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Oled

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Published in: Physics
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organic light emitting diode

Narendra K / Mumbai

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Teaches: Chemistry, Mathematics, Physics, B.Sc Tuition, CTET, KVPY Exam

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  1. Current Balance is made more difficult because of large difference in carrier mobility O 000 0 Cathode LUMO HOMO Alq3 . Anode 88888 88 & =1.9x104cm2/Vs gno=1.9x10-8cm2/Vs
  2. Carrier Profile in Single Layer OLED with widely differing electron and hole mobilities Recombination 0.02 0.04 0.06 0.08 Profile 6x1017 5x1017 E 4x1017 = 311017 211017 O Ixi017 0.00 BN 0.02 0.04 0.06 0.08 0.10 Distance From Cathode (um) 0.12 2.ox1021 0.0 0.00 0.10 0.12 Distance From Cathode (pm)
  3. • large fraction of holes are simply collected at the Cathode. Most of recombination takes place close to Cathode where probability of non-radiative recombination is high. Key Problem: How do we balance electron and hole currents?
  4. Must Keep Excitons away from electrodes PL Quenching in a film of 10011m: Polymer Glass -17.400 Polymer/1TO Glass 12.300 Metal/P01ymer/1TO Glass 8.300 Nature 1999
  5. Use two different organic materials with properties such that electron flo» to Anode and hole flow to cathode is blocked. HTL Cathode ETL o o 000 Anode o oo co
  6. Carrier Profile in Bilayer OLED Recombination Profile 0 0 3.5x1020 3.ox1020 2.5x102Û ?.5x1020 ?.ox102Û 5.ox1019 0.0 -5.ox?o?g 0.00 0.02 0.04 0.06 0.08 0.10 6x? 22 5x?o 4x1022 2x1022 1x10ü -IXI 022 0.00 0.02 0.04 0.06 0.08 0.10 Distance From Cathode (pm) Distance From Cathode (Pm)
  7. Key: Organic Heterostructures Key to all success In th1S area is understanding and engineering amorphous organic heterostructures
  8. Organic Light Emitting Diodes (OLEDs) Single Layer Design Anode Organic layer Cathode Multilayer Design Anode HTL EL ETC Cathode
  9. l) Hole Transport Materials NPD TPD There are several established hole transport materials. The more mature materials are NPD and TPD. Both are used extensively in the current OLED design.
  10. Electron Transport Materials Probably most prominent OLED material in OLED is DAIq3 Not only it is a emissive material, it is also an electron transporter. More recent electron transport material is ADN
  11. External Quantum Efficiency of photons emitted //EQE of carriers injected QEQE = Oint Oc = ( X = ratio of electrons to holes injected from opposite contacts X - fraction of total excitons formed which result in radiative transitions (Singlet:TripIet :: 1:3; NO.25 from fluorescent polymers) Imposes fundamental limit to OLEDs = intrinsic luminescence efficiency of the material = light out-coupling from device
  12. Outcoupling Efficiency For n=l .6, 22% Il out
  13. Maximum EQE? nEQE = nt X nc N 5 - 60/0
  14. A BRIEF HISTORY OF QUANTUM EFFICIENCY 100 3 phosphorescence (small molecule) luetes tent M.A.Baldo 0.01 tgso i 970 ego -2000 Year Improved outcup/ing ( RIG) Euploitjng triplet excjt011$ (z) Improvlng material PL efficfencleö (m) Improving electron-hole ba/ance Through: Device engineering Materials Engineering 'http:/twww.rle.mit.edu/rleonline/People/MarcA.Batdo.html
  15. Dye Doped OLEDs Excitons formed from combination of electrons and holes transparent anode ö00 0000 holes 2.6 ev a-NPD 5.7eV .:ooooo host molecules •.0 0 00008: (charge transport material) 000 oo 0000 0000 dopant molecule electrons 2.7 trap states low work functio cathode AIG exciton 6.0 2. Excitons transfer to luminescent dye (luminescent dye)
  16. Dye Doped OLEDs •Dyes have 100% luminescent efficiency Ways to Improve •Get new colors 'How to employ Dyes in OLED????? ---Use Energy transfer mechanisms
  17. Forster Energy Transfer Initial State Final State DOUOR ACCEPTOR Radiationless Resonannce Transfer of Energy Singlets: Coulombic; Resonance Effective : can compete with non-radiative transfer R— 50 to 100 A (over large distances) Spin Consenration a must
  18. Dexter Energy Transfer Donor Acceptor Dexter Energy Transfer (Singlet to Singlet) Guest Hut Guest Dexter Energy Transfer (Triplet to Triplet) Exchange interactions (tunneling) Orbital overlap needed R— 5-10 A (exponential dependence) Relaxed selection mles (overall multiplicity consen-ed, not spun) Triplet Energy Transfers are allowed. •Gue:t Host Guest
  19. Requirement for energy transfer Spectral overlap Host Dopant Wave length
  20. Dye Doped OLEDs Excitons formed from combination of electrons and holes transparent anode ö00 0000 holes 2.6 ev a-NPD 5.7eV .:ooooo host molecules •.0 0 00008: (charge transport material) 000 oo 0000 0000 dopant molecule electrons 2.7 trap states low work functio cathode AIG exciton 6.0 2. Excitons transfer to luminescent dye (luminescent dye)
  21. Dyes These are several dye examples. They are for two main purposes: For colour tuning. For colour stabilisation. Rubrene is used to dope Alq3 for yellow emission. DCM Il and DCJTB are used to dope AIC13 for red emission. NC CN NC CN Rubrene DCM Il DCJTB
  22. C545 is used for green colour stabilisation. Perlene is used for blue colour stabilisation.
  23. DYE-DOPED OLEDs Cathode(M3Ag) EML/ETL(AIQ3) HTL (Diamine) Substrate coumann 540 Doped AlQ3 H, o o •use Förster energy transfer to separate luminescent and charge transpon functions. •Get new colors. Improve Tang, Van Slyke, Chen Electroluminescence of doped organic thin films. JAP 65 3610 (1989)
  24. Effect of Dopants on the OLED EL Spectrum z 1.0 0.8 0.6 0.4 0.2 0.0 o O a-NPD O 400 N CN DCM2:Alq3 PtOEP:Alq3 500 600 700 800 Wavelength [nm]
  25. Organic Phosphorescent Dyes Heavy metal facilitated tiiplet emission pt N N PtOEP Triplet lifetime T 100 PIs Triplet lifetime T MLCT exciton ligand c*Vexciton c 'ligand 'Nigand o metat ligand metal ligand Strong Spin-orbit coupling due to heavy metal ions leading to --lmproved intersystem crossing from singlet to triplet I pts states --- Reduced life time of triplet states
  26. Optical propenies of Organic Phosphorescent Dyes Absor ion 400 hotolu •nescence 0.4 0.2 1300 PdOEP Singlet 500 Wavelen Triplet 700 20 15 10 800 600 h nm
  27. Blue emitters 7.5±0.80/0 APL 2003, 82, 2422 Flrpic Green emitters 15.4 ± 0/0 Nature, 2000, 403, 750 Red emitters n APL, 2001, 78, 1622
  28. Mg:Ag Alq3 (10nm) Å1q3:PtOEP (40nm) a-NPD (35nm) CuPc (6nm) ITO glass Phosphorescent OLED Alq3 Baldo et al. Nature 395, 151 (1998) PtOEP ISC Forster Dexter
  29. Effect of dye concentration (ppyhlrpic 1.0 0.8 0.6 0.4 0.2 0.0 200 250 300 350 400 450 500 Wavelength (nm) PVK 0 wt0/ 8 wt% 5 wt% 3 wt% 1 wt% 0.5 wtO/0 550 600 1.0 0.6 0.4 0.2 0.0 650 (a) 4 2 PVK 350 s 40.0 - -466 500 550 600 650 Wavelength (nm) Phosphorescent dye doped in PVK matrix
  30. High Efficiency OLEDs from Eletrophosphorescence Doping phosphorescent dyes into an emitting host material 1.8 1.6 1.4 08 04 02 00 PtOEP rvWAg 700Å ooÅ DCM2JAl 350Å a-NPO 60Å cupc ITO Glass Device 1 DCM2 700 Wavelength 400Å 1 coÅ PtOEP/Al 1 ooÅ Al ÅDCWAI 350Å WNPO 6 CuPc ITO Glass Device 2 Device 1: Florescent dye DCM Device 2: DCM + Phosphorescent dye PtOEP Realized for the first time by Mark Baldo (M A Baldo et al, Nature 395 (1998), 151) 25 Host: AIq3
  31. OLED for lighting: White OLED White light is made up of nearly equal intensities of light from all regions (VIBGYOR) of the visible spectrum. Combining three different wavelengths of light - primary colors'. 'complementary colors' where the combination of only two colors can produce white light. Organic molecules generally have very broad emission spectra, therefore, combination of emission from not exactly complementary colors may also give white light.
  32. Various Approaches to obtain white light • Host-guest system Multilayer structure Exciplex emission Microcavity Down conversion phosphorous • Single Molecule
  33. Host-Guest system ITO 4.9-5.1 e o o Emissive layer mixed with RGB dyes Q.) o O 5 400 • Single/multiple dopants • Full/partial energy transfer • Fluorescent/phosphorescent dopants • Colour tunability — Dopant concentration — Dependent on energy transfer rates — Change in colour with current densities 500 600 700 wavelength (nm) 0.7 0.6 0.5 0.3 0.2 0.0 0.0 0.1 0.2 0.4 0.5 0.7 CIE x
  34. Multilayer WOLED White emission Anode Hole Blue Green HTL EML EML Red EML Electron Cathode ETC • Mixing of the emission from different layers • Spectral characteristic by adjusting the recombination-zone
  35. Full Co -0 ? Display ? ? ? ? Arrangement Of RGB pixels
  36. Methods for full-color display fabrication RGB method Color conversion method Cathode Color filter method Cathode Cathode Anode Glass Red Green Blue Red Green Blue
  37. Patterned-Emitting Layer Advanlagcs Layer Smiäi.ng Ley-r Emitting Disadvantages • lypicajJyuses shadow ilJf R', —n"/ PAIR."YYiif'i" • inqofRGB White-Emitting Layer with Color-filter Array Advanlagcs oaaclvanlages (no masks) Fjfr.ur .å'l;.snryliun h'GY pallerninp • by high efficiency white Blue-Emitting Layer with Color-Change Medium Advantages • Unpatemed em•ting • Requj.æs efficiency blue • Compatibility of materials s&.f lilfwgwvfiY

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