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Electronic and Optoelectronic Polymers

Electronic and Optoelectronic Polymers. Wen-Chang Chen Department of Chemical Engineering Institute of Polymer Science and Engineering National Taiwan University. Outlines. History of Conjugated Polymers Electronic Structures of Conjugated Polymers Polymer Light-emitting Diodes

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Electronic and Optoelectronic Polymers

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

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

  3. Optical Absorbance Absorption of light and the excited states of molecules Beer-Lambert Law A = 2 - log10 %T A is absorbance C is concentration I0 is intensity of incident light λ is wavelength of light I1 is intensity after passing through the materials l is path length k is extinction coefficient α is molar absorptivity or absorption coefficient 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

  4. Photophysics Process Jablonski Diagram Non-Radiative 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 ( )

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

  6. Photophysics Process Radiative Process (S1 S0) (T1 S0) >100ns 0.1~10ns 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

  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) Excited States are produced upon light absorption by a conjugated polymers Charge Transfer (CT) Exciton : typical of organic materilas Ground state Excited state Molecular picture binding energy ~1eV Diffusion radius ~10Å 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 1963 First organic electroluminescene based on anthracene single crystal Low quantum efficiency and high operating voltage (>100V) 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) quantum efficiency (~1%) and low operating voltage (~10V) 3 cd/A (green) 1990 Conjugate polymers LEDs (PPV) were first reported by R. H. Friend and coworkers Nature 1990, 347, 539 (Univ. of Cambridge, England) Quantum efficiency ~0.05% Green yellow Light

  11. Progress of Light Emitting Diodes (LEDs) Performance

  12. Geometry & Mechanism of PLEDs

  13. Mechanism of PLEDs Schematic of PLED operations

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

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

  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. Electron Transport Materials (ETM) in PLEDs EL mechanism Energy level diagram Exciton recombination PLED architectures with ETM Control charge injection, transport, and recombination by ETM • lower barrier for electron injection • μe > μh in ETM • Larger △IP to block hole SA Jenekhe et al, Chem Mater2004, 16, 4556

  20. Electron Transport Materials (ETM) an Electrode in PLEDs Cathode Electrode Small work function of metal Electron transport materials Commonly used in Cathode 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) Protective layer Nitrogen-contaning heterocyclic ring Electron withdrawing in main backbone or substituents Anode Electrode Large work function (ITO, φa=4.7~4.8 eV) SA Jenekhe et al, Chem Mater2004, 16, 4556

  21. Electron Transport Materials in OLEDs Benzothiadiazole Polymers Oxadiazole Molecules and Dendrimers Triazines Polymeric Oxadiazole Azobased Materials Polybenzobisaoles Metal Chelates Pyridine-based Materials SA Jenekhe et al, Chem Mater2004, 16, 4556

  22. Electron Transport Materials in OLEDs Quinoline-based Materials Phenanthrolines Anthrazoline-based Materials Siloles Cyano-containing Materials Perfluorinated Materials High EA ~3eV High degree of intermolecular π- π stacking Enhanced EQE & brightness & luminance yield SA Jenekhe et al, Chem Mater2004, 16, 4556

  23. Visible Spectrum & Color & CIE 1931 Coordinate

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

  25. Efficiency Experimental setup for direct measurement of EQE External Quantum Efficiency (EQE) Np phonon number Ne electron number Definition of efficiency

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

  27. Towards Improved PLEDs Better Efficiency (> 5%) High Luminance (>106 cd/cm2) 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. Flexible Internet Display Screen THE ULTIMATE HANDHELD COMMUNICATION DEVICE UDC, Inc.

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

  30. Eletrophosphorescence from Organic Materials Excitons generated by charge recombination in organic LEDs 2P+‧ + 2P-‧1P* + 3P* Singlet :electroluminescence Triplet: electrophosphorescence Spin statistics says the ratio of singlet : triplet, 1P* : 3P*= 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

  31. Eletrophosphorescence from Organic Materials The external quantum efficiency (ηext) is given by ηext = ηint ηph = (γ ηex φp )ηph ηph = light out-coupling from device ηex = 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 φp = intrinsic quantum efficiency for radiative decay If only singlets are radiative as in fluorescent materials, ηext is limited to ~ 5%, assuming ηph ~ 1/2n2~ 20 % for a glass substrate (n=1.5) By using high efficiency phosphorescent materials, ηint can approach 100 %, in which case we can anitcipate ηph ~ 20 %

  32. 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 All emission colors possible by using appropriate phosphorescent molecules From S. R. Forrest Group (EE, Princeton University) Maximum EQE Blue emitters Green emitters Red emitters 7.5 ± 0.8 % 15.4 ± 0.2 % 7 ± 0.5% APL 2003, 82, 2422 Nature, 2000, 403, 750 APL, 2001, 78, 1622

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

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