Introduction. Fang-Chung Chen Department of Photonics and Display Institute National Chiao Tung University. Organic light-emitting diodes

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1 rganic light-emitting diodes Introduction Fang-Chung Chen Department of Photonics and Display Institute National Chiao Tung University rganic light-emitting diodes --The emerging technology LED Displays Pull out travel guide 2 1

2 LED Revenue Forecast by Application Future Star!!

3 Seiko Epson: The largest LED Display using IJP Screen size 40-inch diagonal Number of pixels 1280 x RGB x 768dots (W-XGA) Driving method Active matrix Pixels per inch 38 No. of colors 260,000 commercialization in 2007 Main Specifications Screen size 40-inch diagonal Number of pixels 1280 x RGB x 768dots (W-XGA) 5 Efficiencies of LEDs Performance (lm/w) LEDs PLEDs Candle year 6 3

4 History of organic light-emitting devices (LEDs) First organic electroluminescent (EL) based on anthracene single crystal; (Pope et al.) Problem: low quantum efficiency and high operating voltage (>100V) First organic electroluminescent (EL) based on amorphous organic molecules; (Kodak; C. W. Tang et al.) high quantum efficiency (~1%); low driving voltage bi-layer structure; thin amorphous organic films First organic electroluminescent (EL) based on polymer; (Cambridge University; Burroughes et al. polymer light-emitting diodes (PLEDs) 7 Device structure of LEDs Mg Ag Alq 3 + Diamine IT Glass Devices were fabricated by thermal evaporation Drive voltage ~5V QE: ~1%; 3 cd/a (green) Fast response time (<1 μsec) 8 4

5 perating mechanism of LEDs Mg Ag + Alq 3 Diamine IT Glass Mechanism involves: 1: Charge injection diamine (hole-transporting layer) - Mg/Ag - cathode 2: Charge transport 3: Charge recombination (Exciton formation) IT + anode HTL + + ETL Alq 3 electron-transporting layer Electrical field: >10 5 V/cm perating mechanism of PLEDs MEH-PPV + - Ca/Al MEH-PPV PEDT:PPS IT glass H 3 C n PEDT:PPS Devices were fabricated by spin-coating Single emissive layer was used S S S S + n S n m S 3 - S 3 H 10 5

6 Device mechanism 2 1 Cathode Mechanism involves: 1: Charge injection Polymer LUM 2: Charge transport 3 3: Charge recombination Anode Polymer HM LUM π * (2.8eV) e - 2.9eV Ca eV IT h + π (4.9eV) HM 11 Diodes!!! 10-3 I-V V characteristics IT/MEH-PPV/Ca Au/MEH-PPV/Ca 10-3 Current (A) Radiance (W) Voltage (V)

7 Why PLEDs? Easy and low-cost fabrication Solution processability Light weight and flexible Easy color tuning Spin-coating for mono-color display Ink Jet printing for multi-colors display 40-inch commercialized in Family of light-emitting poly(thiophene) 14 7

8 Molecular engineering of light-emitting polymers PA PPP Eg = 1.4 ev n Eg = 3.0 ev n PPV MEH-PPV n Eg = 2.4 ev H 3 C Eg = 2.1 ev n 15 The merits of LEDs over other display technologies Flat and thin Wider viewing angle (> 160 o ) Saturated emissive color Wide operating temperature High contrast Flexible, Plastic can be used as substrates Light weight Fast response time (~μs) Low temperature processing Low cost 16 8

9 Comparison of /PLED with other display technology < 1μsec 17 LED Displays LCD LED 18 9

10 LED Displays LCD LED 19 LED Displays 20 10

11 LED Displays 21 LED Displays AU world first 4 a-si AMLED (2003) Parameter Display Size Resolution Sub-Pixel Pitch Driving Method Color Number Brightness Power Cons. Substrate Contrast ratio Module thickness Emission Type Features 4-inch 160 (RGB) um 264um 2-TFT Voltage Programming 262K (6 bits) 300 cd/m mw a-si TFT > mm Bottom Emission 22 11

12 Flexible LEDs LED built on flexible substrates Flexibility Ultra-lightweight and Thin 23 rganic light-emitting diodes Chemical and electronic Structure of organic materials Fang-Chung Chen Department of Photonics and Display Institute National Chiao Tung University 12

13 rganic Materials C H N S.. rganic metallics Alq 3 CuPc.. α-npd 25 Comparison of Bohr and wave-mechanical atom models 26 13

14 Atomic rbitals ϕ : one-electron wave function ϕ 2 : electron density 27 Molecular rbitals Anti-bonding Energy levels Δ Bonding Δ: depends on the degree to which the orbitals occupy the same space or overlap 28 14

15 Molecular rbitals 29 Electronic energy vs interatomic separation of an aggregate of 12 atoms 30 15

16 Carbon atom bonding configurations π orbitals single bond double bond triplet bond 31 rbital structure of benzene (Six Carbons) 32 16

17 The π-molecular orbitals and energy levels for benzene 33 Chemical structures of common organic semiconductors E g1 E g2 The lowest electronic transition (band gap, E g ) Ethylene (C 2 H 4 ) : E g1 = 6.9 ev Benzene (C 6 H 6 ) : E g2 = 4.6 ev More delocalized π electrons, the lower the band gap energy 34 17

18 Electron band structures in solids at 0 K Metal (Cu) Metal (Mg) Insulator Semiconductor Empty band Band gap E f Empty band Empty conduction band Band gap Empty conduction band Band gap E f Empty states Filled states Filled states Filled valence band Filled valence band (Eg >2 ev) 10 7 Ω -1 cm Ω -1 cm Ω -1 cm Chemical structures of common organic semiconductors 36 18

19 Conductivity domain of metals, semiconductors, and insulators p-doped polyacetylene 37 rganic (Molecular) Semiconductors Weak bonding (van der Waals force) Low melting point Low conductivity Ω -1 cm -1 Small Molecules Functional Polymers 38 19

20 Conjugation A conjugated system is one having alternating single and double bonds Conjugated Polymer Backbones: alternating single-double bonds PPP PA Delocalized π electron clouds n Eg = 3.0 ev n Eg = 1.4 ev PPV polyacetylene Eg = 2.4 ev n 39 Polymer vs Small Molecular Polymer, Macromolecules Historically, molecules larger than 10k (10000 g/mole) belong to this group Technically, all polymers are mixtures Polymers show isomers, and polymers having the same Chemical formula can show different properties different Regioregular - Polypropylene Random - Polypropylene 40 20

21 Excitons in rganic Materials Electronic excitation is considered as a quasi-particle, capable of migrating. This is termed as Exciton Excitons can be regarded as bounded electron-hole pairs. Also can be viewed as the excited states of molecules lattice constant Charge-transfer exciton Wannier-Mott exciton Frenkel exciton 41 The Nature of Excitons in rganic Materials organic molecules LUM HM + hv Frenkel Exciton Coulombic interaction E q 1 q 2 ε r (binding energy ev Radius ~ 10Å) 42 21

22 Ultraviolet-visible visible (UV-vis vis) ) Spectroscopy σ E + hν σ ground state excited state 43 Ultraviolet-visible visible (UV-vis vis) ) Spectroscopy λ ~ 150 nm, σ σ* transition λ < 200 nm, vacuum ultraviolet, strongly absorbed by the oxygen λ = nm, ultraviolet, λ = nm, visible, π π* transition 44 22

23 π π* * transitions The longer the chain of conjugation The longer the wavelength of the absorption band 45 Ultraviolet-visible visible (UV-vis vis) ) Spectroscopy monochromator hν Sample hν Light source I 0 I detector I A = - log ( ) I 0 n C 8 H 17 C 8 H 17 PF UV-vis Spectroscopy of polyfluorene 46 23

24 UV-vis Spectroscopy of polyfluorene -- another example 47 Photoluminescence (PL) hν Sample Light source hν Monochromator & detector 48 24

25 Typical energy levels and energy-transfer process of a molecule Energy 49 Vector representation of an electron s s spin magnet moment nly two spin states (α, β) are stable 50 25

26 Single and Triplet single excited state S 1 S 1 triplet excited state S 0 X T 1 S 0 S = 0 S = 0 S = 1 ground state Fluorescent Phosphorescent 51 Single and triplet states up state α down state β 52 26

27 rganic light-emitting diodes Basic Device Physics Fang-Chung Chen Department of Photonics and Display Institute National Chiao Tung University Device structure of LEDs Mg Ag Alq 3 + Diamine IT Glass Devices were fabricated by thermal evaporation Drive voltage ~5V QE: ~1%; 3 cd/a (green) Fast response time (<1 μsec) 1

28 perating mechanism of LEDs Mg Ag + Alq 3 Diamine IT Glass Mechanism involves: 1: Charge injection diamine (hole-transporting layer) - Mg/Ag - cathode 2: Charge transport 3: Charge recombination (Exciton formation) IT + anode HTL + + ETL Alq 3 electron-transporting layer Electrical field: >10 5 V/cm 100 perating mechanism of LEDs LUM (Conduction band) hν cathode anode HM (Valence band) 2

29 Typical multilayer-device structures Metal Metal Metal ETL ETL ETL (EML) HTL (EML) EML HTL HTL IT IT IT ETL, electron-transport layer EML, emissive layer HTL, hole-transport layer perating mechanism of PLEDs MEH-PPV + - Ca/Al MEH-PPV PEDT:PPS IT glass H 3 C n PEDT:PPS Devices were fabricated by spin-coating Single emissive layer was used S S S S + n S n m S 3 - S 3 H 3

30 What is PEDT:PSS? PEDT:PSS is a hole-transporting conductive polymer Deposited from an aqueous suspension ρ ~ 1000 to Ω-cm PEDT:PPS Work function ~ 5.0±0.2 ev IT work function depends on the surface treatment IT surface is often full of spikes S S n S S + n m S PEDT:PSS (~ 100 nm) both planarizes the surface and stablizes the work function of the anode of the PLEDs It is one of the keys to reproducible devices S 3 - S 3 H Single layer organic EL device 2 1 Cathode Polymer LUM 3 Anode Polymer HM 1 2 Very common for PLEDs The material should be bi-polar 4

31 Small molecule and Polymer LEDs Metal Metal ETL EML HTL Emitting polymer Hole-injection layer IT IT smleds: Evaporation of a multilayer stack of small organic molecules (Mw ~ several 100) PLEDs: Spincoating/inkjet printing of polymers (Mw ~ 50, ,000) Diodes!!! 10-3 I-V V characteristics IT/MEH-PPV/Ca Au/MEH-PPV/Ca 10-3 Current (A) Radiance (W) Voltage (V)

32 Why PLEDs? Easy and low-cost fabrication Solution processability Light weight and flexible Easy color tuning Spin-coating for mono-color display Ink Jet printing for multi-colors display Efficiency of rganic EL Devices η ext = η int η p = γ η r φ f η p ~100% ~25% ~100% ~20% Maximum external quantum efficiency is ~5% η ext : external quantum efficiency η int : internal quantum efficiency η p : light out-coupling efficiency γ: charge carrier balance factor (e/h) η r : efficiency of exciton production φ f : internal quantum efficiency of luminescence 6

33 η p : light out-coupling efficiency due to total internal reflection loss η p = 1 / (2n 2 ) n : reflection index of the emissive medium If n ~ 1.5 η p = 22% n = 1.0 Front view rganic layer n ~ 1.5 Mirror γ: charge carrier balance factor (e/h) J h J h J J e IT anode J e Metal cathod γ= J r / J J : circuit current J r : current used for charge recombination J = J h + J e = J e + J h J r = J h -J h = J e -J e 7

34 η r : efficiency of exciton production hole (+) electron (-) exciton ( * ) + + or singlet triplet 1/ symmetric states Triplets 1/ antisymmetric state Singlet φ f : internal quantum efficiency of luminescence k other deactivation processes Thermal deactivation S 1 k I intersystem k F crossing fluorescence X T 1 phosphorescent k T k P S 0 φ F = k F kf + k I + k T + k 8

35 Typical I-L-V I V curves of an Alq3-based LED 75 nm NPD/75 nm Alq 3 L. S. Hung and C. H. Chen, Mater. Sci. & Eng. R 39, 143 (2002) Manufacture of LEDs Thermal evaporation substrate Metal mask Cathode material rganics 9

36 Manufacture of PLEDs Spin-coating or ink-jet printing Ink-jet printing to pattern polymers 10

37 Efficiency of organic EL Devices Quantum efficiency: η ext = η int η p = γ η r φ f η p η ext : external quantum efficiency η int : internal quantum efficiency η p : light out-coupling efficiency γ: charge carrier balance factor (e/h) η r : efficiency of exciton production φ f : internal quantum efficiency luminescence Power efficiency: optical power output electrical power input η pow = η ext E p U -1 E p : the average energy of the emitted photons U : the known values of the applied voltage (lm/w), important for engineer and system design Efficiency of organic EL Devices Luminous efficiency: η lum = η pow S S : the eye sensitivity curves Current efficiency (Cd/A), important for material evaluation 11

38 Efficiency of organic EL Devices an Example Device current density : 50 ma/cm 2 Brightness : 3500 cd/m 2 at 10V Current Efficiency : 3500 cd/m 2 1 x 50 ma/cm 2 10 = 7 cd/a Power Efficiency : 7 cd/a x π = 2.2 lm/w 10 V Definitions of Efficiencies of LEDs S. R. Forrest et al. Adv. Mat. 15, 1043 (2003) 12