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.

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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.  

LCD screens are created by stacking layers of different materials atop one another.  Together, it creates a displayed image.

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 layers that make up a basic OLED.  It requires less materials and layers than a traditional LCD.

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.

A visual representation of the principle behind the luminescence of an OLED.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.”