Sébastien FORGET. Laboratoire de Physique des Lasers Université Paris Nord P13. www-lpl.univ-paris13.fr:8088/lumen/

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1 OLEDs Basic principles, technology and applications Sébastien FORGET Laboratoire de Physique des Lasers Université Paris Nord P13 www-lpl.univ-paris13.fr:8088/lumen/

2 Paris Nord University (Paris 13) This course gathers slides taken from various presentations by those guys : S Chenais S Forget «copyright» : Some slides were also illustrated with images from the web. When known, the origin of the pictures is given as a reference 2

3 Outline. Introduction. Basic principles. Technology : state of the art and bottlenecks. Applications : Displays, Lighting, Lasers (?) 3

4 Outline. Introduction. Basic principles. Technology : state of the art and bottlenecks. Applications : Displays, Lighting, Lasers (?) 4

5 L.Hirsch, IMS bordeaux 5

6 L.Hirsch, IMS bordeaux 6

7 L.Hirsch, IMS bordeaux 7

8 What about Organic SCs and OLEDs? Organic Electronics: building basic (opto)electronic components with organic semiconductors : transistors, photovoltaic cells, light-emitting diodes (OLEDs) OLEDs specific properties: Low electric consumption/ high efficiency Emission all over the visible spectrum Compatibility with flexible substrates Low cost (compared to inorganic) Large areas with uniform luminance UDC Applications : ultra-flat displays / lighting Sony Novaled 8

9 1962 : First inorganic LED (General Electrics) 1963 : Electroluminescence in anthracène (Pope) 1977 : Electronic conduction in polyacetylene films A. Hegger A. McDiarmid 2000 Nobel Prize (chemistery) H. Shirakawa Some history : breakthroughs 1987 : First organic light-emitting diode with a several-layer design (C.Tang and S. Van Slyke, Eastman Kodak) 1990 : Electroluminescence in polymers (Cambridge) 1997 : First commercial product(pioneer) 2002 : flat screen 15 (Kodak, Sanyo) 2003 : Camera (Kodak) and Crystals Thin Films 1977 Polymers Heterojonctions Applications

10 As for a LED, several layers are superimposed : What does an OLED look like? Electrons Injection Al, Au, Ag Metalic Cathode Organic Materials (small molecules or polymers) Electron transport, Multilayers Molecules/Polymers Transparent AND conductive Anode = ITO Substrat Light Total thickness ~ 200 nm : high F with reasonable V Hole injection Recombination Ligth emission through ITO 10

11 Organic materials : Materials «Small» molecules Polymers polyethylene 11

12 Organic materials : How can we make it? Can be thermally evaporated Small Molecules only «complex» Very fine thickness control Multilayer possible Can be spin-casted Polymers only Very simple and cheap Multilayer? Control? 12

13 Outline. Introduction. Basic principles. Technology : state of the art and bottlenecks. Applications : Displays, Lighting, Lasers (?) 13

14 Back to basics 1977 : Discovery of the electronic conduction in polyacetylene films A. Hegger A. McDiarmid H. Shirakawa Chemistry Nobel Prize 2000 «Plastic» is a priori an insulator but organic semi-conductors do exist Thoses molecules can conduct electricity (badly!) How? Some basic chemistry is needed 14

15 Back to basics : some chemistry The Carbon Carbon bond E π*, anti-liante 4 valence electrons and 4 atoms around : Sp 3 Hybridation : INSULATOR 4 valence electrons and 3 atoms around : Sp² Hybridation Pz π, liante SP 2 C : 1s² 2s 1 2p x 2p y 2p z SP 2 SP 2 4 valence e - H H C C H H 15

16 What is π-conjugation? Back to basics : some chemistry Benzène C 6 H 6 What happens when a pi-conjugated molecule absorbs an electron? «Classical view» (here on polyacetylene) H H H H H H C C C C C C C C H H H H C H From ISS, B.Wright ( H 6 electrons delocalised over the whole molecule C H H Or a more «quantical» one : the electron is delocalized over the whole molecule like in a quantum well (here with anthracène) 16

17 Energy bands p z HOMO = Highest Occupied Molecular Orbital = highest π orbital occupied by a pair of electrons π* π* = conduction band LUMO π GAP Back to basics : some chemistry π HOMO = valence band LUMO = Lowest Unoccupied Molecular Orbital = lowest unoccupied π* orbitale The emitted photon has ~ the gap energy : mostly in the visible spectrum λ is proportionnal to the length of the polymeric chain LUMO LUMO HOMO 17 HOMO 17

18 Organic luminescence Back to basics : some chemistry Also see the animation at 18

19 Material panel : huge! Back to basics : some chemistry Gap Energies for some polymers 19

20 Tang et VanSlyke, 1987 Classical OLED Structure Vacuum level (E = 0) 2.3 LUMO ITO E (ev) NPB Alq 3 N N HOMO N O O Al N N O nm 60 nm Al X (nm) 20

21 Gaussian disorder Weak electronic coupling between two molecules Random positioning during deposition Energetic and geometric disorder Vacuum level (E = 0) LUMO 4.6 NPB Alq Al ITO HOMO E (ev) 21

22 Contact Vacuum level (E = 0) NPB Alq Al ITO 22

23 Contact Vacuum level (E = 0) V0 Electronic Affinity Work function W - Vapplied + NPB (HTL) Alq 3 (ETL) Al ITO 23

24 MODEL 1 (Richardson-Schottky) Injection F=0 F 0 Thermoelectronic injection : J T² exp(-e/kt) W 1/r Schottky effect : image potential The total energy barrier is lowered by the attractive potential : J T².exp(-(E-bF 1/2 )/kt) + Total potential V=-eFr This model (Richardson-Schottky) is valid essentially when F and T are weak METAL Distance r 24

25 MODEL 2 (Fowler-Nordheim) Injection F=0 F 0 Tunneling injection : J F² exp(-b/f) W The Schottky effect (image potential) is here neglected Tunneling V=-eFr This model (Fowler-Nordheim) is valid essentially when F and T are high METAL Distance r More complex effects can be considered to get more subtle models : still an active research area 25

26 Initiation of a polaron 1) Spatial re-organization 2) Polarisation Transport : «hopping» e - Al - + NPB (HTL) Alq 3 (ETL) Al ITO Molecules are fairly independant of each other and are bonded via weak Van der Waals interactions. Molecules can hence undergo large amplitude vibrations 26

27 Initiation of a polaron 1) Spatial re-organization 2) Polarisation Transport : «hopping» e - Al - + NPB (HTL) Alq 3 (ETL) Al ITO 27

28 Initiation of a polaron 1) Spatial re-organization 2) Polarisation Transport : «hopping» e - Al - + NPB (HTL) Alq 3 (ETL) Al ITO 28

29 Transport : «hopping» Polaron Transport by «hopping» Transport is thermally activated Al e NPB (HTL) Alq 3 (ETL) Al ITO 29

30 Key parameter for transport : mobility ( E T p) F j = p. e. µ,,. Electric field (V/m) Current density (A/m²) Charge carrier density (e- or h+) Mobility (m²/v.s) = average velocity of the charge carriers per unit of electrical field Mobility model H. Bässler, Phys. Stat. Sol. B 175, 15 (1993) σ σ 2 C0 FE 3kT kt T E e e (, F ) = 0 µ µ σ = width (RMS) of the density of state Σ = parameter for geometric disorder Temperature dependence of the zero-field mobility of four PPV derivatives Martens et al, Phys. Rev. B 61, (2000) Orders of magnitude : µ ~ cm².v -1.s -1 Silicium : µ ~ 10 3 cm².v -1.s -1 µ with T (hopping evidence) and with F Generally µ electron << µ (Poole-Frenkel) hole 30

31 Mobilities : Very low/ inorganic semi-conductors Key parameter for transport : mobility electrons and holes exhibit very different mobilities Temperature dependence of the zero-field mobility of four PPV derivatives Martens et al, Phys. Rev. B 61, (2000) 31

32 Mobilities : Very low/ inorganic semi-conductors Key parameter for transport : mobility electrons and holes exhibit very different mobilities 32

33 Recombinaison : exciton formation Eexciton < E polaron because the exciton is «stabilised» by the Coulomb interaction LUMO HOMO + + «electron» «hole» EXCITON electrons - + NPB (HTL) Alq 3 (ETL) Al ITO holes 33

34 Excitons Neutral Quasi-particule : electron-hole pair linked by Coulombic interaction Spatially limited to a single molecule (in a first approach) Excitons INORGANIC 10 nm Wannier-Mott excitons Fundamental hole electron exciton + - ORGANIC 1 nm Frenkel excitons V. M. Agranovich and G. F. Bassani, ed., Electronic Excitations in Organic Based Nanostructures, in Thin Films and Nanostructures Vol. 31, (Elsevier Academic Press, Amsterdam, 2003) 34

35 Photon emission Eexciton < E polaron because the exciton is «stabilised» by the Coulomb interaction LUMO HOMO + + «electron» «hole» EXCITON electrons - Photon + NPB (HTL) Alq 3 (ETL) Al ITO holes 35

36 Differences ISC /OSC Organic Semiconductors / OLEDs Inorganic Semiconductors / LEDs Electrons (holes) localised on ONE molecule (=polarons) : charges are hopping from one molecule to another Very low mobility, increasing with T (hopping) Electrons (holes) delocalised in the crystal : energy bands High Mobility decreasing with T (phonons) No doping needed : charges are directly coming from the electrodes Doping is needed! The free charges are inside the material Wide choice of structures and materials Emission over the whole visible spectrum, possible mixing Limited Heterostructures design (crystalline structure must fit!) Emission only for a given set of λ (gap) 36

37 What is the «OLED efficiency»? Some definitions : External quantum efficiency η ext = number of emitted photons / number of injected e - η ext = η rad..φ PL. η coupling η rad = probability of exciton formation (from one e- and one h+) (~ 1) א = probability that the exciton is emissive (~ 0.25) Φ PL = luminescence quantum yield (> 80%) η coupling = fraction of photons escaping from the OLED (~ 0.20) 37

38 What is the «OLED efficiency»? Electron Polaron - Polaron + Hole transport Cathode Recombinaison Exciton creation Anode η rad ~ 100% S Diffusion T 75% א ~ 25% Φ PL ~ 80% Desexcitation radiative Desexcitation (non-radiative) η couplage ~ 20% Out Coupling Waveguiding by Total Internal Reflection Emitted Photon TOTAL ~ 4% 38

39 What is the «OLED efficiency»? Some definitions : External quantum efficiency η ext = number of emitted photons / number of injected e - η ext = η rad..φ PL. η coupling η rad = probability of exciton formation (from one e- and one h+) (~ 1) א = probability that the exciton is emissive (~ 0.25) Φ PL = luminescence quantum yield (> 80%) η coupling = fraction of photons escaping from the OLED (~ 0.20) 25% of singlets excitons (antiparalleles spins) 75% of triplets excitons (paralleles spins) Emission S 1 T 1 SOLUTION : PHOSPHORESCENCE S 0 No emission 39

40 Phosphorescence S 1 S 1 T 1 Rule (Pauli) Emission S 0 T 1 S 0 No emission Two electrons with same spin CANNOT occupy the same energetic level. S = +1/2-1/2 = Authorised S = +1/2 + 1/2 = Forbidden One electron CANNOT change its spin during a transition Fluorescence Phosphorescence Idea : inserting a heavy element (high Z) to by-pass the selection rule! (The spinorbit coupling becomes non negligible and Triplet-Singulet transitions becomes allowed) 40

41 What is the «OLED efficiency»? Some definitions : External quantum efficiency η ext = number of emitted photons / number of injected e - η ext = η rad..φ PL. η coupling η rad = probability of exciton formation (from one e- and one h+) (~ 1) א = probability that the exciton is emissive (~ 0.25) Φ PL = luminescence quantum yield (> 80%) η coupling = fraction of photons escaping from the OLED (~ 0.20) 41

42 Light extraction Glass Substrat ~ 2 mm ; n = 1,5 Outcoupled Modes Modes guided in the substrat Modes guided in the organic layers+ito cathode Organic layers+ito ~ 300 nm Refractive index ~ 1.7 Localisation of the excitons (~ 10 nm) 42

43 Light extraction Outcoupled Fraction of the light 2 1 4n 2 20% for n = 1.7 With reflection on a perfect mirror and neglecting the losses. Rapid proof : Ω=2π (1- cosθ) ~ π θ² ~ π/n² Then Ω/(4π)=1/4n² Solutions : Optical Microcavity Diffraction gratings, corrugation, microlenses 43

44 What is the «OLED efficiency»? Some definitions : External quantum efficiency η ext = number of emitted photons / number of injected e - η ext = η rad..φ PL. ηcoupling = 1 x 0.25 x 0.8 x 0.2 =4% 1 x 1 x 0.8 x 0.35 =28% η rad = probability of exciton formation (from one e- and one h+) (~ 1) א = probability that the exciton is emissive (~ 0.25) Φ PL = luminescence quantum yield (> 80%) η coupling = fraction of photons escaping from the OLED (~ 0.20) 44