Lecture 7 6.976 Flat Panel Display Devices
Emissive Displays – Organic Electroluminescence Flexible OLED
• • • •
types of organic materials growth of organic materials organic light emitting devices OLED-based displays
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 1
Organic Materials 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
MOLECULAR MATERIALS
Alq3 POLYMERS
n
6.976 Flat Panel Display Devices – Spring 2001
But problems exist: • Stability • Patterning • Thickness control of polymers • Low carrier mobility
PPV
Lecture 7
Slide 2
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 - this constitutes nearly 90% of all known materials 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 3
Device Preparation and Growth substrate holder thickness monitor
• Glass substrates precoated with ITO - 94% transparent - 15 Ω/square • Precleaning Tergitol, TCE Acetone, 2-Propanol • Growth - 5 x 10-7 Torr - Room T
substrate
shutter
TURBO PUMP COLD TRAP
ROUGHING PUMP
source boats
VACUUM CHAMBER
- 20 to 2000 Å layer thickness POWER SUPPLIES 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
GND Slide 4
Materials Growth Laboratory Base Pressure 10-9 ~ 10-11 torr
OMBD II
Sputtering
OMBD I
Metal e-Beam
III-V MBE
Princeton University
Analysis Chamber 6.976 Flat Panel Display Devices – Spring 2001
Load Lock
Lecture 7
Transfer Chamber
Load Lock
Slide 5
Integrated Materials Growth System
Evaporative Deposition • molecular organics (amorphous and crystalline)
• metals Shadow Mask Storage
Probe Station with Cryostat AFM STM
Ante Chamber and Oven Wet N2 Glove Box
Sputtering • ITO • ceramics
Dry N2 Glove Box
Load Lock with Sample Storage Physical & Vapor Phase Dep. • • • •
Laminar Flow Hood
molecular organics nano-dots ** solvated polymers ** colloids **
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 6
Other Growth Methods • Spin-on • Langmuir-Blodgett • Inkjet Printing • Dye Diffusion • Silkscreen
glass substrate
DIFFUSION highly doped polymer film } SOURCE SHADOW MASK doped polymer film ITO glass substrate
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
}
DEVICE
Slide 7
Development of Organic LEDs • Conventional, Transparent, Inverted, Metal-Free, Flexible, Stacked ~ OLED, TOLED, OILED, MF-TOLED, FOLED, SOLED ~
• Displays
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 8
Personal Organizer, Notebook Rugged, high resolution, full-color, video-rate displays
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 9
Automotive
Dashboard displays, external indicator lights, and road signs
Multi-Function Video Watch
Rugged, high resolution, full-color, video-rate displays enable a multitude of applications 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 10
Active Wallpaper Large area displays
Active Clothing
Light, rugged, low voltage, flexible displays
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 11
Why do OLEDs Glow ?
Alq3
PL
OLED
V
Ag Mg:Ag
+
~1000 Å ~500 Å ~500 Å
Alq3 TPD
electrons and holes form excitons (bound e--h+ pairs)
ITO
E
HTL
Alq3
TPD N
Al
6.976 Flat Panel Display Devices – Spring 2001
N O
+ 3
400 500 600 700 800
Wavelength [nm]
recombination region
Glass
N
EL
_
LUMO
ETL HOMO
some excitons radiate Lecture 7
Slide 12
Electroluminescence CARRIER INJECTION
Ca
PPV
Single Layer Device
EXCITON RECOMBINATION
hν ν ITO
CARRIER TRANSPORT
Heterostructure Device Al
CN-PPV 2.1 eV
2.4 eV PPV
hν ν
6.976 Flat Panel Display Devices – Spring 2001
ITO
Lecture 7
Slide 13
Exciton Recombination Zone
C. Adachi, et al. 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 14
Trap Limited Conduction in Organic Materials charge trapping can dominate conduction
_
´ McCarty, Thompson, Forrest, Shen, Burrows, Bulovic, Jpn. J. Appl. Phys. 35, L401 (1996).
_
free molecule molecule distorts
_
Þ∆E
LUMO ∆E
E trap level
molecule distorts
10-3 15
m
10-4
Current [A]
10-5
HOMO
10
Tt = 1780 K
10-6
5
4
6
8
1/T [10-3/K]
-7
10
10-8 T=141K T=180K T=240K T=295K
10-9 10-10 10-11 1
Voltage [V]
6.976 Flat Panel Display Devices – Spring 2001
10 Lecture 7
m -2m-1 m+1 V J ∝ NLUMO µnNt d m = Tt/T
Nt=3.1x1018cm-3, µnNLUMO=4.8x1014/cm-V-s Slide 15
Progress in LED Efficiency
OLEDs PLEDs ol M
ec
S ar l u
id ol
s
after Sheats et al., Science 273, 884 (1996).
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 16
Why Make Organic LEDs WE DEMONSTRATED OLEDs THAT ARE :
• Bright - 100,000 cd/m2 (30,000 ft-L) • Efficient - >30 lm/W • Scalable Emissive Area - from a few µm 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)
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 17
OLED Stability
Relative luminance [L/L0]
1.0 0.9
MQA/Alq QA/Alq
0.8 0.7
Alq 0.6 0.5
1
5 10
50 100
1000
10000
Hours of operation
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 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
(C.W. Tang) Slide 18
Electroluminescence in Doped Organic Films
1.
2.6 eV
Excitons formed from combination of electrons and holes transparent anode
electrons
2.7 eV trap states
a-NPD
low work function cathode
Alq3 exciton
holes
5.7eV 6.0 eV
host molecules (charge transport material)
2. Excitons transfer to luminescent dye
dopant molecule (luminescent dye) 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 19
Effect of Dopants on the OLED EL Spectrum
N
1.0 Normalized EL Intensity
O
0.8
N α-NPD
N NC
DCM2:Alq3
CN
PtOEP:Alq3
0.6 Alq3
N
0.4 Al
0.2
N O
N
Pt
N N
3
0.0 400
500
600
700
800
Wavelength [nm] 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 20
Solid State Solvation Effect
DCM2 in Alq3 Alq3
Bulovic´ et al., Chem. Phys. Lett. 287, 455 (1998); 308, 317 (1999).
low DCM2
high DCM2
EL Spectrum Tuning
0.8
DCM2 in Alq3
10% 5% 2%
0.6 0.4
1%
Alq3
0.2 0.0
500
600 700 Wavelength [nm]
Temporal Response
800
600
host
1.00 ns 0.75 ns 0.50 ns 0.25 ns
dopant (chromophore) with DIPOLE MOMENT µ 6.976 Flat Panel Display Devices – Spring 2001
wavelength shift Lecture 7
650
700
750
10% DCM2 16 1.50 ns in Alq 3 12 2.00 ns 8 5.00 ns 4
35 nm
0
Slide 21
Intensity [a.u.]
EL Intensity [a.u.]
1.0
Influence of µ0 and µ1 on Chromatic Shift Direction solute (chromophore) WITH DIPOLE MOMENT
µ
solvent
excited state
S1
S1
S0
S0
ground state SOLVENT POLARITY
SOLVENT POLARITY
µ0 < µ1 6.976 Flat Panel Display Devices – Spring 2001
∆E
µ0 > µ1 Lecture 7
Slide 22
Solid State Solvation Effect (SSSE)
áµñ > 0 R
Eloc
áµñ → 0
as R → large
polar lumophore
“self polarization” for strongly dipolar lumophores 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
dipolar host with moment µ Slide 23
Thin Film Photoluminescence DCM2 in Alq3
Intensity [a.u.]
1.0 1% 2% 5% 10% 20% 50%
0.8 0.6 0.4
polar host µ ~ 5.5 D
0.2 0.0
DCM2 in Zrq 4
Intensity [a.u.]
1.0 1% 2% 5% 10% 20% 100%
0.8 0.6 0.4
non-polar host
0.2 0.0 500
600
700
800
900
Wavelength [nm] 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 24
1.0 0.6% 1.5% 3% 6%
0.8 Intensity [a.u.]
Tuning Emission of White OLEDs
0.6 0.4
changing DCM2 in α-NPD concentration (with 40A BCP)
0.2
Ag Mg:Ag
0.0
400
1.0
glass
Intensity [a.u.]
ITO
700
120 Å BCP 80 Å BCP 40 Å BCP
0.8
TPD
600 Wavelength [nm]
Alq3 BCP NPD:DCM2
500
0.6
changing BCP layer thickness
0.4
(with 0.6% DCM2)
0.2 0.0
6.976 Flat Panel Display Devices – Spring 2001
400
500 Lecture 7
600 Wavelength [nm]
700 Slide 25
Fluorescence E
singlet S1 excited state
triplet T1 excited state
Phosphorescence E
S1
T1
FLUORESCENCE
S0
state S0 ground (singlet)
triplet exciton
singlet exciton
• triplet to ground state transition is not permitted
• symmetry conserved fast process ~10-9s 6.976 Flat Panel Display Devices – Spring 2001
PHOSPHORESCENCE
slow process ~ 1s Lecture 7
Slide 26
Phosphorescent OLED Performance Ir(ppy)3
6% Ir(ppy)3 in CBP OLED:
N
at 100 cd/m2 : 4.5 V, 19 lm/W at 10,000 cd/m2 : 7.2 V, 8 lm/W
Ir 3
N
N
CBP 30 10,000 cd/m2
10,000
25
1,000
20 100 cd/m2
100
15
10
10
1
5
0.1
0 0
1
2
3
6.976 Flat Panel Display Devices – Spring 2001
4
5 6 Voltage [V] Lecture 7
7
8
9
Power Efficiency [lm/W]
Luminance [cd/m2]
100,000
10
Slide 27
Simulated Power Consumption (5 inch/320x240 pixels monochrome display) 33% pixels “on” 800
D C L M A
700
Power [mW]
600 500
PM
400 300
-
sc e or u Fl
P
Ph M
t n e
nt e c s e r ho p s o
ent c e s ore u l F -
200
AM
100
rescent o h p s o h P AM -
0
0
50
100
150
200
Brightness [nits] 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
UDC,Slide Inc.28
Monochrome Passive-Matrix Polymer-LED Display
Cambridge Display Technologies, Ltd. 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 29
Full-Color OLED Display
Kodak - Sanyo 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 30
Low Cost Potential
Transparent Cathode Organic LED Multi-Color Icons ITO Anode Transparent Substrate: Glass, Plastic, Metal
• Lower cost materials than LCDs • Fewer process steps than LCDs • Less capital cost than LCDs
15” XGA Cost Comparison 400 350
Other Labor
300 US$
250 200
Module Material Cell Material
150
Array Material
100
Equipment Depreciation Building Depreciation
50
AT SAME YIELDS
0 AMLCD
6.976 Flat Panel Display Devices – Spring 2001
LTPS/OLED
Lecture 7
Source: DisplaySearch
Slide 32
Technology Landscape TECHNOLGY/ FEATURES Brightness
AMLCD
PMLCD
LED
PDP
FED
OLED
Good
Good
Good
Good
Resolution
High
High
Very Good Low
Medium
High
Very Good High
Voltage
Low
Low
High
High
High
Low
Viewing Angle
Medium
Poor
Excellent
Excellent
Excellent
Excellent
Contrast Ratio
Good
Fair
Good
Good
Excellent
Response Time
Good
Poor
Fast
PoorGood Very Fast
Very Fast
Very Fast
Power Efficiency
Good
Good
Medium
Temp Range
Poor
Poor
Form Factor
Thin
Thin
FairGood Very Good Wide
Very Good Wide
Very Good Very Good Thin
Light
Light
Heavy
Light
SmallLarge Laptops, Desktop Average
Small to Medium Small Display Low
Above Avg. Small to Large Signs, Indicators High
Very Good Very Good Very Thin Conformable Light
Large
Medium
Large Screen High
Multiple
Weight Screen Size Primary Applications Cost
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Average
Small to Large Multiple New/Existing Below Average Slide 33
Transparent OLEDs
TOLEDs
MF-TOLEDs
EL Light 500 Å ITO
-
50-100 Å Mg-Ag
V
ETL HTL ITO Glass
+
EL Light
Alphanumeric TOLED Display
> 70% transparent Bulovic´ et al., Nature 380, 29 (1996). Parthasarathy et al., Appl. Phys. Lett. 72, 2138 (1998). 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 34
I - V Characteristics of a Metal Free-TOLED 3
10
2
Current Density (mA/cm2 )
10
1
10
0
10
-1
10
ITO CuPc Alq 3 α-NPD CuPc ITO Glass
EL
+
V
I ∝ V9 EL
-2
10
-3
10
I ∝ V2
-4
10
10-5
MF-TOLED TOLED
-6
10
0.1 6.976 Flat Panel Display Devices – Spring 2001
1 Voltage (V) Lecture 7
VT 10 Slide 35
Transparency/Reflection of a Metal Free-TOLED 1.0
Transmission
Intensity (a.u)
0.8
0.6
0.4
Reflection
CuPc Absorption
0.2
0.0 500
550
6.976 Flat Panel Display Devices – Spring 2001
600
650
Wavelength (nm) Lecture 7
700
750
800 Slide 36
Schematic Cross-Sections of Monochrome OLEDs NON-TRANSPARENT DEVICES
TRANSPARENT DEVICES
CATHODE ON THE BOTTOM
ANODE ON THE BOTTOM
Conventional V +
OLED Ag
V +
Mg:Ag
+ V -
6.976 Flat Panel Display Devices – Spring 2001
TOLED ITO Mg:Ag
Alq 3
Alq 3
TPD
TPD
ITO
ITO
Glass
Glass
OILED ITO
+ V -
TOI-LED ITO
PTCDA
PTCDA
TPD
TPD
Alq 3
Alq 3
Mg:Ag Ag Silicon
Mg:Ag ITO Glass
Lecture 7
Slide 37
TOLED Applications
UDC, Inc. 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 38
Microcavity Effects
Stacked Organic LEDs (SOLEDs)
Glass
head-up, high resolution, true-color, high-contrast, brightly-emissive, flexible displays
E5 E4 E3 E2 E1
E5
Θ
k
ITO
WAVEGUIDE Modes
α - NPD
R-G-B light
SURFACE emission RADIATION Modes EDGE emission
Bulovic´ et al., Phys. Rev. B 58, 3730 (1998).
Alq3
- EL Region -
Metal Losses Mg:Ag Electrode
E4
Surface Plasmons
E3
E1
0.8
E2
Red
520
530
GREEN
0.6
540 550 560
YELLOW
500
y
Glass substrate Gu et al., J. Appl. Phys. (1999). Shen et al., Science 276, 2009 (1997).
580ORANGE
0.4 WHITE
490
0.2 480
0.0
RED
700
BLUE PURPLE
450
0.0
PINK
600 650
400
0.2
0.4
0.6
x 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 39
0.8
Example of a Stacked OLED Structure MATERIAL THICKNESS [Å] n
RED OLED BLUE OLED
V3
V2 V1
6.976 Flat Panel Display Devices – Spring 2001
GREEN OLED
MICROCAVITY 2
MICROCAVITY 1
V4
Ag
1000
----
Mg:Ag
1000
----
Alq3
170
1.72
330
1.72
α-NPD
540
1.78
ITO CuPc
490 80
2.02 1.5+0.8i
α-NPD
250
1.77
Alq’2OPh
390
1.66
Alq3 Mg:Ag
50 80
1.72 ----
Alq3
540
1.72
α-NPD
445
1.77
ITO
1600
1.8
Glass
~1 mm
1.45
3% DCM2 in Alq3
Lecture 7
Slide 40
Advantages of Stacked OLEDs
True-Color Pixels
Up to 3 times the resolution of conventional displays 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 41
I SIT PO DE
(a) ANGLE DEPOSITION WITH A STATIONARY SUBSTRATE
D ON
Θ
SIDE VIEW
SiO2 ITO ITO
DEPO
SITION
(b) ANGLE DEPOSITION WITH A ROTATING SUBSTRATE
TION
ITO Contacts
DIR EC
A Method for Depositing a Multilayer SOLED Structure with Fanned Electrodes
PR
ION CT IRE
TOP VIEW
TOP VIEW
SIDE VIEW
ROTATION AXIS
(c) TWO-COLOR STACKED OLED PR
transparent contacts
SiO2
OLED #2 OLED #1 Glass
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
ITO
Slide 42
Organic LED Backlight Integrated with an AMLCD for full-color displays the backlight can consist of a stack of R,G, and B TOLED backlights
AMLCD ~ 1mm OLED backlight R G B
6.976 Flat Panel Display Devices – Spring 2001
}
Lecture 7
TOLED stack
Slide 43
Principle of OLED Backlight Operation Timing Diagram ON OFF
R sub-cycle AMLCD
AMLCD ON OFF
B-TOLED
ON OFF
G-TOLED
ON OFF
R - on G - off B - off
R-TOLED
R
G
B
time
TOLED Stack
sub-cycles 5 ms
Response Times OLED rise time ~ 1µ µs LCD response time ~ 5 ms
one display write cycle
6.976 Flat Panel Display Devices – Spring 2001
reflector
Lecture 7
Slide 44
White Backlight vs. TOLED R,G,B Stack
2-4 %
TRANSMITTANCE
10 - 12 % pixel width (RED fill factor > 0.9)
pixel width (RED fill factor < 0.3)
AMLCD color filters
TOLED Stack
White Light
R - on G - off B - off
ADVANTAGES OF TOLED STACK BACKLIGHTS
- AMLCD with no sub pixels Þ larger fill factor - no color filters Þ more efficient use of backlight emitted light
3 to 6 fold improvement in efficiency 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 45
OLED backlight AMLCD
computer interface cathode
anode 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 46
Other Approaches for Full-Color Displays Advantage
Disadvantage
Patterning of RGB Emitters
Without CF Efficient light usage
Patterning? → Insoluble polymer
Microcavity
Color Purity Without CF Unpatterned LED
Viewing angle Cost of mirror (SiO2 /TiO2 6 layers)
White Emission with CF
Contrast Color Purity Unpatterned LED
With CF Absorption Loss
Blue Emission w/ Color Conversion
Efficient light usage Color purity Unpatterned LED
Need stable and efficient blue dyes
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 47
Flexible OLED (FOLED) - Ultra lightweight - Thin form factor - Rugged - Impact resistant - Conformable Manufacturing Paradigm Shift Web-Based Processing
6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 48
Low-Cost All-Polymer Integrated Circuits Drury et al., Appl. Phys. Lett. 73, 108 (1998).
LAYOUT
~ 3 mm
ID - VD RESPONSE
· 15 bit programmable code generator · 326 all-polymer transistors (2µµm x 1mm gates) · · 6.976 Flat Panel Display Devices – Spring 2001
µchannel
with vertical interconnections = 3 x 10-4 cm2/Vs, 40-200 Hz bandwidth
3” diameter polyimide substrate
Lecture 7
Slide 49
FOLED-based Pixelated, Monochrome Display
Source: UDC, Inc. 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 50
Transparent FOLED-based Pixel
Source: UDC, Inc. 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 51
The PRESENT …
… and the nearby FUTURE …
… of ORGANIC DISPLAY TECHNOLOGY 6.976 Flat Panel Display Devices – Spring 2001
Lecture 7
Slide 52
