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.

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.

The Principle Behind OLEDS

Throughout our exciting journey together, we have explored the wonderfully innovative and unpredictable OLED industry in depth all across the spectrum.  We first focused on the bare bones of the basic layers of an organic light emitting diode.  Subsequently, took a figurative trip to the factories, where we learned about the tedious methods of production that were necessary to create such a complex technology.  This led to a discussion on the current research focuses within OLEDs and the possibilities of the future of OLED being introduced to the everyday household.  Subsequently, we delved into the industry itself, and highlighted a few notable products and applications that the OLED industry is involved in.  Now, we will finally take a close look at what makes an OLED what it is and the underlying principle that allows it to emit light.  After all, we’re interested in just how it identifies itself as a light emitting diode.  In this post, we will explain just how an OLED generates light and makes it such a potential powerhouse technology that we will no doubt look forward to in the future.

The placement of the heterojunction within an OLED

As we stated earlier, the typical OLED structure is composed of layers of organic material between two electrodes which are all deposited on a substrate.  The layers of organic material are considered semiconductors.  This is due to the delocalization of electrons and the structure of the organic molecules; they are electrically conductive throughout the molecule.  What results are levels of conductivity ranging from insulators to conductors.  Like all other molecules, organic material is full of unoccupied molecular orbitals.  In this case, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are two orbitals most similar in energy.  The energy difference between the HOMO and LUMO is known as the HOMO-LUMO gap and allows electrons to move freely within the diode.

 The first OLED was devised by J.H. Burroughes in 1990.  It consisted of a single organic layer of polyphenylene vinylene, a popular conducting polymer.  However, as OLED technology began to evolve, it became clear that a single layer did not provide the efficiency desired.  Thus, contemporary OLEDs now contain two or more layers, usually a conductive layer with an emissive layer.  Further information about this specific structure and the materials used in its production can be found in Blog 2.  The most recent levels OLED efficiency (19%) was achieved by utilizing a graded heterojunction.  A heterojunction is an interface between two layers of semiconductor, such as the organic material being used in the OLED.  The unequal HOMO-LUMO band gaps of a heterojunction allows for greater operating frequencies as well as a high forward gain.  Additionally, the heterojunction structure allows for the hole and electron-transport materials to vary within the emissive layer.  This improves charge injection and balances charge transport at the same time.

In blog 2, we briefly mentioned the purpose of the anode and cathode within the structure of the OLED.  When a voltage is applied across the OLED, a current of electrons begins to flow through the device from the cathode to the anode.  During this process, electrons are injected into the LUMO of the cathode and ejected from the HOMO of the anode.  This can also be described as the injection of electron holes into the HOMO.  As described earlier, an electron hole is the theoretical opposite of an electron and describes a local where an electron can exist.  The ejected electron from the anode is now an electron hole.  Afterwards, electrostatic forces within the OLED bring the electrons and the electron holes together to form anexciton near the emissive layer.  An exciton is considered condensed matter that is able to transport energy without affecting the net electrical charge.  Electrons and electron holes are both fermions with half integer spin.  When they combine to form an exciton, it can be in a singlet or triplet state.  The decay of the singlet state exciton produces the light we see.

A graphical representation of how OLEDs emit light

As mentioned in blog 2, indium tin oxide (ITO) is a popular anode because it is transparent to visible light, high work function, and electrical conductivity.  This reduces the energy required for hole injection within the HOMO.  On the cathode side, barium and calcium are often used due to their low work functions which allows for an easier injection of electrons into the LUMO.  The materials chosen for the anode and cathode for an OLED are fundamental in reaching high efficiency, performance and extension of lifetime.  Thus, it is vital to produce as smooth as possible anode surface to increase adhesion, decrease electrical resistance and reduce dark spots.  While procedures to decrease anode surface roughness have been developed, potential alternate substances for the anode/cathode are also being researched.  Current candidates include single crystal sapphire substrates treated with gold.

The process in which the OLED emits light is the essence of why they exist. This method is simply unrivaled and and absolutely original in regards to anything else in the technology sector, and it brings such a unique and supreme performance experience that the OLED market is predicted to soar to unimaginable heights. It would be unwise to hazard a guess at what holds for the future for such a limitless concept. Over the course of our series covering the OLED technology, we have touched hands with all corners of chemistry, ranging from electrochemistry to organic chemistry and even ventured slightly into materials science.  We hope that by covering such an integral part of the evolving nature of the industry, we have sparked your interest in the technology revolution that has taken over the world. The current rate of innovation requires constant learning in order to stay up to date with each modification.   As Arthur C. Clarke once said, “Any sufficiently advanced technology is indistinguishable from magic.”  OLED technology has the potential to become that magic.

Are our Smartphones getting Smarter?

In the modernized world, almost everyone immerses themselves in the wonders of technology, making use of the increasing advancement of cell phone development, whether they are tweeting about an awesome video, or browsing the internet for presents to buy for the upcoming holidays. A somewhat vexing issue arises from lackluster battery life, which at times can seem mediocre considering how cutting-edge our phones have become. As technology becomes increasingly universal and mobile, the amount of usage a fully charged battery is able to provide is a key concern.  Luckily, recent research into organic LED displays have in turn generated a more energy efficient alternative.

LCDs

Ordinary LCDs (liquid-crystal display), which are in virtually every monitor and laptop, work by choosing to block areas of the backlight to create images that audiences see.  A simple LCD panel can be created with layers of polarized glass, nematic liquid crystals, polarizing film and polymers, as seen in the image below.  

However, for every LCD panel, there must be an external light source since they emit no light on their own.  As a source of light, LCDs commonly utilize LEDs (light-emitting diodes) or built in fluorescent tubes.  However, the light is not always evenly distributed throughout the display.  This results in uneven backlighting and can be witnessed when the monitor displays a black image.  Its overall design limits the viewing angle of the monitor which results in variations in color saturation, contrast and brightness.  Additional characteristics of LCDs can be found described by VarTech Systems Inc here.

OLEDs

How is an OLED different from an LED?  As their name suggests, OLEDs (organic light emitting diodes) is an LED composed of organic material that emit light in response to an electric current.  They do not require an external light source, since they are able to produce their own light.  Instead of LCD backlights dissipating light across several layers of fragile layers of film and glass, an OLED screen only requires a minimum of five basic thin layers: the cathode electrode, anode electrode, emissive and conductive layers, and transparent material.  Additionally, the plastic, organic layers of an OLED panel are thinner, lighter and more flexible than the crystalline coats in a LCD.  While this may seem like a cosmetic superficiality, a more flexible display of plastic allows for a more malleable support than the hard glass of their counterparts.

The OLED display is generated from the cathode and anode layers.  The anode layer expels electrons through the conductive and emissive layers while the cathode applies the electrons through the layers.

This interchanging process of electron transfer happens when a current passes through the OLED screen.  This produces an electric signal in the conductive/emissive layers that gets sent through the substrate, which is the layer that the user sees the eye-popping image through.  The organic product also holds the upper hand in light emissions; due to the OLED’s thin layers, which are far skinnier than the inorganic crystal layers of an LED, the conductive and emissive layers of an OLED can be multi-layered, thus giving off more light and better color. As one might realize, this is an extremely efficient process.  Instead of having a row of energy consuming LCDs spreading light, only a small current would be applied through all the layers of the OLED screen.

Benefits

The underlying principle behind OLEDs makes them drastically different than LCDs.  These differences are usually benefits that make OLEDs a target technology for mobile devices such as cell phones.  Unlike LCDs, OLEDs emit their own light, eliminating the need for a backlight.  This results in a lower energy consumption, since a significant amount of energy within LCDs goes towards backlighting.  The lower energy consumption of an OLED panel makes them an attractive screen for use in mobile devices.  Cell phones that adopt the OLED display would therefore be one step closer to a longer battery life, since the outdated LCD screens will no longer be sucking the electricity from the cell phone.

In addition to their energy efficiency, the structure of the OLED display poses many advantages over LCD screens.  An OLED panel does not require any glass casing, unlike an LCD.  The glass casing in an LCD absorbs some of the light and reduces it efficiency.  The layers of an OLED, which are plastic and organic allow it to be much thinner, lighter, and more flexible than a traditional LCD screen.  This leads to a variety of features and benefits ideal for use in cell phones. It would allow for virtually indestructible and even curved phones to be created. In fact, Samsung has already announced the first OLED curved phone.

Before any company makes a significant investment in research and development, it carefully weighs the costs and potential benefits.  Yes, OLED technology has been developed enough for use in a smartphone– but will it be commercially viable and attractive to customers? In fact, it would be false to suggest that merely because something bends and is lighter will make it a commercial success.  Samsung’s risky foray into the market by creating an OLED sector will determine the future for OLEDs in the near future.  Nevertheless, it is these risks that has the potential to make Samsung a leader in the next generation of smartphones.  As Steve Jobs once said, “Innovation distinguishes between a leader and a follower.”