1. Polymeric Electroluminescent Devices K W Wong Department of Physics The Chinese University of Hong Kong

2. Contents Introduction –Why PLED/OLED? –Business & Academic Historical background Present status in research –Underlying physics –Materials research –Device technology –Device failure Our current status

3. Introduction …why PLED/OLED Simple & robust device structure Good processability Low production cost Theoretically full colour range Workable at low temperature Low electrical requirement Applicable on flexible substrate (e.g. plastics)

4. Metallic cathode Polymeric / Organic layer Transparent ITO anode Glass …typical structure Introduction

5. ITO Hole transport layer Cathode Electron transport layer Hole injection Electron injection …EL mechanism Introduction

6. …applications Introduction

7. Introduction …business

8. Introduction

9. Introduction …academic

10. Introduction During 1990s, there were over 5000 titles registered in Chemical Abstracts and over 500 related US Patents. The publications and patents ranged from the areas of physics, chemistry, materials science, electronic engineering, and etc. …academic

11. History – “High driven voltage” 1950sBernanose- High-voltage AC field to crystalline thin film of acridine orange & quinacrine.  EL in organic materials 1960 Gurnee & Fernandez- AC-driven EL cells using anthracene. (DOW)  emit blue light 1963 Pope- DC-driven EL cells using single crystals of anthracene. REMARKS... During this period, EL with organic materials were demonstrated. The EL cells were firstly driven by AC-field, while DC-field was employed in later stages. However, a HIGH driven voltage (~10 2 V) were required.

12. History – “Use of electrodes” During late 60s and early 70s, the idea of using of metal electrodes to enhance carrier injection. This significantly reduces the driven voltage to the order to ~10 V. REMARKS... 1969 Digby & Schadt- First use of a reactive cathode in organic EL cell to facilitate e - injection.  US Patent 3,621,321 1975 Partridge- First EL cell with polymer: polyvinyl carbozole (PVK) doped with perylene. - First use of alkali metal cathode.  US Patent 3,995,299

13. History – “Turning point” 1987 C. W. Tang- A two-layer structure of vacuum-deposited (Eastman Kodak Co.) small-molecules film; - Use of indium-tin-oxide as hole-injection anode; - Driven voltage < 10 V.  US Patent 4,164,431, 4,356,429 1990 R. H. Friend et. al.- fabricated green-yellow EL cell using poly (Cambridge University) (p-phenylene vinylene), PPV in a single layer structure.  Nature 347, 539 (1990) REMARKS... The pioneering works from Tang and Friend have aroused worldwide interests from physicists, materials scientists and engineers.

14. History – “Commericalization” 1980s Eastman Kodak Co. - actively initiated related intensive research programmes. 1991 UNIAX- a new company spun out of UC Santa Barbara, the company was later acquired by DuPoint. 1992 Cambridge Display Technology - a company established by Friend et. al. based on their patents on material synthesis and technologies. 1997 Pioneer Electronics Co.- manufactured car radio consoles based on organic EL materials. 1999 Covion Organic Semiconductors - a spin-off company of Hoechst. Others: Dow Chemicals, DuPont Chemicals, Philips Electronics, Epson Groups, IBM, and etc.

15. Present status in research – Efficiency > 20 Im/W, Lifetime ~ 10,000 hours at 200 cd/m 2, (target: 100,000 hours) Achieved emission of green, blue, red, yellow and white light, Already have simple commercialized products. …achievement

16. Present status in research – Area of interests: –Insertion of buffer layers C. W. Tang et. al. from Eastman Kodak Co.  Most buffer layers used are inorganic materials, e.g. LiF, SiO 2, and etc. –Surface and interface modification R. H. Friend et. al. from Cambridge University  Most treatments are restricted to plasma and chemical treatment on ITO. –Energy level alignment at the interfaces A. Kahn et. al. from Princeton University Y. Gao et. al. from U. of Rochester, C. W. Tang et. al. from Eastman Kodak Co. S. Forrest et. al. from Princeton University  Experimental findings based on different system leads to very different proposed mechanism. Although lots of studies have been performed, there is still no unified picture on the energy level alignment between the electrode/polymer and polymer/polymer interfaces. Also, the use of buffer layers provides a large platform for research. PENDING ISSUES... …underlying physics

17. Present status in research – …materials research Leading polymeric emissive materials: –Poly (p-phenylene vinylene), PPV –Poly (phenylene), PPP –Polyfluorenes, PFO PPVPFOPPP

18. Present status in research – …materials research Carrier transport materials: –Poly (3,4-ethylene dioxythiophene), PEDOT (HTL) –Polyaniline, PANI (HTL) –Arylamine,  -NPB (HTL) –Copper Phthalocyanine, CuPc (HTL) –Tris-(8-hydroxyquinoline) aluminium, Alq (ETL) –Bathophenanthroline, BPhen (ETL) PEDOTPANI

19. Present status in research – …materials research Dopants: –Tetrephenylporphyrin, TPP (Red dye) –Rubrene (Red dye) –Inorganic dopants, e.g. Li, LiF Composite materials: –Polymer/polymer composite –Polymer/organic composite RubreneTPP

20. Present status in research – Area of interests: –Transparent stacked layer device structure Forrest and Kahn et. al., from Princeton University …device technology

21. Present status in research – Area of interests: –Microcavity EL devices Tokito et. al., from TOYOTA, Inc. –Inkjet printing of polymers Yang Yang et. al., from UCLA –EL devices on flexible substrate Universal Display Co. …device technology

22. Present status in research – Area of interests: –Inter-diffusion between layers –Kato et. al., J. Appl. Phys. 81, 7313 (1997) –Emergence of “dark” spot –Ch. Jonda et. al., J. Appl. Phys. 85, 6884 (1999) –Life-time related problems –Parker et. al., J. Appl. Phys. 85, 2441 (1999) –Thermal breakdown of organic materials –Z. Zhou et. al., Adv. Mater. 12, 265 (2000) –Quenching effects by ambient species (e.g. O 2 ) –J. Yu et. al., Science 289, 132 (2000) –Susceptibility of certain materials towards environment –PEDOT absorbs water quickly from ambient and degrade the device …device failure

23. Our current status …integrated coating & analysis system

24. Our current status …integrated coating & analysis system

25. Our current status …green emitting PLED fabricated here

26. Our current status  On-going research Interface characterization of PEDOT:PSS/ITO, Blocking of In diffusion from ITO to PEDOT:PSS by using “Self Assembly Monolayer (SAM)”, Solvent effect on ITO, Study on polymer / ITO contact under different spin-casting conditions.