Download Organic Thin Film Transistors

Organic Electronics
Yousof Mortazavi
VLSI Course Presentation
December 2004
References
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L. Ficke,M. Cahay, “The bright future of organic LEDs”, IEEE Potentials,
Jan. 2004.
J. N. Bardsley, “International OLED technology roadmap”, IEEE J. Selected
Topics in Quantum Electronics, Vol. 10, No. 1, Feb. 2004.
T. Y. Winarski, “Patenting bright ideas; the current state of patented
technology in the field of organic light emitting diodes”, IEEE Circuits and
Devices Magazine, Apr. 2004.
T. Shimoda, T. Kawase, “All-polymer thin film transistor fabricated by highresolution ink-jet printing”, In Proceedings IEEE International Solid-State
Circuits Conference, 2004.
S. Forrest, P. Burrows, M. Thompson, “The dawn of organic electronics”,
IEEE Spectrum, Aug. 2000.
G. Schmid, et al., “Organic electronics: perspectives towards applications”,
ISSCC 2004.
K. Nomoto, et al., “A bottom-contact organic-thin-film-transistor for flexible
display application”, ISSCC 2004.
M. G. Kane, “Organic electronics: what is it good for?”, ISSCC 2004.
D. Gundlach, et al., “High-mobility, low voltage organic thin film transistors”,
IEDM 1999.
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Outline
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Motivations
OLED Fundamentals
OTFTs
Advantages of Organic Electronics
Applications
OLEDs for Color Displays
Challenges
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Motivations
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Microelectronics vs. “Macroelectronics”:
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Cost/area
Cost/function
Bulk Si ICs
$10K/ft2
100 µcents/
transistor
a-Si TFTs on
glass
$150/ft2
1 mcents/
transistor
Printed
Organic TFTs
$30/ft2
200 µcents/
transistor
Thin Film Transistors:
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Microelectronics: try to make smaller
transistors to reduce cost and boost
performance
Macroelectronics: reduce costs in order build
ever larger devices, with acceptable
performance
Active layer is silicon (a-Si) deposited on
glass .
For high mobilities, a-Si can be crystallized (pSi) by laser-pulses at high temperatures.
Can’t easily use flexible substrates, such as
plastics
Organic Thin Film Transistors
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Organic semiconductors were discovered in
1987.
Organic compounds are a natural match for
plastic substrates.
Use of polymers allows large-areas to be
coated and patterned without conventional
photolithography (e.g. spin-coaters and ink-jet
printers).
Organic TFTs may be made large or small
(30 nm @ Cornell U.)
[Kane (ISSC’04)]
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OLED Fundamentals
• In 1987, Tang, et al.
published “Organic
electroluminescent
diodes”.
• Currently more than 500
U.S. Patents have been
issued on organic
electronics.
• Challenges:
– Choice of anode for ohmic
contact (for low voltage
devices)
– Diffusion of In, O into HTL
 HIL interface between
ITO and HTL
– Protection from oxygen and
water  encapsulation
Cathode
Metal
ETL
HTL
ITO-Covered Substrate
Transparent Anode
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OTFT (OFET)
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Typical OTFT:
– Bottom gate, inverted staggered
structure
– Pentacene (C22H14) active
– Gate dielectric
• SiO2
• PMMA
• PVP
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Pentacene:
Formula: C22H14
Metling Point: 300°C
Optical Bandgap: 2.8 eV
OTFTs operation:
– accumulation
– depletion
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Mobilities as high as 1 cm2/Vs has
been obtained with Ion/Ioff ratio of
108.
Very low fabrication temperature
(<60°C) allows use of cheap
plastics.
Conventional MOSFET equations
are used to model OTFTs
however, mobility is voltage
dependent.
W/L = 240 µm/44 µm
Tgate= 1700 Å.
SAM dielectric to reduce
gate thickness to 2.5 nm
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[Schmid et al.]
Advantages of Organic Electronics
• Thin, lightweight, flexible
displays
• Low voltage, low power,
emissive source
• High brightness
• Broad color gamut
• Wide viewing angle (~180º)
• Good contrast
• High resolution (<5 µm pixel
size)
• Fast switching (1-10 µs)
• Low bill of materials and
fabrication cost
[Bardsley, 2004]
Dupont Thermal Multilayer
Transistor Process
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Applications
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Flexible Displays
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Sensor Arrays
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PM-OLED
AM-OLED
Wearable Displays
Artificial Skin
Gas Sensors
RF ID Tags
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Inductors
Capacitors
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X-ray imaging panels
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Solid-State Lighting
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OLEDs for Color Displays
[Forrest, et al.]
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Challenges
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Choice of electrodes
Encapsulation
Reliability and yield
Lifetime
Brightness control
with feedback
• Particle migration
control with AC driver
A. Giraldo, et al.
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Thank You