Editorial Feature

An Introduction to Materials on Display

Apr 11 2001

Image Credits: GaudiLab/shutterstock.com

While at one time TVs that hang on the wall like a portrait were predicted only to be part of our distant future, they are now a regular sight in most homes.

Now there is a real consumer demand for high information content displays in the 50cm to 100cm diagonal size range. It has been reported that sales of LCD TVs worldwide were worth $106bn in 2016, but the smartphone market exceeded that figure by far and reached over $200bn in 2013.

In many ways, the standard has been set by the once ubiquitous cathode ray tube (CRT), which had long dominated the domestic market. The CRT was an affordable and reliable means of generating bright moving images, only let down by being as deep as it was wide. However, the ever-widening screens became increasingly bulky, heavy and difficult to manufacture, and is it thought the last manufacturer of CRTs closed down in 2015.

Liquid Crystal Displays – LCD’s

The most successful flat panel display (FPD) technology to emerge since is the active matrix liquid crystal display (AMLCD) found in laptop computers, space-saving desktop monitors and many other consumer electronic items. Compared to domestic television, these screens suffer from a poor viewing angle, and their brightness relies on an energy-sapping color filtered backlight.

They require three thin film transistors positioned at each color pixel (one to control each primary color), so the display is akin to a very large area memory chip. Producing a defect-free device over such a broad area, in a clean room where dust particles are present, is a real challenge. Not only that but as the size extends, the capital equipment costs rise very steeply.

Some of the tolerances necessary for the liquid crystal to operate reproducibly are tight, requiring extremely flat glass substrates and good registry between the numerous deposition, photolithography, and etching steps. For example, the metal tracks are deposited by magnetron sputtering, the transparent indium tin oxide electrodes are produced using reactive ion sputtering, and the transistors are made using plasma enhanced chemical vapor deposition at around 300°C.

Solid State Emissive Technologies

The life of the LCD for sub 50cm screens may also be under threat from alternative lower cost color solid-state emissive technologies on the horizon. These include Cambridge Display Technology's light emitting polymers and Opsys's organolanthanide phosphors. They can both be deposited with relative ease over large areas using spinning and printing techniques.

The main problem in most solid-state emissive devices has been efficiently generating a pure and intense primary color to give a good color balance. Achieving this has demanded understanding and tuning the electronic properties of these materials, and being able to deposit them without introducing detrimental changes. The organolanthanides being developed by Opsys overcome these problems because the lanthanide metal determines the color of the emission and provides a highly pure color, while the organic ligands determine the deposition conditions.

Plasma Display Panels – PDP’s

At the other end of the size scale are the plasma display panels (PDPs), now finding application as information boards at major transport hubs such as Paddington Railway station in London (UK). These are essentially an array of ‘neon lamps’, generating ultraviolet radiation that excites a phosphor. Their adoption into the home has been hindered by their high cost, due mainly to the relatively high power electronic circuits needed to control the plasma discharge at each pixel, which make them both expensive to produce and inefficient to operate. Striking a bright and stable plasma also becomes problematic as the screen size drops and the pixels necessarily become smaller. PDPs currently tend to be in the 100cm to 160cm diagonal range, and even then they can suffer from poor color depth and unacceptable image stability.

Despite these limitations, PDPs have driven forward the market for broad area displays, and in doing so have solved the problems of fabricating and sealing thin, broad area vacuum tube-like glass vessels. PDP manufacturers have also pioneered the screen-printing of the electrodes and dielectric materials that make up the devices, moving away from semiconductor processing. Not only does this prove that this alternative approach is viable, but it also shows it to be a much lower cost and higher throughput solution, effective because the large pixel size implied by the large screen greatly eases the lithographic tolerances.

Field Emission Display – FEDs

The field emission display (FED) has successfully replaced the CRT and solved its problem with thinning down while simultaneously needing to become larger. The key lies in giving each pixel separate electron guns situated very close behind the phosphor-coated screen. Conventionally, these guns have been fabricated using the Spindt process, in which arrays of small sharp silicon or molybdenum cones are deposited onto a substrate within an etched hole. The result is a triode structure of between a few and less than one micron in diameter, of which there are thousands per individual pixel. Electrons can leave the sharp tips with relatively low extraction voltages at the gate. The advantages are CRT-like viewing characteristics using mature phosphor technology for the anode, and energy efficient low voltage control at the gate. FEDs have been taken over by LCD displays.

LCD

LCD monitors put out a higher brightness than CRT, and are well suited to brightly-lit environments. They also produce significantly lower electric, magnetic and electromagnetic fields than CRT, however, the resolution and aspect ratio are fixed with LCD screens. LCD screens can suffer from broken or ‘dead’ pixels which are either always on or off, due to being improperly connected.

OLED and QLED

OLED (organic light emitting diodes) is a screen made up of thin sheets of organic electroluminescent material. These sets are extremely expensive, but a growing competitive market means more affordable OLED sets are becoming available. OLED screens operate without the need for a backlight. This is because the carbon film inside the panel emits its own light, and the organic compounds light up individually when powered by electricity. One OLED is the size of one pixel, so a single TV screen has millions of OLEDs. OLED TVs can be manufactured to be thinner than QLEDs.

One drawback of OLEDs is that the backlight can bleed through dark areas of the screen despite the advance in dimming technology made to reduce the visibility of the backlight in TV screens.

QLED (quantum dot LED) displays are designed to display brighter and richer colors than OLED, which has the potential to work well in tandem with HDR content. Quantum dots are particles that are a nanometer in size and act like a filter that produces clearer light than LEDs can on their own. They do not directly emit colors themselves, but through LED backlights where the light is refined to the right color temperature. Manufacturers also claim that QLED TVs can produce a wider range of colors than LED TVs not using quantum dots.

Like LCD screens, QLED screens can suffer from poor angle viewing, and the best view is from the center.

In terms of size, OLED screens can be made as large as 88 inches, but QLED can be made over 100 inches.

The Future in 4K Ultra HD and HDR

The majority of households use TV sets with a pixel resolution of 1920 x 1080. However, there is movement towards 4K Ultra HD, which would see screens of 3840 x 2160. These screens would offer four times the detail if the footage being screened is also filmed at 4K resolutions. The platforms for the content of this resolution have been available for some time, but filming equipment has only just caught up. Additionally, 4K TV sets are becoming more affordable, which will see them enter more and more homes in the near future.

Challenging 4K is HDR (high dynamic range), which is capable of showing a wider range of colors and higher contrast, producing lifelike images. It is argued that HDR will do more for image quality than an increase in resolution. However, HDR content isn’t as widely available as 4K.

It may still be years until 4K or HDR viewing becomes commonplace in households, but a shift in viewing technology should be expected as the competition between manufacturers continues alongside development and innovation in the area.