Liquid-crystal displays

Liquid-crystal displays have become the image-display technology of choice, following a long chain of physics-based R&D initiated by pioneering work in the UK.

What is an LCD?

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A liquid-crystal display is a type of electrically generated image shown on a thin, flat panel. The first LCDs, seen in the 1970s, were tiny screens used mostly in calculators and digital watches displaying black numbers on a white background. Today, the latest LCD flat-panel TVs, which have largely replaced the traditional, bulky cathode-ray-tube kind, can produce high-definition colour images up to 108 inches on the diagonal. LCDs are now found everywhere – in home-electronics systems, mobile phones, cameras and computer monitors, as well as watches and TVs.

The technology is based on remarkable electrically sensitive materials called liquid crystals, which flow like liquids but have a crystalline structure. In crystalline solids, the constituent particles – atoms or molecules – sit in regular geometrical arrays, whereas in the liquid state they are free to move about randomly. Liquid crystals consist of molecules – often rod-shaped – that organise themselves in the same direction but are still able to move about. It turns out that liquid-crystal molecules respond to an electrical voltage, which changes their orientation and alters the optical characteristics of the bulk material. It is this property that is exploited in LCDs.

An LCD display panel, on average, consists of thousands of picture elements (“pixels”), which are individually addressed by a voltage. They have become popular because they are thinner, lighter and have a lower voltage of operation than other display technologies, and they are perfect for battery-powered devices, for example. However, behind the large colour LCD TVs, which are now available in every superstore, are several decades of R&D. Physicists, chemists and technologists working together have had to solve many problems.

The science

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Liquid crystals were discovered by accident in 1888 by Austrian botanist Friedrich Reinitzer. He showed that a plant derivative, cholesteryl benzoate, had two melting points, becoming a cloudy liquid at 145 °C and turning clear at 179 °C. To seek an explanation, he passed his samples to physicist Otto Lehmann. Using a microscope fitted with a heating stage, Lehmann showed that the in-between cloudy state had optical properties typical of some crystals, yet was a liquid – and so the term “liquid crystal” was born.

It is understood that most liquid crystals, like cholesteryl benzoate, consist of molecules with long, rod-like structures. It is the combination of the attractive forces that exist between all molecules coupled with the rod-like structure that causes the liquid-crystal phase to form. However, the interaction is not quite strong enough to hold the molecules firmly in place. Many different kinds of liquid-crystal structures have since been discovered. Some organise further into layers, while others are even disc-shaped and form columns.

Throughout the 1920s and 1930s, researchers studied the effects of electric and magnetic fields on liquid crystals. In 1929, Russian physicist Vsevolod Freedericksz showed that liquid-crystal molecules, in a thin film sandwiched between two plates, changed their alignment when a magnetic field was applied. This was the forerunner of the modern voltage-operated LCD. The first patent for a liquid-crystal device was taken out by the UK Marconi Wireless Telegraph company in 1936. However, it was not until after the Second World War that LCDs generated serious interest. As physicists started to develop ever-smaller electronic devices and integrated circuits for everyday appliances, it became clear there was a need for a compatible display technology. LCDs became a candidate.

The first devices, which were developed in the late 1960s, consisted of a thin film of liquid crystal sandwiched between glass slides coated with transparent electrodes. An applied electric field disrupted the liquid-crystal alignment, transforming its appearance from transparent to opaque. These and subsequent devices were rather sensitive, for example, to temperature and did not last long. However, the breakthrough came in the UK when physicist Peter Raynes at the Royal Signals and Radar Establishment (RSRE) collaborated with chemists George Gray and Ken Harrison of the University of Hull in developing novel LCD materials that worked, were stable at room temperature and were suitable for mass-production. This interdisciplinary collaboration was crucial in advancing LCD technology. The RSRE research programme led by a physicist, Cyril Hilsum, resulted in a number of key device inventions, including the supertwisted nematic LCD, thin-film transistors (TFTs) for driving LCDs, the defect-free twisted nematic device and the zenithal bistable display. TFT LCDs, which incorporate a thin-film silicon transistor, are now the main technology used in TVs and computer monitors.

Current developments

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UK physics research groups have continued to work on improving liquid-crystal technology to produce faster, more stable LCDs with wider viewing angles. So-called ferroelectric liquid crystals promise to produce faster-responding displays, and liquid crystals are being combined with carbon nanotubes with the aim of creating new types of optical device, such as three-dimensional displays. Based on university research, Scottish company Exxelis has developed LCD back-lighting technology that is four times as efficient and will substantially improve picture quality and prolong battery life. Meanwhile, a Japanese electronics company is devising highly reflective LCDs that do not need backlighting at all. Another organisation, ITRI in Taiwan, is making ultrathin, lightweight, flexible LCD displays built on a plastic substrate.

UK physicists have also played a major part in developing newer, competing display technologies. Richard Friend and his team at the Cavendish Laboratory at the University of Cambridge pioneered the development of polymer organic light-emitting diodes (P-OLEDs) in the late 1980s while undertaking basic research into the physics of conducting organic polymers. P-OLEDs are now being produced by the university spin-out company Cambridge Display Technology. They are currently used in small displays, such as mobile-phone screens, but large TVs are on the horizon. They promise to be thinner and more efficient, though may not have the lifetime of LCDs.

Impacts

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The technology behind the evolution of LCDs from simple displays for watches and calculators into fast colour displays for mobile phones, computer monitors and TVs has generated substantial revenue for the UK, through sales of materials and royalty income from device patents. One of the inventions at the RSRE – the so-called “supertwist display” – has resulted in royalty income to the UK in excess of £100 m.

The pace of technological development since the early 1990s has been fast, and it continues to increase. The global market for LCDs is currently approaching $100 bn, having grown from around $60 bn in 2005 ($24 bn in 2003). LCD manufacture is truly global, with display production concentrated in the Far East and growing in central and eastern Europe. US firms lead the way in production technologies.

LCDs now hold a dominant position in the displays market, challenged in only a few niche areas, and this is unlikely to change in the near future.


For further information about this case-study, contact Tajinder Panesor.