Organic Light-Emitting Diodes: Basic Concepts

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

2. Organic Display Technologies Philips Uniax/Dupont CDT/Seiko Epson Pioneer UDC eMagin

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

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

6. Design of Organic LEDs

7. Advantages • Light weight • Structural flexibility • Low power consumption • Low dc drive voltage • High brightness (100,000 cd/m2) • Fast response time (ns) • Thin (< 1 mm) • RGB, white • Large viewing angle • Large operating temperature range

8. Introduction to organic electroluminescence

9. Charge and Energy Transfer

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

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

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 cm2/Vs

15. TOF experiments N2 laser, 337 nm, 6 ns R = 102 –104, C = 10 pF, RC << 

16. Field and temperature dependence Field dependence Temperature dependence

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

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

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

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

21. Fundamentals of radiometry A source is characterized by its radiance Radiometry Optics Energy Q: [Joule] Flux (power) : dQ/dt [Watt] Intensity I: d /d [W/sr] Radiance L: d2 /dAcosd [W/sr.m2] Power: [Watt] Intensity: [Watt/cm2] Energy: [Watt] x [time] = [Joule] Radiance: power per unit area per unit of projected solid angle : angle between the normal of the surface and the line of sight.

22. 2 1 dA2  dA1 Fundamentals of radiometry Formula for radiative transfer: Exitance E = d/dA Incidance M = d/dA Power radiated per unit area Power received per unit area

23. Fundamentals of radiometry For a point source: S Z/r >5 [W/m2] Z S: source area with radius r Z: distance from source to detector L: radiance of the source In both cases, one measures intensity (in the optics definition in W/m2) and deduce the radiance of the source For an area source: (and not 2L)

24. Fundamentals of photometry In radiometry: radiance given in W.m-2. sr-1 lm = lumen In photometry: radiance given in lm.m-2.sr-1 lm.m-2.sr-1 = cd.m-2 = nit With Km = 683 lm/W at 555 nm 1 W of optical power per cm2 per steradian of monochromatic light at 555 nm has a radiance of 683 cd/cm2 = 6.83 x 106 cd/m2

25. Photopic response of the human eye

26. 3,000,000 Upper limit for visual tolerance 30,000 Fluorescent lamp 300 Sky heavily overcast day Neon lamp 3 Snow in full moon 0.03 Moonless clear starlight 0.0003 0.000003 Threshold of vision Examples of luminance levels cd/m2 The sun: 900,000,000 cd/m2

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

28. Light Emission in Organic Solids Selection rules Spin selection rule Parity selection rule Forbids electronic transitions between levels with different spin Forbids electronic transitions between levels with same parity 2Ag T2 S1 1Bu T1 1Ag S0

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: Exchange interaction Coulomb interaction Initial Final

30. Geometrical factor Overlap integral Constant Förster transfer: long range interaction itransition dipole moment pure radiative lifetime fitransition oscillator strength Absorption of A Emission of D 

31. Dexter transfer: short range interaction J: normalized overlap integral Dexter transfer is based on two electron transfer reactions and requires proximity of the two molecules  short range interaction 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. Singlet-singlet transfer: allowed both by Forster and Dexter Triplet-triplet: allowed only by Dexter

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

33. From fluorescence towards phosphorescence Collect all the singlets and triplets: 100% efficiency Baldo et al., Nature 395, 151 (1998), Susuki et al. APL 69 224 (1996) El in benzophenone at 100 K.