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Paul E. Burrows PhD Energy Sciences and Technology Directorate Manager, Nanoscience and Technology Initiative Pacific Northwest National Laboratory OLEDs: A bright opportunity for vacuum technology Slide 2 Disclaimer: this is not the whole story "Never try to tell everything you know. It may take too short a time." - Norman Ford Slide 3 What are they? A sense of history LED : OLED key differences What we dont understand, why its interesting Making OLEDs: Large area and manufacturing The lure of plastic Slide 4 6/19/034 The organic zoo: Phylum Small molecule Polymer Dendrimer This lecture will mostly focus on these Slide 5 6/19/035 Class: Small Molecule Organics Slide 6 The History of Manufacturing 1. Stone Age 3. Molecular Age 2. Micro-Stone Age Intel 4004 Slide 7 Why OLEDs are not LEDs Inorganic LEDs (e.g. InGaN) Crystalline, epitaxial OLEDs Amorphous, flexible, weak adhesion, structural complexity p,n-doping Generally can be either p- or n-doped with substitutional dopant atoms at 10 15 10 20 /cm 3 Materials are either electron or hole conducting. Negligible background charge carrier density. Electronic doping requires 1 5% loading and chemically changes the host molecules mobility up to ~ 1000 cm 2 /VsHoles: 10 -3 cm 2 /Vs Electrons: 0 10 -4 cm 2 /Vs voltage and field dependent excited states Electronic: light generated by band-to-band recombination, weakly bound excitons, weak exciton-phonon coupling Excitonic: correlated e - -h + pairs conduction bands meaningless, strongly bound excitons, strong exciton-phonon coupling Slide 8 W. Helfrich & W.G. Sneider Phys. Rev. Lett. 14(7), 229 (1965) Anthracene (C 14 H 10 ) electrode +- 5 mm 1000V Slide 9 J. Dresner, RCA Rev. 30, 322 (1969) Anthracene (C 14 H 10 ) Thin gold electrode Ag Paste electrode + - 50 m 100-1000V 8% external quantum efficiency Slide 10 Hole transporter Light Transparent conductor Electron transporterCathode C.W. Tang, U.S. Patent # 4,356,429 (1980) Vacuum deposition enabled thin electron transport layer Hole transport layer was spin-coated polymer: 10 20 V, 15cd/m 2 brightness All vacuum device: 10 20 V, 100 cd/m 2 using Alq 3 emission layer C.W. Tang and S.A. VanSlyke Appl. Phys. Lett. 51, 913(1987) 100 nm Slide 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 Slide 12 OLEDs: The Future Kodak/Sanyo active-matrix display features full-color, 1280 x 720 (HDTV) resolution Sony: 13 inches,800 x 600, low temperature poly-silicon TFT active matrix using organic phosphorescence Not shown: Toshiba 17inch AM OLED with resolution of 1280 x 768 pixels. Slide 13 Complexity of Molecular Systems u There has been an alarming increase in the number of things we dont understand u Why we need more research! Slide 14 Slide 15 The effects of traps Slide 16 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 Alq 3 data Are extracted parameters meaningful? MetalOrganic Trap Charge Limited Burrows, et al, J. Appl. Phys. (1996) 79, 7991 Energy Distance LUMO Trap distribution EFEF Interface Limited Injection Baldo & Forrest Phys Rev. B. (2001), 64, 085201 MetalOrganic Interfacial Dipole layer Energy Distance LUMO EFEF MetalOrganic Trap Charge Limited Burrows, et al, J. Appl. Phys. (1996) 79, 7991 Energy Distance LUMO Trap distribution EFEF MOTIVATION: Correlate current conduction w/ molecular structure Slide 17 Alq 3 Do we know what we have? mer-Alq 3 Higher symmetry More polar ( ~ 7D vs. 5.3D) Higher energy (4.7kcal/mol) Trap state for electron ? (Curioni et al. Chem. Phys. Lett. (1998) 294, 263) Several polymorphic phases, all involve interactions of mer enantiomeric pairs Brinkman, et al., JACS, 122, 5147 (2000) C1C1 fac-Alq 3 C3C3 Braun, et al, J. Chem. Phys. (2001) 114, 9625. Amati & Lelj, Chem. Phys. Lett. (2002) 358, 144 Slide 18 6/19/0318 Degrees of Freedom: Dynamical Motions for AlQ 3 Single frame Overlaid Trajectory Frames Dynamical trajectory shows quinolate ring motion about Al coordination Slide 19 Organic Electroluminescence 2. Excitons transfer to luminescent dye 1. Excitons formed from combination of electrons and holes 6.0 eV a-NPD 2.6 eV 5.7eV Alq 3 2.7 eV electrons exciton trap states low work function cathode transparent anode holes dopant molecule (luminescent dye) host molecules (charge transport material) + - Slide 20 Why its important to put the right spin on your excitons: u 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 u 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 Slide 21 Fluorescence ground state (singlet) singlet excited state triplet excited state FLUORESCENCE singlet exciton symmetry conserved triplet exciton PHOSPHORESCENCE Phosphorescence triplet to ground state transition is not permitted fast process ~10 -9 sslow process ~ 1s Slide 22 From fluorescence towards phosphorescence Collect all the singlets and triplets: 100% efficiency Baldo et al., Nature 395, 151 (1998), Susuki et al. APL 69 224 (1996) El in benzophenone at 100 K. N Ir R 3 R = F, OMe,... Slide 23 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 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 M.E. Thompson University of Southern California Slide 24 0.16, 0.37 0.30, 0.63 0.65, 0.35 0.57, 0.43 0.61, 0.38 Phosphorescent OLED Status* 0.70, 0.30 + 0.15, 0.22 + 0.14, 0.23 1931 CIE chart *Subset of PHOLEDs Courtesy Universal Display Corporation Slide 25 Xxxxxx PhOLED Technology (Phosphorescent OLED) Courtesy Universal Display Corporation White PHOLEDs CIE = (0.37, 0.40), CRI = 83 31,000 cd/m 2 at 14V 6.4 lm/W US patents: 6,303,238 6,097,147 * Under development 14 lm/W 6 lm/W Breaking news: lower voltage structures further improve power efficiencies by 20 50% no data Slide 26 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/m 2 Outcoupling x5:70 lm/W Voltage decrease,140 lm/W 2 possible This assumes no further increase in quantum efficiency! Slide 27 Manufacture and Scale-Up Slide 28 Assembling OLEDs at PNNL System by Angstrom Engineering Inc. Andrew Bass et al. 4 substrate, organic deposition (thermal), oxides (sputtering), metal (thermal) Slide 29 People are serious about OLED! Slide 30 Large area? Kodak thermal deposition Society for Information Display Annual Meeting 2002 Slide 31 Alternative: OVPD, The R&D Concept Cooled Substrate Carrier Source 1 (Host) Source 2 (Dopant) Gas Phase Transport by Inert Carrier Gas, ~ 1 Torr Multiple Zone Heater SublimationTransportCondensation "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 Slide 32 OVPD scaleup vs thermal evaporation Substrate Showerhead Shadow Mask Highly efficient deposition Gas phase controlled No bowing of shadow mask Inefficient deposition (wall coating) Temperature controlled Bowing of shadow mask Close Coupled Courtesy Universal Display Corporation Slide 33 - Web-based processing - Cost-effectiveness What about plastic? Supply Roll Product Roll OLED Deposition Patterning Encapsulation Tensioner Slide 34 So Whats the problem? Photos: Courtesy of Dupont Displays Photo: Courtesy of Universal Display Corporation U.S. Patent No. 5,844,363 Slide 35 Light Oxide H 2 O, O 2 Degradation of Organic Devices Slide 36 Rigid OLED Architecture: Stainless steel can Glass ITO OLED layers desiccant Epoxy adhesive membrane Pioneer Patent EP 0 776 147 A1 Flexible (FOLED) Architecture: Flexible moisture barrier substrate Flexible thin film encapsulation Typical lifetimes 5k 100k hours Blue is generally the least stable Slide 37 10 -6 10 -4 10 -2 10 0 2 4 PNB, ArtonPET (hardcoat)Organic CoatingsInorganic Coatings PECVD Barix Limit of MOCON measurement OLED Requirement H 2 O Permeation Rate (g/m 2 /day at 25C) Slide 38 PET High Speed, Large Area Monomer Liquid Cure Ceramic Deposition Multilayer Barrier Deposition: Slide 39 (i) (ii) L 0 = 400 cd/m 2 ITO/CuPc(10nm)/NPD(30nm)/CBP:Irppy[6%](30nm)/BAlq(10nm)/Alq 3 (40nm)/LiF(1nm)/Al(100nm) 1200 hr 3000 hr 2 mm pixel Irppy-based OLED: PET substrate, glass lid Constant current, DC drive Appl. Phys. Lett. 81, 2929 (2002) Slide 40 PNNL Rollcoating 7 web 2 monomer sources 3 inorganic sources UV, ebeam or plasma cure Polymer evaporation Composite extrusion Oxide deposition Slide 41 Latest Flexible Display Results: 2000 hours at L 0 = 600 cd/m 2 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.) Slide 42 Opportunities and Challenges (by way of conclusion) u Flat Panel Displays: $70B worldwide market u OLEDS: $2B by 2006 (by some estimates) u Next Generation Lighting u Practical if we can reach 50 lm/W u 22% of US electricity generation goes for lighting u Luminescent wallpaper? u Dual or multi use windows using transparent OLEDs? u Lifetime, particularly in blue u Large area scale-up at very high yield and low cost u Commercial scale-up production lines with minimal downtime u Supply infrastructure?? Materials purity assay etc. u Still insufficient understanding of basic material structure-property relationships