Organic Photonic Materials

Transcription

1 rganic Photonic Materials onlinear ptics Materials rganic Light Emitting Diode (LED) 1

2 onlinear optics The interaction of electromagnetic fields with various media to produce new electromagnetic fields altered in phase, frequency, amplitude from the incident fields 2

3 Inversion asymmetry materials Second harmonic generation (SHG), the conversion of coherent light of frequency ω into light of frequency 2ω 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. 3

4 The electro-optic effect is a change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light. a) change of the absorption electroabsorption: general change of the absorption constants Franz-Keldysh effect: change in the absorption shown in some bulk semiconductors Quantum-confined Stark effect: change in the absorption in some semiconductor quantum wells electro-chromatic effect: creation of an absorption band at some wavelengths, which gives rise to a change in colour 4

5 b) change of the refractive index Pockels effect (or linear electro-optic effect): change in the refractive index linearly proportional to the electric field. nly certain crystalline solids show the Pockels effect, as it requires inversion asymmetry Kerr effect (or quadratic electro-optic effect, QE effect): change in the refractive index proportional to the square of the electric field. All materials display the Kerr effect, with varying magnitudes, but it is generally much weaker than the Pockels effect electro-gyration: change in the optical activity. 5

6 In harmonic generation, multiple photons interact simultaneously with a molecule with no absorption events. Because n-photon harmonic generation is essentially a scattering process, the emitted wavelength is exactly 1/n times the incoming fundamental wavelength. When the excitation color is changed, the emission color changes also. In contrast the wavelength of fluorescence emission is Stokes-shifted to a longer wavelength; the line shape is determined strictly by the molecular energy levels. 6

7 The polarization P induced in a molecule by a local electric field E P= αe + βe 2 +γe 3 + α linear polarizability (the origin of refractive index) β second order hyperpolarizability (the origin of the second order nonlinear polarization response) 7

8 Push-Pull in a Donor-Acceptor 8

9 βvalues of Some rganic Chromophores (10-30 esu, 1064 nm) 9

10 Charge Transfer Resonance Structures First, the greater the charge separation in the charge transfer state (Dm), the larger the β Second, the closer the frequency of the incident light is to the resonant frequency of the charge transfer, the larger the β 10

11 rganic Electro-ptic Materials A Historical Perspective Statistical mechanical calculations suggested a new paradigm optimization of electrooptic 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 CLD-2 CLD-3 Disperse Red (1995) 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 11

12 For Bulk Materials P = χ (1) E + χ (2) E 2 + χ (3) E

13 Fabrication of organic second order L materials organic crystal growth, inclusion complexes, mono- and multilayered assemblies (e.g. Langmuir-Blodgett films), poled polymers 13

14 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. 14

15 light-emitting diode (LED) Alight-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color of the emitted light depends on the chemical composition of the semiconducting material used. AlGaAs - red and IR AlGaP - green AlGaInP - high-brightness orange-red, orange, yellow, and green GaAsP - red, orange-red, orange, and yellow Ga - green, and blue InGa - near UV, bluish-green and blue Al, AlGa - near to far UV 15

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17 (A) a band diagram and (B) absorption spectrum of a semiconductor. 17

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19 S Zn Diamond structure Zinc blende structure 19

20 Material Table 7.3 Periodic Properties of a Family of Isoelectronic, Tetrahedral Semiconductors Cubic Unit Cell Parameter, Å χ Eg, ev (λ, nm) Ge (1900) GaAs (890) ZnSe (460) CuBr (430) 20

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25 Pauling s Electronegativities 25

26 Increasing ionic character Length of unit cell = ± Å 26

27 Emission Spectra of LED 27

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29 Progress of LED, LED, and PLED 29

30 Units of LED Efficiency Ω External Quantum Efficiency (%) = (Photon# / Electron#) 100% Ω Luminance Efficiency (cd/a) (Photometric Efficiency) Ω Power Efficiency (lm/w) Luminance (Lm) : cd/m 2 Current density (J) : ma/cm 2 ame: Unit: Luminous flux Lumen Luminous Intensity Candela Luminance Candela/m 2 (nit) 30

31 Scale of Light Intensity 300,000, ,000,000-3,000, ,000-30,000-3,000 - cd/m

32 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 inkjet 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. 32

33 rganic Light-Emitting Diodes (LEDs) LEDs operate at substantially lower efficiency than inorganic (crystaline) LEDs. The best efficiency of an LED so far is about 10%. It is much cheaper to fabricate LED than inorganic LEDs, and large arrays of them can be deposited on a screen using simple printing methods to create a color graphic display. LEDs will have most impact on markets for small, high information content display required low to medium brightness (mobile phone, PDA, lap-top computer). 33

34 rganic Light-Emitting Diodes (LEDs) vs. inorganic LEDs Flexibility Simple and easy thin film fabrication and micronscale patterning (vs. wire-bonded epitaxial AlGaAs or group III nitride discrete semiconductor LEDs) 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 34

35 比一比看誰炫有機發光二極體( LE比一比看誰炫 D) 液晶顯示器( LCD) 35

36 Configuration of LCD and LED LCD LED 36

37 吸收光譜與螢光光譜 螢光發光原理 Photoexcitation and Relaxation Stokes shift 37

38 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 vc T 1 vc S 0 : singlet ground state S 2 : second lowest singlet excited state S 1 : lowest singlet excited state T 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 38

39 Competition Among Flat Panel Displays (FPDs) 39

40 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 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. 40

41 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 41

42 Adv. Mater. 2000, 12,

43 Comparison of LEDs with the ther FPDs Item LCD PDP VFD FED Inorg. EL LED View Angle Improving Excellent Excellent Excellent Excellent Excellent Efficiency (lm/w) Full color Excellent good Limited Limited Limited Improving Size (in.) < 21 > 40 Small Voltage TFT: 2 5 BL: 1000 AC DC DC 1000 AC 200 DC <10 Response (µs) ms 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

44 Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51,

45 Indium tin oxide (IT) is a mixture of indium(iii) oxide (In 2 3 ) and tin(iv) oxide (Sn 2 ), typically 90% In 2 3, 10% Sn 2 by weight. IT is mainly used to make transparent conductive coatings for electronic displays, and heatreflecting coatings for architectural, automotive, and light bulb glasses. State of matter Melting point Density Color (in powder form) Solid Physical Properties K ( F) kg/m 3 at 293 K Pale yellow to greenish yellow, depending on Sn 2 concentration 45

46 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

47 hole-transporting layer emitting layer Adv. Mater. 2000, 12,

48 Al Ag Alq 3 o (600 Α) Diamine o (750 Α) anthracene crystal ( 10~20 µm) Mg: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,

49 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 49

50 LED 發光效率 (Efficiency) 50

51 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 ( %) 51

52 Fine Tuning Color of Alq 3 Cl Al Al Cl 532 nm 542 nm Cl 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, Chen, C. H.; Shi, J. Coord. Chem. Rev. 1998, 171, 161.

53 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 53

54 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,

55 Cascade Förster Energy Transfer a through space Coulombic dipole-dipole interaction the overlap of donor emission with acceptor absorption spectra absorption emission excitation by chargerecombination excitation by Forster energy transfer from Alq 3 Alq 3 green emission DCM1 red emission nm 55

56 Color Dopant Materials for LED S Coumarin 6 Rubrene S Eu F 3 C 3 Eu complex C DCJT C perylene in Al BAlq H H Quinacridone DCM C C Pt PtEP nm 56

57 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. 1991, 6,

58 Theoretical Efficiency (η el ) of LEDs η el = α γ η r ψ pl α : Light output coupling factor α= 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 Phosphorescence quantum yields 50% ~100% for most organic compounds Maximumη el is 2.5%~5% for fluorescent materials 7.5%~15% for phosphorescent materilas 58

59 First Polymer-Based LED (PLED) 59

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61 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. 61

62 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. 62

63 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,

64 Issues need to be Solved for LEDs Ψ Reliability (operation lifetime) 壽命 (polymeric film) ~ (molecular film) 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) Ψ Efficiency (photon/electron) 效率 <5% (fluorescence-based) compared to >10% of commercial light bulbs 64

65 Decay of LED Glass Mg : Ag Alq 3 : rubrene α-pd CuPc IT Initial luminance of 100 cd/m 2 65

66 Methods for Full Color LEDs (a) Side-by-side patterning of RGB emitters (b) Color passband filting of white emitters (c) Wavelength down-conversion of blue emitters (d) Microcavity-filtered white emitters (e) Color-tunable of stacked emitters Bulovic, V.; Burrows, P. E.; Forrest, S. R. Semicond. Semimetal. 2000, 64,

67 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. 67