rganic Photonic Materials onlinear ptics Materials rganic Light Emitting Diode (LED)
onlinear optics The interaction of electromagnetic fields with various media to produce new electromagnetic fields altered in phase, frequency, amplitude from the incident fields
Second harmonic generation (SHG), the conversion of coherent light of frequency into light of frequency 2 The electro-optic effect allows one to change the refractive index of a material by simply applying a DC electric field to the material; thus, one can utilize the modulation of an electrical signal to activate an optical switch.
The polarization P induced in a molecule by a local electric field E P= E + E 2 + E 3 + linear polarizability (the origin of refractive index) second order hyperpolarizability (the origin of the second order nonlinear polarization response)
Push-Pull in a Donor-Acceptor
Values of Some rganic Chromophores (10-30 esu, 1064 nm)
Charge Transfer Resonance Structures First, the greater the charge separation in the charge transfer state (Dm), the larger the Second, the closer the frequency of the incident light is to the resonant frequency of the charge transfer, the larger the
rganic Electro-ptic Materials A Historical Perspective Statistical mechanical calculations suggested a new paradigm optimization of electro optic activity: Control chromophore shape! R CLD-1 R = TBDMS R R R C C C C C C CLD-2 CLD-3 R = H R=H 140 120 100 80 60 40 20 0 CLD-2 CLD-3 Disperse Red (1995) fi 0 20 40 60 o. Density (10^19/cc) R R R' S R' C C C FTC-1 FTC-2 R = Ac, R' = H R = Ac R' = CH 2 CH 2 CH 2 CH 3
For Bulk Materials P = (1) E + (2) E 2 + (3) E 3 +...
Fabrication of organic second order L materials organic crystal growth, inclusion complexes, mono- and multilayered assemblies (e.g. Langmuir-Blodgett films), poled polymers
Polymer poling The polymer is heated above the glass transition temperature and placed in a strong external electric field; this process is termed poling. The field serves to orient the chromophore with its dipole moment parallel to the applied field.
Progress of LED, LED, and PLED
Units of LED Efficiency W External Quantum Efficiency (%) = (Photon# / Electron#) 100% W Luminance Efficiency (cd/a) (Photometric Efficiency) W Power Efficiency (lm/w) Luminance (L) : cd/m 2 Current density (J) : ma/cm 2 ame: Unit: Luminous flux Lumen Luminous Intensity Candela Luminance Candela/m 2 (nit)
Scale of Light Intensity 300,000,0000-30,000,000-3,000,000-300,000-30,000 - cd/m 2 3,000-300 - 3-0.3-0.03-0.003-0.0003-0.00003-0.0000003 -
There are two main directions in LED: Small Molecules and Polymers. The first technology was developed by Eastman-Kodak and is usually referred to as "small-molecule" LED. The production of Small-molecule displays requires vacuum deposition which makes the production process expensive and not so flexible. A second technology, developed by Cambridge Display Technologies or CDT, is called LEP or Light-Emitting Polymer, though these devices are better known as Polymer Light Emitting Devices (PLEDs). o vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial ink-jet printing. Recently a third hybrid light emitting layer has been developed that uses nonconductive polymers doped with light-emitting, conductive molecules. The polymer is used for its production and mechanical advantages without worrying about optical properties. The small molecules then emit the light and have the same longevity that they have in the Small-Molecule LEDs.
rganic Light-Emitting Diodes (LEDs) Flexibility (vs. inorganic LEDs) Simple and easy thin film fabrication and micronscale patterning (vs. wire-bonded epitaxial AlGaAs or group III nitride discrete semiconductor LEDs) LEDs will have most impact on markets for small, high information content display required low to medium brightness (mobile phone, PDA, lap-top computer). Flat-panel-display (vs. liquid crystal display, LCD) Wide viewing angle Very bright and highly contrast o back-lighting needed (low energy consumption) Fast switching times (video-rate display) Multicolor emission (RGB) Thin and light weight Foldable, very thin screen possible
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Configuration of LCD and LED LCD LED
Photoexcitation and Relaxation Stokes shift
Jablonski Diagram Illustrating possible electronic process following absorption of S 2 vc IC vc vc a photon with energy hν a IC ISC vc hν a hν a hνf S 1 ISC T 1 vc 0: singlet ground state 2: second lowest singlet excited state 1: lowest singlet excited state 1: lowest triplet excited state S 0 vc : vibrational cascade IC: internal conversion ISC : intersystem crossing hν p hν a : absorption energy hν f : fluorescence energy hν p : phosporescence energy
Competition Among Flat Panel Displays (FPDs)
Thin-film transistor (TFT) From Wikipedia, the free encyclopedia. A thin film transistor (TFT) is special kind of field effect transistor made by depositing thin films for the metallic contacts, semiconductor active layer, and dielectric layer. Most TFTs are not transparent themselves, but their electrodes and interconnects can be. The first transparent TFTs, based on zinc oxide were reported in 2003. The best known application of thin-film transistors is in TFT LCDs. Transistors are embedded within the panel itself, reducing crosstalk between pixels and improving image stability. As of 2004, all but the cheapest color LCD screens use this technology.
CIE 1931 (x, y) Chromaticity Diagram International Commission on Illumination The human eye has receptors for short (S), middle (M), and long (L) wavelengths, also known as blue, green, and red receptors. That means that one, in principle, needs three parameters to describe a color sensation. In the CIE diagram, those parameters are not the M, S, and L stimuli, but rather a more abstract x and y parameter, and an implicit luminosity (brightness) parameter, that is not shown
Comparison of LEDs with the ther FPDs Item LCD PDP VFD FED Inorg. EL LED iew Angle Improving Excellent Excellent Excellent Excellent Excellent Efficiency (lm/w) 2-3 1 0.8-14 7 2 4 5-10 Full color Excellent good Limited Limited Limited Improving Size (in.) < 21 > 40 Small 5-20 2-20 2-20 Voltage TFT: 2 5 BL: 1000 AC 90-150 DC 10-40 DC 1000 AC 200 DC <10 Response (ms) 20 60 ms 2-20 10 1 50 1 Issues Market 1999* Market 2005* View angle Large area Active: 13 Passive: 4.5 Active:31.3 Passive:5.8 Efficiency; Cost; Voltage; Power Resolution; Weight US Billion; source from Standford Resource, 2000, 8 Full-color resolution; Wieght; Voltage Blue podsphor Voltage; Contrast Blue phosphor Power Reliability Full color 0.8 1.4 0.003 5.8 1.3 0.7
Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
Electrochemical and Light-Emitting of LED Element Work Function (ev) Element Work Function (ev) Cs 2.14 Ag 4.26 K 2.30 Al 4.28 Ba 2.70 b 4.30 a 2.75 Cr 4.50 Ca 2.87 Cu 4.65 Li 2.90 Si 4.85 Mg 3.66 Au 5.10 In 4.12
hole-transporting layer emitting layer Adv. Mater. 2000, 12, 1737
Al Ag Alq 3 o (600 Α) Diamine o (750 Α) anthracene crystal ( 10~20 µm) g:ag 1% external quantum efficiency 1.5 lm/w luminous efficiency turn-on voltage < 10 V IT Glass Ag external quantum efficiency ~5% turn-on voltage > 400 V Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. Pope, M.; Kallmann, H. P.; Magnante, P. J. Chem. Phys.1963, 38, 2042.
Layer Structures of LED Unbalanced charge-mobility (10-5 cm 2 /Vs for electron and 10-3 cm 2 /Vs for hole) requires electron- or hole-transporting materials to balance the charges Single-Layer Device Double-Layer Device Metal Metal Metal Metal IT Glass IT Glass Electron-Transporting (Hole-Blocking) Material Light-Emitting Material Hole-Transporting (Electron-Blocking) Material Double-Layer Device Triple-Layer Device Metal Metal Metal Metal IT Glass IT Glass
LED Efficiency
Alq 3 Al Six-Coordinated ctahedron Ai The Magic of Alq 3 1. Ball-Shape Molecule: Hard to crystallize Exciplex formation prohibited: efficient fluorescence in solid state Voltile under reduced pressure High Tg ~ 175 o C: stable glass phase defect-free amorphous film 2. Six-Coordinated Metal : Chemically inert High T d > 350 o C: thermally stable 3. Availability: Very easy to synthesize + Metal stabilizes chelating ligand H toluene Al Aluminium isoproxide 8-Hydroxyquinoline 39 USD/ Kg 79 USD/ 500 g Alq 3 45 USD/ 5 g (99%) 66 USD/ 5 g (99.9995%)
Fine Tuning Color of Alq 3 Cl Al Al Cl 532 nm Cl 542 nm Al Alq 3 LUM Al 522 nm 563 nm Al Al 440 nm Alq 3 HM 580 nm Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S. R.; Thompson, M. E. J. Appl. Phys. 1996, 79, 7991
Tuning of Energy Gap by Donor and Acceptor Red-Shifted Acceptor on LUM Donor on HM LUM Acceptor on HM Blue-Shifted LUM Donor on LUM HM HM
Enhancing Performance of LED by Dopants Highly Fluorescent Green Dopant: Fluorescent Red Dopant: S Coumarin 540 C C DCM1 Mg:Ag IT Mg:Ag/Alq 3 :dopant/diamine/it Glass Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys.1989, 65, 3610
Cascade Förster Energy Transfer absorption a through space Coulombic dipole-dipole interaction the overlap of donor emission with acceptor absorption spectra emission excitation by chargerecombination excitation by Forster energy transfer from Alq 3 Alq 3 green emission DCM1 red emission 400 500 600 700 nm
Color Dopant Materials for LED S Coumarin 6 Rubrene S Eu F 3 C 3 Eu complex C C DCJT perylene in Al BAlq H H Quinacridone DCM C C Pt PtEP 400 500 600 700 nm
The Width of Recombination Region in LED Virtually all radioative recombination occurs in the HTL, within 100 A of the HTL/ETL interfaces CH 3 PBD Electron Transporting Layer (ETL) SD CH 3 Hole Transporting / Light Emitting Layer (EML) H 3 C C 2 H 5 S range Dopant C 2 H 5 Adachi, C.; Tsutsui, T.; Saito, S. ptoelectron. Dev. Technol.
Theoretical Efficiency ( el ) of LEDs el = r pl α : Light output coupling factor = 1/(2n 2 ) 20% n: refractive index of the emission medium (n = 1.7 in Alq 3 -based devices) γ : Probability of carrier recombination maximum γ ~ 100% (balanced hole and electron in LED) η el : Production efficiency of an exciton 25% for singlet-state (fluorescence) 75% for triplet-state (phosphorescence) ϕ pl : Fluorescence or Phorescence quantum yields 50% ~100% for most organic compounds Maximum el is 2.5%~5% for fluorescent materials 7.5%~15% for phosphorescent materilas
First Polymer-Based LED (PLED)
Current-voltage-luminance determinations for two PLED devices: a) employing a green emitter, and b) using a red one. c) EL spectra for the two emitting materials.
Due to the disorder of the polymer matrix, emission peaks will be broad, with a full width at half maximum (FWHM) approaching 60 to 70 nm for monochromatic sources.
arrow Emission Band from PLED with Microcavities Distributed Bragg Reflector (DBR): a stack of layers having alternating high (PPV doped with nanoparticles of Si 2 ) and low refractive indexes Ho, K. H.; Thomas, D. S.; Friend, R. H.; Tessler,. Science, 1999, 285, 233.
Issues need to be Solved for LEDs Y Reliability (operation lifetime) 10000 (polymeric film) ~ 35000 (molecular film) hours @ 200 cdm -2 Encapsulation problems: H 2 and 2 from air kill LED devices Material problems: Crystallization (Low Tg) of molecular materials Electrode problems: Charge-injection interface barrier Diffusion and degradation of IT anode and metal cathode) Y Efficiency (photon/electron) <5% (fluorescence-based) compared to >10% of commercial light bulbs
Decay of LED Glass Mg : Ag Alq 3 : rubrene α-pd CuPc IT Initial luminance of 100 cd/m 2
Methods for Full Color LEDs ) Side-by-side patterning of RGB emitters ) Color passband filting of white emitters ) Wavelength down-conversion of blue emitters ) Microcavity-filtered white emitters ) Color-tunable of stacked emitters
Disadvantages of LED: - Engineering Hurdles LED s are still in the development phases of production. Although they have been introduced commercially for alphanumeric devices like cellular phones and car audio equipment, production still faces many obstacles before production. - Color lifetime The reliability of the LED is still not up to par. After a month of use, the screen becomes nonuniform. Reds, and blues die first, leaving a very green display. 100,000 hours for red, 30,000for green and 1,000 for blue. Good enough for cell phones, but not laptop or desktop displays. - vercoming Commercial development of the technology LCD s have predominately been the preferred form of display for the last few decades. Tapping into the multi-billion dollar industry will require a great product and continually innovative research and development. Furthermore, the basics of LED technology is heavily patented by Kodak and other firms, requiring outside research teams to acquire a license.