From CRT to OLED and Beyond
Flatscreen TVs are ubiquitous, but even tech-savvy readers might not know of some of the engineering feats that continue to push this technology forward. Here, I cover the history of their development, and look at what’s ahead.
We are not far from the centenary of one of the most important inventions in modern history—one that brought moving pictures to living rooms all across the world. On October 2, 1925, the Scottish inventor John Logie Baird, often called “The Father of Television,” succeeded in demonstrating the first proper television. It was a truly groundbreaking invention that laid the foundation for the British Broadcasting Corporation (BBC) in the years that followed. Recognizing the impact of his invention over the course of nearly a hundred years, the UK Royal Mint recently released a coin celebrating this achievement (Figure 1).
Baird’s original invention was an electromechanical device that was not really suitable for long-distance TV transmission. In later years, with the advent of valve-based electronics, Baird and others re-invented the TV in a form that could be beamed through radio waves to reach audiences over large geographic areas. Later still, the technology embraced solid-state electronics, and even the ability to transmit and display images in color. Thereafter, up until the 1980s TV technology remained relatively stagnant, before another wave of innovations took it by storm.
TV display technology changed dramatically from the late 1980s to the early 1990s when the so-called flat panel displays (FPDs) started to appear. Prior to that, the venerable cathode ray tube (CRT) had served that role since the emergence of TVs, themselves, in the 1930s. The FPDs were immediately perceived as a disruptive technology, because of the several clear advantages they offered over the old CRT displays. The main benefit was easy scale-up of display dimensions. Before the advent of FPDs, increasing the size of CRTs was challenging, because of their complicated construction and method of operation.
This is easy to understand if one keeps in mind that CRTs are basically oversized vacuum tubes. Just like any vacuum tube, CRTs are made from fragile materials, painstakingly assembled together into a heavy and bulky image-display device. Flat displays—not based on the use of accelerated electron beams (cathode rays)—are inherently easier to construct, transport, and use. As it happened, the FPD revolution brought a proliferation of display technologies as the years went by. In what follows, I examine the main features of FPD technologies, focusing on both their operating principles and attendant benefits.
NEON STARTS A REVOLUTION
The first FPD technology was based on plasma display panels (PDPs), which are essentially an array of miniature neon lights. A PDP consists of a two-dimensional array of tiny chambers, each coated with a red, green, or blue phosphor, sandwiched between two thin glass sheets. These dielectric sheets are printed with a pattern of parallel conductive lines, with lines on one sheet running orthogonal to lines on the other sheet. These tracks are called “address and display electrodes.” This assembly is enclosed between a pair of glass sheets and hermetically sealed all around. During the panel sealing process it is filled with a mixture of neon and xenon gases and mercury vapor, at a low pressure.
As shown in the schematic cut-away diagram in Figure 2, the chambers or cells are formed by rib partitions, and each is coated with a thin red, green, or blue phosphor layer. A trio of RGB cells forms a single color pixel, where each R, G, and B sub-pixel is individually addressable. To make any sub-pixel light up, its corresponding row and column tracks are energized, such that a glow discharge takes place at their intersection.
The gas composition and pressure are chosen so that most radiation in the discharge is emitted in the UV region. The UV photons strike the phosphor, and visible red, green, or blue light is emitted. This is similar to the way fluorescent neon lamps operate. The intensity of emitted light can be varied by controlling the voltage on the display electrode, through a technique called pulse width modulation. By controlling the intensities of the red, green, and blue sub-pixels, any desired color can be displayed. The entire display is scanned at high speed to show video frames at 50 or 60 frames per second.
Plasma displays were the leading television display technology during the 1990s. PDPs can be made in very large screen sizes and have many advantages over other display technologies. Perhaps the most important is the extremely high contrast exhibited by plasma panels. Since any pixel can be completely dark when not addressed, PDPs have especially high contrast values. PDPs are also “fast,” because their pixel cells can be switched between lit and un-lit states quickly. For this reason, they do not show any motion blur in high-speed action scenes. PDP screens don’t rely on polarization of light to switch pixels on and off, and thus, can be viewed from any angle. While PDPs offer all these benefits, their sealed, gas-containing construction makes them heavy, fragile, and difficult to assemble. These issues caused their ultimate demise, when other displays that were easier to manufacture started to be commercialized in later years.
LIQUID CRYSTALS ENTER THE SCENE
During the late 1990s, PDP technology began to be supplanted by a different FPD technology. It was based on a light valve or switch that could be made transparent or opaque under electrical control, and relied on control of the polarization of light.
Light consists of oscillating electric (and magnetic) fields that are oriented at right angles to the direction of light propagation. These oscillating fields can be oriented at any angle around the light’s propagation direction. If the oscillations take place along only a certain direction perpendicular to the light’s travel direction, then the light is said to be “plane polarized.” This can be easily achieved by passing ordinary unpolarized light through a polarizing material, such as a sheet of Polaroid plastic, which has a well-defined polarizing direction. Another Polaroid sheet placed close to the first sheet will either allow light to pass through it or get blocked, depending on whether the polarization directions of the two Polaroid sheets are parallel or perpendicular to each other.
This basic principle can be used to construct a display device, by using a Polaroid sheet together with an electrically-controllable, polarization-inducing device. Such a device is made by sandwiching a material called a “liquid crystal” between two sheets of electrically-conducting glass plates. Liquid crystals are materials with long, rod-like molecules that can twist under the action of an applied electric field, and rotate the plane of polarization of light passing through them.
A liquid-crystal display (LCD) panel consists of a two-dimensional grid of liquid-crystal cells that can be controlled individually. Applying a voltage to any cell causes the plane of polarization of light passing through that cell to align with the polarization direction of the Polaroid sheet. This allows the arrangement of the liquid-crystal cell and the Polaroid sheet to become locally transparent to the passage of light. A flat, uniformly-lit white screen placed at the back then becomes visible at that point, displaying a picture element (pixel). If the voltage is removed from a chosen liquid-crystal cell, then light is not able to pass through, and that pixel becomes dark. By selectively turning pixels on and off, both still and moving images can be displayed. Color pictures can be displayed by using red, green, and blue plastic filters, uniformly distributed on the LCD panel so that the pixels appear colored (Figure 3).
LCD technology has now matured to the extent that LCD-based TVs have become commodity products—manufactured in large numbers and sold at affordable prices. Early LCD TVs exhibited problems with LCD cell-switching speeds, which caused “motion streaks” to appear in scenes where moving bright lights were shown. This issue was overcome by the use of better liquid-crystal materials and optimized liquid-crystal cell design. These advances also helped to increase the viewing angle of the displays, so that people not seated directly in front of the screen can also see a normal, undistorted image.
The one problem that has taken longer to solve is the limited image contrast delivered by LCDs. This arises because “off” LCD pixels are not completely opaque, and some light passes through them. Thus, black pixels are not entirely black, but are rather a deep shade of gray. Better materials and device design and optimization have greatly improved image contrast in modern LCD panels, but there is still room for improvement.
Traditional LCD technology has spawned a number of more advanced offshoots that feature better image reproduction. While these are still based on the basic LCD panel, the innovations lie in the construction of the backlight, which differentiates various derivative technologies. This fact has been somewhat cleverly exploited by TV manufacturers by advertising them as distinct TV technologies—LED, QLED, Nanocell, and others. Nevertheless, it is a fact that these are definite improvements over the traditional simple LCD technology. So, let’s explore these more modern variants of the LCD technology.
TVs WITH LED BACKLIGHTING
The original LCD TVs made use of a backlight that was basically just a sheet of diffuse white plastic, lit by slim, cold cathode fluorescent light (CCFL) tubes from the sides. CCFL edge-lit backlights were mass produced in several sizes for the TV industry. With the arrival of high-brightness, white LEDs in the late 1990s and early 2000s, fragile CCFL tubes were replaced by lighter, cheaper, and more controllable white LEDs. Thus began the era of LED backlights— with LEDs first positioned around the edges, and later directly behind the light diffuser screen, as shown in Figure 4.
In this role, LEDs offer several advantages. Low power consumption is at the top of the list. LEDs consume only a fraction of the power dissipated by CCFLs. LEDs also provide better color balance, which makes colors more realistic. But perhaps the biggest advantage offered by LED-backlit TVs is the ability to provide local dimming. These devices can be individually controlled, and their brightness can be made to mimic the dark and bright locations of the image being displayed. LEDs behind dark parts of an image can be dimmed appropriately. This greatly enhances the perceived image contrast. Today, most LCD TVs make use of this feature. Because LED brightness can be modulated at high speeds, this works well with modern image displays with high frame rates.
During the past four years, a new generation of TVs has started using dense, two-dimensional arrays of small LEDs to illuminate their backlights. These “mini-LED” TVs offer the ultimate in local contrast control that is possible with LCD technology. This feature, together with their competitive pricing, has made it one of the best-selling TV technologies of recent times.
QUANTUM DOTS AND NANO PARTICLES STEAL THE SHOW
The RGB color combination scheme lies behind all color display systems. This is how it works. Narrow wavelength spread (high color purity) in the red, green, and blue light components enables the creation of a wider palette of displayed colors, or a “wide color gamut.” Traditional white LED TV backlight technology does not allow narrow wavelengths to be selected by simple filtering of white light. However, TV manufacturers have found a way to achieve primary color sources with narrow wavelength spread by the use of quantum dots.
Quantum dots are very pure, nanometer-sized crystals of substances such as cadmium sulfide, cadmium selenide, indium phosphide, and other binary compounds, synthesized through techniques that produce crystals of precise dimensions. By irradiating quantum dots with blue or ultraviolet light, one can obtain other visible colors that depend on the size of the crystal. The smaller the dimension of the quantum dot crystal, the longer its emitted wavelength. Thus, with increasing size, quantum dot emission shifts toward the red end of the spectrum. The same material, grown in different sizes, can produce a range of emission colors, as shown in Figure 5.
Quantum dots have been used in quantum-dot LED (QLED) displays that benefit from the narrow wavelength emission from quantum dots. These displays use a plastic film containing red- and green-emitting quantum dots. The film is placed in front of a backlight illuminated by blue LEDs. Quantum dots absorb blue light and convert it into red and green light. Together with the residual blue light from the backlight, the combination produces white light with narrow red, green, and blue spectral peaks. The QLED light source thus produces the ideal spectral emission for color image reproduction.
QLED TVs, popularized by Samsung, are renowned for their excellent color rendering. They produce shades that are close to natural, for the rendition of challenging colors, such as skin tones. A comparable technology is that of “Nanocell” displays, developed by LG Electronics. Here, too, a polymer sheet with dispersed nanoparticles is used, but in this case the particles are used for filtering light from a white backlight. This produces a spiky RGB spectrum that can be used for more accurate color rendering than is possible from a simple LED-lit white backlight. Both QLED and Nanocell TVs produce exceptional color reproduction, and have gradually become highly successful products.
TV BECOMES ORGANIC
All FPDs based on the LCD technology utilize a backlight as the source of display illumination. It is light from the backlight that is visible at the location of lit pixels. The LCD panel simply acts as a matrix of light valves that make the backlight locally visible, or not.
In contrast, a self-emissive display has a point source of light at each pixel location that can be selectively turned on or off to display brightness or darkness at that point. Widely used LED matrix boards are examples of self-emissive displays, where each pixel location is an LED, or a trio of RGB LEDs. Implementing this scheme in TVs requires much tougher engineering, because of the need to produce miniaturized, light-emitting elements at individual pixel locations. This approach has been successfully developed through the technology of organic, light-emitting diodes (OLEDs), to make a new breed of self-emissive TVs.
OLED TVs make use of special panels manufactured by LG Electronics. The panels are constructed from electrically conducting sheets of organic, light-emitting semiconductors patterned into arrays of organic LEDs—each paired with a driving transistor that can switch the corresponding LED on or off. Using a so-called “thin film transistor” (TFT) to control each OLED is called an “active matrix approach.” This strategy allows fast scanning of the entire OLED panel, leading to high image-refresh rates. The OLEDs can be made from different materials that can emit red, green, or blue light. The most outstanding advantage of this design is that, when not lit, pixels can be completely dark, and thus can reproduce perfect black levels in scenes being displayed. OLED TVs are, therefore, renowned for their high dynamic contrast levels—something that even the best LCD-based TVs cannot match (Figure 6).
The near-perfect black levels, combined with rich color reproduction, wide viewing angles, high frame-rate capability and the possibility of making thin and lightweight curved screens, all have contributed to the widespread popularity of OLED displays.
Quantum dots are also being integrated with OLED displays, to further enhance color reproduction accuracy. Q-OLED displays could be made from a single blue-color, active-matrix OLED array, with red-, green-, and blue-emitting quantum dots printed on adjacent pixels to provide full-color capability.
Compared to LCDs, OLED TVs command a price premium, due to the involved nature of their production and their somewhat low manufacturing yield. Nevertheless, these displays have grown in popularity, and have become available in ever-larger screen sizes—currently approaching 100 inches!
THE ROAD AHEAD
The television industry thrives on introducing new display technologies through continuous technological innovations. Commercially successful technologies are improved and re-marketed, while new technologies are developed and promoted on a regular basis. This is now a well-established trend, and has resulted in ongoing improvements to display picture quality and related performance metrics.
Micro-LED, self-emissive displays based on tiles of gallium nitride LEDs are the most advanced alternative technology at this time. Beyond it, several new technologies are being investigated in corporate and academic R&D labs. These include the use of laser diodes for providing blue illumination for QLED TV backlights. Replacing blue LEDs with blue laser diodes can improve the spectral characteristics of light by narrowing down the blue component, too. New luminescent materials for generating colored light, such as charge transfer compounds, are also being investigated for electronically altering colors produced at each pixel location, instead of using filters to produce component colors.
There is no doubt that we will continue to see more exciting TV display technologies in years to come—some with capabilities that are hard to imagine today.
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • JANUARY 2023 #390 – Get a PDF of the issueSponsor this Article
About the author
Faiz Rahman, PhD, is a materials and device engineer, working on novel materials and device structures for both electronics and photonics. He develops new LEDs and solid-state lighting concepts at Electrospell and is also a visiting professor in Electrical Engineering at Ohio University. Faiz is a senior member of the Optical Society of America and of IEEE. His current research interests include the development of nano LEDs, plasmonic structures, and advanced nanofabrication technologies.