1 Organic Light-Emitting Diodes: Basic Concepts Basic Concepts Bernard Kippelen

2 Organic Display Technologies Uniax/Dupont CDT/Seiko Epson Philips Pioneer eMaginUDC

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4 Flat panel displays Wall-mount TV Computers Car Navigators Replace paper ? $20 billion market LCD 86% Tremendous Market in the information-oriented society

5 Flat panel display technologies Liquid Crystal Displays - Backlight - High power consumption - Limited viewing angle - Slow response - High manufacturing cost Emissive technologies - Plasma - Field Emission - AC thin film EL (ACTFL) - Organic LEDs Source: SHARP

6 Design of Organic LEDs

7  Light weight  Structural flexibility  Low power consumption  Low dc drive voltage  High brightness (100,000 cd/m 2 )  Fast response time (ns)  Thin (< 1  m)  RGB, white  Large viewing angle  Large operating temperature range Advantages

8 Introduction to organic electroluminescence

9 Charge and Energy Transfer

10 Anode Cathode ETL HTL + + Energy TPDX + + AlQ - TPDX + AlQ * Introduction to organic electroluminescence

11 ECEC EAEA hh ee RR SS FF EE Device Quantum Efficiency:  =  R.  S.  F.  E Physics of OLEDs

12 Fundamentals of Charge Transport in Organic Solids Crystals: periodic structures, band model, delocalization, electron in conduction band, hole in valence band Amorphous organic materials: localized charge in the form of a radical ion, intersite hopping through a hopping site manifold

13 Hole transport and electron transport D + + D  D + D + A - + A  A + A -

14 Transport in Organic Semiconductors Benchmark: amorphous silicon 0.5 –1 cm 2 /Vs

15 TOF experiments N 2 laser, 337 nm, 6 ns R = 10 2 –10 4 , C = 10 pF, RC << 

16 Field dependence Temperature dependence Field and temperature dependence

17 The disorder formalism (Bassler and Borsenberger) Transport occurs by hopping through a manifold of localized states with energetic and positional disorder  Energetic disorder: width   Positional disorder width:  Distributions are Gaussian

18 Field dependence Mobility follows field dependence predicted by the disorder formalism

19 Dipolar contribution to the energetic disorder: Random distribution of dipoles generates fluctuations in electrostatic potential   d molecular dipole   vdW van der Waals contribution   D matrix (=0) To appear in Chem. of Mater.

20 TPD * TPD:PC * (50wt.%) * Values measured at 20V/  PMPS * PVK DPQ * AlQ * NTDI * PyPySPyPy * Bphen Hole and Electron Mobility in Non-Crystalline Materials PBD

21 Fundamentals of radiometry Optics Radiometry Power: [Watt] Intensity: [Watt/cm 2 ] Energy: [Watt] x [time] = [Joule] Energy Q: [Joule] Flux (power)  : dQ/dt [Watt] Intensity I: d  /d  [W/sr] Radiance L: d 2  /dAcos  d  [W/sr.m 2 ]  : angle between the normal of the surface and the line of sight. Radiance: power per unit area per unit of projected solid angle A source is characterized by its radiance

22 Fundamentals of radiometry Formula for radiative transfer: 11 22  dA 1 dA 2 Exitance E = d  /dA Power radiated per unit area Incidance M = d  /dA Power received per unit area

23 Fundamentals of radiometry For a point source: [W/m 2 ] S: source area with radius r Z: distance from source to detector L: radiance of the source S Z For an area source: Z/r >5 (and not 2  L) In both cases, one measures intensity (in the optics definition in W/m 2 ) and deduce the radiance of the source

24 Fundamentals of photometry In radiometry: radiance given in W.m -2. sr -1 lm In photometry: radiance given in lm.m -2.sr -1 lm = lumen lm.m -2.sr -1 = cd.m -2 = nit With K m = 683 lm/W at 555 nm 1 W of optical power per cm 2 per steradian of monochromatic light at 555 nm has a radiance of 683 cd/cm 2 = 6.83 x 10 6 cd/m 2

25 Photopic response of the human eye

26 Examples of luminance levels 3,000,000 30, Threshold of vision Moonless clear starlight Snow in full moon Neon lamp Sky heavily overcast day Fluorescent lamp Upper limit for visual tolerance cd/m 2 The sun: 900,000,000 cd/m 2

27 CIE color chart CIE: Commission Internationale de l’Eclairage Tristimulus values x + y + z = 1 Color coordinates 3 kind of sensors in the eye

28 Light Emission in Organic Solids Selection rules Spin selection rule Parity selection rule Forbids electronic transitions between levels with different spin S0S0 S1S1 T1T1 T2T2 1Ag 1Bu 2Ag Forbids electronic transitions between levels with same parity

29 Fundamentals of Energy Transfer D * + A  D + A * D and A molecules separated in space but coupled by the electric field associated with the excited molecule. Interaction hamiltonian has two contributions: Coulomb interaction Exchange interaction Initial Final

30 Förster transfer: long range interaction  i transition dipole moment pure radiative lifetime f i transition oscillator strength Geometrical factor Overlap integral Constant Emission of D Absorption of A

31 Dexter transfer: short range interaction Dexter transfer is based on two electron transfer reactions and requires proximity of the two molecules  short range interaction Singlet-singlet transfer: allowed both by Forster and Dexter Triplet-triplet: allowed only by Dexter J: normalized overlap integral Not that both Förster and Dexter transfer rates depend on the overlap integral. However, in the case of Dexter the rate is independent of the amplitude of the extinction coefficient of A.

32 Spin considerations ECEC EAEA hh ee RR SS FF EE P + + P -  P + P * + + Singlet state Triplet states  S = 0.25 ? No because singlet and triplet wavefunctions are different

33 From fluorescence towards phosphorescence Collect all the singlets and triplets: 100% efficiency Baldo et al., Nature 395, 151 (1998), Susuki et al. APL (1996) El in benzophenone at 100 K.