Brief Review of OLED Technology Cheah Kok Wai ( 謝國偉 ) Department of Physics and Centre for Advanced Luminescence Materials ( 先進發光材料中心 ) Hong Kong Baptist University ( 香港浸會大學 ) Kowloon Tong ( 九龍塘 ), Hong 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.
OLED History Dr. Fred CH Chen HKBU/NJTU Rochester University
Green Materials Wanted
Display Light Source
OLED LCD View Angle >160 0 Limited Contrast high Low Response Time µs ms Light Emission Active Passive ( Backlighting) Temperature Range ~ -50 0 C to ~80 0 C ~ -0 0 C to ~100 0 C Mfg. Processes ~6 steps ~9 steps
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 Alq 3. Not only it is a emissive material, it is also an electron transporter. N O Al N O O N More recent electron transport material is ADN.
Hole Transport Materials N NPD N There are several established hole transport materials. The more mature materials are, NPD and TPD. H 3 C N N Both are used extensively in the current OLED design. CH 3 TPD
Dyes These are several dye examples. Rubrene They are for two main purposes: For colour tuning. NC CN For colour stabilisation. Rubrene is used to dope Alq 3 for yellow emission. H 3 C O N DCM II DCM II and DCJTB are used to dope Alq 3 for red emission. NC CN tbu DCJTB N
Dyes colour stabilisation C545 is used for green colour stabilisation. N S N O O Perlene is used for blue colour stabilisation.
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:
CIE Color Coordinate vs Power Consumption in Full Color OLEDs 0.55 0.50 Power (W) 0.45 0.40 0.35 Power ratio Blue-y 0.30 0.00 0.05 0.10 0.15 0.20 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) 427 nm 2. Idemitsu s Blue Host ADN Solution PL 350 400 450 500 550 600 650 Wavelength (nm) C. Hosokawa, H. Higashi, H, Kakamura, and T, Kusumoto, Appl. Phys. Lett. 67, 3853 (1995) 429 nm Solution PL DPVBi 350 400 450 500 550 600 650 Wavelength (nm)
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) CH 3 452 nm MADN PL 350 400 450 500 550 600 650 Wavelength (nm)
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 1 Substrate Preparation 2 ITO/Cr Pattern 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 a Passive Dot Matrix Display
Passive Matrix OLED Cathodes Organic layers Substrate Anodes
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-thin Power Efficient High Contrast Motion
Early Applications of OLEDs: Importance of Display Attributes 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 H = High; M = Medium; L = Low
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 mm Two 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 inroad 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
Hanheld Devices Palm size MP3 with radio Diesel watch; blue and green colour
Mobile Phones and Laptop Nokia N85 Motorola U9
OLED TV Large Display SONY XEL-1, 11 TV with luminance of 600 cd/m 2 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 Double Stacked OLED Summary 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 LiF/Al OLED 2 Inter connection OLED 1 ITO BPhen Emitting Layer NPB 2TNATA:F4TCNQ
Robust OLEDs Material Summary Table Material s Tg ( ) BUHI-3 126 BUHT-6 145 BUBH-4 BUEI-3 Nil Nil Device structure ITO\BUHI- 3:1.5%F4TCNQ(150nm) / NPB(10nm)/ Alq(60nm)/ LiF(1nm)/ Al ITO/ BUHT-6(60nm)/ Alq(60nm)/ LiF(1nm)/ Al ITO/ CF X / TPD(70nm)/ 5%BUBD-1: BUBH- 4(40nm)/ Alq (15nm)/ LiF(1nm)/ Al ITO/ CF X / NPB(70nm)/ Alq(55nm)/ BUEI- 3(5nm)/ Mg:Ag Voltage (V) Yield (cd/a) Half-life (hrs) 6.10 4.60 510 6.38 4.30 265 Others Patent applied (DOT: 200720178162.X ) 5.78 8.17 976 T d =545.5 4.39 2.75 1913 T d =331 Patent in preparation
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.