Most mid-range TVs and computer monitors today are called LED displays, but this is an inaccurate shorthand. The LED, or light-emitting diode, is just the light source. An array of LEDs sits behind the picture, or around the edges with special diffusers, to illuminate the image.
The pixels themselves are still generated by an old-fashioned liquid crystal display (LCD). This is because traditional semiconductor LEDs are much too big to work as individual screen pixels in an average TV.
Organic LED (OLED) technology is different. The organic chemicals that generate the light can be produced in very thin films. So thin, in fact, that they can actually be printed into the screen matrix using a machine like an inkjet printer – except that it prints pixels instead of ink dots. The limitation on how small you can make the pixels depends not on the size of the OLEDs themselves, but on how tightly you can pack together the grid of transistors to switch them on and off. This is the same grid that LCD screens use, so the manufacturing process already exists. OLED screens have the same pixel resolution as LCDs, however they are drastically thinner. Not only are the OLED pixels themselves thinner, but you don’t need to allow room for a backlight; the OLEDs are their own light source.
These two factors mean that OLED screens are up to a third of the thickness of today’s slimmest TVs. Early OLED displays suffered from relatively short life spans – especially in the blue pixels – but new organic molecules that produce brighter, truer colours and last far longer are being developed. As far as TVs go, the future isn’t just bright – it’s ultra-thin too.
Most OLED displays are made by depositing the different layers on a sheet of glass that forms the front of the screen. But if you swap the glass for a plastic, like PET, you can make OLED screens that are even thinner, and flexible enough to wrap around a pencil. It’s a bit more complicated than that, obviously, because the electronics that control the OLED pixels have to be flexible as well. The organic molecules themselves also degrade when exposed to air and moisture so a flexible screen must be very tightly sealed. Nevertheless flexible screen technology opens up the possibility of manufacturing displays in a continuous roll, with the components ‘printed’ directly onto the plastic like an inkjet printer. And soon, quantum-dot (QD) displays -which use nanocrystals of cadmium selenide instead of organic polymers – will make it even easier to print the thinnest, most flexible displays yet.
TV tech translated
Cathode ray tube (CRT)
Old-style TVs use a heavy glass vacuum tube chamber. Electrons are generated at one end by heating a metal filament – just like a light bulb. Electromagnets accelerate and steer the beam of electrons (cathode rays) so that they strike a grid of phosphor dots which causes them to glow.
Liquid crystal display (LCD)
Liquid crystals change the polarization of light passing through them. Pixels are activated or deactivated by rotating the liquid crystal molecules with an electric field. A polaroid filter at the front blocks out all the light shining through the pixels, except the ones that have been rotated correctly.
Organic light-emitting diode (OLED)
Certain organic compounds and polymers will emit light when you run an electric current through them. Sandwiching a layer of this compound between two electrodes enables you to turn them on and off. Different materials illuminate in different colours.
Plasma display panel (PDP)
Each pixel is essentially a tiny fluorescent light bulb. A low-pressure mixture of xenon and neon gases is ionized to a plasma by an electric field and the colliding gas atoms emit a small amount of ultraviolet light. This strikes a fluorescent coating at the front to convert the UV into visible light.
Facts about OLED
Miniature electron guns – FEDs use electron beams, like a cathode ray tube, generated from thousands of tiny semiconductor guns, rather than a single big one.
Ultra-thin – Without a bulky vacuum layer or a separate backlight, OLED screens can be much slimmer.
Cathode – Highly reactive metals, like barium and calcium, are used for the negative electrode. An aluminium layer protects them.
Emitted light – Each pixel is made of three subpixels – one for each of the RGB colours.
Anode – Indium tin oxide provides the positive electrode for the OLED. It’s transparent so it can go at the front.