Download Alternating-Current Thin-Film Electroluminescent Lamp

Electroluminescent Lamps
OU NanoLab/NSF NUE/Bumm & Johnson
The Luxprint Electroluminescent Inks
for this activity were donated by DuPont.
Outline
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Motivation
History
Final Schematic
Useful Physics
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How to make one:
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Thin film Capacitors (AC)
Luminescence from phosphors
Overview
Lithography: patterning ITO
Applying the phosphor
Power up/ testing/ trouble shooting
Definitions/ Glossary
OU NanoLab/NSF NUE/Bumm & Johnson
Motivation
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Electroluminescence is the direct conversion of
electricity to light.
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Solid state lighting.
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Electroluminescence is cool light, unlike incandescent lamps
where light is generated by heating a filament to high
temperatures.
The heat from the lamps barely increase by 1° C above
ambient temperature.
Unlike incandescent lighting there is no filament and therefore
no critical failure. Light output decays with age.
EL lamps are probably the most rugged lighting technology
available.
A promising future
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Thanks to recent advances in electronics and materials
chemistry, EL lamps have re-emerged as an innovative and
exciting lighting technique.
OU NanoLab/NSF NUE/Bumm & Johnson
Facts taken from An Introduction to Dupont's Screenprintable EL Material System
History of Electroluminescence
1936: EL was discovered by a G. Destriau.
1940's: Chrysler tested EL for Automotive Applications.
1950's: Sylvania developed and sold EL night lights.
1960's: The industry saw decline.
1970's: Acceptance of EL lamps in the aircraft industry.
1980's: EL hit the automotive market and held on to aviation.
1990's: EL continues in automotive, aviation, and is entering
consumer markets.
OU NanoLab/NSF NUE/Bumm & Johnson
Taken from An Introduction to Dupont's Screenprintable EL Material System
Types of Electroluminescent Devices
from ETRI
OU NanoLab/NSF NUE/Bumm & Johnson
ACTFEL Lamps: Schematic of Final Lamp
glass
hn
Phosphor (30µm)
ITO
Dielectric
(1-3 µm)
Silver Layer
(Rear Electrode)
AC current
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The ITO and Silver layers act as two plates of a capacitor. The ITO is
transparent, so the photons can pass through the layer.
The AC current produces a changing electric field in the capacitor that
excites the phosphor. The excited phosphors emit light.
The dielectric evens out the E field, reflects light, and prevents the
capacitor from shorting.
OU NanoLab/NSF NUE/Bumm & Johnson
ACTFEL Lamps: Cross-section of Final Lamp
Silver Layer
(Rear Electrode)
Dielectric Layer
(1-3 µm particles)
Phosphor Layer
(30µm particles)
ITO coated Glass
OU NanoLab/NSF NUE/Bumm & Johnson
ACTFEL Lamps: Basic Physics
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Alternating Current Thin Film
Electroluminescent Lamps
are essentially just
capacitors.
The electric field found
inside a parallel plate
capacitor is used to excite
phosphor molecules.
The excited phosphor emits
light.
OU NanoLab/NSF NUE/Bumm & Johnson
ACTFEL Lamps: Basic Physics, Continued
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Small green circles are
manganese atoms.
Large blue circles are
excited manganese atoms.
phosphor particle
ZnS:Mn
electrodes
The horizontal dashes
represent mobile electrons
in the phosphor particle.
Electrons in the phosphor particles are driven by the electric field.
These electrons slam into manganese atoms in the phosphor and
excite them.
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The excited manganese atoms relax by emitting a visible photon.
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The motion of the electrons is proportional to the electric field.
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The electric field is proportional to the applied voltage and inversely
proportional to the electrode separation. Thus the brightness will
increase by raising the voltage or thinning the phosphor and the
dielectric layers.
OU NanoLab/NSF NUE/Bumm & Johnson
Energy Band Diagram for ACTFEL
OU NanoLab/NSF NUE/Bumm & Johnson
Making an EL Lamp: Overview
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Photolithography: patterning ITO
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Applying the phosphor, dielectric, and silver layers
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Power up/ testing/ trouble shooting
OU NanoLab/NSF NUE/Bumm & Johnson
Patterning the ITO by Photolithography
One way to shape the EL lamp is
by patterning the ITO electrode.
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Only the phosphor under the ITO
electrode will be excited.
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Photolithography is used to
transfer a pattern.
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The ITO coated glass is covered with a
photo resist
The resist is exposed under a mask of
the desired pattern.
The resist is developed. The exposed
sections of the resist dissolve while the
unexposed sections harden (positive
type resist).
1.
2.
3.
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See the photolithography slideshow for
further details.
OU NanoLab/NSF NUE/Bumm & Johnson
Patterning ITO coated slides
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After a pattern has been transferred, the
ITO layer of the ACTFEL lamp can be
etched.
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A solution of hydrochloric acid and nitric acid
will oxidize and remove the conductive metal
oxide.
The etched pattern shown below was
created by photolithography using the mask
shown to the right.
Other lithographic techniques (such as
molecular beam epitaxy) can be used to
etch the ITO
Note: The pattern is reversed because the
lamp will be viewed from the opposite side
of the glass.
OU NanoLab/NSF NUE/Bumm & Johnson
Notes on Etching:
What type of patterns don’t work?
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The phosphor under the ITO electrode will only be excited if the ITO
has current running through it.
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Notice that the ITO inside the capital "D" is not connected to the rest of
the ITO.
This section of ITO lacks current.
The pattern to the right represents an
etched ITO pattern on glass. The black
parts are where ITO is present.
(positive resist)
The ITO connects to a power source
that makes contact along the right edge
of the display (the red bar).
OU NanoLab/NSF NUE/Bumm & Johnson
What type of patterns work?
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The design problem in the last example can be fixed by
modifying the etched pattern.
To illuminate the pattern, all the ITO must be connected
to the power source.
The pattern to the right is the
same as the pattern in the last
slide, but the inside of the “D”
has been connected to the rest
of the ITO. Now this section of
the ITO will have power.
OU NanoLab/NSF NUE/Bumm & Johnson
Applying Thin Films
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After the ITO is patterned the ACTFEL lamp can made.
Each layer comes packaged separately as a thick paste (stir
before using).
The thickness of each layer is controlled by using scotch tape as a
spacer.
Apply scotch tape along 3-5mm on two parallel sides of the plate.
Apply the pastes in sequence using a spatula.
Thin them by scraping a microscope slide
across the layer.
Dry and cure each layer before application of
the next
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Each layer is dried in an oven at 130°C for ~15
minutes.
1st phosphor (Luxprint 8152)
2nd dielectric (Luxprint 8153)
3rd conductive silver rear electrode
(Luxprint 9145)
OU NanoLab/NSF NUE/Bumm & Johnson
Applying Thin Films
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Cross-section of TFEL display
The thin films must be applied to the
substrate within defined boundaries to avoid
shorting the capacitor.
Layer Constraints
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The phosphor layer should be as thin as
possible
The dielectric layer should cover all of the phosphor layer and be as thin as possible
without risking a short in the capacitor.
The silver layer must not touch the ITO. Parts of the ITO layer are removed in order
to extend the silver layer to the edge of the glass. This makes it easier to connect the
lamp to a power source.
Phosphor Layer
Dielectric Layer
Silver Layer (Rear Electrode)
The black lines mark the etched ITO pattern, and are used to accurately place the scotch tape; they’re later removed with acetone.
OU NanoLab/NSF NUE/Bumm & Johnson
Power Up
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After the thin films are dry, the lamp needs
a power source.
Copper tape is used to make good
contacts without damaging the lamp.
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The phosphor requires a changing electric
field in order to fluoresce.
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Small pieces of tape are attached to the ITO
layer and the silver layer separately.
Front and back of device
A DC voltage will only produce a changing
electric field in a capacitor as it charges.
In order to produce continuous lighting an
AC voltage is required.
Normal 110V 60Hz AC power can be used to
light your lamp. In the lab we use a high
frequency power supply 60-2000 Hz and a few
hundred volts, which gives a brighter light.
OU NanoLab/NSF NUE/Bumm & Johnson
Device with
leads on,
powered, and
in darkness.
Trouble Shooting: Non-uniformity of Lighting
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Notice the dark regions along the
bottom and upper left corner of the
display.
This non-uniformity is caused by an
irregularity in the thickness of the thin
films.
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The difference in thickness between the
center of the display and the dark band
at the bottom is about 16 microns.
Areas where the film is thinner will be
brighter because the electric field is
larger here. Thicker areas will be
dimmer.
OU NanoLab/NSF NUE/Bumm & Johnson
Definitions/ Glossary:
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ACTFEL – alternating current thin film electroluminescence; gives off light when influenced
by electrical current.
Electroluminescence – the direct conversion of electrical energy into light.
Thin layer - a very thin deposition of a colloidal substance (phosphor, dielectric, silver)
onto the ITO coated glass plate.
ITO – Indium Tin Oxide (In203:Sn02) A thin layer of indium oxide that has been doped with
tin; transparent, conductive coating on glass plate.
Phosphor – powders made of materials such as zinc sulfide, doped with either copper or
manganese to achieve the emission colors when exposed to an electric field.
Dielectric layer – an insulating layer that serves to even out the electric field across the
phosphor layer and prevents short circuits. The dielectric in this case is barium titanate.
Electrodes – form the plates of the capacitor; one front electrode of transparent ITO and
one back electrode of silver.
Acknowledgements
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The Luxprint Electroluminescent Inks for this lab were donated by DuPont Microcircuit
Materials. http://www.mcm.dupont.com
Initial development of this lab activity was performed by James Dizikes and Lloyd Bumm
with the support of a Nanotechnology Undergraduate Education program grant.
NSF DMR-0304664
OU NanoLab/NSF NUE/Bumm & Johnson