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OLEDs: A bright opportunity for vacuum technology

OLEDs: A bright opportunity for vacuum technology. Paul E. Burrows PhD Energy Sciences and Technology Directorate Manager, Nanoscience and Technology Initiative Pacific Northwest National Laboratory. Disclaimer: this is not the whole story….

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OLEDs: A bright opportunity for vacuum technology

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  1. OLEDs:A bright opportunityfor vacuum technology Paul E. Burrows PhD Energy Sciences and Technology Directorate Manager, Nanoscience and Technology Initiative Pacific Northwest National Laboratory

  2. Disclaimer:this is not the whole story… "Never try to tell everything you know. It may take too short a time." - Norman Ford

  3. OLED: Organic Light Emitting Device • What are they? • A sense of history • LED : OLED… key differences • What we don’t understand, why it’s interesting • Making OLEDs: Large area and manufacturing • The lure of plastic

  4. Small molecule This lecture will mostly focus on these Polymer The organic “zoo”: Phylum Dendrimer

  5. Class: “Small Molecule” Organics

  6. 1. Stone Age 2. Micro-Stone Age Intel 4004 3. Molecular Age The History of Manufacturing

  7. Why OLEDs are not LEDs

  8. 1000V electrode + - electrode W. Helfrich & W.G. SneiderPhys. Rev. Lett. 14(7), 229 (1965) 5 mm Anthracene (C14H10)

  9. 100-1000V Thin gold electrode - Ag Paste electrode + J. Dresner, RCA Rev. 30, 322 (1969) 50 mm Anthracene (C14H10) 8% external quantum efficiency

  10. 100 nm Cathode Light Electron transporter Hole transporter C.W. Tang, U.S. Patent # 4,356,429 (1980) Transparent conductor • Vacuum deposition enabled thin electron transport layer • Hole transport layer was spin-coated polymer: 10 – 20 V, 15cd/m2 brightness • All vacuum device: 10 – 20 V, 100 cd/m2 using Alq3 emission layer • C.W. Tang and S.A. VanSlyke Appl. Phys. Lett. 51, 913(1987)

  11. OLED products available: Kodak LS633 Camera 2.2 inch, 512x218 OLED screen, ~ $500 (in partnership with Sanyo) Not yet available in USA Optrex Instrument Cluster BMW 7 series $85,000 (car included) Not shown: Philips OLED-equipped electric shaver

  12. OLEDs: The Future… Sony: 13 inches,800 x 600, low temperature poly-silicon TFT active matrix using organic phosphorescence Kodak/Sanyo active-matrix display features full-color, 1280 x 720 (HDTV) resolution Not shown: Toshiba 17inch AM OLED with resolution of 1280 x 768 pixels.

  13. Complexity of Molecular Systems • There has been an alarming increase in the number of things we don’t understand… • Why we need more research!

  14. The effects of traps…

  15. Trap Charge Limited Burrows, et al, J. Appl. Phys. (1996) 79, 7991 Trap Charge Limited Burrows, et al, J. Appl. Phys. (1996) 79, 7991 Interface Limited Injection Baldo & Forrest Phys Rev. B. (2001), 64, 085201 LUMO LUMO LUMO EF EF EF Trap distribution Trap distribution Energy Energy Energy Metal Metal Organic Organic Metal Organic Interfacial Dipole layer Distance Distance Distance • Assumes bulk effects limit current conduction • Assumes trap energies are exponentially distributed below LUMO • Neglects voltage and temperature dependence of mobility (secondary to trap effects) • Assumes charge separation at the metal-organic interface, which creates dipole layer • Assumes dipolar disorder in the bulk Both models only fitted to Alq3 data Are extracted parameters meaningful? MOTIVATION: Correlate current conduction w/ molecular structure

  16. mer-Alq3 Higher symmetry More polar (m ~ 7D vs. 5.3D) Higher energy (4.7kcal/mol) Trap state for electron ? (Curioni et al. Chem. Phys. Lett. (1998) 294, 263) C1 Several polymorphic phases, all involve p-p interactions of mer enantiomeric pairs Brinkman, et al., JACS, 122, 5147 (2000) fac-Alq3 C3 Alq3 – Do we know what we have? Braun, et al, J. Chem. Phys. (2001) 114, 9625. Amati & Lelj, Chem. Phys. Lett. (2002) 358, 144

  17. Degrees of Freedom: Dynamical Motions for AlQ3 Single frame Overlaid Trajectory Frames • Dynamical trajectory shows quinolate ring motion about Al coordination

  18. - + host molecules (charge transport material) dopant molecule (luminescent dye) Organic Electroluminescence 1. Excitons formed from combination of electrons and holes 2.6 eV electrons 2.7 eV trap states low work function cathode exciton a-NPD Alq3 holes transparent anode 5.7eV 6.0 eV 2. Excitons transfer to luminescent dye

  19. Electrical excitation is spin-random • Simple statistics  25% singlets, 75% high spin triplet state (vertical recombination to ground state “forbidden”) • e-h correlation may change this ratio • some evidence of > 25% singlets in polymers • remains a controversial area Why it’s important to put the right spin on your excitons: • Optical excitation is spin-conserved • a spin zero ground state produces a spin zero excited state which can vertically relax back to the ground state with unit quantum efficiency

  20. Fluorescence Phosphorescence singlet excited state triplet exciton triplet excited state   FLUORESCENCE PHOSPHORESCENCE ground state (singlet) singlet exciton symmetry conserved triplet to ground state transition is not permitted fast process ~10-9s slow process ~ 1s

  21. From fluorescence towards phosphorescence Collect all the singlets and triplets: 100% efficiency N Ir 3 R Baldo et al., Nature 395, 151 (1998), Susuki et al. APL 69 224 (1996) El in benzophenone at 100 K. R = F, OMe, ...

  22. PL eff. = 0.35 • = 4 sec (77K) • max = 525 nm • PL eff. = 0.4 • = 2 sec max = 555 nm • PL eff. = 0.05 • = 2 sec max = 590 nm • PL eff. = 0.2 • = 2 sec max = 605 nm Phosphorescent molecules enable triplet state recombination • Heavy metal ion causes spin-orbit coupling with organic ligand • Symmetry broken  allowed phosphorescent recombination • Color tuning by ligand choice M.E. Thompson University of Southern California

  23. Phosphorescent OLED Status* 1931 CIE chart 0.57, 0.43 0.61, 0.38 0.30, 0.63 0.65, 0.35 0.16, 0.37 + + 0.70, 0.30 0.15, 0.22 0.14, 0.23 *Subset of PHOLEDs Courtesy Universal Display Corporation

  24. PhOLED Technology (Phosphorescent OLED) Courtesy Universal Display Corporation Xxxxxx no data 6 lm/W 14 lm/W * Under development • White PHOLEDs • CIE = (0.37, 0.40), CRI = 83 • 31,000 cd/m2 at 14V • 6.4 lm/W US patents: 6,303,238 6,097,147 Breaking news: lower voltage structures further improve power efficiencies by 20 – 50%

  25. What is the limit of the possible?20% of the light from a simple OLED escapes a planar device Existing: 14 lm/W green at 250 cd/m2 Outcoupling  x5: 70 lm/W Voltage decrease, 140 lm/W ÷ 2 possible This assumes no further increase in quantum efficiency!

  26. Manufacture and Scale-Up

  27. Assembling OLEDs at PNNL System by Angstrom Engineering Inc. Andrew Bass et al. 4” substrate, organic deposition (thermal), oxides (sputtering), metal (thermal)

  28. People are serious about OLED!

  29. Large area? Kodak thermal deposition Society for Information Display Annual Meeting 2002

  30. Multiple Zone Heater Sublimation Transport Condensation Alternative: OVPD, The R&D Concept Source 1 (Host) Cooled Substrate Carrier Source 2 (Dopant) Gas Phase Transport by Inert Carrier Gas, ~ 1 Torr "Low Pressure Organic Vapor Phase Deposition of Small Molecular Weight Organic Light Emitting Device Structures.“ Appl. Phys Lett. 71, 3033 (1997) Courtesy Universal Display Corporation

  31. OVPD scaleup vs thermal evaporation Substrate Close Coupled Shadow Mask Showerhead Substrate • Highly efficient deposition • Gas phase controlled • No bowing of shadow mask • Inefficient deposition (wall coating) • Temperature controlled • Bowing of shadow mask Courtesy Universal Display Corporation

  32. OLED Deposition Encapsulation Tensioner Patterning Supply Roll Product Roll What about plastic? - Web-based processing - Cost-effectiveness

  33. U.S. Patent No. 5,844,363 So… What’s the problem? Photos: Courtesy of Dupont Displays Photo: Courtesy of Universal Display Corporation

  34. Light Light Degradation of Organic Devices Oxide H2O, O2

  35. Glass OLED layers ITO Stainless steel can Epoxy adhesive membrane desiccant Rigid OLED Architecture: Pioneer Patent EP 0 776 147 A1 Typical lifetimes 5k – 100k hours Blue is generally the least stable Flexible (FOLED) Architecture: Flexible moisture barrier substrate Flexible thin film encapsulation

  36. Limit of MOCON measurement OLED Requirement Inorganic Coatings Organic Coatings PET (hardcoat) PNB, Arton Barix™ PECVD -6 -4 -2 0 2 4 10 10 10 10 10 10 H2O Permeation Rate (g/m2/day at 25ºC)

  37. Multilayer Barrier Deposition: Monomer Liquid Ceramic Deposition Cure PET High Speed, Large Area…

  38. Irppy-based OLED: PET substrate, glass lid Constant current, DC drive L0 = 400 cd/m2 (i) (ii) 1200 hr 3000 hr 2 mm pixel Appl. Phys. Lett. 81, 2929 (2002) ITO/CuPc(10nm)/NPD(30nm)/CBP:Irppy[6%](30nm)/BAlq(10nm)/Alq3(40nm)/LiF(1nm)/Al(100nm)

  39. PNNL Rollcoating • 7” web • 2 monomer sources • 3 inorganic sources • UV, ebeam or plasma cure • Polymer evaporation • Composite extrusion • Oxide deposition

  40. Latest Flexible Display Results: 2000 hours at L0 = 600 cd/m2 for green phosphorescent OLED display on plastic (passive matrix 128 x 64) (A. Chwang et al. Materials Research Society Conference, April 2003 Collaboration between Universal Display Corporation, Pacific Northwest National Laboratories and Vitex Systems Inc.)

  41. Opportunities and Challenges(by way of conclusion) • Flat Panel Displays: $70B worldwide market • OLEDS: $2B by 2006 (by some estimates) • Next Generation Lighting • Practical if we can reach 50 lm/W • 22% of US electricity generation goes for lighting • Luminescent wallpaper? • Dual or multi use windows using transparent OLEDs? • Lifetime, particularly in blue • Large area scale-up at very high yield and low cost • Commercial scale-up… production lines with minimal downtime • Supply infrastructure?? Materials purity assay etc. • Still insufficient understanding of basic material structure-property relationships

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