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- Motivation
- History
- Final Schematic
- Useful Physics
- Thin film Capacitors (AC)
- Luminescence from phosphors
- How to make one:
- Overview
- Lithography: patterning ITO
- Applying the phosphor
- Power up/ testing/ trouble shooting
- Definitions/ Glossary
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- Electroluminescence is the direct conversion of electricity to light.
- 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.
- Solid state lighting.
- 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
- Thanks to recent advances in electronics and materials chemistry, EL
lamps have re-emerged as an innovative and exciting lighting technique.
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- 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.
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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.
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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.
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- Small green circles are manganese atoms.
- Large blue circles are excited manganese atoms.
- 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.
- The excited manganese atoms relax by emitting a visible photon.
- The motion of the electrons is proportional to the electric field.
- 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.
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- Photolithography: patterning ITO
- Applying the phosphor, dielectric, and silver layers
- Power up/ testing/ trouble shooting
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- One way to shape the EL lamp is by patterning the ITO electrode.
- Only the phosphor under the ITO electrode will be excited.
- Photolithography is used to transfer a pattern.
- 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).
- See the photolithography slideshow for further details.
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- After a pattern has been transferred, the ITO layer of the ACTFEL lamp
can be etched.
- 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.
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- The phosphor under the ITO electrode will only be excited if the ITO has
current running through it.
- Notice that the ITO inside the capital "D" is not connected
to the rest of the ITO.
- This section of ITO lacks current.
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- 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.
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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
- 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)
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- 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.
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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.
- Small pieces of tape are attached to the ITO layer and the silver layer
separately.
- The phosphor requires a changing electric field in order to fluoresce.
- 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.
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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.
- 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.
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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
- 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
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