Flexible organic light-emitting diode
Encyclopedia
A flexible organic light emitting diode (FOLED) is a type of organic light-emitting diode
(OLED) incorporating a flexible plastic substrate
on which the electroluminescent
organic semiconductor
is deposited. This enables the device to be bent or rolled while still operating. Currently the focus of research in industrial and academic groups, flexible OLEDs form one method of fabricating a rollable display
.
of thin films of organic semiconductor
s approximately 100 nm thick. Regular OLEDs are usually fabricated on a glass substrate
, but by replacing glass with a flexible plastic such as polyethylene terephthalate
(PET) among others, OLEDs can be made both bendable and lightweight.
Such materials may not be suitable for comparable devices based on inorganic semiconductor
s due to the need for lattice matching and the high temperature fabrication procedure involved.
In contrast, flexible OLED devices can be fabricated by deposition of the organic layer onto the substrate using a method derived from inkjet
printing, allowing the inexpensive and roll-to-roll fabrication of printed electronics
.
Flexible OLEDs may be used in the production of rollable display
s, electronic paper
, or bendable displays which can be integrated into clothing, wallpaper or other curved surfaces. Prototype displays have been exhibited by companies such as Sony
, which are capable of being rolled around the width of a pencil.
of materials in the device, in addition to the external stress from the bending of the device.
Stress introduced into the organic layers may lower the efficiency or brightness of the device as it is deformed, or cause complete breakdown of the device altogether. Indium tin oxide
(ITO), the material most commonly used as the transparent anode
, is brittle. Fracture of the anode can occur which can increase the sheet resistance
of the ITO or disrupt the layered structure of the OLED. Although ITO is the most common and best understood anode material used in OLEDs, research has been undertaken into alternative materials that are better suited for flexible applications including carbon nanotube
s.
Encapsulation
is another challenge for flexible OLED devices. While the common method of encapsulation for regular OLEDs is to seal the organic layer between glass, this is inappropriate for flexible OLEDs. The materials in an OLED are sensitive to air and moisture which lead to degradation
of the materials themselves as well as quenching
of excited states within the molecule. Flexible encapsulation methods are generally not as effective a barrier to air and moisture as glass, and current research aims to improve the encapsulation of flexible organic light emitting diodes.
Organic light-emitting diode
An OLED is a light-emitting diode in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes...
(OLED) incorporating a flexible plastic substrate
Substrate (semiconductor)
Substrate is a solid substance onto which a layer of another substance is applied, and to which that second substance adheres. In solid-state electronics, this term refers to a thin slice of material such as silicon, silicon dioxide, aluminum oxide, sapphire, germanium, gallium arsenide , an alloy...
on which the electroluminescent
Electroluminescence
Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field...
organic semiconductor
Organic semiconductor
An organic semiconductor is an organic material with semiconductor properties. Single molecules, short chain and organic polymers can be semiconductive. Semiconducting small molecules include the polycyclic aromatic compounds pentacene, anthracene, and rubrene...
is deposited. This enables the device to be bent or rolled while still operating. Currently the focus of research in industrial and academic groups, flexible OLEDs form one method of fabricating a rollable display
Rollable display
A rollable display is a flexible display device which can be rolled up into a scroll.Technologies involved in building a rollable display include electronic ink, Gyricon, and OLED.Electronic paper displays which can be rolled up have been developed by E Ink...
.
Technical Details and Applications
An OLED emits light due to the electroluminescenceElectroluminescence
Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field...
of thin films of organic semiconductor
Organic semiconductor
An organic semiconductor is an organic material with semiconductor properties. Single molecules, short chain and organic polymers can be semiconductive. Semiconducting small molecules include the polycyclic aromatic compounds pentacene, anthracene, and rubrene...
s approximately 100 nm thick. Regular OLEDs are usually fabricated on a glass substrate
Substrate (semiconductor)
Substrate is a solid substance onto which a layer of another substance is applied, and to which that second substance adheres. In solid-state electronics, this term refers to a thin slice of material such as silicon, silicon dioxide, aluminum oxide, sapphire, germanium, gallium arsenide , an alloy...
, but by replacing glass with a flexible plastic such as polyethylene terephthalate
Polyethylene terephthalate
Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
(PET) among others, OLEDs can be made both bendable and lightweight.
Such materials may not be suitable for comparable devices based on inorganic semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s due to the need for lattice matching and the high temperature fabrication procedure involved.
In contrast, flexible OLED devices can be fabricated by deposition of the organic layer onto the substrate using a method derived from inkjet
Inkjet printer
An inkjet printer is a type of computer printer that creates a digital image by propelling droplets of ink onto paper. Inkjet printers are the most commonly used type of printer and range from small inexpensive consumer models to very large professional machines that can cost up to thousands of...
printing, allowing the inexpensive and roll-to-roll fabrication of printed electronics
Printed electronics
Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment or other low-cost equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography and...
.
Flexible OLEDs may be used in the production of rollable display
Rollable display
A rollable display is a flexible display device which can be rolled up into a scroll.Technologies involved in building a rollable display include electronic ink, Gyricon, and OLED.Electronic paper displays which can be rolled up have been developed by E Ink...
s, electronic paper
Electronic paper
Electronic paper, e-paper and electronic ink are a range of display technology which are designed to mimic the appearance of ordinary ink on paper. Unlike conventional backlit flat panel displays, electronic paper displays reflect light like ordinary paper...
, or bendable displays which can be integrated into clothing, wallpaper or other curved surfaces. Prototype displays have been exhibited by companies such as Sony
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
, which are capable of being rolled around the width of a pencil.
Disadvantages
Both the flexible substrate itself as well as the process of bending the device introduce stress into the materials. There may be residual stress from the deposition of layers onto a flexible substrate, thermal stresses due to the different coefficient of thermal expansionThermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
of materials in the device, in addition to the external stress from the bending of the device.
Stress introduced into the organic layers may lower the efficiency or brightness of the device as it is deformed, or cause complete breakdown of the device altogether. Indium tin oxide
Indium tin oxide
Indium tin oxide is a solid solution of indium oxide and tin oxide , typically 90% In2O3, 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey...
(ITO), the material most commonly used as the transparent anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
, is brittle. Fracture of the anode can occur which can increase the sheet resistance
Sheet resistance
Sheet resistance is a measure of resistance of thin films that are namely uniform in thickness. It is commonly used to characterize materials made by semiconductor doping, metal deposition, resistive paste printing, and glass coating. Examples of these processes are: doped semiconductor regions...
of the ITO or disrupt the layered structure of the OLED. Although ITO is the most common and best understood anode material used in OLEDs, research has been undertaken into alternative materials that are better suited for flexible applications including carbon nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s.
Encapsulation
Encapsulation
- Chemistry :* Molecular encapsulation, in chemistry, the confinement of an individual molecule within a larger molecule* Capsule , in pharmacy, the enclosure of a medicine within a relatively stable shell for administration...
is another challenge for flexible OLED devices. While the common method of encapsulation for regular OLEDs is to seal the organic layer between glass, this is inappropriate for flexible OLEDs. The materials in an OLED are sensitive to air and moisture which lead to degradation
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...
of the materials themselves as well as quenching
Quenching (fluorescence)
Quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisional quenching. As a consequence, quenching is often heavily dependent on...
of excited states within the molecule. Flexible encapsulation methods are generally not as effective a barrier to air and moisture as glass, and current research aims to improve the encapsulation of flexible organic light emitting diodes.
See also
- Active-matrix OLEDActive-Matrix OLEDAMOLED is a display technology for use in mobile devices and televisions. OLED describes a specific type of thin-film display technology in which organic compounds form the electroluminescent material, and active matrix refers to the technology behind the addressing of pixels.As of 2011, AMOLED...
- Flexible electronicsFlexible electronicsFlexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on...
- Organic light-emitting diodeOrganic light-emitting diodeAn OLED is a light-emitting diode in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes...
- Phosphorescent organic light-emitting diode
- Rollable displayRollable displayA rollable display is a flexible display device which can be rolled up into a scroll.Technologies involved in building a rollable display include electronic ink, Gyricon, and OLED.Electronic paper displays which can be rolled up have been developed by E Ink...