Electroluminescent Lamps The Luxprint Electroluminescent Inksfor this activity were donated by DuPont.
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
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
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
Types of Electroluminescent Devices from ETRI
ACTFEL Lamps: Schematic of Final Lamp glass hn Phosphor (30µm) ITO Dielectric (1-3 µm) Silver Layer (Rear Electrode) AC current • 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.
ACTFEL Lamps: Cross-section of Final Lamp SilverLayer (Rear Electrode) Dielectric Layer (1-3 µm particles) Phosphor Layer (30µm particles) ITO coated Glass
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.
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. phosphor particle ZnS:Mn electrodes
Making an EL Lamp: Overview • Photolithography: patterning ITO • Applying the phosphor, dielectric, and silver layers • Power up/ testing/ trouble shooting
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.
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.
Notes on Etching: What type of patterns don’t work? • 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). • 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.
What type of patterns work? • 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. • 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.
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 130°C for ~15 minutes. 1st phosphor (Luxprint 8152) 2nd dielectric (Luxprint 8153) 3rd conductive silver rear electrode (Luxprint 9145)
Applying Thin Films Cross-section of TFEL display • 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. • 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 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.
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.
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.
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 (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