1. Electronic and Optoelectronic Polymers Wen-Chang Chen Department of Chemical Engineering Institute of Polymer Science and Engineering National Taiwan University

2. History of Conjugated Polymers Electronic Structures of Conjugated Polymers Polymer Light-emitting Diodes Polymer-based Thin Film Transistors Polymer-based Photovoltaics Outlines

3. Optical Absorbance Absorption of light and the excited states of molecules A is absorbance I 0 is intensity of incident light I 1 is intensity after passing through the materials l is path length C is concentration λ is wavelength of light k is extinction coefficient α is molar absorptivity or absorption coefficient Beer-Lambert Law α is a measurement of the chromophore’s oscillator strength or the probability that the molecule will absorb a quantum of light during its interaction with a photon A = 2 - log10 %T

4. Photophysics Process Internal conversion (IC): electron conversion between states of identical multiplicity Intersystem conversion (ISC): electron conversion between states of different multiplicity singlet state : all electrons are paired ( )with opposite spins Triplet state : same spins pairing of electrons ( ) Jablonski Diagram Non-Radiative Process

5. Photophysics Process 1/√2 Singlet state (anit-symmetric) Triplet state (symmetric) Spin unpaired, S=1 Spin paired, S=0 From Quantum Statistics 25% 75%

6. Photophysics Process Absorption or excitation spectroscopy is used to probe ground state electronic structure and properties Emission or luminescence spectroscopy is used to probe excited state electronic structure and properties Radiative Process (S 1 S 0 )(T 1 S 0 ) 0.1~10ns >100ns

7. Photophysics Process Fluorescence: spontaneously emitted radiation ceases immediately after exciting radiation is extinguished Phosphorescence: spontaneously may persist for long period mirror image

8. Excitons (bounded electron-hole paies) binding energy ~1eV Diffusion radius ~10Å Charge Transfer (CT) Exciton : typical of organic materilas Excited States are produced upon light absorption by a conjugated polymers Molecular picture Ground stateExcited state Treat excitions as chargeless particles capable of diffusion and also view them as exited stated of the molecules

9. Why PLEDs ? Easy and low-cost fabrication Solution processibility Light and flexible Easy color tuning Spin coating and inject printing

10. History of Organic Light Emitting Diodes First organic electroluminescene based on anthracene single crystal 1963 1987 The first efficient, bright, and thin film organic light emitting diode (OLED) was reported by C. W. Tang et al. Appl Phys Lett 1987, 51, 913 (Kodak Research Labs, Rochester, NY) 1990 Conjugate polymers LEDs (PPV) were first reported by R. H. Friend and coworkers Nature 1990, 347, 539 (Univ. of Cambridge, England) Low quantum efficiency and high operating voltage (>100V) Quantum efficiency ~0.05% quantum efficiency (~1%) and low operating voltage (~10V) 3 cd/A (green) Green yellow Light

11. Progress of Light Emitting Diodes (LEDs) Performance

12. Geometry & Mechanism of PLEDs

13. Schematic of PLED operations Mechanism of PLEDs

14. Mechanism and Design of PLEDs Single-layer LED Structure Energy Level Diagram The problem of charge injection

15. Scheme of Multilayer PLEDs

16. Fabrications of Organic Light Emitting Diodes Electron Transport Layer: Vacuum Evaporation of Dyes/Oligomers Spin Coating of Polymers Emissive Layer: Vacuum Evaporation of Dyes/Oligomers Spin Coating of Polymers Layer-by-layer Self- assembly Hole Transport Layer: Vacuum Evaporation of Dyes/Oligomers Spin Coating of Polymers Cathode: Metal (Al, Mg, Ca) by Vacuum Evaporation Transparent substrate Plastic Glass Anode ITO (sputter) Conducting Polymer (spin coating) Emitters 50~150nm CTL 5~50nm Cathode 100~400 nm ITO 100~500 nm

17. Glass substrates precoated with ITO - 94% transparent - 15 Ω/square Precleaning Tergitol, TCE Acetone, 2-Propanol Growth - 5 x 10 -7 Torr - Room T - 20 to 2000 Å layer thickness Device Preparation and Growth (use thermal coater)

18. Hole Transport Materials (HTM) in PLEDs Triarylamine as functional moiety Poly (9,9-vinlycarazole) (PVK) IP between ITO (φ=4.7) and emitters Typically IP~ 5.0eV

19. SA Jenekhe et al, Chem Mater 2004, 16, 4556 Electron Transport Materials (ETM) in PLEDs EL mechanism Exciton recombination PLED architectures with ETM Energy level diagram Control charge injection, transport, and recombination by ETM  lower barrier for electron injection  μ e > μ h in ETM  Larger △ IP to block hole

20. Electron Transport Materials (ETM) an Electrode in PLEDs Cathode Electrode Small work function of metal Anode Electrode Large work function (ITO, φ a =4.7~4.8 eV) Electron transport materials Reversible high reduction potential Suitable EA & IP for electron injection and hole block High electron mobility High Tg and thermal stability Processability (vacuum evaporation or spin casting) Amorphous morphology (prevent light scattering) Nitrogen-contaning heterocyclic ring Electron withdrawing in main backbone or substituents Commonly used in Cathode Materials SA Jenekhe et al, Chem Mater 2004, 16, 4556 Protective layer

21. Electron Transport Materials in OLEDs Oxadiazole Molecules and Dendrimers Polymeric Oxadiazole Metal Chelates Azobased Materials Triazines Polybenzobisaoles Benzothiadiazole Polymers Pyridine-based Materials SA Jenekhe et al, Chem Mater 2004, 16, 4556

22. Quinoline-based Materials Anthrazoline-based Materials Phenanthrolines Siloles Cyano-containing Materials Perfluorinated Materials Electron Transport Materials in OLEDs High EA ~3eV High degree of intermolecular π- π stacking Enhanced EQE & brightness & luminance yield SA Jenekhe et al, Chem Mater 2004, 16, 4556

23. Visible Spectrum & Color & CIE 1931 Coordinate

24. Emissive Materials in PLEDs Blue emitters Green emitters Red emitters White emitters ~436nm (0.15,0.22) ~546 nm (0.15,0.60) ~700nm (0.65,0.35) (0.33,0.33) cover all visible region

25. Efficiency Experimental setup for direct measurement of EQE External Quantum Efficiency (EQE) N p phonon numberN e electron number Definition of efficiency

26. Mechanism and Design of PLEDs Double Charge (electrons and holes) Injection (At interface) Charge Transport/Trapping Excited State Generation by Charge Recombination Radiative Decay of Excitons γ = injection efficiency if ohmic contact, γ = 1 η = singlet exction generation efficiency~ 0.25? φ = Fluorescence efficiency Key Process in EL Devices

27. Towards Improved PLEDs Better Efficiency (> 5%) High Luminance (>10 6 cd/cm 2 ) Stability with Packaging (5000~25000 hrs) Low operating Voltage (3~10V) Charge Injection (choose suitable work function electrode) Charge Transport (choose high electron and hole mobility)

28. THE ULTIMATE HANDHELD COMMUNICATION DEVICE UDC, Inc. Flexible Internet Display Screen

29. Full color display- Active matrix - 200 x 150 Pixels - 2 inch diagonal Cambridge Display Technology (CDT)

30. Eletrophosphorescence from Organic Materials Excitons generated by charge recombination in organic LEDs Spin statistics says the ratio of singlet : triplet, 1 P* : 3 P*= 1 : 3 To obtain the maximum efficiency from an organic LED, one should harness both the singlet and triplet excitations that result from electrical pumping 2 P + ‧ + 2 P - ‧ 1 P* + 3 P* Singlet :electroluminescenceTriplet: electrophosphorescence

31. Eletrophosphorescence from Organic Materials The external quantum efficiency (η ext ) is given by η ext = η int η ph = (γ η ex φ p )η ph η ph = light out-coupling from device η ex = fraction of total excitons formed which result in radiative transitons (~0.25 from fluoresent polymers) γ = ratio of electrons to holes injected from opposite contacts φ p = intrinsic quantum efficiency for radiative decay If only singlets are radiative as in fluorescent materials, η ext is limited to ~ 5%, assuming η ph ~ 1/2n 2 ~ 20 % for a glass substrate (n=1.5) By using high efficiency phosphorescent materials, η int can approach 100 %, in which case we can anitcipate η ph ~ 20 %

32. All emission colors possible by using appropriate phosphorescent molecules Maximum EQE Blue emittersGreen emittersRed emitters 7.5 ± 0.8 %15.4 ± 0.2 % 7 ± 0.5% Nature, 2000, 403, 750APL 2003, 82, 2422APL, 2001, 78, 1622 From S. R. Forrest Group (EE, Princeton University) High Efficiency LEDs from Eletrophosphorescence Organometallic compounds which introduce spin-orbit coupling due to the central heavy atom show a relatively high ligand based phosphorescence efficiency even at room temperature

33. http://www.cibasc.com/pic-ind-pc-tech-protection-lightstabilization2.jpg As DCM2 acts as a filter that removes singlet Alq 3 excitons, the only possible origin of the PtOEP luminescence is Alq 3 triplet states that have diffused through the DCM2 and intervening Alq3 layers.