Relaxation of Singlet & Triplet Excitonsocw.snu.ac.kr/sites/default/files/NOTE/5464.pdf · 3 Organic Semiconductor EE 4541.617A 2009. 1 st Semester 0.20 0.15) N N Pt Phosphorescent

1

Organic SemiconductorEE 4541.617A

2009. 1st Semester

2009 5 2

M. Baldo and M. Segal, phys. stat. sol. (a) 201, 1205 (2004)

Changhee Lee, SNU, Korea1/30

Changhee LeeSchool of Electrical Engineering and Computer Science

Seoul National Univ. [email protected]

2009. 5. 2.


2009. 1st SemesterRelaxation of Singlet & Triplet Excitons

)(2

121 ↓↑−↑↓=χχ

Singlet exciton S=0

)(2

121 ↓↑+↑↓=χχ

↑↑=21χχ

Triplet exciton S=1

+–

e-h pair

Ground State S=0

S0

Excited States

Singlet exciton S=0

Spin-allowed transitionfast, efficientFluorescence

Triplet exciton S=1

Spin-forbidden transitionslow, inefficient

Phosphorescence

↓↓=21χχ


Ground State, S=0 Singlet exciton S 0 Triplet exciton S 1

Fluorescence : Radiation restricted to singlet excitons, η ~25%Phosphorescence : Radiation is from triplets η ~ 100%.


If the excited state is formed from the combination of two uncorrelated electrons, then in a completely random formation process the relative degeneracies of the singlet and triplet states result in a 1: 3 singlet : triplet ratio, i.e. the fraction of singlet excitons is χS = 0.25.

2


2009. 1st SemesterQE of OLEDs and a brief history


M. Baldo, IMID/IDMC '06 DIGEST, 645-674 (2006)


2009. 1st SemesterOrganic Phosphorescent Dyes

M

Ligand-centeredExciton (LC)

Metal-Ligand Charge TransferExciton (MLCT)

ICISC

1LC

3LC1MLCT MM

IC

3LC

3MLCT

Phosphorescence

Ground state Ir(ppy)3

NIr

3

N N

N NPt

M


• The emissive state is a mixture of a LC exiton and a MLCT exciton• The MLCT state has stronger singlet - triplet mixing, due to the overlap with the heavy metal atom.• For strong spin-orbit coupling, the IC and ISC rates are very fast.

Ground state S0

Ir(ppy)3Triplet lifetime τ ~ 1 μs

3PtOEP

Triplet lifetime τ ~ 100 μs4

322

2

SO Z SL12

H ∝⋅=rcm

Ze

3


2009. 1st Semester

0.20

0.15

)

N NPt

Phosphorescent Dye OLED

0.10

0.05

EL (a

rb. u

nits

)

N NPt

PtOEP


0.00800700600500

Wavelength (nm)

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

Efficient red EL emission from triplet excitons: ηext ~ 2.2% @ 100 cd/m2


2009. 1st SemesterTriplet energy of R & G phosphors


M. A. Baldo and S. R. Forrest, Phys. Rev. B 62, 10958–10966 (2000).

4


2009. 1st Semester

Firpic = iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2']picolinate, Et=2.65 eV

Triplet energy of a blue phosphor and its hosts

Host4,4'-N,N'-dicarbazole-biphenyl (CBP), Et=2.56 eV, a maximum EQE= 5.7% C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo, M. E. Thompson, and S. R. Forrest, Appl. Phys. Lett. 79, 2082 (2001)

3,5'-N,N'-dicarbazole-benzene (mCP), Et=2.9 eV, EQE=7.5%R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong, J. J. Brown, S. Garon, and M E Thompson Appl Phys Lett 82 2422 (2003)


and M. E. Thompson, Appl. Phys. Lett. 82, 2422 (2003).

4,4'-bis(9-dicarbazolyl)-2,2'-dimethyl-biphenyl (CDBP), Et=3.0 eV, EQE=10%S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, and F. Saito, Appl. Phys. Lett. 83, 569 (2003).

R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong, J. J. Brown, S. Garon, and M. E. Thompson, Appl. Phys. Lett. 82, 2422 (2003).


2009. 1st SemesterBlue phosphorescent OLED

C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo, M. E. Thompson, and S. R. Forrest, Appl. Phys. Lett. 79, 2082 (2001)

Changhee Lee, SNU, Korea8/30S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, and F. Saito, Appl. Phys. Lett. 83, 569 (2003).

5


2009. 1st Semester

Blue Green Red

DopantFI i

Ir(ppy)3

Ir(ppy)2acac

Btp2Ir(acac)NIr

O

2

ON

F

F

IrN

O O

2

NIr

N

O O

CF3F3C

2N N

NIr

O

2

O

S

NIr

3

N

H3C

Host and Phosphorescent Dopant Materials

Host

CN-PPV

CBP

mCP

UGH2 TAZ

PVK

FIrpic(CF3ppy)2Ir(pic) Ir(mpp)3

PtOEP

n

O

CN

CHCH2 nN

N N

NN

N

N N

N NPt

2

Si CH3H3C

NIr

3


Hole/ExcitonBlockingMaterials

BCP C60F42

mCP V

NNH3C CH3

FF

F

F

FF F

FFF F

FFF F

F F

FF

F

F

F

F

F FF

FF

F

F

FF

F

F

F

F

FF

FF

FF

Dr. H. N. Cho (InkTek)

NO

N

O AlO

BAlq


2009. 1st SemesterOrganic Electrophosphorescence devices

Princeton group


6


2009. 1st SemesterEnergy transfer in blue phosphorescent OLEDs

• endothermic energy transfer• exothermic energy transfer• charge trapping



2009. 1st SemesterExothermic and endothermic energy transfer

2.4 eV

2.56 eV

2.56 eV2.3 eV

Slow endothermic energy transfer from TPD to Ir(ppy)3



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2009. 1st SemesterExothermic energy transfer from Alq3 to PtOEP and triplet exciton diffusion


M. Baldo and M. Segal, phys. stat. sol. (a) 201, 1205 (2004) ; M. A. Baldo and S. R. Forrest, Phys. Rev. B 62, 10958–10966 (2000).

The delay in the PtOEP phosphorescence after an electrical excitation pulse is due to triplet diffusion through the Alq3 host. The delay is observed to decrease to zero as the exciton formation zone is moved closer to the PtOEP layer, confirming that excitons are formed initially in the Alq3 host. These experiments were used to estimate the triplet exciton lifetime in Alq3 to be (25 ± 15) μs.


2009. 1st SemesterCharge trapping

trapping

No trapping

pp g

trappingNo trapping


M. A. Baldo and S. R. Forrest, Phys. Rev. B 62, 10958–10966 (2000).

8


2009. 1st SemesterQuantum Efficiency vs Current

Triplet – Triplet (T – T) Annihilation

Q.E. decay as current is increased is due to

T-T annihilation


Ref. M. A. Baldo, C. Adachi, and S. R. Forrest, Phys. Rev. B 62, 10967 (2000)

decrease the effects of T-T annihilation by:

• short triplet lifetime will decrease T-T annihilation

• decrease dopant aggregation in the thin film


2009. 1st SemesterReduced Efficiency Roll-off with Short Triplet Exciton Lifetime

Ir(ppy)3

triplet exciton τ: 500ns (doped in


~500ns (doped in CBP)

9


2009. 1st Semester

Jndn TT 2

Rate equation for the triplet-triplet annihilation

Triplet – Triplet (T – T) Annihilation

qdJnkn

dtdn

TTTT +−−= 2

τ

1) transient 0)(,0 => tJt

τ)(nk TT

1 0 i) << τt

TT enn−

≈ )0(

)(nk TT10ii) >> 2

TTT nkdn

−≈


τ)(nk TT 0 ii) >>

BAtnT +

≈1

TT nkdt

22 )()( BAtk

BAtA T

+−

≈+−

)0(1 ,

TT n

BkA ==∴

tknn

ntkBAt

nTT

T

TT

T )0(1)0(

)0(1

11+

=+

=+

≈∴


2009. 1st Semester

trial solution

BAetn tT

+=

τ

1)(22 )()(

1

)( BAe

k

BAeBAe

eA

tT

tt

t

+−

+−=

+

−

τττ

τ

ττ

Transient Solution

τTkB −=∴T

tt

kBAeeA−+−=− )(1 ττ

ττ

BAnt T +

==1)0( ; 0 τT

TT

kn

Bn

A +=−=∴)0(

1)0(

1

)0()( tT

Tntn =∴


)0()]0(1[ TT

t

TT nkenk ττ τ −+

Light emission intensity

τττ τ KeK

LtntL tT

−+==

)1(

)0()()(

M. A. Baldo, C. Adachi, and S. R. Forrest, Phys. Rev. B 62,10967 (2000)

))0( ( Knklet TT =

10


2009. 1st SemesterT – T Annihilation: Transient behavior

qdJnkn

dtdn

TTTT +−−= 2

τ

Rate equation for the T-T annihilation

Transient behavior: J=0




2009. 1st Semester

0=dt

dnT 02 =−+gdJnnk T

TT τ

(근의공식)

⎤⎡⎤⎡++− T

JJkqdJk

81414)1(1

22

τττ 2k

T – T Annihilation: Steady-state solution

⎥⎦

⎤⎢⎣

⎡++−=

⎥⎥⎦

⎤

⎢⎢⎣

⎡++−==

TT

T

TTT J

Jkqd

Jkkk

qdn 811

21411

21

2 ττ

τττ

)2

( 12

−= TT Jqd

k τQ

τTnL = QE :

τη

Jn

JL T==

nL

T1

0η : 0=Tk 인경우, 즉 , T-T annihilation이없는경우이므로 gdJnT =

τ

Light emission intensity


qdJJL 1

0 ===∴ τη

⎥⎦

⎤⎢⎣

⎡++−=⎥

⎦

⎤⎢⎣

⎡++−=

T

T

TT JJ

JJ

JJ

Jkqd 811

4811

2 20 τηη


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2009. 1st SemesterEfficiency Roll-off

Steady-State: d[3M]/dt=0

t d it i d t itcurrent density required to excite every phosphorescentmolecule (i.e., the onset of saturation)



2009. 1st SemesterTriplet-polaron annihilation

Assuming bulk limited transport, then [nt] is proportional to the applied potential V,

1+∝ lVJ


12


2009. 1st SemesterTriplet-polaron annihilation


S. Reineke, K. Walzer, and K. Leo, Phys. Rev. B 75, 125328 (2007)

Documents

Relaxation of Singlet & Triplet Excitonsocw.snu.ac.kr/sites/default/files/NOTE/5464.pdf · 3 Organic Semiconductor EE 4541.617A 2009. 1 st Semester 0.20 0.15) N N Pt Phosphorescent