Effects of Nano-Structure and Refractive Index on Light Extraction in OLEDs

Franky So

Department of Materials Science and Engineering

North Carolina State University

Optical Modes in an OLED

Simulated Mode Distribution

kx (μm-1)

Air cone

2

Glass

ITO

Organic

Cathode

• Corrugated substrate works as diffraction grating• kG = 2π/Ʌ

• Reduce kx of trapped modes with corrugated structure• k’ = k - kG

Light Extraction in Corrugated OLEDs

Simulated Mode Distribution

kx (μm-1)

Air cone

kG

Metal

Organic

ITO

SPP

WG

Air Mode

Glass

3

Efficiency at 1000 cd/m2

• Planar : 50.8 cd/A, 30.3 lm/W

• Buckling : 79.2 cd/A, 56.7 lm/W

56% enhancement in cd/A and

87% enhancement in lm/W

Corrugated OLEDs

Adv. Opt. Mater. 1, 404 (2013)4

10 100 1000 100000

20

40

60

80

100

Pow

er

effic

iency (

lm/W

)

Curr

ent effic

iency (

cd/A

)

Luminance (cd/m2)

0

20

40

60

80

100

15 nm Al

100 nm ITO

LiF & Al cathode

50 nm TAPC

70 nm Bphen

15 nm CBP:Irppy3

15 nm TCTA:Irppy3

0 1 2 3 4 5 6 7 81E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10 Buckling

Reference

Lu

min

ance

(cd

/m2)

Cu

rre

nt d

en

sity (

mA

/cm

2)

Voltage (V)

10-1

100

101

102

103

104

105

Substrate

Refractive index effects

Effect of ETL refractive index

6

π phase shift upon reflectionMetal

𝑘𝑖𝑛_𝑆𝑃𝑃 =2π

𝜆(𝜀𝑐𝑎𝑡ℎ𝑜𝑑𝑒 . 𝜀𝐸𝑇𝐿𝜀𝑐𝑎𝑡ℎ𝑜𝑑𝑒 + 𝜀𝐸𝑇𝐿

)12

Effect of ETL refractive index

7

WG +substrate

Outcoupling Efficiency (χout)Optical mode distribution

Effect of ETL and HTL refractive indices

9

ETL HTL

Air

Air + substrate

Effect of refractive index of ETLs

Emitter Hole-transport material Host

Cul-iBuPyrPHOS PLEXCORE UT-314 PYD2

Electron-transport material

T2T NBphen 3TPYMB

Emitter Hole-transport material Host

Cul-iBuPyrPHOS PLEXCORE UT-314 PYD2

Electron-transport material

T2T NBphen 3TPYMB

3TPYMB

T2T

NBPhen

Em

itter H

ole-tra

nsp

ort m

ateria

l H

ost

Cul-iB

uP

yrP

HO

S

PL

EX

CO

RE

UT

-314

PY

D2

E

lectron

-tran

sport m

ateria

l

T2T

N

Bphen

3T

PY

MB

OLED using T2T/NBPhen as ETL

• Maximum EQE of 12% achieved. Close to the 14% expected EQE based on 70% PLQY and 20% light outcoupling.

11

1 10 100 10000

5

10

15

20

25

30

Exte

rna

l Q

ua

ntu

m E

ffic

ien

cy (

%)

Luminance (cd m-2)

Em

itter

Hole-tran

sport m

aterial

Host

Cul-iB

uPyrPHO

S

PLEX

CO

RE U

T-314

PYD

2

Electron-tran

sport m

aterial

T2T

NB

phen

3TPY

MB

Device using 3TPYMB

• A maximum EQE of 21% was achieved using 3TPYMB. Nearly 75% higher than the T2T/NBPhen device.

12

1 10 100 10000

5

10

15

20

25

30

Exte

rnal Q

uan

tum

Effic

ien

cy (

%)

Luminance (cd m-2)

Comparing two devices

13

0 1 2 3 4 5 610

-5

10-4

10-3

10-2

10-1

100

101

Voltage (V)

Curr

en

t D

en

sity (

mA

cm

-2)

100

101

102

103

104

105

106

T2T/NBPhen

3TPYMB

Lu

min

ance

(cd m

-2)

1 10 100 10000

5

10

15

20

25

30

T2T/NBPhen

3TPYMB

Exte

rnal Q

uan

tum

Effic

ien

cy (

%)

Luminance (cd m-2)

Refractive indices of ETLs

14

• 3TPYMB has the lowest refractive index.

Amin Salehi

Index Effect on Optical Mode Profile

15

T2T/NBPhen 3TPYMB

Wav

egu

ide

~ 5

%

A. Salehi, Adv Opt Mater 5, 1700197 (2017)

Emitter Hole-transport material Host

Cul-iBuPyrPHOS PLEXCORE UT-314 PYD2

Electron-transport material

T2T NBphen 3TPYMB

Index Manipulation by Oblique Angle Deposition

Barranco, Angel, et al. Progress in Materials Science (2016)

Plawsky et al. Materials Today (2009)

Lowering the index by OAD

Oblique Angle Deposition of Alq3

OLED devices using the OAD Alq3 ETL

*The last 50 nm of Alq3 was doped with Cs2CO3

1 10 100 1000 100000

5

10

15

20

25

30

35

40 OAD

Control

EQ

E

luminance (cd m-2)

0 1 2 3 4 5 6 7 810

-5

10-4

10-3

10-2

10-1

100

101

102

OAD

Control

Voltage (v)

Cu

rre

nt

De

nsity (

mA

/cm

2)

100

101

102

103

104

105

106

107

Lu

min

an

ce

(cd

/m2)

Control OAD

A. Salehi, ACS Appl Mater Interfaces 10, 9595 (2018)

Power Dissipation and Mode Analysis

Summary

• Reducing refractive index of the OLED layers cansignificantly increase light extraction.

• The refractive index of ETL has the largest impact onlight extraction.

• OLED device using OAD Alq3, showed nearly a 30%enhancement in external quantum efficiency.