Smartphone Futurology: The science behind your phone's next display

Welcome to Smartphone Futurology. In this new series of science-filled articles, Mobile Nations guest contributor Shen Ye walks through current technologies in use within our phones, as well as the cutting-edge stuff still being developed in the lab. There's quite a bit of science ahead, as a lot of the future discussions are based on scientific papers with a vast amount of technical jargon, but we've tried to keep things as plain and simple as possible. So if you want to dive deeper into just how the guts of your phone function, this is the series for you.

A new year brings the certainty of new devices to play with, and so it's time to look ahead at what we might see in the smartphones of the future. The first instalment in the series looked at what's new in battery tech. The series' second part looks at what's perhaps the most important component of any device — the screen itself. On a modern mobile device, the screen acts as the main input and output device. It's the most visible part of the phone, and one of its most power-hungry components. Over the past few years we've seen screen resolutions (and sizes) reach into the stratosphere, to the point where many phones now pack 1080p displays or higher. But the future of mobile displays is about more than just size and pixel density. Read on to find out more.

Shen Ye is an Android developer and MSci graduate in Chemistry from the University of Bristol. Catch him on Twitter @shen and Google+ +ShenYe.

Be sure to check out the first instalment of our Smartphone Futurology series, covering the future of battery technology. Keep watching for more in the coming weeks.

Only 5 years ago did the leading flagship Android phone have a 3.2-inch, 320×480 HVGA screen, with a pixel density of 180 PPI. Steve Jobs proclaimed the "magic number is right around 300 pixels per inch" when the iPhone 4, with its Retina Display, was released in 2010. Now we have 5.5-inch QHD screens with 538 PPI, way beyond the resolution of the human eye when held 20cm away. However with VR accessories like the Google Cardboard and Samsung Gear VR which use our phones — not to mention the bragging rights that go with sharper screens — manufacturers continue to seek for higher resolutions for their flagship devices.

Right now the three most popular types of screens on the market are LCD, AMOLED and E-ink. Before talking about the upcoming improvements for each of these technologies, here's a brief explanation of how each of them work.

LCD (Liquid-crystal display)

LCDs have been around for decades — the same type of technology used in modern laptop and smartphone displays powered the screens of pocket calculators back in the 1990s. Liquid crystals (LCs) are exactly as their name states, a compound which exists in the liquid phase at room temperature with crystalline properties. They are unable to produce their own color, but they have a special ability to manipulate polarized light. As you may know, light travels in a wave, and when light leaves a light source the waves are in every degree of orientation. A polarizing filter is able to filter out all waves not aligned to it, producing polarized light.

The most common phase of LCs is known as the nematic phase, where the molecules are essentially long cylinders which self-align into a single direction like bar magnets. This structure causes polarized light passing through it to be rotated, the property which gives LCDs their ability to display information.

When light is polarized, it will only be able to pass a polarizing filter if the two are aligned on the same plane. A century ago the Fréedericksz Transition was discovered, it provided the ability to apply an electric or magnetic field on a LC sample and change their orientation without affecting the crystalline order. This change in orientation is able to alter the angle of which the LC is able to rotate polarized light and this was the principle which allows LCDs to work.

In the diagram above, the light from the backlight is polarized and passes through the liquid crystal array. Each liquid crystal subpixel is controlled by its own transistor which adjusts the rotation of the polarized light, which passes through a color filter and a second polarizer. The angle of polarization of light leaving each subpixel determines how much of it is able to pass through the second polarizer, which in turn determines the subpixel's brightness. Three subpixels make up a single pixel on a display — red, blue and green. Due to this complexity, a variety of factors affect the screen's quality such as color vibrancy, contrast, frame rates and viewing angles.

AMOLED (Active-Matrix Organic Light-Emitting Diode)

Samsung Mobile has been one of the main innovators in bringing AMOLED screens to the mobile industry, with all of its screens made by its sister company Samsung Electronics. AMOLED screens are praised for their "true blacks" and the color vibrancy, though they can suffer from image burn-in and oversaturation. Unlike LCDs, they don't use a backlight. Each subpixel is an LED which produces its own light of a specific color, which is dictated by the layer of material between the electrodes, known as the emissive layer. The lack of a backlight is why AMOLED displays have such deep blacks and this also brings the benefit of power saving when displaying darker images.

When a subpixel is activated, a current specific to the intensity required is passed through the emissive layer between the electrodes, and the component of the emissive layer converts the electrical energy to light. As with LCD, a single pixel is (usually) made from three subpixels red, blue and green. (The exception here is PenTile displays, which use a variety of irregular subpixel matrix patterns.) With each subpixel producing its own light the high energy can cause deterioration in the subpixels, which leads to lower light intensity which can be observed as screen burn. Blue LEDs have the highest energy and our sensitivity to blue is lower, so they have to be turned up even brighter which speeds up this deterioration.

E-ink (Electrophoretic Ink)

E-ink has been doing phenomenally in the e-reader industry, most notably Amazon's Kindle. (Pebble's e-paper display is slightly different.) Russian firm YotaPhone has even made phones with a rear e-ink display.

There are two main advantages of E-ink over LCD and AMOLED. The first is purely aesthetic, the appearance and lack of glare is appealing to readers as it is close to the appearance of printed paper. The second is the amazingly low power consumption — there is no need for a backlight, and the state of each pixel does not need energy to maintain, unlike LCD and AMOLED. E-ink displays are able to keep a page on the screen for vastly long periods of time without the information from becoming unreadable.

Contrary to popular belief, the "E" doesn't stand for "electronic", but its "electrophoretic" mechanism. Electrophoresis is a phenomenon where charged particles move when an electric field is applied to it. The black and white pigment particles are negative and positively charged, respectively. Like magnets, alike charges repel and opposite charges attract. The particles are stored inside microcapsules, each half the width of a human hair, filled with an oily fluid for the particles to move through. The rear electrode is able to induce either a positive or negative charge on the capsule, which determines the visible color.

The Future

With a basic understanding of how these three displays work, we can look at the improvements coming down the line.

Cascaded LCD

NVIDIA published a paper detailing its experiments in quadrupling screen resolutions with cascading displays, a fancy term for stacking a pair of LCD displays on top of each other with a slight offset. With some software wizardry, based on some serious mathematical algorithms, they were able to turn each pixel into 4 segments and essentially quadruple the resolution. They see this as a potential way of making cheap 4K displays from merging two 1080p LCD panels together for use in the VR industry.

The group 3D-printed a VR headset assembly for their prototype cascaded display as a proof of concept. With phone manufacturers racing to make thinner and thinner devices, we may never see cascaded displays in our future smartphone, but the promising results may mean we will get cascaded 4K monitors at a very reasonable price. I highly recommend checking out NVIDIA's paper, it's an interesting read with several comparison pictures.

Quantum Dots

Most of current commercially available LCD displays use either a CCFL (cold cathode fluorescent lamp) or LEDs for the backlight. LED-LCDs have started becoming the preferred choice as they have better color gamuts and contrast vs CCFL. Recently quantum dot LED-LCD displays have begun rolling into the market as a replacement for LED backlight, with TCL recently announcing their 55" 4K TV with quantum dots. According to a paper from QD Vision¹ the color gamut from a QD backlit LCD display exceeds that of OLED.

You can actually find QD enhanced displays in the tablet market, most notably the Kindle Fire HDX. The advantage of QDs is that they can be tuned to produce the specific color which the manufacturer wants. After numerous companies showing off their quantum dot TVs at CES, 2015 may be the year QD enhanced displays reach the mass market in phones, tablets and monitors.

Liquid Crystal Additives

Research groups all around the world are actively looking for things to add to liquid crystals to help stabilize them. One of these additives is carbon nanotubes (CNTs)³. Just adding a small quantity of CNTs was able to reduce the Fréedericksz Transition, explained above, so it led to both lower power consumption and faster switching (higher frame rates).

More discoveries in additives are being made all the time. Who knows, maybe eventually we'll have liquid crystals stabilized so well that they won't need a voltage to maintain their state, and with very little power consumption. Sharp's Memory LCDs are most likely using similar technology with their low power consumption and "persistent pixels". Despite this implementation being monochrome, the removal of the backlight makes it a competitor with E-ink displays.

Transflective LCDs

A transflective LCD is an LCD that both reflects and transmits light. It eliminates the need for a backlight under sunlight or bright conditions, thus significantly reducing power consumption. The backlight is also dim and low powered as it is only needed in the dark. The concept has been around for a few years, now and they have been used in LCD watches, alarm clocks and even a small netbook.

The main reason why you may have not heard about them is their prohibitively high upfront cost to the manufacturer compared to standard TFT LCDs. We have yet to see transflective displays used in smartphones, possibly because they would have a tough time being sold to the general consumer. Live phone demos and display units are one of the best ways to attract a customer so retailers tend to ramp up the brightness settings on the demo units to grab the attention of potential buyers, the low powered backlight in transflective screens would have a hard time competing. It'll become increasingly hard for them to get into the market with LCD backlights becoming more efficient, and color E-ink displays already patented.

Vision-Correcting Displays

Some readers may know someone long-sighted who has to hold their phone at an arm's length, or set their display font to enormous just to read it (or both). Teams at UC Berkeley, MIT and Microsoft teamed up to produce vision correcting displays using light field technology, similar concept to that found in Lytro cameras. Light field is a mathematical function which describes the amount of light travelling in every direction through every position in space, which is how the sensor in Lytro cameras work.

All the vision correcting display needs is the optical prescription to computationally alter the way light from the screen enters the eyes of the user to achieve perfect clarity. The great thing about this technology is that conventional displays can be modified to achieve vision correction. In their experiments, an iPod Touch 4th Gen screen (326 PPI) was fitted with a clear plastic filter. Spread throughout the filter is an array of pinholes slightly offset to the pixel array, with the holes small enough to diffract the light and emit a light field wide enough to enter both eyes of the user. The computational software can alter light leaving from each of the holes.

The display does come with a few downsides however. For starters, the brightness is slightly dimmer. The viewing angles also are very narrow, similar to that of glasses-less 3D displays. The software is only able to sharpen the display for a single prescription at a time, so only one user can use the display at any one time. The current software used in the paper does not work in real time, but the team have proved that their display works with the still images. The technology is suitable for mobile devices, PC and laptop monitors, and TVs.

Crystal IGZO Transistors

IGZO (indium gallium zinc oxide) is a semiconducting material only discovered in the last decade. Initially proposed in 2006³, it has recently started being used in thin film transistors for controlling LCD panels. Developed at Tokyo Institute of Technology, IGZO has been shown to transport electrons up to 50× faster than standard silicon versions. As a result these thin film transistors can achieve higher refresh rates and resolutions.

The technology has been patented and Sharp has recently used its licensing to produce a 6.1-inch LCD panels with 2K resolution (498 PPI). Sharp has been supplying high resolution LCD IPS displays across the mobile industry, and its crystal IGZO panels will only increase the company's share of this market, especially in light of past partnerships with Apple to supply LCD panels for iOS devices. Recently Sharp released the Aquos Crystal, showing off a high resolution IGZO display with shrunken bezels. Expect 2015 to be the year where IGZO displays start taking over in various flagship devices.

Nanopixels

Scientists from Oxford University and the University of Exeter recently patented and published a paper⁴ on using phase-change material (PCM) for displays, achieving 150× the resolution of conventional LCD displays. PCM is a substance whose phase can be easily manipulated, in this case changing between a transparent crystalline state and an opaque amorphous (disorganised) state.

Similar to LCD technology, an applied voltage is able to dictate whether a subpixel is transparent or opaque, however it does not require the two polarizing filters and so allows paper-thin displays. The PCM layer is made of germanium-antimony-tellurium (GST), the same ground-breaking substance used in rewriteable DVDs. Particles of GST are bombarded onto an electrode, producing a thin flexible film which allows the screen to be flexible. Manufacturers are also able to manually tune the color of each nanopixel, as GST has a specific color depending on its thickness — similar to the technology of interferometric modulator displays (or trademarked as Mirasol).

PCM displays are highly power efficient. Similar to E-ink the pixels are persistent, thus only requiring power when the pixel state requires changing. We may never need a 7000 PPI display on our phones, but the team see them being useful in applications where the devices require magnification, e.g. VR headsets. Phase-changing materials can also change in electrical conductivity, a highly researched area in NAND technology which we'll save for a future article in this series.

IMOD/Mirasol displays

Interferometric modulator displays (IMOD) use a phenomenon which occurs when a photon (light particle) interacts tiny structures of matter causing light interference, inspired by the way butterfly wings are colored. Similar to other displays, each subpixel has its own color that is determined by width of the air gap between the thin film and reflective membrane. Without any power, the subpixels retain their specific colored states. When a voltage is applied, it induces an electrostatic force that collapses the air gap and the subpixel absorbs light. A single pixel is made up of several subpixels, each with a different brightness for each of the three RGB colors, as the subpixels cannot change in brightness like LCD subpixels.

Mirasol displays are in slow production, targeting the e-reader market and wearable technology. Qualcomm recently released their Toq smartwatch which uses the display. Mirasol's low energy persistent pixels and lack of backlighting make it a serious competitor in the colored e-reader industry. The costs of manufacturing the microelectromechanical systems (MEMS) required are still a bit high, however they are rapidly becoming cheaper.

Similar to transflective displays, Mirasol's lack of backlight would make it hard to sell to the general consumer in the current smartphone market. That said, the technology has been used in devices like the Qualcomm Toq, to varying degrees of success.

Flexible OLED

Samsung and LG have been actively racing to advance OLED technology, with both companies putting a lot of investment in the technology. We've seen their curved OLED displays on their TVs and even their phones − the LG G Flex and G Flex 2, Samsung Galaxy Note Edge, etc. Both companies have shown off their translucent flexible displays with LG displaying an 18-inch flexible OLED that can be rolled up into a tight tube just over an inch in diameter.

Despite this display only being 1200×810, LG believe confidently they can develop 60-inch 4K flexible displays by 2017. The scientific breakthrough shown off by this is the flexible polyimide film used as the backbone for the display. Polyimide is a strong yet flexible material that is resistant to heat and chemicals. It's used extensively in electrical cable insulation, ribbon cables and medical equipment. Expect to see more and more of these flexible displays being shown off, but we'll have to wait and see if the costs of production are low enough to be viable in the mobile market.

For more on the most compelling flexible OLED implementation we've seen thus far in a phone, check out Android Central's LG G Flex 2 preview.

The bottom line

By the end of 2015 we should see IGZO LCD panels in some of the Android flagship devices, possibly using quantum dot enhanced backlights. We may also see Mirasol panels become more widely adopted in wearables, giving us the extended battery life we need — however those who prefer the vibrancy of an LCD or OLED panel may not be convinced. There is certainly great variety in the display market — bright, vibrant, high resolution displays on one end and low power, persistent displays on the other.

The mobile display industry continues to progress at breakneck speed, and expanding screen size and pixel densities are only part of the equation.

1. J.S. Steckel, R. Colby, W. Liu, K. Hutchinson, C. Breen, J. Ritter, and S. Coe-Sullivan, 68.1: Invited Paper: Quantum Dot Manufacturing Requirements for the High Volume LCD Market, SID Symposium Digest of Technical Papers, 2013. 44(1): p. 943-945. ↩
2. R. Basu, Effect of carbon nanotubes on the field-induced nematic switching, Applied Physics Letters, 2013. 103(24): p. -. ↩
3. J.H. Ko, I.H. Kim, D. Kim, K.S. Lee, T.S. Lee, J.H. Jeong, B. Cheong, Y.J. Baik, and W.M. Kim, Effects of ZnO addition on electrical and structural properties of amorphous SnO2 thin films, Thin Solid Films, 2006. 494(1–2): p. 42-46. ↩ ↩
4. P. Hosseini, C.D. Wright, and H. Bhaskaran, An optoelectronic framework enabled by low-dimensional phase-change films, Nature, 2014. 511(7508): p. 206-211. ↩