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Yvan BONNASSIEUX 06/02/2014
OLED: Organic Light Emitting Diode
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Organic Light Emitting Diode OLED
yvan.bonnassieux@polytechnique.edu
Yvan BONNASSIEUX 06/02/2014
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SUMMARY
ØIntroduction !
ØOrganic material !
ØOLED: Organic Light Emitting Diode !
ØOLEDs today !
ØOLED futur ?
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Introduction (I)Organic materials
• Organic:)Rela-ng)or)belonging)to)the)class)of)chemical)compounds)having)a)carbon)basis)
• Include)polymers)and)small)molecules)
• Polymer:*Large*molecule*composed*of*a*repea4ng*structural*unit*(monomer)*
• Small*molecules*M<800
• Until recently, polymers were known as electrical insulators • In 1977, discovery of electrical conductivity in doped polyacetylene
• Nobel prize in chemistry in 2000 to Alan Heeger, Alan McDiarmid and Hideki Shirakawa • Organic electronic devices came out ten years later
• 1986. Organic photovoltaic cell OPV (Ching W Tang, Kodak) • 1986. Organic field-effect transistor OFET (H Koezuka, Mitsubishi) • 1987 Organic light-emitting diode OLED (Ching W Tang, Kodak)
Conducting polymers
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Introduction (II)Why organic?
• Organic(materials(
• Large'diversity'
• Low'cost,'large'area,'flexible'substrates'
• Manufacturable'via'prin;ng'techniques'
• �Green��technology'
• Drawbacks(
• Low'performance'
• Poor'stability'
• Small'market'(except'OLEDs)'
Organic electronic devices
Organic field-effect transistors (OFET)
Organic light-emitting diode (OLED)
Organic light-emitting diode (OLED)
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Conjugated molecules (I)Benzene Conjugated = alternation of simple and double bond
benzene: C6H6
6 delocalize electrons on all the molecule
Frontier orbitals • HOMO:%Highest%Occupied%Molecular%Orbital%
• LUMO:%Lowest%Unoccupied%Molecular%Orbital
Yvan BONNASSIEUX 06/02/2014
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C CH H
HH
*
*
σ bond ⇒ ensure the maintenance of the molecule
π bond ⇒ electrical properties
Double bond: hybridization sp2 (superposition orbitals s et pz)
Ethylene
Conjugated molecules (II)
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• HOMO:%Highest%Occupied%Molecular%Orbital%
• LUMO:%Lowest%Unoccupied%Molecular%Orbital
n Ethylène Octatétraène Butadiène Polyacéthylène
Energy
π* (LUMO)
π (HOMO)
π* (LUMO)
π (HOMO)
π* (LUMO)
π (HOMO)
Number of motif
π* (LUMO)
π (HOMO)
Gap
Frontier orbitals: energy bands (I)
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Frontier orbitals: energy bands (II)
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Main properties (I)
Organic(solids! Inorganic(solids!
Binding(energy! weak((Van(der(Walls)! strong((covalent)!
Mechanical(strength! weak! strong!
Charge(carriers! localised! delocalised!
Polarisa=on! strong! weak!
Charge(transport! hopping! band!
Charge(mobility! ~(1(cm2/Vs! ~(1000(cm2/Vs!
Exciton! FrenkelItype! WannierItype!
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Low mobility: 10-4 to 1 cm2.V-1.s-1
Doping is a complexe process No free charge (~1010) in intrinsic material Always in full depleted mode Current: electrode injection or photo-generation P and N type only defined by electrode WF (Work function) Gaussian DOS (Density of states) Transport mechanism GVRH (Gaussian Variable Range Hopping)
Main properties (II)
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W. Helfrich et al. Phys. Rev. Lett. 14, 229 (1965)
Electroluminescence was observed from single crystals of anthracene.
5 mm thick crystalEl 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)
History
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Multilayers interest
OLEDS principle (II)
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Metallic electrodes
OLEDS principle (III)
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OLEDs conduct in forward bias and do not conduct under reverse bias. The impedance drops exponentially with V for V>Vth.
Brightness is proportional to the current flowDiode behavior
Electrical and optical characteristic
OLEDS principle (IV)
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Pope (1963) Helfrich (1965)
Thick crystals
Efficiency & stability
Heterostructures
1985 20021965
HBL Hole Blocking layer: Exciton confinementPLED
Doped transport layers K. Leo, U. Dresden
Cathode
Anode
EML
Monolayer
~~~~
2-layers
EK US patent #4539507
Cathode
Anode
EML
HTL
Cathode
AnodeHTL
EML
Doped-EML
EK US patent #4769292
Cathode
Anode
EML/dopedHTL
ETL
HIL
HBL
Multilayers
Cathode
Anode
EML/dopedHTL
ETL
P-doped
N-doped
PIN
OLEDS principle (V)
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No really standardized measurement method (DC vs pulsed constant current, brightness level …)
Device lifetime usually defined as : Mean time to half-brightness
Degradation of OLED devices is one of the main issues. Degradation phenomena occur both under operating condition as well as under storage.
Encapsulation (II)
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Source : Vitex
Substrate
OLED
Barrier coating
Encapsulation (III)
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OLED display (I)
! Very thin ! Light weight ! Fast response time ! High brightness ! Large viewing angle ! Low power consumption
OLEDs display strengths
Active matrix pixel
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OLED display (III)
Samsung Galaxy Note 3. 5.7-inch HD Super AMOLED display 1920 x 1080 resolution
LG G Flex 6 inch HD AMOLED display 1280 x 720 resolution thickness 0.44 mm, 7.2g
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OLED display (IV)
55-inch class WRGB OLED TV Only 4 millimeters (0.16 inches) thin and weighing less than 10 kilograms (22 pounds) near 8.000 €
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Pliable display
rollable displays
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Micro & transparents displaysMicro écrans (lunettes)
Lens electronics
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