OLED Organic Light Emitting Diodes - Dieeannalisa/didattica/OLEDs_2007.pdf · Electroluminescence (EL) • The emission of light by electric current • Historical development a

OLED

Organic Light Emitting Diodes

Electroluminescence (EL)

• The emission of light by electric current

• Historical development

a. 1962 (Holonyak and Bevacqua)

• Inorganic semiconductors for light-emitting diodes

(LEDs)

e.g. GaPxAs1−x (x = 1.00–0.40) and GaN-based materials

• The energy of the light emitted can be changed by

adjusting the composition of the material

Electroluminescence (EL) (2)

BlueGaN

OrangeGaAsP:N

GreenGaP:N

RedGaP:ZnO

ColorLED material

+ − hv

Charge recombination

Basic Theory

Valence band

+

−−Conduction band

+ −Applied voltage

p-type n-type


• p-type semiconductors : the added material has

fewer electrons than the host and adds positive

holes e.g. Si doped with B

• n-type semiconductors : the added material has

one more electrons in the valence shell than the

host material e.g. Si doped with P


• Light emission arises from the recombination of

holes and electrons in a p-n junction made up of

p- and n-type semiconductors

• However, only single-crystal material is capable

of exercising this kind of recombination radiation

efficiently enough for practical use

Visible LEDs have been available commercially

since 1960s


Crystal substrate

(transparent)

n-type

p-type

+

−

Light

output

Device structure for common LEDs


b. 1987 (Tang and VanSlyke in Kodak)

• Fluorescent organic dyes as electroluminescent

materials for organic light-emitting devices

(OLEDs) (Small-molecule-based OLEDs)

e.g. tris(8-hydroxyquinoline)aluminum (Alq or

Alq3) and N,N’-diphenyl-N,N’-bis(3-

methylphenyl)-1,1’-biphenyl-4,4’-diamine (TPD)


The first double-layer OLED

N

CH3

N

H3C

TPD

N

O

Al

3

Alq3

Ag

VMg:Ag

TPD

ITO

Glass

Alq3

−

+

EL light


– 1990 (Friend et al. in Cambridge University)

• Luminescent conjugated polymers for polymer

light-emitting devices (PLEDs)

e.g. poly(1,4-phenylene vinylene) (PPV)

poly[2-(2’-ethylhexyloxy)-5-methoxy-1,4-

phenylene vinylene] (MEH-PPV)

nMeO

O

n

PPV MEH-PPV


• Important features of PLEDs

– Inorganic semiconducting and small molecule organic dyes

have to be deposited as thin films by the relatively

expensive techniques of sublimation or vapour deposition,

which are not well suited for large-area displays

– Luminescent polymers can be deposited from solution as

thin films over larger area by spin-coating techniques

– The physical properties (e.g. colour, emission efficiency) of

conjugated polymers can be fine-tuned by manipulation of

their chemical structures


Normalized photoluminescence spectra of side-chain-modified PPV with the general

formula shown on the top: (1) R1, R2, R3, R4 = alkyl, R5, R6 = CN; (2) R1, R2 = alkoxy,

R3, R4 = alkyl, R5, R6 = CN; (3) R1, R2 = alkoxy, R3, R4 = alkoxy, R5, R6 = H; (4) R1, R2

= alkoxy, R3, R4 = alkoxy, R5, R6 = H annealed; and (5) R1, R2 = alkoxy, R3, R4 =

alkoxy; R5, R6 = CN

R1

R2

R3

R6

R5

R4

l

1-l

Theory of OLEDs or PLEDs

• The simplest OLED or PLED configuration

consists of an electroluminescent layer

sandwiched between an anode and a

cathode, one of which has to be semi-

transparent

• Under an applied bias, injection of holes

takes place at the anode whereas injection

of electrons occurs at the cathode

Theory of OLEDs or PLEDs (2)

• Some of the electrons and holes combine within the emissive material to form singlet and triplet excited states. Such an electron-hole pair (exciton) may then result in the emission of a photon

Schematic drawing of single-layer

EL device

++

+ ++

+

+

− −

hv

+

−Al, Ca, or Mg

Cathode

light-emitting

polymer

glass or polymer substrate

ITO anode


a) Irradiation of a fluorescent layer excites an

electron from HOMO to LUMO. In a typical

conjugated polymer, two new energy states are

generated upon relaxation within the original

HOMO−LUMO energy gap and are each filled with

one charge of opposite sign (singlet excited state).

The excited polymer may then relax to the ground

state with emission of light at a longer wavelength

than that absorbed (photoluminescence). b) In a

polymer LED, electrons are injected into the LUMO

(to form radical anions) and holes into the HOMO

(to form radical cations) of the electroluminescent

polymer. The resulting charges migrate from

polymer chain to polymer chain under the influence

of the applied electric field. When a radical anion

and a radical cation combine on a single conjugated

segment, singlet and triplet excited states are formed,

of which the singlets can emit light.

Anode: high work-function electrode e.g. indium tin oxide (ITO)

Cathode: low work-function electrode e.g. Ca, Al, MgAg alloy

HOMO

LUMO

hv hv'

singlet excited state

a)

singlet excited stateradical anion

hv'

−

+ e -

Cathodeb)

+Anode

radical cation

− e -


• Unfortunately, the mobility of electrons and

holes in most organic materials are

considerably different, leading to exciton

formation in the vicinity of one of the two

contacts (For example, since holes migrate

much more easily through PPV than

electrons, electron-hole recombination takes

place near the cathode)


• Since non-radiative exciton recombination,

or quenching, is enhanced at the electrode-

organic interface, the single-layer structure

typically exhibits a low quantum efficiency

⇒ Heterojunction OLEDs

Layers in OLEDs

• Hole-Transporting Layer (HTL)

– Transports holes from the anode to the EML or

ETL

• Electron-Transporting Layer (ETL)

– Transports electrons from the metal cathode to

the EML or HTL

Layers in OLEDs (2)

• Emission Layer (EML)

– Transports both holes and electrons

– The layer where the recombination of holes and

electrons takes place

Cathode

ETL/EML

HTL

Anode

Substrate

−

+

hvCathode

HTL/EML

ETL

Anode

Substrate+

−

hv

Double layer membrane

Either ETL or HTL behaves as EML

Layers in OLEDs (3)

Triple-layer device

Cathode

ETL

EML

HTL

Anode

Substrate

−

+

hv

The recombination of holes and electrons occurs in an

independent EML

Common layer materials in

OLEDs

i. HTL materials e.g. PPV, TPD, NPD

NN

H3CCH3

NN

TPD α-NPD

Common layer materials in OLEDs (2)

ii. ETL materials, e.g. PBD, Alq3

NN

ONN

O

PBD

Oxadiazoles are

electron-deficient

Color tuning

– Varying the HOMO−LUMO Gap

S

R

nH13C6 C6H13

n

OR

RO

OR

RO

CN

NC

(R = C6H13)

n

Yellow-orangeCO2C8H17

Yellow-orangeC10H21OMe

RedC8H17

RedC12H25

ColorR

Blue

Red

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OLED Organic Light Emitting Diodes - Dieeannalisa/didattica/OLEDs_2007.pdf · Electroluminescence (EL) • The emission of light by electric current • Historical development a