What is the relationship between the grayscale and brightness of the LED display?

According to Korean media reports, Samsung Electronics’ operating profit in the second quarter of this year may reach a record high of US$11.6 billion, which will help Samsung surpass Apple’s top spot in the world. Even more frightening is that Samsung Q2's operating profit is even higher than the combined operating profits of Facebook, Amazon, Netflix and Google. The total operating profit of the four companies in the first quarter of this year is only 11.15 billion US dollars.

This time we prepared a long article for students who are looking to learn more about the technical details of OLED. Let's talk briefly about one component of the OLED manufacturing process: evaporation. And how vendors like Panasonic can make further cost savings in this part of the process, and how Sony has improved this step.

do you know? Each pixel "bulb" of the OLED screen is steamed up.

So what exactly is evaporation? This has to start with the structure of OLED. As shown in the above figure, the typical structure is to make a layer of tens of nanometers thick luminescent material on ITO glass - that is, people usually say OLED screen pixel self-luminous material, there is a layer of metal electrode above the luminescent layer, the electrode is applied with voltage, and the illuminating The layer generates light radiation; electrons and holes are injected from the yin and yang respectively, and the injected electrons and holes are transported in the organic layer, and are combined in the light-emitting layer to excite the light-emitting layer molecules to generate singlet excitons, and the singlet exciton radiation is attenuated. Glowing.

This is a bit more complicated to explain, but in general, the three sub-pixels you see, red, green and blue will shine on your own. Of course, the structure is much more complicated when it comes to the entire panel, including isolation columns, insulation layers, and the like between sub-pixels. AMOLED also has a TFT backplane that controls each pixel switch.

This complicated structure is impossible to use for a micro-carving with a knife. If these structures are put into practice, it is a problem with the manufacturing process. The manufacturing process of OLED involves ITO glass cleaning, lithography processing, etc., all of which require high-tech, technologies that we have never seen before. In short, ITO can form electrode patterns and ITO on the substrate. Patterns, isolation column patterns, etc. - you can imagine that there are small people carved with a knife...

Subsequent process parts are crucial in the manufacture of OLED panels, namely evaporation. Inside the vacuum chamber, place the ITO glass substrate on a heated rotating sample holder, then put the fire under it (of course not really put the fire), and the luminescent material you see is steamed up. Yes, the red, green and blue tri-color bulbs (of course not the real ones) are steamed up.

Speaking taller, evaporation is the method of heating in the vacuum, electron beam bombardment heating and laser heating to evaporate the vaporized material into atoms or molecules, which then move linearly with a large free path, colliding the substrate. The surface is condensed to form a film.

It can be said that evaporation is the essence of the OLED manufacturing process, and not only luminescent materials, metal electrodes and the like are also steamed up. Although we have the same evaporation method as steamed steamed buns, the actual operation is very complicated, such as how to control the pixel area, how to align the pixels, and how to control the thickness of the film that is steamed up, what pretreatment, and the vacuum degree of the evaporation chamber. Wait, it is not something that most of us can penetrate. In addition to evaporation, there are subsequent processes such as dispensing, encapsulation, aging, cutting, testing, and the like.

In fact, evaporation is indeed an important reason for the high cost of OLED screens, so LG can't buy too many evaporation machines, and naturally it can't make an order for iPhone 8. And even Canon Tokki, it is not easy to digest Samsung's orders.

(This is a typical process flow of PMOLED, PMOLED is also an OLED, but the structure is simpler than AMOLED , no TFT)

However, in addition to "steaming" the bulb, you can choose to "print" it out.

The above-mentioned high-end atmospheric grade "evaporation" method is mainly applied to a typical OLED screen of RGB three-color arrangement. Many of Samsung's OLED TV products are steamed based on this method. The effect is very good. The three primary colors are very pure, but the cost is very high. The technique used in this type of evaporation is called FMM, a fine metal mask, which is to cover the pixels and cover a mask when vapor deposition, so how to align, and the mask material itself will become a technical difficulty.

In fact, in order to control costs, humans have more than one kind of OLED TV. There is a type of blue light + color conversion layer: this scheme only needs to evaporate blue OLED components, and converts the light into RGB three colors through the conversion layer. The limitations of converter development have not been adopted on a large scale.

There is also a class of OLED TVs that are white light + three color filters, which are similar in principle to LCD liquid crystal panels, with white backlighting and color filters - this method is obviously much lower in cost, LG OLED TVs have been produced in this way, and white OLED+color filters have also been considered as a solution for further low cost of OLEDs. Only with the addition of filters, the light color purity is more problematic, so the brightness, contrast, color, and energy-saving performance are theoretically inferior to RGB OLED. As for LG's WRGB OLED (four sub-pixels), there is no more detailed discussion here.

Matsushita believes that the last method has a shrinking image quality, and the cost is not too low, or RGB OLED is reliable. But the cost of FMM evaporation is very high, so what should I do? So at the CES show in 2013, they showed a self-developed “printing” process, and according to them, it was the world's largest 4K OLED TV (56 inch).

In fact, the "printing" technology is not new, LCD LCD screens are widely used in this technology, like the color filters used in LCD, RGB three color blocks are printed like inkjet printers. Just need to determine the dot pitch between the three sub-pixels of RGB, you can start printing: Panasonic's printing technology for OLED is directly painted on a large-sized glass substrate, which does not require a high-temperature vacuum environment compared to FMM evaporation. The manufacturing process requirements are much lower and the cost is naturally lower.

This process is actually not perfect, especially the efficiency and longevity of blue materials is a big problem. This is also why many people think that Panasonic's TV will have a yellowish appearance. At the CES show that year, someone raised this question.

Many companies in Japan seem to be interested in this technology. Mitsubishi Chemical is developing new materials for "printing" production of OLED panels. Although it will not be ready for mass production until 2017, Mitsubishi Chemical has already provided samples to panel makers in Japan and abroad. It is said that their "printing" method can control the material cost to 1/10 compared with "evaporation", which is undoubtedly a significant reduction in the finished product - and there is no micro-component micro-coating technology before, Mitsubishi Chemical developed A new material that can be thinly and accurately applied to a small area.

Imagine that the cost of OLED TV can really come down, and there is still a little excitement.

Sony's improvement: combining evaporation and printing

In fact, FMM evaporation and printing (or wet preparation) is not the only solution in this process, but also the laser transfer solution (although evaporation is also required). However, printing is currently the most tempting technology, because of its low cost and high efficiency, it is also the ideal technology for the production of flexible OLEDs in the future.

Sony has made this point, but it has clearly noticed the problem of blue luminescent materials. So Sony thought of a combination of evaporation and printing. The red, green and blue luminescent materials also have the strange three-dimensional arrangement of the above picture.

To put it simply, the blue paint is first coated to form a common layer, and then the red and green colors are first printed and then vapor-deposited. According to the above design, the life and luminous efficiency of the blue material can be ensured. So at the CES 2013 show, Sony also released the world's largest OLED TV (it seems that Panasonic has added a first word in front of it).

It is said that Sony TV, which is achieved by this process, has no problem of color cast, and even in terms of perception, it can surpass the more expensive, Samsung-based and LG-like Korean TV manufacturers. Panasonic and Sony had cooperation in the TFT part as early as 2012, so it is not surprising that both of them are printing OLED TVs, and they are the largest in the world.

With the efforts of Japanese manufacturers such as Mitsubishi Chemical mentioned above, the single organic film-forming technology may reduce costs as printing technology is further improved in terms of ink materials, printing equipment and process control. Even though the price of current OLED TV products is still quite high, organic film formation is not the only part of OLED manufacturing. For example, packaging plays a vital role in this. This is another chapter.

In the human vision, OLED products that have been put into the prototype product level have at least appeared on the booth with a curlable, paper-thin screen product, which is the future of considerable screen technology. It is worth mentioning that the organic film-forming technology mentioned in this article is only part of the improved development of OLED technology. In the future, we have the opportunity to explain more technical details. For example, Panasonic has changed the cathode in the OLED structure to a transparent material, and developed a technology called “top emission”, which naturally improves the luminous efficiency; Sony will use the TFT part. Move to the bottom of the OLED, and add a microcavity structure. There are different wavelengths between the three colors of RGB, and the light of each color is enhanced by the reflective film on the cathode/anode, and then the color purity is further improved by the color filter, and the like. This is Sony's "super top lighting structure."

Although OLED is widely regarded as the next generation of display technology , it is not perfect at present, and there is still too much improvement in technology maturity compared with LCD . The reason why OLED TV is so expensive is not difficult to understand.