Different OLED Variants

In a market where consumers demand innovation and novelty products with each passing quarter, some ideas flop while others soar to unimaginable amounts of profit. The technology industry is commanded by the need to satisfy the ever changing needs of the techno-savvy community. Right now, exploration into OLED’s has taken the big name corporations by storm, as they consider the plethora of different paths they can take with such an applicable concept. Whether it be in the lighting, cellular phone, or television, OLED’s are sure to make it into the homes of Americans soon. Yes, OLEDS are the next big thing.

AMOLED

Although not prominently advertised as OLED displays, there are multiple variants of OLEDs that have been in the market for many years. One of those is AMOLED, the active-matrix organic light emitting diode, often seen specifically in the smartphones that several Americans have today.  It is also present in televisions and its market is perfect for affordable and efficient devices.

The AMOLED holds the active matrix, which generates the light the thin-film transistor (TFT) array is electrically activated.  In an AMOLED, the TFT serves as a series of switches that controls the current flowing to each pixel.  As its name suggests, TFT is a field-effect transistor that deposits thin films on an active semiconductive layer onto a substrate usually made of glass. Commonly made from silicon, the characteristics of a silicon-TFT are dependent on the crystal structure of the silicon.  As previously mentioned the TFT layer can be composed of indium tin dioxide to create a transparent semiconductor for use in displays such as OLED and AMOLED.  The TFT array plays a significant role in AMOLED function due to its duality.  The continuous current to a pixel is controlled simultaneously by two TFTs.  One TFT starts the charging of the storage capacitor while the other provides a voltage that maintains a constant current.  This process allows for a lower required current to run, making the AMOLED more ideal in smartphone use.

The integration of TFTs is fundamental to the function of AMOLED displays.  The two main TFT technologies in commercial use are polycrystalline silicon (poly-Si) and amorphous silicon (a-Si).  Amorphous silicon does not contain the normal long range order of a tetrahedrally bounded silicon atom.  Thus, it can be passivated by hydrogen which allows a-Si to be deposited in low temperatures.  On the other hand, polycrystalline silicon is composed of a homogenous crystalline framework.  The entire layer is continuous and deposited easily onto a semiconductor wafer.  In the end, both methods allow the active-matrix backplanes to be fabricated in low temperatures for flexible AMOLED displays.  Further information on TFT displays can be found here.

Figure 1: Different variants of TFT layers used for various applications

 

AMOLED displays and phones were most commonly developed by Samsung and Motorola.  Like all technologies, there are various variations within the AMOLED family as well.  Samsung has incorporated the AMOLED displays into their Galaxy S range quite extensively, as the powerful Samsung Galaxy Note 3 was fitted with a Super AMOLED screen.  The Super AMOLED Plus was later introduced with the Samsung Galaxy S II.  It is an improvement from the Super AMOLED screen by replacing the PenTile 2 subpixel RGBG matrix with the three subpixel RGB RGB matrix. Upgrading from a two subpixel RGBG matrix with the three subpixel RGB RGB matrix allows for a crisper image, and cleaner, smoother looking text.  This replacement made the screen much brighter and energy efficient than its predecessor while giving a clearer picture due to the increase in subpixels.  The HD Super AMOLED would then follow in the Samsung Galaxy Note.  Although the Galaxy S III uses a 2 subpixel RGBG matrix HD Super AMOLED, the screen was upgraded for the Galaxy Note II by using a 3 subpixel RBG matrix.  The Samsung Galaxy Round also uses the AMOLED screen, as a part of the curved phone fad that has started to hit the market.  This screen, the Super Flexible AMOLED capacitive touchscreen is paramount curved handsets, since it is able to be made transparent and flexible, which is required for a phone that wants to achieve wider viewing angles through bending screens.

Figure 2: Samsung Galaxy phones using an AMOLED display

 

PMOLED

       OLEDs can also be made using passive-matrix addressing schemes.  PMOLEDs are fundamentally the opposite from an AMOLED.  They were used in early displays and are not commonly seen anymore.  They function by controlling each line of pixels sequentially without the use of a capacitor.  The lack of a capacitor makes PMOLEDs different from AMOLEDs in that they do not use a TFT layer to keep the pixels constantly on.  This results in most of the pixels being off for the majority of the time.  To adjust for this, more voltage is required for brightness.  Although this principle makes PMOLEDs easy to manufacture, the quality and lifetime of PMOLEDs are severely lower than AMOLEDs.  The fact that they require more voltage for each line of pixels also restricts the size of PMOLED displays.

 

Figure 3: Transparent TDK PMOLED screens

Conclusion

       Although PMOLED displays were a good foray for many companies when the OLED market was still in its infancy, it is now clear that they are less desirable than AMOLED displays.  By using the technology similar to old CRT displays, PMOLED pixels were controlled by switching on a row and a column.  The intersection of the row and column was then lit up.  Although they were easy to build, the restrictions in size severely limited PMOLED applications.  They also consumed power at a higher rate.  On the other hand, AMOLEDs used a unique principle where each pixel is controlled individually.  This allows for larger displays and power efficiency at the cost of ease of production.  Thus, as the full capabilities of PMOLED and AMOLEDs were discovered, each fit into their own niche market.  PMOLEDs are now integrated more in small MP3 players while AMOLEDs dominate the smartphone market.

Evonik’s ULTRASIL Tires increase fuel efficiency

Company Profile:

Evonik Industries is one the world’s lead specialty chemical companies. One of the main goals of the company is to provide product that solve problems and provide a maximum benefit to customers and society. Recently, they have developed a more fuel efficient tire that does not compromise on performance through the implementation of a silica-silane system, known as ULTRASIL.

ULTRASIL:

The greenhouse effect is a natural process in which radiant heat from the sun is captured in the lower half of the atmosphere, directly resulting in higher temperatures and thus global warming. In order to reduce this greenhouse effect, most companies are working towards minimizing carbon dioxide emissions from transportation. Carbon emissions from combustion of energy fuels has accounted for 81.5% of total greenhouse gas emissions over the last several years, and global warming is quickly becoming a major problem throughout the world. Transportation contributes to this on a large scale, and it is responsible for 31% of the CO₂ emissions from the United States. However, Evonik’s silica-silane system (ULTRASIL) is a unique approach to this problem. ULTRASIL is created in several different forms and is applicable in many different situations, however its primary purpose is to serve as a coating for tires. This advanced tire technology can reduce the rolling resistance of tires, increase traction in wet conditions, and reduce carbon dioxide emissions. In general, tires have been targeted as quick way to reduce carbon dioxide emissions, as simple changes in size and shape can increase fuel efficiency by up to 15%.

ULTRASIL is able to reduce rolling resistance between tires and wet or icy road conditions due to the presence of intermolecular forces (IMFs), which can determine whether a solid will be hydrophobic (resists water) or hydrophilic (attracts water). This is an important concept to the concept of ULTRASIL because it is produced with hydrophobicity in mind. Being hydrophobic, water will adhere to the ULTRASIL coated tires, resulting in increased traction between the tires and the road. The major types of intermolecular forces that impact hydrophobicity include dipole-dipole forces, hydrogen bonding, ionic interactions, and London dispersion forces.

Dipole-dipole forces, hydrogen bonding, and ionic interactions are all known to be hydrophilic interactions. The larger presence of these forces in a molecule, the more the solid will attract water molecules. Dipole moments in a molecule are dictated by the polarity of a molecule. Polarity is the sum of all of the bond polarities in a molecule, resulting in dipole moments. The dipole moment is measured in a vector as the sum of the individual vector movements. For example, CO₂, a linear and non-polar molecule, has no dipole moment. Hydrogen bonds are the interactions of a hydrogen atom with a nitrogen, oxygen, or fluorine atom. They are a much more powerful force than dipole-dipole forces, resulting in a larger increase on the hydrophilicity of the molecule. Similarly, the presence of ionic bonds (interactions between positive and negative ions) can have the same effect.

London dispersion forces, the weakest of the intermolecular forces, are the sole forces that can raise the hydrophobicity of a molecule. This force, also called an induced dipole-dipole force, is a temporary attractive force that results when the electrons in two adjacent positions occupy positions that make the atoms form temporary dipoles. These forces occur in all molecules. In the production of ULTRASIL, Evonik has created a silica-silane system, where the hydrophobic regions of the molecule dominate, causing adhesive forces to arise and increase the tension between tires and wet/icy road conditions. More information about intermolecular forces can be found here, or

.

Also, the chemical structures of the silica helps contribute to its unique properties. Silicon dioxide can exhibit one of the largest varieties of crystal structures among the compounds commonly available and used. These many different crystalline forms allow silica to be used in a broad range of applications, including ULTRASIL. Precipitated silica, which is key to this product’s functionality, is a specially prepared form that has an amorphous structure, similar to silica gel or glass, both of which are predominantly silicon dioxide, or silica. As already discussed, adding these silicon dioxide granules to the surface of rubber tires has many beneficial effects on vehicle performance, but binding this hydrophilic molecular solid to the long, continuous, and hydrophobic polymer chains that make up vulcanized rubber can be difficult. It is up to sulfur, linking the polymers of vulcanized rubber to make it more resistant to temperature extremes, to act as a coupling agent for silica, since the hydrocarbon polymers will not bond to it by themselves.

From what we know, however, ULTRASIL production takes a rather different approach to solving the problem of coupling silica to rubber: the silica-silane system. By treating the original rubber  material with various organosilanes, a surface that silica particles can easily bond to is created, making it possible to form the desired composite with more cross-links to the silica granules and a higher overall thermal stability than without the treatment. Organosilanes usually have both a nonpolar and polar end and can not only bond with vulcanized rubber, but also with the silicon dioxide particles, through dehydration synthesis of their hydroxyl groups with the hydroxyl groups that cover the surface of the silica particles.

While many of the specific details of the ULTRASIL manufacturing process are trade secrets of Evonik, the company does share the basic concept of how it obtains the very pure amorphous silica needed for its products: precipitation from solution. Precipitated silica is widely used in industrial processes around the world, and Evonik Industries is its largest producer. Just like in ULTRASIL, these fine silica grains are often used in rubber products like tires and shoe soles for benefits similar to those of Evonik’s products. Generally, an aqueous silicate salt is reacted with an inorganic acid (like H₂SO₄) to form insoluble silica in the following reaction:

Na₂SiO_3(aq) + H₂SO_4(aq) → SiO_2(s) + Na₂SO_4(aq) + H₂O_(l)

    After the silica precipitate has been dried, it still contains no more than 88% silicon dioxide according to Evonik, with most of the rest being water. The ensuing treatment to purify the product varies depending on the desired size and quality of the particles obtained, but eventually a fine powder consisting of 99% silica can be obtained. The precipitated silica used in the ULTRASIL product line consists of miniscule, porous granules often of nanoparticle size to allow a high surface area to volume ratio, with the 7000 GR variant having a surface area of 170 m² per gram. It is this kind of fine silica that allows for the reinforced rubber of the emerging “Green Tire” that advances in silica rubber have created.

Further Reading:

If you are interested in the chemistry behind Evonik’s ULTASIL, there is a lot of in depth reading available in scientific journals. A thorough account of this technology and the chemistry that drives it can be found

• In this study by Brinke, Debnath, Reuvkamp, and Noordermeer
• And this article by Park and Cho

The Manufacturing Processes of OLEDs

At the recent CES convention in Las Vegas, many new products were unveiled and demonstrated.  Both the public and professional communities were captivated by the new OLED technologies.  However, as the prices were announced, it soon became clear to the techno-savvy consumers at home that it would be many years before OLEDs were a common sight in the average household.  Although the prices seemed daunting, the manufacturing methods used to produce these organic screens have been steadily improving.  As a revolutionary advance in display technology, OLED displays are expected to overtake the LCD market in the near future.  Thus, many researchers have worked tirelessly to develop a more efficient and quicker method in order to make OLEDs a more accessible technology.

Patternable OLEDs operate through a light or heat activated electroactive layer.  This is a polymer that exhibits a physical change when stimulated with an electric field.  A popular polymer for this layer is PEDOT-TMA, which becomes a highly efficiently hole injection layer.  It is a p-type conducting polymer and a modification of the PEDOT monomer, which is based off the EDOT monomer and can be seen below.  PEDOT-TMA is a popular material for this layer because it is noncorrosive and dispersible in organic solvents.  This is due to the methacrylate groups at both ends of the polymer chain.  These caps limit the length of the polymer, making it more soluble in organic solvents and ideal for usage in OLEDs.  These OLEDs are conventionally produced through the use of two main patterning technologies, organic vapor phase deposition (OVPD) and inkjet etching (IJE).

The PEDOT-TMA chain.  The two methacrylate caps at the end are clearly visible.

Organic Vapor Phase Deposition 

The organic vapor phase deposition manufacturing process works by transporting organic molecules to a cold substrate through the use of a hot inert carrier gas.  The final film product can be fine-tuned by altering the gas flow rate and source temperature.  Uniform films are produced by lessening the carrier gas pressure, which creates a larger velocity and mean free path of the gas.  This leads a decrease in the thickness of the boundary layer.  OVPD is also popular for OLED production because the process naturally prevents issues related to contaminants from the walls of the chamber.  This is because the walls are naturally warm, which prevents molecules from sticking to and producing a film.

However, maintaining a uniform film thickness and dopant concentrations over the large surface area needed for many applications is a demanding task in the context of vacuum evaporation, which is currently the most prevalent method of depositing organic molecular solids.   In addition, there is a relatively low yield with OVPD production, which explains the massive production costs.  As a remedy, low pressure organic vapor phase deposition has been proposed as a substitute technique.  This process has a notably improves command over doping and offers a potential method to rapid, particle-free, uniform deposition of organics on large-area substrates.  Further information detailing the uses and advantages of OVPD can be found here.

Inkjet Etching

The most basic method of producing OLEDs incorporates old technology originating from 1976, when the first inkjet printer was created.  Inkjet printing is a cheap and efficient alternative to traditional OLED production and is used by several start-up OLED companies such as Kateeva.

The Kateeva system for inkjet etching.

Kateeva’s system of inkjet printing involves a movable platform capable of holding screen substrates of up to 55” diagonal length.  The movable platform is able to transfer a selected substrate under various print heads, which are then able to spray droplets of organic material from different nozzles.  These drops of organic material are placed precisely (picoliter scale) on a selected substrate to form the pixels on the screen.  Additionally, the inkjet printing system is a versatile procedure, as it can be adapted from large television screens to small handheld devices such as smartphones and tablets.

This process is not as accurate or efficient without the use of a shadow mask on the substrate.

Shadow mask used for consistency and quality of the final display.

A shadow mask is a fine metal stencil drawn on the substrate that directs the material spray into its correct place, ensuring the consistency and quality of the final screen.  However usage of the shadow mask, while convenient and economical, also has its downfalls.  The shadow mask may lead to contamination of the organic materials of the OLED screen.  Because the OLED screen is composed of organic products, it is extremely sensitive to outside particles that may cause a drop in the vibrant image quality.  Nevertheless, inkjet printing is still a viable method for producing OLED displays and further improvements will reduce the risks of contamination.

New Developments in the OLED Industry

Recently, researchers have discovered that utilizing a molecule of a certain shape and structure allows anew type of OLED that has the potential to emit as much light as a commercial OLED, but without the need for costly rare metals normally demanded to make the OLEDs effective. Rare earth metals such as iridium or platinum are currently vital to OLED displays because it increases the efficiently of the display to commercial standards.  The removal of these expensive metals from the production of OLEDs has the potential to significantly reduce the costs of OLEDs.  This improvement in OLED structure is vital to advance the technology and reduce prices as the OLED industry of smartphones, televisions, and solid-state lighting continues to grow.

New developments regarding OLED production also includes metal oxide deposition systems.  These have potential to become leading production methods because they are more efficient and consistent in their output than the currently costly production methods.  For example, the AKT-55KS, a plasma enhanced chemical vapor deposition (PECVD) system that handles 8.5-Gen substrates, is a good option to consider based on its defect-free nature and its ability to keep out unwanted gases.

AKS-55KS

Two systems that hold potential for widespread use is the new physical vapor deposition (PVD) deposition systems, which comes in 2 models: SKT PiVot 25K which controls Gen-6 substrates, and the SKT PiVot 55K which controls 8.5-gen substrates.  The advantages of these systems are derived from the utilization of tubular cathodes of donor material, where the cathodes rotate because of the process of deposition.

Conclusion

Constant innovation is a necessity of a demanding market, especially in an industry that is still attempting to establish itself as a viable consumer market.  The production of OLEDs is still in its infancy, as there are several obvious and monumental obstacles that present itself in the first phases of starting such a market.  The most pressing issue is the costly and inefficient manner in which these organic LED’s are created, which has prompted researchers to find ways to reduce these expenses. This had led to the introduction of the inkjet printer formatted specifically for the production of OLEds as well as the new deposition systems offer viable alternatives that hold promise for a market that contains endless possibilities. One can only speculate the boundless paths that the OLED industry can take, whether it be into solid-state lighting, a lucrative smartphone market, or both.  Consumers can only wait to see what the raw potential of this revolutionary technology can manifest into.