1. 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.

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

3. 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

4. Organic Semiconductors (cont.)
Similar to LEDs, an OLED is a solid state semiconductor
nm 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

5. 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

6. 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

7. 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

8. 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

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

10. 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

11. 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

12. 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

14. 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

15. 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

16. 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

17. 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

18. References https://lcp.elis.ugent.be/tutorials/tut_oled

19. Questions/Comments?