recombination region

• types of organic materials

• growth of organic materials

• organic light emitting devices

• OLED-based displays

Lecture 7

6.976 Flat Panel Display Devices

Emissive Displays –

Organic Electroluminescence

Flexible OLED

Organic Materials

Alq ₃

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 mobility •

n

Scientific Interest in Organic Materials

• 1828 - Wöhler first synthesized urea

without the assistance of a living organism

• 1950’s - steady work on crystalline organics starts

• 1970’s - organic photoconductors (xerography)

• 1980’s - organic non-linear optical materials

• 1987 - Kodak group published the first

efficient organic light emitting device (OLED)

• Since then, the field has dramatically expanded both commercially and scientifically

(OLEDs, transistors, solar cells, lasers, modulators, ... )

to date, about two million organic compounds have been made to 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 -

VACUUM CHAMBER TURBO

PUMP COLD TRAP

ROUGHING PUMP

substrate holder thickness

monitor shutter

GND

substrate

POWER SUPPLIES

source boats

Device Preparation and Growth

• Glass substrates precoated with ITO - 94% transparent

- 15 Ω/square

• Precleaning Tergitol, TCE

Acetone, 2-Propanol

• Growth

- 5 x 10

Torr - Room T

- 20 to 2000 Å

layer thickness

OMBD II OMBD I Sputtering

Analysis Chamber

Load Lock

Materials Growth Laboratory

III-V MBE

Load Lock Transfer

Chamber Metal

e-Beam

Base Pressure 10

~ 10

torr

Princeton

Princeton University University

Integrated Materials Growth System

Evaporative Deposition

• molecular organics

(amorphous and crystalline)

• metals

Sputtering

• ITO

• ceramics

Wet N2 Glove Box

Load Lock

with Sample Storage Ante Chamber

and Oven

Laminar Flow Hood

Dry N2 Glove Box Shadow Mask Storage

Probe Station with 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

}

}

DIFFUSION SOURCE

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,

Notebook Notebook

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

displays

Multi-Function Multi-Function

Video Watch Video Watch

Rugged, high resolution, full-color, video-rate displays enable a multitude

of applications

Automotive Automotive

Dashboard displays, external indicator lights,

and road signs

Active Wallpaper Active Wallpaper

Large area displays

Active Clothing Active Clothing

Light, rugged, low voltage,

flexible displays

Why do OLEDs Glow ?

+

electrons and holes form excitons excitons (bound e ^- -h ⁺ pairs)

some excitons radiate some excitons radiate

HOMO LUMO

rec ombination regi on

ETL HTL

E

_

V +

-

Glass ITO Alq

TPD Mg:Ag

Ag

400 500 600 700 800 Alq

PL

OLED EL

Wavelength [nm]

Alq ₃

Al N

O 3

TPD

N N

~1000 Å

~500 Å

~500 Å

Electroluminescence

Single Layer Device

Heterostructure Device

ITO Al

2.1 eV 2.4 eV CN-PPV

PPV

h νννν

ITO Ca

h νννν

PPV

EXCITON RECOMBINATION

CARRIER TRANSPORT CARRIER

INJECTION

Exciton Recombination Zone

C. Adachi, et al.

1 10 10

10

10

10

10

10

10

10

10

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

Cu rren t [ A ]

Voltage [V]

4 6 8

5 10 15

T_t = 1780 K

m

1/T [10^-3/K]

HOMO LUMO

trap level

molecule distorts

E

Trap Limited Conduction in Organic Materials

charge trapping can dominate conduction

free molecule

molecule distorts

Þ Þ

Þ Þ ∆∆∆∆ E _

_

_

∆∆∆∆ E

N _t =3.1x10 ¹⁸ cm ^-3 ,

µµµµ _n N _LUMO =4.8x10 ¹⁴ /cm-V-s J ∝ N _LUMO µµµµ _n N _t ^m d ^-2m-1 V ^m+1

m = T _t /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).

M ol ec ul ar S ol id s

OLEDs

PLEDs

WE DEMONSTRATED

WE DEMONSTRATED OLEDs OLEDs THAT ARE : THAT ARE :

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

• Efficient - >30 lm/W

• Scalable Emissive Area - from a few µµµµ m 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

Alq ₃ devices driven at 20 mA/cm ² Initial luminance for Alq ₃ is 510 cd/m ²

for QA doped Alq ₃ devices is 1600 cd/m ²

and for MQA doped Alq ₃ devices is 1400 cd/m ² (C.W. Tang)

QA/Alq

MQA/Alq

Alq

Hours of operation Rela tive l u mina nce [L /L

]

1.0

0.9

0.8

0.7

0.6

0.5 1 5 10 50 100 1000 10000

OLED Stability

Electroluminescence in Doped Organic Films

2.

Excitons transfer to luminescent dye

1.

Excitons formed from combination

of electrons and holes

6.0 eV a-NPD

2.6 eV

5.7eV

Alq

2.7 eV electrons

exciton

trap states

low work function cathode

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 m alized EL I n tensit y

Wavelength [nm]

Alq

PtOEP:Alq

DCM2:Alq

N Pt N N N Al N

O 3

O NC CN N

αααα -NPD

N N

Effect of Dopants on the OLED EL Spectrum

Solid State Solvation Effect

Bulovic ´et al., Chem. Phys. Lett. 287, 455 (1998); 308, 317 (1999).

Alq Alq

DCM2 in Alq DCM2 in Alq

low DCM2

low DCM2 high DCM2 high DCM2

500 600 700 800

0.0 0.2 0.4 0.6 0.8

1.0 10%

5%

2%

1%

EL Intens ity [a.u. ]

Wavelength [nm]

Alq

DCM2 in Alq

EL Spectrum Tuning EL Spectrum Tuning

Temporal Response Temporal Response

0.25 ns 0.50 ns

0.75 ns

1.00 ns 1.50 ns 2.00 ns

5.00 ns

4 8 12 16

0

600 650 700 750

35 nm 35 nm wavelength shift

10% DCM2 in Alq

10% DCM2

in Alq

In te n s it y [a .u .]

host

dopant (chromophore)

with DIPOLE MOMENT µµµµ

Influence of µ ₀ ^and µ ₁

on Chromatic Shift Direction

solvent solute (chromophore)

WITH DIPOLE MOMENT µµµµ

µ ₀ < µ ₁ µ ₀ < µ ₁

SOLVENT POLARITY

S ₀ S ₁

ground state excited

state

µ ₀ > µ ₁ µ ₀ > µ ₁

SOLVENT POLARITY

S ₀ S ₁

∆∆∆∆ E

Solid State Solvation Effect (SSSE)

E _loc R

dipolar host with moment µ

polar lumophore

áááá µµµµ ññññ > > > > 0 áááá µµµµ ññññ ^→ → → → 0

as R → large

“self polarization”

for strongly dipolar lumophores

500 600 700 800 900 0.0

0.2 0.4 0.6 0.8 1.0

1%

2%

5%

10%

20%

In te n s it y [a .u .] 50%

Wavelength [nm]

0.0 0.2 0.4 0.6 0.8 1.0

Intensit y [a. u .] 1%

2%

5%

10%

20% 100%

DCM2 in Alq Alq _{3 3}

DCM2 in Zrq Zrq _{4 4}

polar host µµµµ ~ 5.5 D

non-polar host

Thin Film Photoluminescence

400 500 600 700 120 Å BCP

80 Å BCP 40 Å BCP 0.0

0.2 0.4 0.6 0.8 1.0

Wavelength [nm]

400 500 600 700

In ten s ity [a .u.]

0.6%

1.5%

3%

6%

0.0 0.2 0.4 0.6 0.8 1.0

In ten s ity [a .u.]

changing DCM2 in α -NPD

concentration

(with 40A BCP)

changing BCP layer

thickness

(with 0.6% DCM2)

TPD

Ag Mg:Ag

ITO glass Alq ₃

BCP

NPD:DCM2

Tuning

Emission of

White OLEDs

• symmetry conserved fast process ~10 ^-9 s

• triplet to ground state transition is not permitted

slow process ~ 1s

Fluorescence

E

ground state (singlet) singlet excited state

triplet excited state

S ₁ T ₁

S ₀

FLUORESCENCE FLUORESCENCE

singlet exciton singlet exciton

E

Phosphorescence

PHOSPHORESCENCE PHOSPHORESCENCE

triplet exciton triplet exciton

S ₁ T ₁

S ₀

0 1 2 3 4 5 6 7 8 9 10 0.1

1 10 100 1,000 10,000 100,000

Lumina n ce [cd/m

]

Voltage [V]

0 5 10 15 20 25 30

P o w e r E ffi c ie n c y [l m/ W]

10,000 cd/m

Phosphorescent OLED Performance

N N

CBP

Ir

3 N

Ir(ppy) ₃ 6% Ir(ppy) ₃ in CBP OLED:

at 100 cd/m ² : 4.5 V, 19 lm/W at 10,000 cd/m ² : 7.2 V, 8 lm/W

100 cd/m

Simulated Power Consumption

(5 inch/320x240 pixels monochrome display) 33% pixels “on”

UDC, Inc.

0 100 200 300 400 500 600 700 800

Po wer [mW ]

0 50 100 150 200

Brightness [nits]

PM - Fl uo re sc en t

PM - P hos pho res cen t

AM - Phosp horescent

AM - Flu ores ecen t

A ML CD

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$

Other Labor

Module Material Cell Material Array Material

Equipment Depreciation Building Depreciation

Source: DisplaySearch

AT SAME YIELDS

TECHNOLGY/

FEATURES

AMLCD PMLCD LED PDP FED OLED

Brightness Good Good Very

Good Good Good Very

Good

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 Very

Good Very

Good Temp Range Poor Poor Very

Good

Very Good

Very Good

Very Good

Form Factor Thin Thin Wide Wide Thin Very Thin

Conformable

Weight Light Light Above

Avg.

Heavy Light Light

Screen Size Small-

Large Small to

Medium Small to

Large Large Medium Small to Large Primary

Applications Laptops,

Desktop Small

Display Signs,

Indicators Large

Screen Multiple Multiple New/Existing

Cost Average Low High High Average Below

Average

Technology Landscape

Transparent OLEDs

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

-

V

+

EL Light

500 Å

500 Å ITO ETL HTL ITO Glass

EL Light

50-100 Å

50-100 Å Mg-Ag

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

> 70% transparent

Alphanumeric TOLED Display Alphanumeric TOLED Display

TOLEDs MF-TOLEDs

TOLED Applications

UDC, Inc.

E1 E2 E4 E3 E5

Glass substrate

Red

E1 E2 E3

E4 E5

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 0.8

0.0 0.2 0.4 0.6 0.8

y

x

YELLOW YELLOW

BLUE BLUE

RED RED ORANGE ORANGE GREEN

GREEN

WHITE WHITE

PURPLE PURPLE

PINK PINK

450 400 480 490

500 520

550 560 540

580 600

650 700 530

Microcavity Effects Microcavity Effects

Bulovic et al.,

Phys. Rev. B 58, 3730 (1998).

´

Glass

ITO

Mg:Ag Electrode ααα

α - NPD

RADIATION RADIATION

Modes Modes

WAVEGUIDE WAVEGUIDE

Modes Modes

Surface Surface Plasmons Plasmons Metal

Metal Losses Losses

emissionEDGE SURFACE

emission

Alq₃

Θ

- EL Region -

MATERIAL THICKNESS [Å] n

Mg:Ag 1000 ----

Ag 1000 ----

αααα -NPD 540 1.78

ITO 490 2.02

3% DCM2 in Alq

330 1.72

Alq

170 1.72

Alq’

OPh 390 1.66

αααα -NPD 250 1.77

CuPc 80 1.5+0.8i

Alq

50 1.72

Mg:Ag 80 ----

Alq

540 1.72

αααα -NPD 445 1.77

ITO 1600 1.8

Glass ~1 mm 1.45

V ₄

V ₁ V ₂ V ₃

MI CR O C A V IT Y 1 MI CR O C A V IT Y 2 RE D OLED BLUE OLED GREEN OLED

Example of a Stacked OLED Structure

Up to 3 times the resolution of conventional displays

Advantages of Stacked OLEDs

True-Color Pixels

AMLCD

OLED backlight

TOLED stack

}

G R B

~ 1mm

for full-color displays for full-color displays the backlight can consist of a the backlight can consist of a stack

stack of R,G, and B TOLED backlights of R,G, and B TOLED backlights

Organic LED Backlight Integrated with an AMLCD

TOLED Stack

R - on G - off B - off

Principle of OLED Backlight Operation

R G B sub-cycles

one display write cycle

ON OFF ON

OFF ON

OFF

R sub-cycle

time

ON OFF

Timing Diagram

AMLCD

B-TOLED G-TOLED R-TOLED

reflector

5 ms Response Times

OLED rise time ~ 1 µµµµ s

LCD response time ~ 5 ms

AMLCD

AMLCD

color filters

White Light TOLED Stack

pixel width (RED fill factor < 0.3)

R - on G - off B - off pixel width

(RED fill factor > 0.9)

White Backlight vs. TOLED R,G,B Stack

ADVANTAGES OF TOLED STACK BACKLIGHTS ADVANTAGES OF TOLED STACK BACKLIGHTS

- AMLCD with no sub pixels

- AMLCD with no sub pixels Þ Þ Þ Þ Þ Þ Þ Þ larger fill factor larger fill factor - no color filters

- no color filters Þ Þ Þ Þ Þ Þ Þ Þ more efficient use of backlight emitted light more efficient use of backlight emitted light

2 2 - - 4 % 4 % TRANSMITTANCE TRANSMITTANCE 10 10 - - 12 % 12 %

3 to 6 fold

3 to 6 fold improvement in efficiency improvement in efficiency

OLED backlight

AMLCD

anode ^cathode

computer

interface

Flexible OLED (FOLED) - Ultra lightweight

- Thin form factor - Rugged

- Impact resistant - Conformable

Manufacturing Paradigm Shift

Web-Based Processing

Low-Cost All-Polymer Integrated Circuits

Drury et al., Appl. Phys. Lett. 73, 108 (1998).

~ 3 mm

· 15 bit programmable code generator

· 326 all-polymer transistors (2 µµµµ m x 1mm gates) with vertical interconnections

· µ µ µ µ

= 3 x 10

cm

/Vs, 40-200 Hz bandwidth

· 3” diameter polyimide substrate

L AYOUT

I

- V

R ESPONSE

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