Active matrix organic light-emitting diode (AMOLED) displays have been available on high-end smartphones for a while now, and there has been a lot of speculation about when we’ll start to see tablet devices equipped the same screen technology. I would like to take a closer look at why AMOLED technology is so hotly anticipated.
OLED displays use an alternative pixel-lighting mechanism compared to liquid crystal display (LCD) - a mechanism that is simpler in concept and offers advantages over LCD, but introduces numerous technological challenges that display manufacturers are working to overcome.
How LCDs and OLEDs Work
An LCD pixel works by applying different voltages to three electrodes, which each align a small area of liquid crystal material to allow a certain amount of backlight to shine through a red, green and blue color filter. These three sub-pixel colors combine to form the final light output for one pixel, with the liquid crystal material acting as a variable shutter for each of these color components.
An OLED pixel works in a different way. Instead of selectively blocking light, light is generated directly in each sub-pixel by applying different currents to three diodes. Each diode subsequently emits red, green, or blue colored light. Many OLED devices vary the sub-pixel dimensions and layout to be more visually appealing, such as the phone in the photograph above which has twice as many green sub-pixels in at roughly half the area. Leaving these quality-enhancing nuances aside, the final OLED light output is the combination of the sub-pixel colors, similar to LCD. Besides the direct light generation, other differences with OLED displays are the elimination of the LCD material, the color filter, the backlight, a polarizer, and other layers. As a result, an OLED display can offer many advantages over an LCD.
• Energy Efficiency: OLED displays have theoretical energy efficiency advantages over LCD displays. There are fewer layers through which to transmit the material, and no light is wasted by shining against non-transmitting liquid crystal material. In practice the actual energy consumption depends largely on what is being viewed. White-on-black text will use little power, while black-on-white text will drain the battery faster than an LCD would. Overall OLED displays have an average energy savings between 0 and 40% for TV and smartphone usage, depending on what content and brightness is being viewed. The savings projection was larger several years ago when all LCD displays used fluorescent backlights. LCD display energy efficiency has improved by incorporating LED backlights, and improved further with more efficient locally-dimmed LED backlights. These locally-dimmed backlights reduce brightness (and therefore power consumption) in darker regions of the screen.
• Enhanced colors: OLED materials give off truer and wider variety of color than the LCD method of shining a backlight through a color filter. The technical term for this is that OLED displays have a wider color gamut than LCD displays, enabling richer, more vibrant colors on your screen. You can see this clearly when comparing an LCD TV or handset device to an OLED device, side by side. OLED displays have much higher contrast ratios as a result of much richer blacks. Further, the color quality is the same regardless of viewing angle. Depending on the LCD technology, the color mix will change at wider viewing angles. This is because the OLED material is emitting directly without having to go through liquid crystal and other layers.
• Motion and 3-D adaptability: The refresh rates (the number of times per second a display redraws data) required for some 3-D technologies are twice that of LCD to achieve the same image motion quality. Since the OLED lighting element is directly driven, while the LCD lighting element passes through the liquid crystal “shutter”, the OLED pixel has a faster response rate, and thus is easier to drive at higher refresh rates. As a result, OLED is capable of superior viewing quality for several leading 3-D technologies.
• Thinner is better: Eliminating the liquid crystal, color filter, and associated layers required for LCD means OLED displays can be much thinner. I saw a 46” TV prototype at a recent industry show that was only 2.6mm thick. It was so thin that it couldn’t support itself- it was bolted to a backing plate which appeared thicker than the panel to provide structural integrity.
• Flexible and transparent: The thinness, and lack of glass layers required to hold liquid crystal intact, open up whole new applications beyond traditional displays. OLED displays can be built on flexible substrates, and can also be built transparent. You can read more about these devices of the future in a recent blog post.
Most applications for OLED are in mobile devices including smartphones such as Samsung’s Galaxy line, Google Nexus S, and the HTC Incredible, game consoles such as the Sony Play Station, and even some mp3 players and cameras. Additionally, many tablet makers today are evaluating the benefits of OLED technology. All the specs and even a You Tube video are available online for the first Samsung tablet using an OLED screen, the “Galaxy Tab 7.7”, but it is not available commercially as I write this. You might think today’s devices are already pretty vibrant, but there is room for improvement in vibrancy if willing to sacrifice resolution. The best resolution available in commercial devises today with OLED technology is around 260 ppi, so there is a trade-off. The color quality, resolution, and power efficiency trade-offs between OLED and LCD are evolving and will largely dictate their proliferation throughout the consumer space. Of course manufacturing costs will also play a large role, with OLED currently costing more due to yields and economies of scale, but can potentially be driven below LCD costs due to less overall material usage and fewer manufacturing steps.
Applied Materials’ role in OLED technology is similar to our role in LCD: our equipment deposits conductive and insulating thin films for the “backplane”. This is analogous to the “interconnect” for an integrated circuit (IC). Both the OLED (or LCD) backplane and the IC interconnect are responsible for carrying electrical signals to all the various devices. We also manufacture test equipment for displays. Our largest released product for OLED processes 1.3m x 1.5m sheets of glass, which is half the area of a king-sized bed, over 250 smartphone panels from a single sheet of glass. Products in the pipeline run glass even larger than a king-size bed.
Most OLED devices today are higher-end mobile devices and therefore include a touch screen, requiring additional thin transparent conductive films. These films can also be deposited on Applied Materials equipment, similar to equipment we originally designed for LCD processing. Our touch screen customers have driven a lot of new business opportunities for us, and the whole industry, even as they have helped revolutionize the end-users’ experience.
So we consumers can now enjoy smartphones and eventually tablets with OLEDs, all with touch screens... this is getting really exciting. You’ll have to ask our customers what upcoming visually interactive products our tools will enable them to make next!