Transcript OLEDs * Theory and Fabrication

Tanner Mariucci
4/22/16
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Organic Semiconductors
 Working Principle
 Fabrication
 Types of OLEDs
 Advantages/Disadvantages
Made from organic (carbon based) polymers
 Emit Light when electricity is applied through them
 Martin Pope-1960
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› Developed Ohmic dark-injection electrode contacts for organic crystals
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Basis for charge injection in all modern OLED devices
Researchers started work on the technology over the past
20 years
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Similar to LEDs, an OLED is a solid state semiconductor
› 100-500nm thick
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OLEDs use organic molecules to produce holes and
electrons
› Instead of using layers of n-type and p-type semiconductors
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Simple OLEDs are made up of six different layers
Modern OLEDs use many more layers for efficiency
› Basic functionality remains the same
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Substrate (clear plastic, glass, foil)
› Supports OLED
Anode
› Removes electrons
Conducting layer (P-type material)
› Organic plastic molecules that transport "holes" from the anode
› Ex-polyaniline
Emissive layer (N-type material)
› Organic plastic molecules (different than the conducting layer) transport electrons
from the cathode
› Light is generated here
› Ex-polyfluorene
Cathode
› Injects electrons
Electro-phosphorescence
(electron-hole
recombination)
 Voltage is applied across OLED
 Current flows from cathode to anode through organic
layers
 Cathode gives electrons to the emissive layer
 Anode removes electrons from the conductive layer
 At the boundary between the emissive and the
conductive layers, electrons find holes and fill them
 The electron then gives up energy in the form of a
photon
 The intensity or brightness of the light depends on the
amount of electrical current applied
Recombination of charges leads to the
creation of a photon
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Frequency given by the energy gap between
the LUMO and HOMO
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(E = hν)
Most organic materials are intrinsically p-type
Higher hole mobility than electron mobility
› Opposite of non-organic semiconductors
 Trap states
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Electrons more susceptible to trapping
Impurities frequently have empty orbitals (which
trap electrons) below -3eV
› Filled orbitals (which trap holes) above -5eV are
not as common
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Operating voltage: 2-10V
Very dynamic
› Can emit all steps between 0% and 100% light
depending on the current
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The color of the light depends on the
type of organic molecule in the
emissive layer
Several types of organic films on the
same OLED to make color displays
General layout
 Three Types:
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1. Vacuum Deposition/Vacuum
Thermal Evaporation(VTE)
2. Organic Vapor Phase
Deposition
3. Inkjet Printing
Ideally in vacuum chamber
Very low pressure (10-6 or 10-5 Torr)
Organic molecules gently heated until
evaporation
 Condensed as thin films on a cooled substrate
 Thickness of each layer can be precisely
controlled
 Disadvantages
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› Evaporant condensed on cold walls can flake off,
contaminating the system and substrate
› Very difficult to control uniformity and doping
concentration over large areas
› Very expensive and inefficient
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Process
› Under low-pressure and in a hot-walled reactor
chamber
› Carrier gas transports evaporated organic
molecules onto cooled substrates
› Condensed into thin films
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Improves control over doping
› Controlled by both temperature and carrier
gas flow rate
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Better for large-area substrates
Advantage
› Use of a carrier gas increases the efficiency
› Reduces cost
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Process
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Organic materials diluted into a liquid and sprayed onto
substrates
 Similar to a standard inkjet printer
Organic Vapor Jet Printing
Developed at Princeton
› Uses vaporized organics instead of the liquid based jets
of other inkjet printers
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Current Equipment Manufacturer
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MIT spinout Kateeva
Advantages
Drastically reduces manufacturing costs
› Allows OLEDs to be printed onto very large films
 Examples - 80 inch TV screen or electronic billboard
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Passive-matrix OLED (PMOLED)
› Strips of cathode, organic layers, and strips of anode
› Anode strips are arranged perpendicular to cathode strips
› Intersections of cathode and anode make up the pixels where light is emitted
› External circuitry applies current to selected strips of cathode and anode
› Easy to make but consume more power than other types (still less than LCDs)
 Active-matrix OLED (AMOLED)
› Full layers of cathode, organic molecules, and anode
› Anode layer overlays a thin film transistor (TFT) array that forms a matrix
› Consume less power than PMOLEDs and are very efficient for large displays
› Faster refresh rates
 Transparent OLED
› Transparent components (substrate, cathode and anode)
› Up to 85 percent as transparent as their substrate when turned off
› Allow light to pass in both directions when turned on
› A transparent OLED display can be either active or passive matrix
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Foldable OLED
› Substrates made of very flexible metallic foils or plastics
› Very lightweight and durable
› Potentially could be attached to fabrics to create "smart"
clothing
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White OLED
› Emit white light that is very bright
› More energy efficient than fluorescent lights
› Reduce energy costs for lighting
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Relatively easy to produce
Drastically reduce power consumption
Flexible, foldable, and transparent
Brighter than conventional LEDs and LCDs
Low Voltage and fast switching
Lifetime
› Red and green OLED films have longer lifetimes (46,000 to
230,000 hours)
› Blue organics currently have much shorter lifetimes (14,000
hours)
Manufacturing processes are expensive right now
Water can easily damage OLEDs
OLEDs are made of organic semiconductors
 Electrophosphorescence
 Fabrication
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› Vacuum Thermal Deposition
› Organic Vapor Phase Deposition
› Inkjet Printing
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Multiple types of OLEDs
› AMOLED, PMOLED, Transparent, Foldable, White
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Drastically reduce power consumption
https://lcp.elis.ugent.be/tutorials/tut_oled
http://www.oled-info.com/oled-technology
http://www.oled-info.com/flexible-oled
http://www.oled-info.com/transparent-oleds
http://electronics.howstuffworks.com/oled1.htm
http://www.explainthatstuff.com/how-oleds-and-lepswork.html
 https://en.wikipedia.org/wiki/OLED
 https://www.technologyreview.com/s/521656/ink-jetprinting-could-be-the-key-to-next-generation-oled-displays/
 http://www.princeton.edu/~benziger/OVPD.pdf
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