Emissive Displays – Organic Electroluminescence MIT OLED.pdfInitial luminance for Alq3 is 510...

Lecture 76.976 Flat Panel Display Devices – Spring 2001

types of organic materials growth of organic materials organic light emitting devices OLED-based displays

Lecture 76.976 Flat Panel Display Devices

Emissive Displays – Organic Electroluminescence

Flexible OLED


Organic Materials

Alq3

PPV

MOLECULAR MATERIALS

POLYMERS

Attractive due to:Attractive due to: Integrability with inorganic semiconductors Low cost (fabric dyes, biologically derived materials)

Large area bulk processing possible Tailor molecules for specific

electronic or optical properties Unusual properties not easily attainable with conventional materials

But problems exist:But problems exist: Stability Patterning Thickness control of polymers Low carrier mobilityn


Scientific Interest in Organic Materials

1828 - Wöhler first synthesized ureawithout the assistance of a living organism

1950s - steady work on crystalline organics starts

1970s - organic photoconductors (xerography)

1980s - organic non-linear optical materials

1987 - Kodak group published the firstefficient organic light emitting device (OLED)

Since then, the field has dramatically expandedboth commercially and scientifically(OLEDs, transistors, solar cells, lasers, modulators, ... )

to date, about two million organic compounds have been madeto date, about two million organic compounds have been made- this constitutes nearly 90% of all known materials -- this constitutes nearly 90% of all known materials -


VACUUMCHAMBER

TURBOPUMP

COLDTRAP

ROUGHINGPUMP

substrateholder

thicknessmonitor

shutter

GND

substrate

POWERSUPPLIES

sourceboats

Device Preparation and Growth

Glass substrates precoated with ITO- 94% transparent- 15 Ω/square

PrecleaningTergitol, TCEAcetone, 2-Propanol

Growth- 5 x 10-7 Torr- Room T

- 20 to 2000 Ålayer thickness


OMBD II OMBD ISputtering

AnalysisChamber

Load Lock

Materials Growth Laboratory

III-V MBE

Load LockTransferChamber

Metale-Beam

Base Pressure 10-9 ~ 10-11 torr

PrincetonPrinceton University University


Integrated MaterialsGrowth SystemEvaporative Deposition

molecular organics (amorphous and crystalline) metals

Sputtering

ITO ceramics

Wet N2 Glove Box

Load Lockwith Sample Storage

Ante Chamberand Oven

Laminar Flow Hood

Dry N2 Glove Box

Shadow Mask Storage

Probe Stationwith Cryostat

AFM

STM

Physical & Vapor Phase Dep.

molecular organics nano-dots ** solvated polymers ** colloids **


Spin-on

Langmuir-Blodgett

Inkjet Printing

Dye Diffusion

Silkscreen

Other Growth Methods

glass substrate

ITO

doped polymer film

highly doped polymer film

glass substrate

DIFFUSIONSOURCE

SHADOW MASK

DEVICE


Development of Organic LEDs

Conventional, Transparent, Inverted,Metal-Free, Flexible, Stacked

~ OLED, TOLED, OILED, MF-TOLED, FOLED, SOLED ~

Displays


Personal Organizer,Personal Organizer,NotebookNotebook

Rugged, high resolution,full-color, video-rate

displays


Multi-FunctionMulti-FunctionVideo WatchVideo Watch

Rugged, high resolution,full-color, video-rate

displays enable a multitudeof applications

AutomotiveAutomotiveDashboard displays,

external indicator lights,and road signs


Active WallpaperActive WallpaperLarge area displays

Active ClothingActive ClothingLight, rugged, low voltage,

flexible displays


Why do OLEDs Glow ?

+

electrons and holesform excitonsexcitons

(bound e--h+ pairs)

some excitons radiatesome excitons radiate

HOMO

LUMO

recombination region

ETLHTLE

_

V

+

-

GlassITO

Alq3

TPD

Mg:AgAg

400 500 600 700 800

Alq3 PLOLED EL

Wavelength [nm]

Alq3

AlN

O3

TPDN N

~1000 Å

~500 Å~500 Å


Electroluminescence

Single Layer Device

Heterostructure Device

ITO

Al2.1 eV 2.4 eV

CN-PPV

PPV

hνννν

ITO

Ca

hνννν

PPV

EXCITONRECOMBINATION

CARRIERTRANSPORT

CARRIERINJECTION


Exciton Recombination Zone

C. Adachi, et al.


1 10

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

T=141KT=180KT=240KT=295K

Cur

rent

[A]

Voltage [V]

4 6 85

10

15

Tt = 1780 K

m

1/T [10-3/K]

HOMO

LUMO

trap level

moleculedistorts

E

Trap Limited Conduction in Organic Materials

charge trapping candominate conduction

free molecule

molecule distorts

∆∆∆∆ E

_

_

_

∆∆∆∆ E

Nt=3.1x1018cm-3,µµµµnNLUMO=4.8x1014/cm-V-s

J ∝ NLUMO µµµµnNtmd-2m-1Vm+1

m = Tt/T

Shen, Burrows, Bulovic, McCarty, Thompson, Forrest,Jpn. J. Appl. Phys. 35, L401 (1996).

´


Progress in LED Efficiency

after Sheats et al., Science 273, 884 (1996).

Molecular

Solids

OLEDsPLEDs


WE DEMONSTRATED WE DEMONSTRATED OLEDsOLEDs THAT ARE : THAT ARE :

Bright - 100,000 cd/m2 (30,000 ft-L)

Efficient - >30 lm/W

Scalable Emissive Area - from a few µµµµm to a few cm in size

Colors - fluorescent R,G,B and phosphorescent R,G

Low Voltage - 3 to 10 V

Low Cost Materials

Low Cost Substrates

Wide Viewing Angle - >160 deg

Reliability - 1,000,000 hrs (phosphorescent R half-life)

Why Make Organic LEDs


Alq3 devices driven at 20 mA/cm2

Initial luminance for Alq3 is 510 cd/m2

for QA doped Alq3 devices is 1600 cd/m2

and for MQA doped Alq3 devices is 1400 cd/m2 (C.W. Tang)

QA/Alq

MQA/Alq

Alq

Hours of operation

Re

lati

ve lu

min

an

ce

[L

/L0]

1.0

0.9

0.8

0.7

0.6

0.51 10 100 1000 100005 50

OLED Stability


Electroluminescence in Doped Organic Films

2.Excitons transfer to

luminescent dye

1.Excitons formed

from combinationof electrons and

holes

6.0 eV

a-NPD

2.6 eV

5.7eV

Alq3

2.7 eV electrons

exciton

trap states

low work functioncathode

transparent anode holes

host molecules(charge transport

material)

dopant molecule(luminescent dye)


400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

EL

Inte

nsity

Wavelength [nm]

Alq3

PtOEP:Alq3

DCM2:Alq3

PtN N

NN

AlN

O3

O

CNNC

N

αααα-NPDN N

Effect of Dopants on the OLED EL Spectrum


Solid State Solvation EffectBulovic et al., Chem. Phys. Lett. 287, 455 (1998); 308, 317 (1999).´ AlqAlq33

DCM2 in AlqDCM2 in Alq33

low DCM2low DCM2 high DCM2high DCM2

500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0 10%

5%

2%

1%

EL In

tens

ity [a

.u.]

Wavelength [nm]

Alq3

DCM2 inAlq3

EL Spectrum TuningEL Spectrum Tuning

Temporal ResponseTemporal Response

0.25 ns

0.50 ns

0.75 ns

1.00 ns 1.50 ns

2.00 ns

5.00 ns4

8

12

16

0

600 650 700 750

35 nm35 nmwavelength shift

10% DCM2in Alq3

10% DCM2in Alq3

Inte

nsity

[a.u

.]

host

dopant (chromophore)with DIPOLE MOMENT µµµµ


Influence of µ0 and µ1on Chromatic Shift Direction

solvent

solute (chromophore)WITH DIPOLE MOMENT µµµµ

µ0 < µ1µ0 < µ1

SOLVENT POLARITY

S0

S1

groundstate

excitedstate

µ0 > µ1µ0 > µ1

SOLVENT POLARITY

S0

S1

∆∆∆∆E


Solid State Solvation Effect (SSSE)

Eloc

R

dipolar hostwith moment µ

polarlumophore

µµµµ > > > > 0

µµµµ → → → → 0as R → large

self polarizationfor strongly dipolar lumophores


500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.01%

2%5%

10%20%

50%

Inte

nsity

[a.u

.]

Wavelength [nm]

0.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

[a.u

.] 1%2%

5%10%

20%100%

DCM2 in Alq3Alq3

DCM2 in Zrq4Zrq4

polar hostµµµµ ~ 5.5 D

non-polar host

Thin Film Photoluminescence

Lecture 76.976 Flat Panel Display Devices – Spring 2001400 500 600 700

120 Å BCP80 Å BCP40 Å BCP

Wavelength [nm]

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength [nm]

400 500 600 700In

tens

ity [a

.u.]

0.6% 1.5%

3% 6%

0.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

[a.u

.]

changingDCM2 in α-NPDconcentration

(with 40A BCP)

changingBCP layerthickness

(with 0.6% DCM2) TPD

AgMg:Ag

ITO

glass

Alq3

BCP

NPD:DCM2

TuningEmission ofWhite OLEDs


symmetry conserved

fast process ~10-9s

triplet to ground state transition is not permitted

slow process ~ 1s

Fluorescence

E

ground state(singlet)

singletexcitedstate

tripletexcitedstate

S1 T1

S0

FLUORESCENCEFLUORESCENCE

singlet excitonsinglet exciton

E

Phosphorescence

PHOSPHORESCENCEPHOSPHORESCENCE

triplet excitontriplet exciton

S1 T1

S0


0 1 2 3 4 5 6 7 8 9 100.1

1

10

100

1,000

10,000

100,000

Lum

inan

ce [c

d/m

2 ]

Voltage [V]

0

5

10

15

20

25

30

Pow

er E

ffici

ency

[lm

/W]

10,000 cd/m2

Phosphorescent OLED Performance

N N

CBP

Ir3

NIr(ppy)36% Ir(ppy)3 in CBP OLED:

at 100 cd/m2 : 4.5 V, 19 lm/Wat 10,000 cd/m2 : 7.2 V, 8 lm/W

100 cd/m2


Simulated Power Consumption(5 inch/320x240 pixels monochrome display)

33% pixels “on”

UDC, Inc.

0

100

200

300

400

500

600

700

800

Pow

er [m

W]

0 50 100 150 200Brightness [nits]

PM -

Fluor

esce

nt

PM - Phosphorescent

AM - Phosphorescent AM - Fluoresecent

AMLC

D


Monochrome Passive-Matrix Polymer-LED Display

Cambridge Display Technologies, Ltd.


Full-Color OLED Display

Kodak - Sanyo

Transparent Substrate:Glass, Plastic, Metal

Low Cost Potential

ITO Anode

Multi-Color Icons

Organic LED

Transparent Cathode

• Lower cost materials than LCDs• Fewer process steps than LCDs• Less capital cost than LCDs


15” XGA Cost Comparison

0

50

100

150

200

250

300

350

400

AMLCD LTPS/OLED

US$

OtherLabor

Module MaterialCell MaterialArray Material

Equipment DepreciationBuilding Depreciation

Source: DisplaySearch

AT SAME YIELDS


TECHNOLGY/ FEATURES

AMLCD PMLCD LED PDP FED OLED

Brightness Good Good VeryGood

Good Good VeryGood

Resolution High

High Low Medium High High

Voltage Low

Low High High High Low

Viewing Angle Medium

Poor Excellent Excellent Excellent Excellent

Contrast Ratio Good

Fair Good Poor-Good

Good Excellent

Response Time Good Poor Fast Very Fast Very Fast Very Fast

Power Efficiency Good Good Fair-Good

Medium VeryGood

VeryGood

Temp Range Poor Poor VeryGood

VeryGood

VeryGood

VeryGood

Form Factor Thin Thin Wide Wide Thin Very ThinConformable

Weight Light Light AboveAvg.

Heavy Light Light

Screen Size Small- Large

Small toMedium

Small toLarge

Large Medium Small toLarge

Primary Applications

Laptops, Desktop

SmallDisplay

Signs,Indicators

LargeScreen

Multiple MultipleNew/Existing

Cost Average Low High High Average BelowAverage

Technology Landscape


Transparent OLEDs

Parthasarathy et al., Appl. Phys. Lett. 72, 2138 (1998).

-

V+

EL Light

500 Å500 Å ITO

ETLHTLITO

Glass

EL Light

50-100 Å50-100 Å Mg-Ag

Bulovic et al., Nature 380, 29 (1996).´

> 70% transparent

Alphanumeric TOLED Display Alphanumeric TOLED Display

TOLEDs MF-TOLEDs


0.1 1 10

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

VT

MF-TOLED TOLED

Voltage (V)

Cur

rent

Den

sity

(mA

/cm

2 )

ITOCuPcAlq 3α-NPDCuPcITOGlass

EL

EL

V-+

I ∝ V9

I ∝ V2

I - V Characteristics of a Metal Free-TOLED


500 550 600 650 700 750 8000.0

0.2

0.4

0.6

0.8

1.0

CuPc Absorption

Wavelength (nm)

Inte

nsity

(a.u

)

Reflection

Transmission

Transparency/Reflection of a Metal Free-TOLED


CA

TH

OD

EO

N T

HE

BO

TT

OM

ITO

TPD

Alq3

Mg:Ag

Glass

-V+

ITO

TOLEDTOLEDAg

TPD

Alq3

Mg:Ag

Glass

-V+

ITO

ConventionalConventionalOLEDOLED

TOI-LEDTOI-LEDOILEDOILEDITO

TPD

Alq3

PTCDA

Glass

+V-

ITOMg:Ag

ITO

TPD

Alq3

PTCDA

Silicon

+V-

AgMg:Ag

AN

OD

EO

N T

HE

BO

TT

OM

TRANSPARENTDEVICES

NON-TRANSPARENTDEVICES

Schematic Cross-Sections of Monochrome OLEDs


TOLED Applications

UDC, Inc.


E1 E2

E4

E3

E5

Glass substrate

RedE1

E3E2

E4E5

R-G-B light

Stacked Organic LEDs (SOLEDs)

head-up, high resolution, true-color, high-contrast,brightly-emissive, flexible displays

Shen et al., Science 276, 2009 (1997).Gu et al., J. Appl. Phys. (1999).

0.0 0.2 0.4 0.6 0.80.0

0.2

0.4

0.6

0.8

y

x

YELLOWYELLOW

BLUEBLUE

REDRED

ORANGEORANGE

GREENGREEN

WHITEWHITE

PURPLEPURPLE

PINKPINK

400450

480

490

500

520

550560

540

580

600650

700

530

Microcavity EffectsMicrocavity EffectsBulovic et al.,Phys. Rev. B 58, 3730 (1998).

´

Glass

ITO

Mg:Ag Electrode

αααα - NPD

RADIATIONRADIATIONModesModes

WAVEGUIDEWAVEGUIDEModesModes

SurfaceSurfacePlasmonsPlasmons

MetalMetalLossesLosses

EDGEemission

SURFACEemission

Alq3

Θk

- EL Region -


MATERIAL THICKNESS [Å] n

Mg:Ag 1000 ----

Ag 1000 ----

αααα-NPD 540 1.78

ITO 490 2.02

3% DCM2 in Alq3 330 1.72

Alq3 170 1.72

Alq’2OPh 390 1.66


CuPc 80 1.5+0.8i

Alq3 50 1.72Mg:Ag 80 ----

Alq3 540 1.72


ITO 1600 1.8

Glass ~1 mm 1.45

V4

V1

V2

V3

MIC

RO

CA

VIT

Y 1

MIC

RO

CA

VIT

Y 2

RE

DO

LE

DB

LU

EO

LE

DG

RE

EN

OL

ED

Example of a Stacked OLED Structure


Up to 3 times the resolutionof conventional displays

Advantages of Stacked OLEDs

True-Color Pixels


(b) ANGLE DEPOSITION WITH A ROTATING SUBSTRATE

DE

PO

SIT

ION

DIR

EC

TIO

N

ROTATIONAXIS

TOP VIEW SIDE VIEW

OLED #1

OLED #2

(c) TWO-COLOR STACKED OLEDPR

SiO2

ITOGlass

transparent contacts

(a) ANGLE DEPOSITION WITH A STATIONARY SUBSTRATE

DE

PO

SITIO

N D

IRE

CTIO

NTOP VIEW SIDE VIEWΘΘΘΘ

ITO Contacts

ITO

PR

SiO2

ITO

A Method forDepositing a MultilayerSOLED Structure with

Fanned Electrodes


AMLCD

OLED backlight

TOLED stackRGB

~ 1mm

for for full-color displaysfull-color displays the backlight can consist of a the backlight can consist of astackstack of R,G, and B TOLED backlights of R,G, and B TOLED backlights

Organic LED Backlight Integrated with an AMLCD


TOLED Stack

R - onG - offB - off

Principle of OLED Backlight Operation

R G Bsub-cycles

one display write cycle

ONOFF

ONOFF

ONOFF

R sub-cycle

time

ONOFF

Timing Diagram

AMLCD

B-TOLED

G-TOLED

R-TOLED

reflector

5 msResponse Times

OLED rise time ~ 1µµµµsLCD response time ~ 5 ms

AMLCD


AMLCD

colorfilters

White Light TOLED Stack

pixel width(RED fill factor < 0.3)

R - onG - offB - off

pixel width(RED fill factor > 0.9)

White Backlight vs. TOLED R,G,B Stack

ADVANTAGES OF TOLED STACK BACKLIGHTSADVANTAGES OF TOLED STACK BACKLIGHTS

- AMLCD with no sub pixels - AMLCD with no sub pixels larger fill factorlarger fill factor- no color filters - no color filters more efficient use of backlight emitted light more efficient use of backlight emitted light

22 -- 4 %4 % 1010 -- 12 %12 %TRANSMITTANCETRANSMITTANCE

3 to 6 fold3 to 6 fold improvement in efficiency improvement in efficiency


OLED backlight

AMLCD

anode cathode

computerinterface


Advantage Disadvantage

Patterning ofRGB Emitters

Without CFEfficient light usage

Patterning?→→→→ Insoluble

polymer

Microcavity Color PurityWithout CFUnpatterned LED

Viewing angleCost of mirror(SiO2 /TiO2 6 layers)

White Emission with CF

ContrastColor PurityUnpatterned LED

With CFAbsorption Loss

Blue Emission w/Color Conversion

Efficient light usageColor purityUnpatterned LED

Need stable andefficient blue dyes

Other Approaches for Full-Color Displays


Flexible OLED (FOLED)

- Ultra lightweight- Thin form factor- Rugged- Impact resistant- Conformable

Manufacturing Paradigm Shift Web-Based Processing


Low-Cost All-Polymer Integrated CircuitsDrury et al., Appl. Phys. Lett. 73, 108 (1998).

~ 3 mm

· 15 bit programmable code generator· 326 all-polymer transistors (2µµµµm x 1mm gates)

with vertical interconnections· µ µ µ µchannel = 3 x 10-4 cm2/Vs, 40-200 Hz bandwidth· 3” diameter polyimide substrate

LAYOUT

ID - VD RESPONSE


FOLED-based Pixelated, Monochrome Display

Source: UDC, Inc.


Transparent FOLED-based Pixel

Source: UDC, Inc.


The PRESENT …

… and the nearby FUTURE …

… of ORGANIC DISPLAY TECHNOLOGY

Emissive Displays – Organic Electroluminescence MIT OLED.pdfInitial luminance for Alq3 is 510...

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Transcript of Emissive Displays – Organic Electroluminescence MIT OLED.pdfInitial luminance for Alq3 is 510...