oled materials - department of energy · requires less current for same luminance as a single unit...

OLED Materials

Mark Thompson

University of Southern California

Outline

• OLED background

• Challenges in Blue and White OLED materials

• Enhancing blue lifetime

• Enhanced out-coupling through molecular design

+

metal cathode

(-)

(+)

substrate

MOx anode

Organics

< 1000 Å

Heavy metal facilitated triplet emission

metal character in the emissive

state can be < 1%

• To get good efficiency you MUST collect both singlet and triplet excitons

• Strong spin-orbit-coupling mixes singlet and triplet states, M = Ir, Pt, Os, Re, etc.

• Mixing of triplet states with 1MLCT makes phosphorescence a largely allowed transition

• Can lead to lifetimes in sec regime and high PL efficiency, esp. for M = Ir and Pt

1MLCT

3MLCT

3LC

Ground state

Metal Complex Dopant

+

h o l e e l e c t r o n

o r

si ngl et

t r i pl et

Organometallic Ir complexes in OLEDs

0.0 0.2 0.4 0.6 0.8

0.0

0.2

0.4

0.6

0.8

y

x

• Optimized OLEDs give external efficiency = 15-20%

Internal eff = 70-100% (abs., waveguiding)

• Luminance efficiency should provide an upper limit to OLED

Phosphorescence

efficiency of

Ir emitters must

be 100%

SiO2

ITO

organics

cathode

External

Efficiency

15-20%

0.16, 0.37

0.33, 0.63

0.67, 0.33

0.62, 0.38

0.68, 0.32

0.15, 0.22

0.14, 0.13

0.35, 0.61

0.65, 0.35

0.42, 0.57

0.16, 0.10

0.17, 0.38

0.16, 0.29

+ +

1931 CIE chart

Performance for PHOLEDs made by VTE Process

Color Mixing to Achieve White Emission

• Color mixing

• Side-by-side arrangement of RGB elements

• Transparent devices can be stacked

– Pixels on top of pixels with a common substrate

– Large sheets of transparent R, G and B OLEDS

can be stacked to achieve white

• Mixed emitters in a single device

– Simplifies device

– Color balance achieved automatically

– Several possible architectures

• In all cases the White OLED lifetimes

will be limited by the blue components

white

HTL ETL EML

[host + 5-10wt% phosphor]

drec

HOMOhost

LUMOhost

tE

Guest

Defect J

qJ

q

holes electrons

N. Giebink, et al., J. Appl. Phys. 2008

Dopants are typically stable under optical excitation, but degrade under

electrical excitation: the high energy exciton does not degrade alone

Intrinsic Lifetime of OLEDs

1 1/S T

0S

* */n nS T

energy

transfer

0D

*

nD

energy

transfer

R

Exciton Localization

Exciton-Exciton Annihilation

Exciton-Polaron Annihilation

1 2

2

1

1 1/S T

0S

R

1 1/S T

0S

0S

1 1/S TR

E

r

Degradation Routes

• Energetically Driven

- Lifetime: R>G>B

• Two particle interactions lead to

luminance loss

-Exciton on phosphor,

polaron on host

- Exciton-exciton also

possible

0.0

0.2

0.4

0.6

0.8

1.0

Lum

inance

(norm

)

10 10

-110

010

110

210

30.0

0.2

0.4

0.6

0.8

1.0

t' (hrs)

Lum

inance (

norm

)

0.0

0.2

0.4

0.6

0.8

1.0

Lum

inance

(norm

)L0 = 1000cd/m2

drec = 12nm

L0 = 4000cd/m2

drec = 5nm

L0 = 1000cd/m2

L0 = 4000cd/m2

L0 = 1000cd/m2

L0 = 4000cd/m2

Exciton

Localization

Exciton-Exciton

Annihilation

Exciton-Polaron

Annihilation

Luminance Decay vs Time

, '

'

dQ x t

dt

, 'XK N x t

2 , 'XK N x t

, ' , 'XK N x t n x t , ' , 'XK N x t p x t

, 'XK n x t , 'XK p x t

Defect Generation Rates

P

E

E-E

E-P

• Blue PHOLED

• Prepared and packaged using industry std.

11

8

9

10

11

12

13

14

Voltage

10-1

100

101

102

103

8

9

10

11

12

13

14

Vo

lta

ge

t' (hrs)

8

9

10

11

12

13

14

Volta

ge

L0 = 4000cd/m2

drec = 5nm

L0 = 1000cd/m2

L0 = 4000cd/m2

L0 = 1000cd/m2, drec = 12nm

L0 = 1000cd/m2

Exciton

Localization

Exciton-Exciton

Annihilation

Exciton-Polaron

Annihilation

L0 = 4000cd/m2

Drive Voltage Drift with Aging

•Q~1018 cm-3 50% increase in quenching

•At 1000 cd/m2, formation rate = 1012cm-2s-1

-1 in 5 x108 E-P encounters leads to defect

-Increasing recombination zone width

extends lifetime

- Guest triplets/host polarons most active

Brighter with extended lifetime:

Phosphorescent SOLEDs

Io , Vo

Lo

Conventional OLED

- +

2xLo

Io , 2xVo

Stacked OLED (SOLED)

- +

- +

Lifetime and efficiency of OLEDs can be increased by

vertically stacking multiple OLED units in series

Requires less current for same luminance as a single unit device

• Longer lifetime at same luminance

Panel 15 cm x 15 cm 82% fill factor

Single

Unit

WOLED

2 Unit

WSOLED

Luminance

[cd/m2] 3,000 3,000

Efficacy [lm/W] 49 48

CRI 83 86

Voltage [V] 4.3 7.4

1931 CIE (0.471,

0.413)

(0.454,

0.426)

CCT [K] 2,580 2,908

LT70 [hrs] 4,000 13,000

P.Levermore et al, SID Digest, 72.2, p.1060, 2011.

Y. Zhang, et al., Nature Comm. (2014), DOI: 10.1038/ncomms6008

Spreading the recombination zone: Dopant/Host Grading

Fig.2 (a) (b)

a

b

0 5 10 15 2010

-3

10-2

10-1

100

101

102

D1 D2 D3 D1S D3S

Voltage (V)

Curr

ent D

ensi

ty (

mA

/cm

2)

0

1000

2000

3000

4000

5000

Lum

inance

(cd/m

2)

0.1 1 10 1000

5

10

15

20

EQ

E (

%)

Current Density (mA/cm2)

400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

Inte

nsi

ty (

norm

.)

D1= Conventional/Uniform doping

D2= No HTL/Uniform doping

D3= Graded doping

DxS=Stacked

0 10 20 30 40 50 600.00

0.01

0.02

0.03

0.04

0.05EML of D1

Exciton d

ensity (

arb

. units)

Distance to anode (nm)

D1

D2

D3

D1S

D3S

EMLs of D2, D3 and D4

Exciton Sensing:

• Very dilute red phosphor slab

• 1.5 nm thick

• Placed at 5 nm intervals in EML

• Measure red emission intensity

Performance of Graded Devices

Fig.2 (a) (b)

a

b

0 5 10 15 2010

-3

10-2

10-1

100

101

102

D1 D2 D3 D1S D3S

Voltage (V)

Curr

ent D

ensi

ty (

mA

/cm

2)

0

1000

2000

3000

4000

5000

Lum

inance

(cd/m

2)

0.1 1 10 1000

5

10

15

20

EQ

E (

%)

Current Density (mA/cm2)

400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

Inte

nsi

ty (

norm

.)

3000 cd/m2 1000 cd/m2

0.5

0.6

0.7

0.8

0.9

1.0

0.5

0.6

0.7

0.8

0.9

1.0

0 150 300 450 6000.0

0.5

1.0

1.5

2.0

0 150 300 450 6000.0

0.3

0.6

0.9

1.2

D1

D2

D3

D1S

D3S

TPA model

EXP model

Lu

min

an

ce

(n

orm

.)

Lu

min

an

ce

(n

orm

.)

V

(V

)

Time (hrs)

V

(V

)

Time (hrs)

10X

Grading: 10 X Lifetime Improvement Over Standard

Y. Zhang, et al., Nature Comm. (2014), DOI: 10.1038/ncomms6008

Summing it All Up

dopant

host

Con

ce

ntr

atio

n

Position

Conventional Graded

Exciton density

LUMO

HOMO

Emission

+

-

+

-

• To increase lifetime: decrease bimolecular collisions/processes

– Lower [exciton] and [polaron], but this increases voltage

• Grading is good, but how do we improve on it?

– New, more stable blue phosphors and host materials (on going)

– Relax the hot-polaron before it decays (managers)

Summing it All Up

exciton-polaron annihilation:






exciton-exciton annihilation:

The Problem of TTA or TPA

• Desirable blue emission

① Electrical excitation

② Blue Emission

③ TTA / TPA

③’ Vibronic relaxation

④ Dissociative reaction (Bond cleavage)

④’ High energy particle management

⑤’ Non-radiative/radiative decay

① ②

• Dissociative reaction (degradation)

① ③ ④ Excited state

“manager” • High-energy states management

① ③ ④’ ⑤’

T1

S0

T2/P2

Host

Blue

Phosphor

① ②

③

④

Tx/Px ④’

⑤’

⑤’

③’






Measured at initial luminance of 1,000 cd/m2

Device Doping [vol %]

Dopant /

Manager

Driving J

[mA/cm2]

EQE

[%]

LT70

[hr]

ΔV

[V]

CTRL 18 − 8 5.4 9.1 207 1.18

S1 16 − 6 / 2 5.3 9.2 334 (+61%) 1.69

S2 18 − 8 / 5 5.5 8.5 301 (+45%) 1.10

0 50 100 150 200 250 300 3500.0

0.5

1.0

1.5

2.0

2.5

3.0

CTRL

S1

S2

Volta

ge

(V

)

Time (hr)

0 50 100 150 200 250 300 3500.70

0.75

0.80

0.85

0.90

0.95

1.00

CTRL

S1

S2

Lu

min

an

ce (

a.u

.)

Time (hr)

Control Scheme 1 (manager replaces dopant)

Scheme 2 (manager replaces host)

Host

Manager

Dopant

erPLEL

Phosphorescent OLED Efficiency:

EL/PL luminescent quantum efficiencies

fraction of usable excitons

r carrier recombination efficiency

e out coupling efficiency

erPLEL

Phosphorescent OLED Efficiency:

- Phosphorescent emitter: PL, = 1

- good devices can have r 1

- EL limited by e

Non-Isotropic Emitter Orientation

• Anisotropy factor:

T.D. Schmidt et al., Appl. Phys. Lett. 99, 163302 (2011)

pz strongly couples to plasmon modes

= 0 0.33 1

Consider the orientation of

the transition dipole relative

to the substrate.

Light Outcoupling in OLEDs

B. Scholz et al., Opt. Exp. 2012, 20, A205

Only ~15-20% of light

directly extracted to air!

Dopant randomly oriented in the matrix.

Consider the orientation of

the transition dipole relative

to the substrate.

Orientation and EQE

isotropic

Ir(ppy)3

non-isotropic

Ir(ppy)2(acac)

()

S.-Y. Kim et al., Adv. Funct. Mater. 2013, 23 3896

0.3

3

0.2

5

0.2

3

0.2

1

0.0 0.5 0.4 0.3 0.2 0.1 1.0

acac

tmd

Oriented Emitters: acac derivatives

Emitter Host Orientation (pz/(px+py+pz) vertical)

Ir(dhfpy)2(acac) NPD 0.25

Ir(ppy)2(acac) CBP 0.23 (EQE reported = 19 and 25%)

CBP 0.23

TCTA/ B3PYMPM 0.24 (EQE reported = 30%)

Ir(ppy)2(tmd) TCTA/ B3PYMPM 0.22 (EQE reported = 32%)

Ir(MDQ)2(acac) NPD 0.24 (EQE reported = 22%)

NPD/ B3PYMPM 0.20 (EQE reported = 33%)

NPD 0.24

Ir(bt)2(acac) BPhen 0.22

Ir(mphmq)2(tmd) NPD/ B3PYMPM 0.18 (EQE reported = 37%)

Ir(mphq)2(acac) NPD/ B3PYMPM 0.23

Ir(bppo)2(acac) CBP 0.21

Ir(ppy)3

Ir(chpy)3

CBP

CBP

0.33

0.22

Graf, A; Journal of Materials Chemistry C 2014, 2, 10298. Liehm, P; Applied Physics Letters 2012, 101. Kim, K.-H.; Advanced Materials 2014, 26,

3844. Kim, S.-Y.; Advanced Functional Materials 2013, 23, 3896. Kim, K.-H.; Nat Commun 2014, 5. Flämmich, M.; Organic Electronics 2011, 12,

1663. Lee, J.-H.; Advanced Optical Materials 2014, 3, 211.

All dopants reported here have PL 100% in doped films

Orientation Measurement

J. Frischeisen, et. al., Org. Electron. 2011, 12, 1663

= 0.6/2.6 = 0.24 = 1.0/3.0 = 0.33

Fortunately we do not need to

rely on OLED measurements

to determine the net

orientation of the TDV.

Mechanism 1: dopant dipole driven alignment

A. Graf, et. al., J Mater Chem C, 2, 10298-10304, (2014)

Large dipole induces dopant aggregation

Decreases dopant-host interaction

Leads to isotropic orientation

[U(,r) -2/r3]

Ir(phq)3Ir(mphq)2acac

Ir(MDQ)2acac Ir(mphmq)2tmd

NPB (host) B3PYMPM (host)

Mechanism 2: electrostatic dopant-host interactions

Kim, K.-H. et al. Nat. Commun. 2014, 5, 4769

Dopant induces order in the host.

M.J. Jurow, et al., Nat. Mater., 2015, doi:10.1038/nmat4428

New material design: polar dopant

“bppo”

• Optical measurement DOESN’T measure molecular orientation directly, only TDV

• Orientation of the transition dipole moment vector (TDV) measured in (ppy)Re(CO)4:

Re-N-TDV = 18.5 *

* Vanhelmont, F. W. M., et. al, J. Phys. Chem. A, 1997, 101, 2946-2952

N

C

Re(CO) 4

N

C

IrL 4

O

O


“bppo”

86

Angle(s) between the permanent (red)

and TDV (green), illustrated for 20°

78,93 68



Similar

electrostatics,

varied # emissive

ligands

“bppo”

electrostatic surface potentials (kcal/mol)

Observed Anisotropy

• PL measurements for doped CBP films

• Only the acac species orients

– It orients just like all the other acac species

– Independent of doping concentration

= 0.22 = 0.33 = 0.32

Mechanism of alignment

• Dipole-aggregation?

– Orientation independent

of dipole moment and

degree of aggregation

• Electrostatics?

– acac-like heteroleptics

do not orient

= .22 = .33 = .32

86 78,93 68

CBP is amorphous, some host materials are anisotropic and some are not,

but they all give nearly identical levels of alignment of (C^N)2Ir(acac) based

emitters. HOW???


Hypothesis: C2 axis by aliphatic/aromatic discrimination

• Vacuum/Organic boundary induces molecular orientation with C2 axis

perpendicular to substrate

• the aromatic ligands prefer the organic material/film, the acac is left

exposed to the vacuum


Oriented Emitters: tris

Emitter Host Orientation (% vertical)

Ir(dhfpy)2(acac) NPD 25%

Ir(ppy)2(acac) CBP 23%

CBP 23%

TCTA/ B3PYMPM 24%

TCTA/ B3PYMPM 23%

Ir(ppy)2(tmd) TCTA/ B3PYMPM 22%

Ir(MDQ)2(acac) NPD 24%

NPD/ B3PYMPM 20%

NPD 24%

Ir(bt)2(acac) BPhen 22%

Ir(chpy)3* NPD 23%

Ir(mphmq)2(tmd) NPD/ B3PYMPM 18%

Ir(mphq)2(acac) NPD/ B3PYMPM 23%

Ir(phq)3* NPD/ B3PYMPM 30%

Ir(piq)3* NPD 22%

Ir(bppo)2(acac) CBP 22%

Ir(ppy)3 CBP 31%

CBP 33%

* Graf, A. et al., J. Mater. Chem. C, 2014, 2, 10298-10304

Ir(ppy)3

Conclusions: Ideal Orientations

0 10 20 30 40 50 60 70 80 90

0.0

0.1

0.2

0.3

0.4

0.5

(C^N)2Ir(O^O)

fac-Ir(C^N)3

Plot of vs assuming C2 and C3 axes

oriented perpendicular to the substrate.

= 0 EQE > 40%

• With our mechanism the

most planar possible host

film is needed

• Substitute ligands to

achieve high

aromatic/aliphatic contrast

in metal complex

• Lower values of are

possible!

acac: move TDV to Ir-N

bond axis

tris: move TDV to 45


Conclusions: Ideal Orientations

Plot of vs assuming C2 and C3 axes

oriented perpendicular to the substrate.

= 0 EQE > 40%


fac-Ir(C^N)3 (C^N)2Ir(acac)

• With our mechanism the

most planar possible host

film is needed

• Substitute ligands to

achieve high

aromatic/aliphatic contrast

in metal complex

• Lower values of are

possible!

acac: move TDV to Ir-N

bond axis

tris: move TDV to 45

oled materials - department of energy · requires less current for same luminance as a single unit...

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