Liquid-crystal display

• 1. Introduction

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  A liquid-crystal display does something strange for a screen. It makes no light of its own. The liquid crystals at its heart do not glow. Instead they twist, untwist, and bend light that comes from somewhere else, a backlight behind the glass or a reflector that borrows the light already in the room. Combine those crystals with polarizing filters, and you can hold an image, a word, a digit, or a full-color television picture in a panel thin enough to hang on a wall. The story begins with cholesterol pulled from carrots and a chemist who noticed it melted twice. It runs through a Swiss patent filed in 1970, a milky display invented at RCA, and a price-fixing scandal that drew fines on three continents. It ends with China holding 40 percent of the world market. Along the way, an everyday object that replaced the heavy cathode-ray tube reveals a far stranger lineage than its flat, quiet surface suggests. How does a material that emits no light come to show you everything? Who fought over the patents? And why does a screen full of chemicals matter to the climate?

• 2. Twist And Untwist

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  Each pixel of an LCD holds a layer of molecules pressed between two transparent electrodes, often made of indium tin oxide, and squeezed between two polarizing filters whose axes usually sit perpendicular to each other. Without the liquid crystal in the gap, light through the first filter would simply be stopped by the second. The crystal is what lets light slip through. In a twisted nematic device, the surface alignment at the two electrodes runs perpendicular, so the molecules coil into a helix. That twist rotates the polarization of incoming light, and the pixel looks gray. Apply enough voltage and the molecules in the center of the layer untwist almost completely. The light is no longer rotated, so it meets the second filter crossed and is blocked, and the pixel turns black. By dialing the voltage up and down across each pixel, the display lets through varying amounts of light, producing every level of gray in between. There is a hidden hazard in this arrangement. The liquid crystal and the alignment layer both contain ionic compounds. Hold a field of one polarity too long and those ions drift to the surfaces and degrade the device. Engineers dodge this by feeding the panel alternating current, or by flipping the polarity each time a pixel is addressed, since the crystal responds the same either way. The display can be built normally on or normally off depending on how the polarizers are set. A character positive LCD with a backlight shows black lettering on a lit background. A character negative LCD inverts that, with letters glowing in the backlight color against black.

• 3. Painting In Subpixels

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  Color arrives in an LCD not from the crystals but from filters laid down red, green, and blue. Those filters are built by a photolithography process on large glass sheets, later glued to other sheets carrying a thin-film transistor array, spacers, and the liquid crystal itself. Several panels are formed at once, then cut apart and laminated with polarizer sheets. The pigments are extraordinarily fine. Each colored photoresist carries ground powder with particles just 40 nanometers across. The black resist goes down first, forming a grid the industry calls a black matrix. That grid walls off the red, green, and blue subpixels from one another, lifting contrast and stopping light from one subpixel bleeding into its neighbors. After the black resist dries in an oven and is exposed to ultraviolet light through a photomask, the unexposed areas wash away, leaving the grid. The same steps repeat for each color, filling the holes. The black matrix has its own history. In the 1980s and 1990s, when most color LCDs went into laptops, the matrix was made of chromium for its high opacity. Environmental worries pushed manufacturers to switch to a black photoresist using carbon pigment instead. Not every early color screen worked this way. Some color personal digital assistants and calculators varied the voltage in a super-twisted nematic display, where the changing twist produced varying birefringence and shifted the hue. Those were usually limited to three colors per pixel: orange, green, and blue.

• 4. Carrots And Williams Domains

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  In 1888, Friedrich Reinitzer found that cholesterol extracted from carrots behaved oddly, melting at two points and throwing off colors, and he published what he saw. Otto Lehmann named the phenomenon in 1904 with his work "Flussige Kristalle," or Liquid Crystals. Charles Mauguin trapped liquid crystals between plates in thin layers in 1911, and in 1922 Georges Friedel sorted them into nematics, smectics, and cholesterics. The leap from curiosity to switch came in 1927, when Vsevolod Frederiks devised an electrically switched light valve, the Freedericksz transition that underlies all LCD technology. The Marconi Wireless Telegraph company patented the first practical use, "The Liquid Crystal Light Valve," in 1936. In 1962, Dr. George W. Gray published the first major English-language book on the subject, and that same year Richard Williams of RCA found that liquid crystals had electro-optic tricks worth chasing. Williams generated stripe patterns in a thin layer by applying a voltage, an effect rooted in an electro-hydrodynamic instability. The stripes still carry his name as Williams domains. RCA was busy on another front too. Building on early MOSFETs, Paul K. Weimer developed the thin-film transistor there in 1962, a device that would later let each pixel carry its own switch.

• 5. The Milky Screen And The First Watch

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  George H. Heilmeier, working at the RCA laboratories on Richard Williams' effect, switched colors in 1964 by field-induced realignment of dichroic dyes in a homeotropically oriented liquid crystal. Practical trouble pushed him toward scattering effects, and he arrived at the first working liquid-crystal display, driven by what he called the dynamic scattering mode. Apply a voltage and a clear transparent layer turns milky and turbid. These displays could run in transmissive or reflective mode, but they drank a heavy current. Heilmeier was inducted into the National Inventors Hall of Fame and credited with inventing the LCD, and his work stands as an IEEE Milestone. The dynamic scattering mode did not last. On the 4th of December 1970, Hoffmann-LaRoche in Switzerland filed for patent on the twisted nematic field effect, naming Wolfgang Helfrich and Martin Schadt as inventors. Hoffmann-La Roche licensed it to Brown, Boveri and Cie, which produced TN displays for wristwatches through the 1970s, feeding the Japanese electronics industry and its first digital quartz watches. James Fergason, working with Sardari Arora and Alfred Saupe at the Kent State University Liquid Crystal Institute, filed an identical patent in the United States on the 22nd of April 1971. His company ILIXCO produced TN displays that soon beat the poor dynamic scattering types, thanks to lower operating voltages and lower power draw. The first wristwatch with a TN-LCD reached the market in 1972, the Gruen Teletime, a four digit display. Behind the watches sat a crucial chemistry breakthrough. The Royal Radar Establishment at Malvern, England, supported George William Gray and his team at the University of Hull, who discovered the cyanobiphenyl liquid crystals with the right stability and temperature behavior for real displays.

• 6. Building The Active Matrix

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  The idea of a TFT-based liquid-crystal display was conceived by Bernard Lechner of RCA Laboratories in 1968. That year Lechner, F.J. Marlowe, E.O. Nester, and J. Tults demonstrated it with an 18x2 matrix dynamic scattering display using standard discrete MOSFETs. The active-matrix concept took shape in 1972, when T. Peter Brody's team at Westinghouse in Pittsburgh prototyped a thin-film transistor liquid-crystal display panel. In 1973, Brody, J.A. Asars, and G.D. Dixon demonstrated the first TFT LCD, and in 1974 Brody and Fang-Chen Luo showed the first flat active-matrix version. Brody coined the term "active matrix" in 1975. Japan turned these prototypes into an industry. Sharp Corporation patented TFT-LCDs in 1976 with a team of Fumiaki Funada, Masataka Matsuura, and Tomio Wada, improved in 1977 by Kohei Kishi, Hirosaku Nonomura, Keiichiro Shimizu, and Tomio Wada, though material problems kept them out of products for a time. The first color LCD televisions appeared as handheld sets in Japan. In 1982, Seiko Epson released the Epson TV Watch, a wristwatch carrying a small active-matrix LCD television. In 1984 came the ET-10, the first full-color pocket LCD television, and the same year Citizen Watch introduced the 2.7-inch Citizen Pocket TV with the first commercial TFT LCD. In 1988, Sharp demonstrated a 14-inch active-matrix full-color full-motion TFT-LCD, the step that launched a full Japanese LCD industry. Epson's VPJ-700, released in January 1989, was the world's first compact full-color LCD projector. In an active-matrix display, a transistor sits at every pixel. When a row line is selected, the column lines drive picture voltages onto a whole row of pixels, the row deactivates, and the next is chosen, sweeping through every row in sequence during a refresh. Such displays look brighter and sharper than passive-matrix screens of the same size and switch faster.

• 7. Wider Angles And Sharper Pictures

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  In 1983, researchers at the Brown, Boveri and Cie Research Center in Switzerland invented the super-twisted nematic structure for passive matrix displays, with H. Amstutz among the named inventors on applications filed that July and October. The same era saw Philips solve the motion problem. In 1984, Philips researchers Theodorus Welzen and Adrianus de Vaan devised a video speed-drive scheme that cured the slow response of STN-LCDs. In 1985, the pair cracked driving high-resolution STN-LCDs with low-voltage CMOS electronics, opening the door to notebook computers and mobile phones. In 1990, inventors looked past the twisted nematic family entirely. One approach placed interdigital electrodes on a single glass substrate to create a field running parallel to the glass, the In Plane Switching technique. Guenter Baur and colleagues at the Fraunhofer Institute ISE in Freiburg published the advantageous designs, with patents assigned to Merck KGaA in Darmstadt. In 1992, engineers at Hitachi worked out the practical wiring of IPS and curbed stray fields between pixels, and Sharp introduced the first wall-mountable LCD TV that year. Hitachi pushed further with Super IPS by reshaping the electrodes, and in 1996 Samsung developed the optical patterning that enabled multi-domain LCDs. The competition between panel types runs deep. Twisted nematic displays are cheap and fast but suffer narrow viewing angles and limited color accuracy. IPS aligns the crystals parallel to the glass, costs more, and needs two transistors per pixel, yet it became the benchmark for color accuracy and reliability, and now drives nearly all LCD smartphone panels. Vertical alignment displays let the crystals stand perpendicular to the glass at rest, giving deeper blacks and a higher contrast ratio than twisted nematic, at the cost of slower response. In 2016, Panasonic developed IPS LCDs with a contrast ratio of 1,000,000:1, rivaling OLEDs, later mass-produced as dual-layer panels using two liquid crystal layers.

• 8. Prices, Power, And A Missing Gas

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  From 2001 to 2006, Samsung and five other major companies held 53 meetings in Taiwan and South Korea to fix prices across the LCD industry. The six firms were fined 1.3 billion dollars by the United States, 650 million euro by the European Union, and 350 million RMB by China's National Development and Reform Commission. The market itself kept moving. In 2007, the image quality of LCD televisions surpassed that of cathode-ray-tube sets, and in the fourth quarter of that year LCD TVs outsold CRTs worldwide for the first time. The industry that began in Switzerland and matured in Japan drifted to South Korea and Taiwan in the late 1990s, then to China. By the 2020s, China was the largest LCD maker, with firms like BOE Technology, TCL-CSOT, TIANMA, and Visionox holding a 40 percent global share, aided by local government investment. The efficiency story is enormous. LED dynamic backlight control and reflective polarizers, including the DBEF films supplied by 3M, made LCD televisions far more efficient than CRT sets. The source ties this to a worldwide energy saving of 600 TWh in 2017, equal to 10 percent of household electricity consumption worldwide, or twice the output of every solar cell on Earth. There is a darker chemical footnote. LCD production uses nitrogen trifluoride as an etching fluid for the thin-film components, and NF3 is a potent greenhouse gas with a long half-life. A report in Geophysical Research Letters called it the missing greenhouse gas, since it was left out of the Kyoto Protocol before being added for the second compliance period during the Doha Round. Critics noted the report assumed all NF3 reached the atmosphere, when two earlier studies found only 2 to 3 percent escapes destruction, and that it replaced perfluorocarbon, of which 30 to 70 percent escapes in typical use.