1

E L E C T R O P H O S P H O R E S C E N C E

1. OLED efficiency

2. Spin

3. Energy transfer

4. Organic phosphors

5. Singlet/triplet ratios

6. Phosphor sensitized fluorescence

7. Endothermic triplet energy transfer

April 10, 2003 – Organic Optoelectronics - Lecture 17

Organic LEDs – part 4

Lecture by Prof. Marc Baldo

2

POWER EFFICIENCY

400 500 600 700

0

200

400

600

800

Pho

topi

c re

spon

se (l

m/W

)

Wavelength (nm)

The eye’s response is wavelength dependent.The unit of perceived optical poweris defined as the lumen (lm).

Power efficiency of LED’s is thereforemeasured in lumens/Watt.

The maximum photopic response is:Φ ~ 20 lm (blue)Φ ~ 680 lm (green)Φ ~ 100 lm (red)

Power efficiency = photopic response of eye x quantum efficiency x electrical efficiency

For an OLED of given color, we must maximize:

- quantum efficiency (ηQ = photons out/electrons in)

- electrical efficiency (Vλ/V = photon energy/operating voltage)

P QVL

VI Vλη η= = Φ

3

Q out PLη χη φ=

φPL is fundamental luminescence efficiency of organic material.

Quantum efficiency

is the measure of a luminescent dye’s performance.ηQ= the number of photons emitted per electron injected.

ηout is output coupling fractionreduced by absorption losses

and wave guiding within the device and its substrate.

glass

OLED

χ is fraction of luminescent molecularexcitations

(defined by spin selection rules)typically ~ 25%

remaining energy is wasted

Imposes a fundamental limitto OLED efficiencies

4

EXCITON SPIN AND SYMMETRY

Lowest unoccupied (LUMO)molecular orbital

Molecular ground state(spatially symmetric under exchange of electrons)

1st excited state(spatially anti-symmetric (triplet) or spatially symmetric (singlet))

LUMO

HOMO

ψ

ψψ

ψ

S=1, triplet

S=0, singlet

}

S=0, singlet

Highest occupied (HOMO)molecular orbital

E

E

Molecular excited states (or excitons) are typically mobile, with binding energy ~ 1eV, and spin S = 0,1

5

Intersystemcrossing (ISC)S1

T1

PhosphorescenceFluorescence

Molecular ground state

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.

Triplet has lower energybecause it is spatiallyantisymmetric under exchange of electrons.(e--e- repulsion lower).

Fluorescence and Phosphorescence

S0

6

Efficient phosphorescenceNeed to mix singlet and triplet states:

- make both singlet and triplet decay allowed.

Pt, Ir, etc...

Ligand molecular orbitalSpin orbit coupling mixes states: proportional to atomic number

Z4

-+ligand

exciton

metalligand

+-

MLCT exciton

metalligand

Use metal-organic complexes with heavy transition metals:

PtN N

NN

PtOEP

NIr

3

Ir(ppy)3

Type I phosphorExciton localized on organic

Type II phosphorMetal-ligand charge transfer exciton

most mixing ~ 1µs triplet lifetimeless mixing ~ 100µs triplet lifetime

7

Structure and operation of OLEDs

- must transfer energy from host material to luminescent dopant. This determines the quantum efficiency of luminescence.

A heterostructure OLED

Excitons form in host at interface.Ideally energy is transferred to luminescent molecules

electronsholes

Cathode

TransparentAnode

Assume balanced currents:every hole combines with an electron

Luminescentdopant molecules

Electrontransport layer

Hole transport layer

Light

8

Donor* Acceptor Donor Acceptor*

Acceptor(dye )

Donor

Long range(up to ~ 100Å)

Exciton non-radiatively transferred by dipole-dipole coupling if transitions are allowed (usually singlet-singlet).

(a) Förster energy transfer

Donor* Acceptor Donor Acceptor*

Acceptor(dye )Donor

Short range~ 10Å

Exciton hops from donor to acceptor.

(b) Dexter energy transfer

Energy transfer

9

A red phosphor:Platinum octaethylporphyrin

Wavelength (nm)400 500 600 700 800 900

Inte

nsity

(a.u

.)

0

1Emission spectrum

0.1 1 10 1000.1

1

1030020010050301042

Qua

ntum

Effi

cien

cy (%

)

Current Density (mA/cm2)

1% PtOEP6% PtOEP

20% PtOEP

Luminance of 6% device (cd/m2)

• Triplet lifetime ~100ms• Peak external quantumefficiency in Alq3 ~ 4%

triplet

triplet groundstate

singlet

efficiency roll-off due to triplet-triplet annihilation

Loss proportionalto square of triplet lifetime

Mg:Ag cathode

Indium tin oxide

a-NPD

PtOEP in Alq3

Devicestructure

PtN N

NN

PtOEP

10

500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8In

tens

ity (a

rb. u

nits

)

Wavelength (nm)

350Å a-NPD

700Å Alq3

Mg/Ag

Ag

Glass

ITO

60Å CuPc

100Å DCM2/Alq3

Device 1Device 1 Device 2Device 2Alq3

DCM2

PtOEP

100Å Alq3

100Å PtOEP/Alq3

350Å a-NPD

500Å Alq3

100Å DCM2/Alq3

Mg/Ag

Ag

Glass

ITO

60Å CuPc

DOES PTOEP CAPTURE ALQ3TRIPLETS?

Put fluorescent dye DCM2 in exciton formation zone.

11

The singlet/triplet ratio in Alq3

Glass

ITO60Å CuPc

450Å a-NPD

200Å Alq3

400Å PtOEP/Alq3

Mg/Ag

Ag

Device 2emits from

singlets and triplets

Device 1emits from

singlets only

Glass

ITO60Å CuPc

450Å a-NPD

200Å Alq3

400Å DCM2/Alq3

Mg/Ag

Ag

Compare ratio of EL emission from devices to get singlet fraction.After correction for PL efficiency of DCM2 and PtOEP, get:

χ=(22±3)%

12

IMPROVED PHOSPHORS

Must reduce triplet lifetime to minimize T-T annihilation.Increase MLCT component of excited state.

The first high efficiency green phosphor was iridium tris(2-phenylpyridine)

Broad green emission at λ ~ 515 nm.Phosphorescent lifetime, τ ~ 1 ms.

NIr

3

Voltage (V)0 2 4 6 80.1

1

10

102

103

104

105

Lum

inan

ce (c

d/m

2 )

1

2

4

10

2030

Pow

er e

ffici

ency

(lm

/W)

Ir(ppy)3

10

+-

MLCT exciton

metalligand

13

TRIPLET-SINGLET ENERGY TRANSFER

S

T

S

T

host fluorescentdye

There are many more fluorescent than phosphorescent materials.Can we get high efficiency from a fluorescent dye?

?

Need to prevent exciting triplet state in fluorescent dye.Want mechanism for triplet-singlet energy transfer.

excitonformation

excitonformation

14

1. Triplet-singlet hopping transfer is disallowed2. Triplet-singlet Förster transfer permitted if triplet

relaxation on donor is allowedi.e. triplet-singlet transfer is possible from a

phosphorescent donor

Predicted by Förster in 1959 (†)Observed by Ermolaev and Sveshnikova in 1963 (§)

e.g. for triphenylamine as the donor and chrysoidine as the acceptor, in rigid media at 77K or 90K the interaction

length is 52Å

(†) Förster, Th. Dicussions of the Faraday Society 27, 7-17 (1959).(§) Ermolaev, V.L. & Sveshnikova, E.B. Doklady Akademii Nauk SSSR

149, 1295-1298 (1963).

Phosphor sensitized fluorescence

15

S

T

S

T

ISC S

T

host phosphorescentsensitizer

fluorescentdye

IMPLEMENTATION OF TRIPLET-SINGLET ENERGY TRANSFER

N

O CH3

NC CN

DCM2absorbs in the greenN

Ir

3Ir(ppy)3

N N

CBP

16

0.01 0.1 1 10 100 10000.50.60.70.80.9

1

2

3

45678

10

Ext

erna

l qua

ntum

effi

cien

cy (%

)

Current density (mA/cm2)

0.00.10.20.30.40.50.60.70.80.91.0

EL

Inte

nsity

(arb

itrar

y un

its)

Wavelength (nm)400 450 500 550 600 650 700 750 800

0.2% DCM2 and 8% Ir(ppy)3in CBP

DCM2 fluorescence sensitized by Ir(ppy)3

DCM2

Ir(ppy)3

Roll-off in efficiency is due to chargetrapping on DCM2 molecules

Nearly complete energy transfer from Ir(ppy)3 to DCM2

0.2% DCM2 in CBP

Note: factor of 4difference

17

Delayed DCM2 fluorescence confirms sensitizing action of Ir(ppy)3

OLED TRANSIENT RESPONSE

0 100 200 300 400 500 600 700 800 900 1000

1

10

100In

tens

ity (a

rbitr

ary

units

)

Time (ns)

DCM2

Ir(ppy)3

Examine transient response ofdifferent spectral componentsof OLED luminescence.

Natural lifetime of DCM2only ~5ns.

18

B L U E

FIrpic (blue phosphor)Peak wavelength 470nmTriplet lifetime 10 µs

Problem: the most energetic charge transport hosts currently availablehave blue-green triplets.

So to transfer energy to a blue phosphor, exciton transfer must be endothermic.

NIrF

F

O

N

O

2

(2.62±0.10) eV

CBPFIrpic

kF

kR

kGkH

(2.56±0.10) eV

0.0 0.1 0.2 0.3

0.00.10.2

0.30.4

CIE

y c

oord

inat

e

CIE x coordinate

6% FIrpicin CBP

FIrpicin CHCl3

FIrpicat 460nm

Inen

tsity

(arb

. uni

ts)

450 500 5500.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

6% FIrpic in CBPFIrpic in CHCl3FIrpic at 460nm

Host CBP triplet lifetime ~ 1s.N N

CBP

Possible to get efficient endothermic transfer

if decay of host triplet disallowed.

19

Transient response of endothermic transfer

102

103

104

PL

inte

nsity

(arb

itrar

y un

its)

50 100 150 200 250 3000.0

0.5

1.0

PL

inte

nsity

(arb

itrar

y un

its)

Temperature (K)

(b)(c)(a)(d)

(a)T=300K(b)T=200K(c)T=100K(d)T= 50K

0 10 20 30 40 50Time (µs)

Efficiency of luminescence decreases belowT=200K as energy transfer to phosphor is frozen out.

Transient response is also consistent with endothermic transfer:

At T=200K, apparent transient lifetime of FIrpic(includes endothermic transfer) >> natural lifetime

(~10µs)

0 10 201

10

100

1000

EL

Inte

nsity

(arb

. uni

ts)

0 20 40 60 80 1001

10

100

1000

τ= (80±10)µs

τ= (15±2)µs

Time (µs)

PL

Inte

nsity

(arb

. uni

ts)

T=200K

T=292K

τ= (15±2)µs

6% Ir(ppy)3:TPD

6% Ir(ppy)3:TPD

Time (µs)

Firpic in CBP

Ir(ppy)3 in TPD

20

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.00.10.20.30.40.50.60.70.80.9

CIE

y c

oord

inat

e

CIE x coordinate

btp2Ir(acac)

PtOEP

ppy2Ir(acac)

FIrpic

Organic materials have broad luminescent spectra.Overcome in red and blue by shifting toward non-visible

wavelengths.

COLOR PURITY

21

J = 1 µA/cm2

phosphor host Φ (lm/W) V VληP (lm/W) ηQ ext ηQ int V VληP (lm/W) ηQ ext ηQ int

J = 1 mA/cm2

ppy2Ir(acac)

btpIr(acac)

FIrpic

PtOEP

TAZ

CBP

CBP

CBP

530

170

260

60

60

4

1.3

0.3

0.19

0.07

0.006

0.056

0.87

0.32

0.027

0.23

0.60

0.34

0.83

0.09

20

2.2

5.0

0.2

0.15

0.06

0.057

0.042

0.68

0.27

0.23

0.19

0.25

0.22

0.34

0.08

O L E D S U M M A R Y

Peak power efficiency in green is 60 lm/WPeak quantum efficiency is 19%(corresponds to ~100% internal)

Largest remaining problem is operating voltage

Table of phosphorescent device operating parameters

22

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)450 500 550

0.0

0.2

0.4

0.6

0.8

1.0

PHOSPHORESCENT DEVICE PERFORMANCE

btp2Ir(acac)

S

N

Ir

2

O

OCH3

CH3

10-2 10-1 100 101 102 1030.1

1

10

0.1

1

10

Pow

er e

ffici

ency

(lm

/W)

Qua

ntum

effi

cien

cy (%

)

Current density (mA/cm2)

btp2Ir(acac) in CBP

600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

R E D

NIr

F

F

O

N

O

2

FIrpic

B L U E

10-3 10-2 10-1 100 101 1020.1

1

10

0.1

1

10

Pow

er e

ffici

ency

(lm

/W)

Qua

ntum

effi

cien

cy (%

)Current density (mA/cm2)

FIrpic in CBP

ppy2Ir(acac)

500 550 6000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

N

IrO

O

CH3

CH32

G R E E N

Pow

er e

ffici

ency

(lm

/W)

Qua

ntum

effi

cien

cy (%

)

Current density (mA/cm2)10-3 10-2 10-1 100 101 102 103 1041

10

100

1

10

100

23

PHOSPHORESCENT STABILITY

Phosphors should improve OLED stabilityby rapidly removing triplet excitonsand lowering drive current required.

Courtesy Ray Kwong and UDC

1 10 100 1000 100000.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

opt

ical

pow

er

Time (hours)

50 cd/m2

100 cd/m2

200 cd/m2500 cd/m2

200 cd/m2

1 10 100 1000 100000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

opt

ical

pow

er

Time (hours)

500 cd/m2

1000 cd/m2

2000 cd/m2

Red: BtpIr(acac) in CBP Green: (ppy)2Ir(acac) in CBP