NTC connecté entre deux électrodes métalliques

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J. P. Bourgoin et coll. Phys. Rev. Lett. 95, 185504 (2005)

Plan

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

The outcome of the response is determined by the maturation state of the cell, the nature of the antigen, the magnitude and duration of BCR signaling, and signals from other receptors such as CD40, the IL-21 receptor, and BAFF-R. Many other transmembrane proteins, some of which are receptors, modulate specific elements of BCR signaling. A few of these, including CD45, CD19, CD22, PIR-B, and FcγRIIB1 (CD32), are indicated here in yellow.

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

H .

E

?* 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

Intersystem crossing (ISC)

10-6-1s

Phosphorescence

(decay of excited state T₁)

Lifetimes and quantum yields

Effect of molecular structure on fluorescence

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

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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

E_F

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

(%)