHI-3:1.5%F4TCNQ(150nm)

/ NPB(10nm)/ Alq(60nm)/ LiF(1nm)/ Al

6.10 4.60 510Patent applied (DOT: 200720178162.X )

BUHT-6 145ITO/ BUHT-6(60nm)/ Alq(60nm)/ LiF(1nm)/ Al

6.38 4.30 265

BUBH-4 Nil

ITO/ CFX/ TPD(70nm)/ 5%BUBD-1: BUBH-4(40nm)/ Alq (15nm)/

LiF(1nm)/ Al

5.78 8.17 976 Td=545.5℃

BUEI-3 NilITO/ CFX/ NPB(70nm)/

Alq(55nm)/ BUEI-3(5nm)/ Mg:Ag

4.39 2.75 1913Td=331℃

Patent in preparation

Summary TableSummary Table

Other On-going ITC Projects

1. Novel white light source for LCD backlight application

2. Design of high efficient conformable OLED

The Future

• OLED is very likely to become part of the commercial display products.

• For example, Sony, Toshiba, Matsushita, Idemitsu Kosan and Sumitomo Chemicals have joint together to develop OLED HD TV.

• Other types of OLED based display are likely to emerge, e.g. transparent display, conformable display.

• The Return of OLED has just began

Conformable Display

Transparent Display

German Effort

Effort by Universal Display

Other Interesting Designs

Conclusion

• The OLED work can be exciting but also fraught with pitfall.

• HK universities have many years of OLED research experience with many projects supported by ITC.

• HKBU is the earliest group in HK to work in OLED.

• Our centre, Centre for Advanced Luminescence Materials with its director, Prof. Fred C. H. Chen, one of the Kodak OLED inventors, is happy to work with the industry.