DOE SSL R&D Workshop 2017

OLED Emissive Materials Beyond

Phosphorescence

Jian Li

Arizona State University

3rdGeneration

Delayed Fluorescence(TADF)

IQE~100%

1st Generation

FluorescenceIQE~25%

2ndGeneration

PhosphorescenceIQE~100%

1987~ 2000~ 2012~

3.5thGeneration

TADF AssistedFluorescence (TAF)

IQE~100%

TADF →Fluorescence

2014~

Singlets Triplets

N

N

N

Ir

N

O

O

N

ON

Al

4CzIPN

Fluorescence

Alq3 Ir(ppy)3

Delayedfluorescence

PhosphorescenceUnlimited Molecular Design

Progress of organic emitters in OLEDs

FRET

TADF Material Intro - Courtesy of Prof. Adachi

ES=(EH-EL)+KET=(EH-EL)-K

DEST=2KEH :HOMO Energy

EL : LUMO Energy

K :Exchange Energy

・ Small DEST

1 2

12

1(1) (2) (2) (1)L U L UK d d

r

(LUMO)(HOMO) (LUMO)(HOMO)

Donor-Acceptor backbone

X:Introduction of steric hindrance

Donor AcceptorX

・Large DEST

m = fHOMOòò er f

LUMOdtｆ ∝m2

Small:DEST

ModerateOscillatorStrength:f

MolecularDesign

Basis for TADF molecular design

Moderate radiative decay rate with small DEST

Transition dipole moment

HOMO

LUMO

4CzIPN

⊿EST：0.01.eV

λ max：513nm

EQE:19%

4CzPN

⊿EST：0.12.eV

λ max：531nm

EQE ：18%

4CzTPN

⊿EST：0.06.eV

λ max：544nm

EQE：17%

Nature, 492, 234 (2012, Dec.)

Unlimited Molecular Design for TADF

HAP-3TPA

⊿EST：0.17.eV

λ max：610nm

EQE：17.5%

Adv. Mat.,25, 3319 (2013)

Spiro-CN

⊿EST：0.06eV

λ max：540nm

EQE：4.4%

Chem. Com.,

48, 9580 (2012)

DMAC-DPS

⊿EST：0.08eV

λ max：465nm

EQE：20%

Nat. Photo.

8, 326 (2014)

ACRFLCN⊿EST：0.10eV

λ max：485nm

EQE：10%

Spiro-AN

⊿EST：0.025eV

λ max：495nm

EQE：16.5%

Chem. Comm.

49, 10385 (2013)

a)

Angew. Chem.

2012, 51, 11311

⊿EST：0.1eV

λ max：466nm

EQE：5.3%

PIC-TRZ2

⊿EST：0.01eV

λ max：505nm

EQE：14%Phys. Rev. Lett.

110, 247401 (2013)

PXZ-TRZ

⊿EST：0.0084.eV

λ max：522nm

EQE：15.5%

CC2TA

⊿EST：0.07eV

λ max：493nm

EQE：11%

Appl. Phys. Lett.,

98, 83302 (2011)

Appl. Phys. Lett.,

101, 93306 (2012)

Chem. Com.,

48, 11392 (2012)

PIC-TRZ

Melem

Sulphone

Molecular DesignSeparation of HOMO and LUMO, but rather gentle decrease of HOMO and LUMO distribution for large transition dipole moment.Use of molecular units having a high localized triplet state (3LE)Small DEST for short transient time ~ms order

DOE SSL R&D Workshop 2017

A new route to harvest triplets in OLEDs with fluorescence emitters

High color purity (narrow spectrum)

Long lifetimes of operational stability

Unlimited molecular design

Theoretical limitation of hr

25% - 62.5% (even TTA process)

Long lifetimes of operational stability High color purity Flexibility of material design Theoretical limitation of hr- 100 %

TADF as assistant & Fluorescence as emitter

Fluorescence based OLEDs

TADF based OLEDs

High efficiency up to 100%

Unlimited molecular design

Broad spectra due to CT emission

(not appropriate for display applications)

Fusion oftwo

Functions

TADF:Exciton-

generation

Fluo:

Light-

emission

400 500 600 700

2CzPN

4CzIPN

4CzPN

4CzTPN

4CzTPN-Me

4CzTPN-Ph

No

rmali

zed

PL

in

ten

sit

y

Wavelength (nm)

T1

T1

S0

S1

S0

1/8

3/8

S0

3M*

4/8

3M*

Interamolecular: T. Uoyama et al., Nature, 492, 234 (2012)Intermolecular: K. Goushi et al., Nature Photo. 6, 253 (2012)

H. Nakanotani, et al., Nature Comm. 5, 4016 (2014)

DOE SSL R&D Workshop 2017

TADF Performance

Hyperfluorescence Performance

EQE[Max] EQE LT95(h)

Green 21.0% 20.5%(@100cd/m2)

20.0%(@1000cd/m2)

2,500(@1000cd/m2)

EQE[Max] EQE LT95(h)

Yellow 15.4% 14.8%(@100cd/m2)

13.4%(@1000cd/m2)

1,060(@1000cd/m2)

Development Status

A remarkable progress has been achieved for the performance and stability of

TADF materials. With the better understanding of TADF mechanism and the

incorporation of better suited host materials, the TADF device performance

can be improved further.

DOE SSL R&D Workshop 2017

Blue TADF OLEDs

Q. Zhang, et al., Nature Photonics, 8, 326 (2014)

• High EQE of 19.5% with emission peak at 450nm

The choice of donor and accepter type materials will

be less for blue TADF materials due to the high

triplet energy requirement.

DOE SSL R&D Workshop 2017

0.1 1 10 100 1000

0

5

10

15

20

25

EQ

E (

% p

h/e

- )

Luminance (cd/m2)

Metal-Assisted Delay Fluorescent Emitters

N

N

N

N

P d

P d N 3 N

400 500 600 700 800

0.0

0.5

1.0

EL

In

ten

sity (

a.u

)

Wavelength (nm)

ITO/HATCN/NPD(30 nm)/TAPC(10 nm)/6%

PdN3N:CBP (25 nm)/DPPS(10 m)/BmPyPB

(30 nm)LiF/Al.

Zhu et.al., Adv. Mater. (2015)

S1 ISCT1

phosphorescence

S0

S1 ISCT1

Thermal Activated Delayed Fluorescence(TADF)

S0

S1 ISC

T1

Metal Assisted Delayed Fluorescence(MADF)

S0

PdN3N devices can be efficient with improved device operational

stability (EQE ~15%, LT90 >1000 hrs @ 1000 cd/m2).

DOE SSL R&D Workshop 2017

Why MADF materials?

Magic of MADF?

77 K RT

S1 ISC

T1

Metal Assisted Delayed Fluorescence(MADF)

S0

S1 ISCT1

phosphorescence

S0

Limit set

by

carbazole

host

S1 ISCT1

Thermal Activated Delayed Fluorescence(TADF)

S0

Blue-emitting MADF emitter with green triplet energy

could be most energetically favorable.

DOE SSL R&D Workshop 2017

Semiconductor Colloidal Quantum Dots

• Size- and composition-dependent tunable

emission through visible spectrum

(quantum confinement effect)

• Narrow emission peaks (minimum width

~20 nm)

• High luminescence quantum yield (~90%)

• Solution processible; good stability

CIE

Y

CIE X

NTSC Standard

▲ QLED

Versatile for solid-state lighting;

potential for high CRI and high efficacy

CRI=90, LER= 360 lm/W

DOE SSL R&D Workshop 2017

Cd1-xZnxSe1-ySy

x,y↑6-7nm

Solution-Processed QD-LEDs

GlassITO

PEDOT:PSS

TFB

CdSe-ZnS

QDs

ZnO NPs

Aluminum

Light Out

Colorλmax

(nm)

FWHM

(nm)

Max. Lum.

(cd/m2)

ηEQE

(%)

ηP

(lm/W)

ηA

(cd/A)

Blue 455 20 4,000 10.3 2.7 4.3

Green 537 29 14,000 14.3 58 62

Red* 625 28 21,000 12.0 17.4 15

10-1

100

101

102

103

104

0

2

4

6

8

10

12

14

16

B

G

R

hE

QE (

%)

L (cd/m2)

EQE > 10% achieved in QLEDs for all three colors

QLEDs operate more efficiently at high luminances

Low operation voltage good lm/W efficiency

Possible lifetime improvement (PEDOT:PSS, HTL, etc.)

L. Qian et al., Nat. Photon. 5, 543 (2011); Y. Yang et al., Nat. Photon. 9, 259 (2015).

* All efficiencies @ 1000 cd/m2

DOE SSL R&D Workshop 2017

• Future Outlooks for Single-doped WOLEDsFuture Outlooks

• The development of stable blue emitters is key;

• The choice of emissive materials should include phosphorescent

materials, thermal activated delayed fluorescent (TADF) materials

and metal-assisted delayed fluorescent (MADF) materials;

• The recent progress of QLEDs has demonstrated enough promise

as a future solution for solution processed organic solid state lighting

devices.