Electronics go everywhere

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1 The Chemistry, Physics and Engineering of Organic Light Emitting Diodes George G. Malliaras Department of Materials Science and Engineering Cornell University Electronics go everywhere Pioneer e-ink & Lucent Electrolux 1

2 Outline Materials Device Principles Device Physics Degradation Applications Organic semiconductors Molecularly Dispersed Functional Small Molecules Polymers (MDP) Polymers 2

3 Materials for OLEDs Carbon as a semiconductor Hybridization: sp 2 and p Z CH 2 =CH 2 Particle in a box: ħ 2 π 2 E n = n 2 2mL 2 n=1,2,3,... E G LUMO HOMO ħ E G 2 π 2 2maN 3

4 Tuning of optical properties Blue Red R.E. Gill et. al., Adv. Mater. 6, 132 (1994). Covion Optical properties of polythiophenes R.E. Gill et. al., Adv. Mater. 6, 132 (1994). 4

5 PC:TPD hole mobility 10-3 Mobility (cm 2 /Vsec) %TPD 80%TPD 60%TPD 50%TPD 40%TPD %TPD 1000 E 0.5 ((V/cm) 0.5 ) Materials for OLEDs (II) 5

6 Materials for OLEDs (III) Advantages Ease of processing large area films flexible substrates Optoelectronic properties trap free transport tunable energy gap high luminescence efficiency large absorption coefficient 6

7 Applications Light emitting diodes Thin film transistors Photovoltaic devices Summary I: Materials Different families of organic semiconductors. Main difference is in processing. Major advantages: Processing, taylor-made properties. 7

8 Outline Materials Device Principles Device Physics Degradation Applications OLED structure and operation Ca e - ITO h + 8

9 Model for single layer devices 2 1 Polymer LUMO Cathode 3 Mechanism involves: Anode 1 2 Polymer HOMO 1: Charge injection 2: Charge transport 3: Charge recombination OLED characteristics 10-3 Au/MEH-PPV/Ca 10-3 Current (A) Radiance (W) Voltage (V)

10 OLED characteristics (2) Ca ITO Ag 10

11 Bilayer devices Hole-transport layer Cathode Anode Electron-transport layer Other architectures Emissive Layer Dopants 11

12 Summary II: Device principles Electroluminescence involves injection, transport and recombination of opposite charges. High and low work-function metals needed for electrodes. Outline Materials Device Principles Device Physics Introduction Built-in potential Charge transport Charge injection Ionic space charge effects Degradation Applications 12

13 Charge injection vs. transport Pedagogical analogue: Water hose and valve Is the flow limited by the valve or the hose? Is the performance injection limited? 10-3 Au/MEH-PPV/Au I/Vappl 2 (A/V 2 ) 10-4 Fowler-Nordheim: I=!V 2 exp("/v) After I.D. Parker, J. Appl. Phys. 75, 1656 (1994). 1/V appl (V -1 ) 13

14 ..or is it trap limited? Trap limited conduction: I=!V " I (A) V appl (V) After A.J. Campbell et.al., J. Appl. Phys. 82, 6326 (1997). Au/MEH-PPV/Au Bipolar current more complicated J (ma/cm 2 ) Au Al Ca V appl (V) 14

15 Electron-hole recombination J h /J J e /J b Anode x Cathode External Quantum Efficiency: η = b Φ/2n 2 Outline Materials Device Principles Device Physics Introduction Built-in potential Charge transport Charge injection Ionic space charge effects Degradation Applications 15

16 The built-in potential Ca Au MEH-PPV Contact It controls the I-V characteristics of OLEDs! Electroabsorption!T /T (arb. units) ITO/MEH-PPV/Ca V appl (V) ΔT /T V bi -V appl After I.H. Campbell et.al., Phys. Rev. Lett. 76, 1900 (1996). 16

17 Energy level diagram LUMO Ca M g Al Ag V bi (V) Au HOMO -0.5 Outline Materials Device Principles Device Physics Introduction Built-in potential Charge transport Charge injection Ionic space charge effects Degradation Applications 17

18 Charge transport in semiconductors v=µ E Conduction band - Shallow trap Deep trap + ε Valence band x Figure of merit: mobility, µ (cm 2 /V sec) Charge transport in semiconductors (II) Question: What is the maximum current that can flow through a (trap-free) semiconductor? J V Log(J) J SCL L V 0 Lower voltages: Ohm s law J OHM = e Ν 0 µ V/L Higher voltages: Space charge limited current J SCL = (9/8) ε ε 0 µ V 2 /L 3 J OHM Lampert and Mark, Current Injection in Solids (Academic Press,1970). Log(V) 18

19 Time-of-flight (TOF) Light pulse V R 20x10-6 OSC 15 80% TPD µ = L 2 t TR V Current (A) t tr µ=l 2 /Vt tr Great book on transport: P. M. Borsenberger, D. S. Weiss, Organic photoreceptors for Xerography (Marcel Decker, Inc., New York, 1998) Time (sec) 100x10-6 Hopping transport ν ij ~ exp-(r ij ) W.D. Gill, J. Appl. Phys. 43, 5033 (1972). 19

20 Dispersive transport and universality t -(1-a) a=0.66 t -(1+a) F.C. Bos and D.M. Burland, Phys. Rev. Lett. 58, 152 (1987). Disorder formalism (I) Gaussian transport Disorder formalism ψ(t) ~ e -t/τ <l> ~ t σ ~ t ½ σ/<l> ~ t -½ ψ(t) ~ t -(1+a) <l> ~ t a σ ~ t a σ/<l> ~ const. Universality! (0<a<1 ) H. Scher and E.W. Monrtoll, Phys. Rev. B 12, 2455 (1975). 20

21 Disorder formalism (II) H. Scher and E.W. Monrtoll, Phys. Rev. B 12, 2455 (1975). Transport in MPDs Decrease distance Increase mobility P.M. Borsenberger et al., Jpn. J. Appl. Phys. 37 (1998) 21

22 Gaussian disorder model LUMO ε ε σ = 0.1 ev x HOMO Energetic disorder Positional disorder DOS Gaussian disorder model (II) Density of states: DOS(ε) = (2 π σ 2 ) -0.5 exp[-(ε 2 /2σ 2 )] ε i ε j R ij Hopping rate: { exp[-(ε j-ε i )/kt] ; ε ν ij = ν 0 exp-(2 γ a ΔR ij /R ij ) j >ε i Mobility: µ=µ 0 exp[-(2σ/3kt) 2 ] exp{c [(σ/kt) 2 -Σ 2 ] E 0.5 } H. Bässler, Phys. Stat. Sol. (b) 175, 15 (1993). 1 ; ε j >ε i 22

23 Gaussian disorder model (III) Carriers relax at: σ 2 /kt µ ~ exp[-(σ/kt) 2 ] H. Bässler, Phys. Stat. Sol. (b) 175, 15 (1993). Comparison with experiment H. Bässler, Phys. Stat. Sol. (b) 175, 15 (1993). 23

24 Correlated disorder (I) Molecules carry a large dipole moment. Charge dipole interaction causes spatial correlation in the energy of hopping sites. Correlated disorder (II) ε LUMO x HOMO Deeper valleys are also wider. µ=µ 0 exp[-(σ/kt) (σ/kt) (e a E/kT) 0.5 ] D.H. Dunlap, P.E. Parris and V.E. Kenkre, Phys. Rev. Lett. 77, 542 (1996). 24

25 Correlated disorder (III) Black/white: sites with energy above/below the mean. S.V. Novikov, J. Polym. Sci. Part B: Polym. Phys., to appear. Hole mobility in MEH-PPV µ (10-6 cm 2 /Vsec ) 1 µ 0 =(2+/-1) 10-7 cm 2 /Vsec!=(5+/-1) 10-4 (m/v) 0.5 µ=µ 0 exp(!e 0.5 ) E 0.5 ((V/m) 0.5 ) 25

26 Charge transport 0.1 ev µ=µ 0 exp(γ E) J SCL (9/8)εε 0 µ 0 V 2 exp[0.89γ(v/l) 0.5 ]/L 3 P.N. Murgatroyd, J. Phys. D: Appl. Phys. 3, 151 (1970). Electrical characteristics of OLEDs J SCL (9/8)εε 0 µ 0 V 2 exp[0.89γ(v/l) 0.5 ]/L 3 JL 3 /(Vappl -Vbi ) 2 (macm/v 2 ) Ca Al Au (98 nm) Au (168 nm) G.G. Malliaras et.al., Phys. Rev. B 58, R13411 (1998). ((V appl -V bi )/L) 0.5 ((V/cm) 0.5 ) 26

27 Efficiency of MEH-PPV OLEDs !q (%) Ca Al V appl -V bi (V) Outline Materials Device Principles Device Physics Introduction Built-in potential Charge transport Charge injection Ionic space charge effects Degradation Applications 27

28 Quantifying the injection process Injection efficiency: η = J INJ / J SCL (Contact supply / Bulk demand) J SCL = (9/8) ε ε 0 µ V 2 /L 3 J INJ =??? (Thermionic emission, Tunneling) Injection efficiency measurements Measure mobility Calculate SCLC Measure injected current Time-of-flight Injection _ + + _ Contact under test Blocking contact M. Abkowitz et.al., J. Appl. Phys. 83, 2670 (1998). 28

29 Hole injection in PC:TPD % TPD J SCL J (A/cm 2 ) % TPD J INJ 50% TPD J SCL 50% TPD J INJ Voltage (V) A simple injection model J = Cexp(-ϕ B /kt) -en 0 S(E) Surface recombination as a hopping process in the image charge potential. No current flow at zero field. C = 16πεε 0 N 0 (kt) 2 µ/e 2 S(0) = 16πεε 0 (kt) 2 µ/e 3 + _ J.C. Scott et al., Chem. Phys. Lett. 299, 115 (1999). 29

30 Outline Materials Device Principles Device Physics Introduction Built-in potential Charge transport Charge injection Ionic space charge effects Degradation Applications Chelated complexes of Osmium 2 + LUMO - π* of ligand HOMO t 2g of metal S. Bernhard et. al., Adv. Mater. 14, 433 (2002). 30

31 Device model Cathode Cathode e - + h + Anode t = 0 sec Anode t >> 0 sec Device characteristics ITO/[Os(bpy) 2 (L)] 2+ (PF 6- ) 2 /Au Current (A) At 3V: η QE = 1% Time (min) 80 4V 6V Radiance (W) Slow response, indicative of ionic motion 300 cd/m 2 S. Bernhard et al., Adv. Mater. 14, 433 (2002). 31

32 Summary III: Device physics Built-in potential important for understanding OLED characteristics. Physics of charge injection and transport is different than in crystalline semiconductors. Ionic space charge can lead to ohmic contacts. Outline Materials Device Principles Device Physics Degradation Applications 32

33 Degradation of the cathode (a) 2 min (b) 10 h (c) 20 h (d) 30 h (e) 40 h Courtesy of Dr. Homer Antoniadis CsF/Al cathodes Current (A) Radiance (W) 10-6 ITO/MEH-PPV/CsF(2Å)/Al ITO/MEH-PPV/Al Voltage (V) Voltage (V) 33

34 Thickness dependence 10 0 Quantum Efficiency (%) Ca cathode ITO/MEH-PPV/(xÅ)CsF/Al CsF Thickness (Å) Degradation of the organic Layered device Mixed emission layer H. Aziz et. al., Science 283, 1900 (1999). 34

35 Encapsulation glass OLED sealant glass Outline Materials Device Principles Device Physics Degradation Applications 35

36 RGB schemes Patterned emitters Fluorescent converters Microcavities Color filters Stacked Organic light emitting diodes (OLEDs) Pioneer (1997) Pioneer ( demo) Kodak (2004) Sony (2004) Motorola (2001) 36

37 OLEDs vs. liquid crystals OLEDs vs. Liquid Crystals Low power consumption High intensity/low voltage 180 o viewing angle Very flat No backlights/polarizers/color filters 37

38 OLEDs for lighting GE Summary IV: Prospects After ~15 years of development OLEDs are on the threshold of widespread commercialization. Key areas for fundamental research: New materials Interfaces 38