NTC connecté entre deux électrodes métalliques
J. P. Bourgoin et coll. Phys. Rev. Lett. 95, 185504 (2005)
Plan
Download as PDF or read online from Scribd. Flag for inappropriate content. Exercice electronique de puissance.
Introduction
Basic device structure
Electroluminescence : Generation of light with electric field
consists of:
3.A transparent electrode (ITO)
4.An emissive layer
5.A reflective electrode (metal)
Oxide)
Thin layer devices from organic dyes or conjugated polymers
Organic layer thickness : ~ 150 nm
History of organic electroluminescence
History of organic electroluminescence
Electroluminescence was observed from single crystals of anthracene.
W. Helfrich et al.
Phys. Rev. Lett. 14, 229 (1965)
5 mm thick crystal
El quantum efficiency ~ 1-5% High driving voltage
Good understanding of the basic physical processes involded in electroluminescence like double injection, charge carrier migration, electron-hole capture (exciton formation), and light emission (fluorescence)
OLEDs roadmap
Forecast display production
Strong increase of OLEDs displays production
OLED unit forecast
2008
Plan
Les diodes électroluminescentes organiques
Electronic structure of carbon
Isolated carbon atom: 1s2 2s1 2p3 à valence of 4
Hybridized spn orbitals (superposition of s & 2p orbitals)
Sp2hybridization (double bond)
Molecules with delocalized ? orbitals
Semiconducting properties
HOMO-LUMO Bands
HOMO : Highest Occupied Molecular Orbital
(The highest energy molecular orbital that contains a pair of electrons)
LUMO : Lowest Unoccupied Molecular Orbital
(The lowest energy molecular orbital that contains no electrons)
Organic semiconductors
Small molecule organic semiconductors
Polymer organic semiconductors p>
Source:
Electron affinity & ionization potential
Electron affinity
Ionization potential
2.5 – 3 eV EAIP 4.5 - 6 eV
Evaluated by cyclic or photoelectron spectroscopy voltametry in solution
Electronic transitions
Polyatomic molecule | H | C | O. |
H .
E
Ground state | ? à ?* | n(p) à ?* à * Excited states | n(p) à * |
?* Â
LUMO ?*
HOMO n(p)
?
?
? ? ?
Optical properties of molecules
PHBN : R=n-hexyl
Organic materials are characterized by a large Stockes shift between absorption and emision spectra à they are almost transparent to their own emitted light
Singlet – triplet states
Excitons
Singlet excited Triplet excited state state
S=0 Â S=1
25 % 100 %
Singlet decay (radiative) is calledfluorescence
Triplet decay (forbidden process) is calledphosphorescence
Ir(ppy)3
Strong spin-orbit coupling mixes singlet and triplet states H3C
Heavy metals (Ir, Pt…) impove triplet emission
Characteristic times
Absorption Vibrational relaxation Internal conversion Fluorescence (decay of excited state S1) | 10-15s 10-12-10-10s 10-11-10-9s 10-10-10-7s 10-10-10-8s |
Intersystem crossing (ISC)
10-6-1s
Phosphorescence
(decay of excited state T1)
Lifetimes and quantum yields
Effect of molecular structure on fluorescence
Molecule | ?f | ?p 010915 779 caribou coffee. Gazzetta Sports Awards gli sportivi dellanno scelti dai lettori della. Gazzetta dello Sport. | ?T (s) |
Naphthalene | 0.55 | 0.051 | 2.3 |
1-Fluoronaphthalene | 0.84 | 0.056 | 1.5 |
1-Chloronaphthalene | 0.06 | 0.30 | 0.29 |
1-Bromonaphthalene | 0.0016 | 0.27 | 0.02 |
1-Iodonaphthalene | < 0.0005 | 0.38 | 0.002 |
Source Wehry 1990
Charge transport in organic solids
Periodic lattice Amorphous lattice
Delocalized Localized chargescharges
Crystals : periodic structures band model (conduction & valence bands) delocalized charges (electrons in CB, holes in VB)
Amorphous organic materials : band model ?
localized charges (radical ions) transport through intersite hopping charge traps (defects)
Charge transport in conjugated polymers
In conjugated polymers the charges are partially transported via delocalisation along the HOMO and LUMO levels.
Transport properties are usually determined by defects in the 1D-chains (intra molecular) or by hopping from chain to chain (inter molecular)
Charge transport in small molecules
Charge transport in small molecules is via hopping, i.e. the charges have to jump from one molecule to the neighbouring one to be transported.
Charge transport
Charge transport via hopping Low mobility (disorder) µh+# µe-
Challenge for
High EL efficiency :
Charge Carrier Balance
Plan
Les diodes électroluminescentes organiques
Organic Light Emitting Diode : Principle
1 à Charge carrier injection
2 à Charge carrier transport
3 à Charge recombination (exciton formation)
4 à Exciton diffusion
5 à Exciton recombination and photon emission
I-V-L characteristics
Diode behavior
Brightness is proportional to the current flow
OLEDs conduct in forward bias and do not conduct under reverse bias. The impedance drops exponentially with V for V>Vth.
OLEDs : 2 main technologies
Charge injection : holes
Anode : ITO
Small barrier for holes injection into HOMO level of HTL organic material
Use of materials with high work function (ideal ~ 5 eV)
Typically use of transparent ITO as anode
Need ITO surface treatment to enhance holes injection (i.e. Oxygen plasma treatment), ITO fermi level stabilization around 5 eV.
Réf.: Kim et al., Appl. Phys. Lett., 74, N°21 (1999) 3084
Charge injection : electrons
Cathode
Small barrier for electrons injection into LUMO level of ETL organic material (ideal ~ 2.5 to3 eV) Use of metals with low work function (Ca, Mg…)
But such metals are very sensitive to oxidation
Use alloys such as Mg/Ag or Al in combination with alkali metals like Li, Cs,
K, Na…
Barrier, dipole vs injection
EF
Metal-organic interfaces are varied and complex
Interface chemistry and interdiffusion can play key roles
- change with interface processing (deposition sequence)
- affect interface barriers (gap states, doping effects, dipoles)
Source: A. Kahn, Summer school, Aussois, 2005
Quantum efficiency
External quantum efficiency
?qext = Number of emitted photonsNumber of injected electrons = ?r. ?????PL.?ext |
(%)