Electroluminescent Lamps

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Electroluminescent Lamps
The Luxprint Electroluminescent Inksfor this
activity were donated by DuPont.
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Outline

• 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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Motivation

• 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.

Facts taken from An Introduction to Dupont's
Screenprintable EL Material System
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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.

Taken from An Introduction to Dupont's
Screenprintable EL Material System
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Types of Electroluminescent Devices

• from ETRI

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ACTFEL Lamps Schematic of Final Lamp

• 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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ACTFEL Lamps Cross-section of Final Lamp
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ACTFEL Lamps Basic Physics

• 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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ACTFEL Lamps Basic Physics, Continued

• 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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Energy Band Diagram for ACTFEL
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Making an EL Lamp Overview

• Photolithography patterning ITO
• Applying the phosphor, dielectric, and silver
  layers
• Power up/ testing/ trouble shooting

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Patterning the ITO by Photolithography

• 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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Patterning ITO coated slides

• 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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Notes on Etching What type of patterns dont
work?

• 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.

• 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).

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What type of patterns work?

• 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.

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Applying Thin Films

• 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 130C for 15
  minutes.
• 1st phosphor (Luxprint 8152)
• 2nd dielectric (Luxprint 8153)
• 3rd conductive silver rear electrode (Luxprint
  9145)

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Applying Thin Films
Cross-section of TFEL display

• The thin films must be applied to the substrate
  within defined boundaries to avoid shorting the
  capacitor.
• Layer Constraints
• 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
theyre later removed with acetone.
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Power Up

• 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.

Front and back of device
Device with leads on, powered, and in darkness.
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Trouble Shooting Non-uniformity of Lighting

• 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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Definitions/ Glossary

• 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 (In203Sn02) 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