organic light emitting diodes (leds) - gistppl/2004ppl/lecture/2009-02/2009_2nd... · organic light...

Organic Light Emitting Diodes (LEDs)

Organic materials for electronics and photonics

Advantages of OLEDs


•Low cost: simple processing, vacuum deposition, inkjet printing,spin coating, roll-to-roll (web) processing

• Superior viewing performance:emissive bright colors, wide viewing angle, fast response time, and high contrast

• Excellent operating characteristics:low operating voltage, power efficient, and wide temperature range

• Good form factor:Thin, light-weight, rugged, and flexible

OLED Structures


Examples of OLED Displays


Progress of OLED Performance


Energy Levels and Band Gap


Molecular Configuration


Energy Band Gap for Organic Materials


Excitons


Role of exciton in OLEDs


1.do radiative decay in luminescent materials2.transfer energy from host material to luminescent dopant.

A heterostructure OLED

Excitons form in host at interface.Ideally exciton decay to ground state or it’s energy is transferred to luminescent

molecules

Molecular Excited States After Electrical ExcitationRelaxation of Singlet & Triplet Excitons


Molecular excited states after electrical excitation

Singlet Triplet

Spin symmetric

χ1 χ2>= > >

χ1 χ2>= > >+

χ1 χ2>= >

χ1 χ2>= >

Spin anti-symmetric

Relaxation allowed fast, efficientFluorescence

Relaxation disallowed slow, inefficient

PhosphorescenceMolecular ground State

Spin anti-symmetric

Singlet-triplet formation ratio


Sσ :Formation cross section of singlet exciton

Tσ :Formation cross section of triplet exciton

:Singlet exciton formation ratioSηTS

SS σσ

ση3+

=

Small molecule

TS σσ = 25.0=Sη

π-conjugated polymer

TS σσ 3≈ 5.0≈Sη

Need to utilize wasted triplet exciton!

Electronic Processes in Molecules


Spin selection rules require that ∆S = 0 during an electronic transition, thus S →T and T →S radiative transitions are forbidden.

Absorption and PL Spectra of Organic Materials


Electroluminescence in Doped Organic Films


Effect of Dopants on the OLED EL Spectrum


Excimer or Exciplex


Excimer: exited state dimerExciplex: exited state complex

Excimer


Exciplex Emission


Various EL mechanisms


Electroluminescence Processes

Organic materials for electronics and photonicsEfficiency of device = injection efficiency*transport efficiency*recombination efficiency*

efficiency of radiative decay

Dye doped OLED


– Easy color tuning by just changing the dopant

– Improvement of the quantum efficiency

– Improvement of the device stability

Host moleculesCharge transfer material

HTLHTL

Transparentanode

holes

Low work function cathode

ETLETLEMLEML

Dopant molecules(Phosphorescent or

Fluorescent molecules)

Carrier Recombination


Langevin Recombination of carriers: P. Langevin, Ann. Chem. Phys. 28, 289 (1903).Carriers are statistically independent, therefore, the e-h recombination is arandom process and kinetically bimolecular.

Total current attracted by a charge into the sphere ofCoulombic capture radius, rc.

The emitting layer must thicker than ~ 20 nm.

Exciton Recombination Zone



Carrier Injection at the electrodes

For a small potential barrier, the current is limited by the space-charge-limited current in a trap-filled insulator, rather than limited by the injection. [P. E. Burrows and S. R. Forrest, APL 64, 2285 (1994)].

Recently, the model for charge injection mechanism considering transporting mechanisms has been developed.

Carrier mobility


Low carrier mobility of the organic semiconductor layers in amorphous state:µ~ 10-4~10-6cm2/Vs for holes and much lower µ for electrons due to disorder, trapping at localized states (defects), and poor wave function overlap between molecules.

⇒ Build-up of space-charge density (bulk-limited current).

Temp. Mobility Temp. Mobility

Very high temp over BG. Mobility

Mobility of amorphous organic thin films


Temperature dependence


Effect of intermolecular distance on the field effect mobility in organic Semiconductor thin films

Intermolecular distance may be controlled by the change of organic semiconductor materials

)2

exp()exp()(0 aR

kTEE

Ef jijiiji −

−−= γγ

Mob

ility

(µ)

Intermolecular distance

i state j state

Hopping Rate between i and j state

EiEj

Rij

Intermolecular or intramolecular mobility


Intermolecular mobility = soliton propagation~1000 cm2/Vs along polymer backbone

E field

Intramolecular mobility = hopping transport~10-3~10-6 cm2/Vs

Mobility measurement


Time-of-Flight Photoconductivity

Beam splitter

PMT or Photodiode

Steady state or pulsesource of voltage

Resistor

0

v

t

Oscilloscope

Etd

T

=µWe can get it from TOF measurement.

D: thickness of sampleE: electrical field

Time-of-Flight Photoconductivity


Phot

ocur

rent

Time

Ideal case

Nondispersive behaviorRelatively ordered

Dispersive behaviorRandom distribution of hopping state

Log I vs log t

Information obtained from transient EL measurement.


1. Charge injection.

2. Accumulated charge.Instantaneous peak. (double voltage pulse)

3. Transport properties.

)(

2

onr VVtd−

=µ

voltageonTurnVvoltageAppliedV

filmpolymerofthicknessdmobilitydrift

−−−

−−−−

:::

.:µ

Transient EL (time resolved EL measurement)


Voltage pulse in

Reponses EL out

Voltage pulse in

Reponses EL out

ITO/PEDOT/PVK-PtOEP/TAZ/Alq3/Mg:Ag/AgGaAs diodeY.-Y. Noh, J. Chem. Phys. 2003, 118, 2853

EL Quantum Efficiency


ηint= ηinjection x ηrecombination x ηPL

Material PL Quantum yieldOhmic contact or not?

ηext~ 0.2 x ηint

Charge balanceExciton binding probability

Coupling-Out Efficiency


Doubling Coupling-Out Efficiency in Organic Light-Emitting Devices Using a Thin Silica Aerogel LayerT.Tsutsui, M.Yahiro, H. Yokogawa, K. Kawano, M. Yokoyama, Adv. Mater. 13, 1149-1152(2001).

Theoretical Limit of the EL Efficiency


EL: Exciton formed by Injected electron and hole recombination- 4 spin combinations

S = 1 triplet states → no EL (forbidden process, different symmetric)S = 0 singlet state → EL

Triplet: 3

Singlet: 1

Theoretical Limit of the EL Efficiency


PL: Exciton formed by excitation of ground state exciton→singlet configuration. (A ground state exciton is usually singlet state)

∴EL ≤0.25 PL (?)

Singlet/Triplet Formation Cross Section

In OLEDs with Small Molecules Experimentally determined fraction = 22±3% singlets

That is, formation cross section of singlet (σS)and triplet (σT) is samein small molecules

M.A.Baldo, et.al., Phys. Rev. B (1999)

However, Polymer is a slightly different

Spin-dependent recombination spectroscopyNot totally confirm


M. Wohlgenannt, et al, Nature 409, 494 (2001)

Dye doped OLED


– Easy color tuning by just changing the dopant

– Improvement of the quantum efficiency

– Improvement of the device stability

Host moleculesCharge transport material

HTLHTL

Transparentanode

holes

Low work function cathode

ETLETLEMLEML

Dopant molecules(Phosphorescent or

Fluorescent molecules)

Molecular Excited States After Electrical ExcitationMolecular Excited States After Electrical Excitation


Molecular excited states after electrical excitation

Relaxation disallowed slow, inefficient

Phosphorescence

Singlet Triplet

Spin anti-symmetric Spin symmetric

Relaxation allowed fast, efficientFluorescence

Molecular ground StateSpin anti-symmetric

χ1 χ2>= > >

χ1 χ2>= > >+

χ1 χ2>= >

χ1 χ2>= >

Fluorescence and Phosphorescence


Triplet has lowerenergybecause it is spatiallyantisymmetric underexchange ofelectrons.(e--e- repulsionlower).

Fluorescence:Decay from singlet allowed by symmetry: fast (109 s-1) and often efficient.Phosphorescence:Decay from triplet disallowed by symmetry: slow ( > 1 s-1) and inefficient.


Possible paths for the emission of light from phosphorescent dye doped polymer LED’s

GuestGuestHostHost①

②③

1S1S3T

3MLCT

① Singlet-singlet or triplet ET from photo and electrical excitation.② Triplet-triplet ET from electrical excitation.③ Charge confinement on dopant site from electrical excitation.

Red Electrophosphorescence


M. A. Baldo, et al, Nature 395, 151 (1998)

Organic Phosphors: R, G, Y, and B


Mark Thompson, IMID 2001

Quantum Efficiency vs Current


Triplet –Triplet (T –T) Annihilation

Q.E. decay as current is increased due to T-T annihilation

To decrease the effects of T-T annihilation by:

•short triplet lifetime will decrease T-T annihilation (platinum complex -> iridium complex)

•decrease dopant aggregation in the thin film

B L U E


Problem: the most energetic charge transport hosts currently available have blue-green triplets.So to transfer energy to a blue phosphor, exciton transfer must be endothermic.

Polymer host for Blue Electrophosphorescence


Most conjugated polymers have large single-triplet gap (0.8~2.6) unlike small molecule host. So, host polymer, which has a relative highTriplet energy level, for blue electro-Phosphorescent LED is rare.

Monkman, Phys. Rev. Lett. 2001, 86, 1358. Y.-Y. Noh, J. Chem. Phys. 2003, 118, 2853

Device Degradation


Change of Electroluminescence depending on time under humid atmosphere

ITO/TPD/Alq3+Quinacridone/Mg

Encapsulation

OLED devices

organic light emitting diodes (leds) - gistppl/2004ppl/lecture/2009-02/2009_2nd... · organic light...

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