Quantum dots have played an important role for some time in the search for screen technology that enables our televisions to display increasingly realistic images. We show you how quantum dots in a television work, what their advantages are and how they are used today and in the future.
What are those quantum dots in a television?
Quantum dots are semiconductor crystals with a size of 2 nm to 6 nm. How small is that? A human hair is about 20 to 100 µm in size, that is 20,000 to 100,000 nm! Those tiny crystals have a unique property: they convert incident light into light of a different color. The generated color depends on the size of the quantum dot. The smallest ones provide blue light, and the largest ones provide red light.
Moreover, those colors are very pure. In other words; if we plot them on a spectrum diagram (as above), they have the shape of a narrow peak, the light contains only a limited selection of frequencies. Because the size of those quantum dots can be determined very precisely, it is also possible to choose very precisely which (very pure) colors are created. And that is a very popular feature for our televisions.
Why do we want pure colors?
Those pure colors are a requirement if we want televisions with a larger color range. If you want to read what a color range now means correctly, reread the article about color range and Wide Color . Each pixel on your television is divided into three sub-pixels, a red, green and blue. These basic colors can be found on the chromaticity diagram as the points of the triangle that determine the color range. The television can create all the colors that lie within the triangle.
If we take a look at the required base colors of Rec2020, the color standard that we want to move towards, you will notice that the three base colors are much closer to (or even on) the edge of the horseshoe-shaped area. And that edge is like monospectral colors, or colors that contain exactly one frequency. However, the spectrum of a typical white LED in an LCD backlight does not contain three beautiful peaks, but rather a blue peak and a large green-yellow-red peak. The color filters at the front of the screen convert that to blue, red and green, but the latter two are less sharply defined peaks (and therefore less pure). The figure below makes that clear. The filtered out part of the spectrum is highlighted in light gray. The broad green and red peak indicate less pure colors.
To make truly monospectral light we need a laser. But with quantum dots we can in any case already make much purer light (much clearer peaks in the next figure), and we are taking a big step towards the Rec2020 color range.
Advantages and disadvantages of quantum dots
The most important asset of quantum dots is of course the color range they provide. But they are not unique in that regard. OLED also provides a larger color range. And even with traditional white LED backlighting, a wider color gamut can be achieved. The ordinary white LEDs (actually blue LEDs with a yellow phosphor layer) are then replaced by blue LEDs with a red and green phosphor layer (we call these ‘wide color LEDs’). For the time being, quantum dots seem to have the best chances of achieving the full Rec2020 color range. But which technology will ultimately deliver the richest colors depends on further developments.
A second advantage: quantum dots are relatively easy to install in current LCD screens. This can be done in various ways, more about that later. It is expected that the price of installing quantum dots will drop significantly in the coming years, so that they will not only be used in premium screens. Quantum dots have an excellent lifespan. Finally, they allow a high light output, which is not an unimportant point in these times of HDR TVs, and in that area they have an advantage over OLED.
Of course, no technology has any advantages. In the first place, a TV with quantum dots remains an LCD TV, with all known weaknesses. That means a limited viewing angle and not perfect black levels. These are points where LED scores better on.
How are quantum dots put in a TV?
In today’s televisions there are two ways to use quantum dots: in a tube or in a film. In both cases, the LCD TV uses a modified backlight, but remains identical in construction to other LCD TVs.
We already saw the implementation with a tube in 2013, in the then Sony W9 and X9 Triluminos screens. The technique can only be used with edge LED backlighting. Instead of white LEDs, they now use blue LEDs. A tube with red and green quantum dots (hence the yellow color) is mounted above the LEDs. That combination then provides the desired white light.
The implementation with a film can be used on an edge LED as well as direct LED backlighting. This is used by Samsung, for example. Again, blue LEDs are used instead of white LEDs. The foil with quantum dots (QDEF: quantum dot enhancement foil) is applied over the entire surface of the backlight and again consists of red and green quantum dots. Together with the blue LEDs, it generates the desired white light.
To explain the function of the full model, and especially to clarify the difference with future implementations, we provide a schematic representation of a pixel below. This illustrates the operation of the LCD TV. The backlight creates blue light and activates the red and green quantum dots. The resulting white light passes through a polarizing filter. The LCD panel (controlled by the transistors) determines whether or not the light passes the last polarizing filter. The color filter converts the white light per sub-pixel to red, green or blue. In essence, the light is created in the backlight and has to pass through all the layers above.
Future use of quantum dots
There have been other methods of using quantum dots for some time now and these could yield enormous benefits. The first option is to use quantum dots as a color filter (abbreviated as QDCF: quantum dot color filter). In that case, the quantum dot layer in the backlight is omitted and the color filter at the front is replaced by quantum dots that are placed in the pattern of the color filter.
That has enormous advantages. For one thing, this isn’t a traditional LCD TV anymore. After all, the light that the viewer sees is generated at the front of the screen by the quantum dots, and not by the backlight. It does generate light, and the LCD panel still determines which pixels emit light, but as soon as the blue light from the backlight reaches the front of the panel it activates the quantum dot layer, which eventually generates red and green (in the blue sub-pixels allow the light from the background lighting to pass through).
Each (sub) pixel therefore emits light itself, which improves the viewing angle enormously. In that respect, the screen can be compared to an OLED screen. It is also much more energy efficient and could deliver even purer colors. The effect on the black value is less easy to predict. After all, it remains an LCD panel, and it cannot deliver perfect black.
QDCF TVs still have to overcome a hurdle: the second polarizer is essential to the operation of the LCD panel. But quantum dots provide non-polarized light, so the last polarizer has to go one step down in the structure. In practice, this means that it has to be processed in the LCD cell. Still, there are indications that such screens would be on the market as early as 2018. Jason Hartlove, CEO of Nanosys, already showed a prototype of such a display at CES 2017.
But quantum dots may go one step further. What if we no longer activated the quantum dots with light, but directly with electricity? In that case, the result is an almost ideal display, which will show even more similarities with OLED. After all, in both cases it concerns an emissive display (each pixel itself emits light) and in both cases this is directly determined by the luminous material, without the intervention of an LCD panel. We call these types of screens QD-LED screens. Such screens would be about five years in the future.
For some extra background, check out this interview with Jaso Hartlove, CEO of Nanosys, at CES2017.
Conclusion
Quantum dots deliver more intense, pure colors that we absolutely need to achieve a wider color range. Current implementations in television use a tube or foil in the backlight. Still, they are essentially LCD TVs. But the future beckons, and color filters based on quantum dots or even electrically controlled quantum dots would create a whole new category of TVs.
