OLEDs Theory & Fabrication Tanner Mariucci 4/22/16 ABSTRACT: Organic Light Emitting Diodes are rapidly becoming the cutting edge in display technology. This presentation will introduce OLEDs, explain the working principle behind them, and then go into fabrication techniques and different types of OLEDs. The advantages and disadvantages of OLEDs will also be examined.

OUTLINE Organic Semiconductors Working Principle Fabrication Types of OLEDs Advantages/Disadvantages

Organic Semiconductors Made from organic (carbon based) polymers Emit Light when electricity is applied through them Martin Pope-1960 Developed Ohmic dark-injection electrode contacts for organic crystals Basis for charge injection in all modern OLED devices Researchers started work on the technology over the past 20 years

Organic Semiconductors (cont.) Similar to LEDs, an OLED is a solid state semiconductor 100-500nm thick OLEDs use organic molecules to produce holes and electrons Instead of using layers of n-type and p-type semiconductors Simple OLEDs are made up of six different layers Modern OLEDs use many more layers for efficiency Basic functionality remains the same

Working Principle 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

Working Principle (cont.) 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

Organic Semiconductors(cont.) Recombination of charges leads to the creation of a photon Frequency given by the energy gap between the LUMO and HOMO (E = hν) Most organic materials are intrinsically p-type Higher hole mobility than electron mobility Opposite of non-organic semiconductors Trap states 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

Working Principle (cont.) Operating voltage: 2-10V Very dynamic Can emit all steps between 0% and 100% light depending on the current 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

Fabrication of OLEDs General layout Three Types: Vacuum Deposition/Vacuum Thermal Evaporation(VTE) Organic Vapor Phase Deposition Inkjet Printing

Vacuum Thermal Evaporation 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 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

Organic Vapor Phase Deposition Process Under low-pressure and in a hot-walled reactor chamber Carrier gas transports evaporated organic molecules onto cooled substrates Condensed into thin films Improves control over doping Controlled by both temperature and carrier gas flow rate Better for large-area substrates Advantage Use of a carrier gas increases the efficiency Reduces cost

Inkjet Printing Process Organic Vapor Jet Printing 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 Current Equipment Manufacturer 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

Types Of OLEDs 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

Types of OLEDs (cont.) Foldable OLED White OLED Substrates made of very flexible metallic foils or plastics Very lightweight and durable Potentially could be attached to fabrics to create "smart" clothing White OLED Emit white light that is very bright More energy efficient than fluorescent lights Reduce energy costs for lighting

Advantages Disadvantages Relatively easy to produce Drastically reduce power consumption Flexible, foldable, and transparent Brighter than conventional LEDs and LCDs Low Voltage and fast switching Disadvantages 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

5 Key Concepts OLEDs are made of organic semiconductors Electrophosphorescence Fabrication Vacuum Thermal Deposition Organic Vapor Phase Deposition Inkjet Printing Multiple types of OLEDs AMOLED, PMOLED, Transparent, Foldable, White Drastically reduce power consumption

References 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-leps-work.html https://en.wikipedia.org/wiki/OLED https://www.technologyreview.com/s/521656/ink-jet-printing-could-be-the-key-to-next-generation-oled-displays/ http://www.princeton.edu/~benziger/OVPD.pdf

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