Liquid crystal display (LCD) screens are omnipresent in our technologically saturated lives. Having largely displaced the older CRT screens, LCDs make up a large portion of the digital imaging we see all around us. They are present in everything from our ubiquitous smartphones and computer screens to digital billboards, wristwatches and vehicle and aircraft instrumentation. How do flat panel LCD screens work? One can often find videos and articles documenting eager teardowns of new devices, but few delve beyond the hardware and into the materials and design behind the screen itself. For this series of blog posts, we will be review some aspects of display technology , why they work, and how they are assembled. To illustrate my points, I’ve disassembled a legacy Sony Ericsson® S500i phone and subjected it to optical microscopy with a at various levels of magnification. Figure 1 shows the S500i displaying a solid-white screen at 400 times magnification.
Figure 1 shows the red, green, and blue subpixels that are in the screens on our devices (a “pixel” usually refers to the set of colored subpixels). My S500i was old enough that one could make out the different subpixels with the naked by peering closely; these days individual pixels are too small to see by visual inspection. One might be curious about why screens have red, green, blue and blue pixels? Most people are probably familiar with the color wheel that uses red, yellow and blue (RYB) as primary colors, taken as common knowledge and still used by many painters and artists. One the other hand, lighting, display, and online graphics industries use red-green-blue (RGB), as it produces a larger range of colors than RYB. In color theory parlance, this set of colors is called the gamut. RGB is specifically designed to capitalize on the three types of color receptors (cones) in our eyes, which detect red, green and blue light. The most effective way to cause our eyes to see colors is to trigger those cones directly with their specific wavelengths.
Since LCDs displays are casting light to create an image, they use the RGB color combination as an additive color model. You might recall that white light is composed of all colors. For additive coloring, one starts with darkness and adds primary colors together in certain ratios and intensities to recreate the color spectrum. RGB displays function just like this, adding just red, green and blue light in various intensities to recreate a complex image. For instance, figure 2 shows a typical smiley emoticon 🙂 at 100x magnification. One can see that where the yellow face is, only the red and green subpixels are lighting up. Where the image is black, the light is cut off. Where the image is white, all three colors are shining equally.
The colors visible on an unpowered LCD screen (like Figure 3) are not due to the eponymous liquid crystals themselves, instead coming from color filters placed in front of a white backlight. The liquid crystals (LC) are somewhat unique from a materials science perspective, and serve a more interesting purpose in the display. These substances exhibit a mixture of characteristics of both liquids and solid crystals. For instance, the molecules in an LC arrange themselves in a predictable order, like a crystalline solid. However, those molecules can also flow and reform their order in response to external stimuli (like force, temperature or electric charge), like a liquid. That changing molecular structure can affect the material’s properties.
A common type of LCD screens, called “twisted nematic” displays, use a type of liquid crystal that “twists” the polarization of light passing through, to a degree which changes in response to an externally supplied voltage. These displays mount the liquid crystal between two polarized filters arrayed with perpendicular polarization. Without the LC in place, light would not be able to pass through both. However, in the uncharged state, the LC’s electrodes are arranged such that the LC forms a helix and twists the polarized backlight by 90°, so that light can pass through the front polarized filter without obstruction. As the driver circuit applies voltage to the subpixel, however, the crystal’s helix structure breaks down as the crystals align themselves parallel with the electric field. At a certain voltage, the helix breaks down completely. With nothing to twist the polarized backlight, terminates at front filter, leaving a black area. By applying different voltages, the circuit driver can open and close the light gate to admit any amount of light through the color filter. This method requires a single transistor for controlling voltage to the subpixel, and is the kind of display my old S500i used.
A similar type of LCD method called “in-plane switching” (IPS) breaks down the helix in the same fashion, but with an electric field perpendicular to that used in twisted nematic. IPS requires a more complicated circuit, but allows for better display quality. Other variations on the type of liquid crystal, the direction of electric field applied, and the mechanical structure of the electrodes and substrates have yielded various types of improved LCD displays with better blacks, better viewing angles, and faster refresh rates.
Complex Materials Science in a Tiny Package
The pixels in the 10-year-old Sony S500i™ are about 120 x 120 microns (0.12 mm x 0.12 mm). For reference, a single one of the pixels visible in the above images could fit about 150-200 8-micron-diameter red blood cells on it. The pixels on my current smartphone are each about 4.7 x 4.7 microns, covering a quarter of a red blood cell. The traces, transistors and capacitors that power each subpixel are even tinier than that, as one can see in Figure 4. Display manufacturing uses technology that is similar to semiconductor fabrication techniques of a few decade past. Semiconductor processing achieved features below 1 micro-meter over 20 years ago. However, displays are manufactured on enormous substrates, much larger than the silicon wafers used in semiconductor processing. In the next blog, I’ll be covering the materials and methods used to assemble such delicate components.