Why 8K OLEDs and QD-OLEDs are So Difficult to Make
A recent news item from Korea caused some excitement in the ‘gadget’ press. A press release from a U.S. company called Kateeva, that makes large inkjet printers, said that it had supplied the printers to a leading manufacturer of QD-OLEDs to be used for 8K TVs and high resolution monitors. The press release mysteriously disappeared from the internet, but the cat, as the saying has it, was out of the bag! There is only one company making QD-OLED display panels, Samsung Display Corp (SDC), so no great detective work was needed to work out who they had supplied.
This encouraged a number of websites to suggest that making 8K QD-OLEDs was getting closer. That’s desirable as 8K sets give the highest resolution and sense of immersion, and QD-OLED is widely regarded as having the best image quality in contrast and color volume. But up to now, SDC has only made QD-OLED TV panels at 4K. Why have they stopped at that resolution and why, in general, are there very few 8K OLED panels at all? And will the 8K printer change things?
OLEDs are Very Hard to Make
Because of the success of OLED displays in the premium smartphone and TV markets it is not always appreciated that making them is very difficult. That is one of the main reasons that only one company (SDC) dominates the small OLED market and one company, LG Display (LGD), dominates the large OLED market. In contrast, LCDs are (relatively) easier to make.
Almost 20 years ago, NEC of Japan said that it was going to drop out of the race to build a business based on making larger and TV-sized panels as it was ‘just about money’ and not technology. Of course, the engineers have really improved LCD performance in the last two decades, but broadly speaking NEC was right. If you have enough capital, it’s not technically difficult to get into the LCD business, as a number of Chinese makers have shown.
That’s a big contrast to OLEDs, where just SDC and LGD are very dominant. The key reason is not related to the OLED emissive layers or the structure of the LCD cells, but more to do with the backplane – the transistors that control the display pixels – and the way they have to be configured.
You Need Transistors
Although you can make LCDs and OLEDs without transistors on the back, the displays then have to be small and low resolution. If you want to make large or high-resolution displays, you need to control each pixel all the time and that means you need some kind of switch controlling the pixel. The switch is the transistor, but the type of switch/transistor that you need for LCDs and OLEDs is very different.
In both cases, you can use thin film transistors (TFTs), but the type and number of TFTs is significantly different in the two display architectures.
LCDs work by using the optical properties of the liquid crystals (LCs) to control the amount of light that is allowed to pass through the LCD cell from the backlight. Crucially, the LC cell layer does not produce the light, it just acts like a switch or dimmer. The energy to produce an image comes from the backlight.
The alignment of the liquid crystals is controlled by the voltage between the anode and cathode of the LCD cell, which can be on one side or both sides of the cell, depending on the type of LCD. The TFT has to be able to accurately control the voltage. However, it doesn’t need to supply much current and, in transistor terms, it doesn’t have to switch very quickly. That means that it can be made of amorphous silicon.
The Nature of Silicon
This is not the place for a detailed deep dive into the nature of different forms of silicon, but a useful analogy is carbon. Carbon can be very unstructured to produce something like soot, it can be more ordered (graphite), or it can be a full crystalline structure (diamond). Silicon can also be in three forms, amorphous (like soot), polysilicon (like graphite) or as a crystal (like diamond and used in most silicon chips).
Engineers were able to create large arrays of amorphous transistors on huge sheets of glass to allow the manufacture of low cost LCD backplanes. That was a key development that enabled the huge success of LCDs in displacing most other display technologies in many applications.
The transistors made of amorphous silicon are not that good, in terms of current capacity or speed but they could be made by relatively low temperature processes, so manufacture on glass was possible. The transistors were (relatively) big, but if you only have one for each pixel, you do not block too much of the light going through the LCD. So amorphous silicon is used on the vast majority of LCDs in notebooks, monitors and tablets.
However for some applications that needed high resolution or special performance, the amorphous TFTs were not good enough. Designers wanted to have TFTs that were smaller, to allow more to be packed in and increase resolution, or to supply more current, or to switch more quickly. The problem was that to create better transistors, you need better silicon. To get better silicon means some type of crystallization process being performed on the silicon. Unfortunately, the melting point of silicon is higher than the glass used for the display substrates, so there is a challenge to melt the silicon to allow it to cool and re-crystallize without melting the glass substrate underneath.
The Challenge Was Met
This challenge was met by delivering a very precise level of heat for a very precise (and short) period of time to the silicon on the glass. This is done in production using an excimer laser that scans the silicon, providing just enough heat to melt the silicon and allow it to cool to become polysilicon. The silicon created is known as Low Temperature PolySilicon (or LTPS). It allows much better performance from much smaller transistors than amorphous silicon and LTPS can have up to 1,000 times better electron mobility.
There’s a Catch!
However, there is a catch. First, the laser can only scan accurately over a limited distance. Scanning a larger area means multiple passes and it’s extremely difficult to get the scanned areas to match, especially at the edges or at joins. Nobody wants to see a ‘join’ on a display surface. For this reason, as far as the author knows, nobody has put an LTPS LCD into mass production bigger than around 25” diagonal (and that only happened for a brief period). Current polysilicon LCDs are pretty well all below 20”.
The second challenge is that the lasers to make LTPS are very expensive and have a limited life, so the cost of the displays goes up substantially. For these two reasons, LTPS has generally only been used for smartphone, tablet and notebook displays. A decade ago, when SDC launched its first development to try to make large OLED panels for TVs, it announced that it would use LTPS as the backplane. However, the cost and difficulty of making the backplanes was one of the reasons that that particular development was later shelved.
Plan B – Oxides
As large area LTPS displays were not commercially viable, other approaches were researched and scientists came up with alternative materials for the transistors to silicon. One of the key materials is indium gallium zinc oxide (IGZO), which was developed in Japan. Amorphous IGZO has around 20-50 times the electron mobility of amorphous silicon so could be used to make much better transistors, without the challenges posed by the LTPS process.
IGZO technology has been around for about 10 years from Sharp and others. It was seen as a really good alternative, but has been much harder to use in mass production than was hoped for when it was first introduced. Nevertheless, there are plenty of LCD fabs now that use IGZO or other oxides rather than silicon today.
LCDs are Simple
LCDs can use a single amorphous silicon, oxide or LTPS transistor to control each RGB subpixel. That means that on a 4K LCD there are usually 24 million transistors (one for each RGB element). On an 8K LCD that goes up to close to 100 million. (There have been advanced TV LCDs that have sub-divided each subpixel in two and actively controlled each and they have double the number of transistors).
OLEDs are More Complicated
Turning to OLEDs, we have different things to consider. First, rather than the energy of the display being from a backlight, all the energy for the image has to go through the emissive pixel. The emissive structure means great viewing angles, response times and speed, but because all the current goes through the transistors in the backplane, the transistors have to be much higher quality than amorphous silicon. (Amorphous silicon has been demonstrated in an OLED but not, as far as the author knows, been put into mass production). The transistors need to be Oxide or LTPS to be able to control the higher current compared to LCD.
The second problem is that the pixel is current driven – that is to say that the brightness of the OLED emitter is proportional to the current of the power supply, not the voltage, as it is with LCD. That makes the control more complicated and means more than one transistor.
OLEDs are much more sensitive to the quality of individual transistors because of this and so there needs to be compensation circuitry built into the subpixel. That means that OLEDs will typically use three to five transistors to control each sub-pixel. In turn, that makes the backplane substantially harder to manufacture, with 4K panels now needing 75 – 125 million transistors (the same as an 8K LCD) and with 8K needing up to 500 million.
Figure 3 A typical AMOLED driving circuit – this one is from Ignis Innovations that has licensed its technology to OLED makers Note the three transistors as well as the capacitor. Fundamentals of TFTs and Circuits for Advanced Signal Processing, Arokia Nathan, SID Display Week Seminars 2022
The combination of the need for oxide or LTPS and the bigger number of transistors are the key reasons that 8K OLEDs are quite rare and when they are available (LG Display makes some), they tend to be quite expensive compared to 4K. There is not much more material used (apart from driver chips), but the difficulty of manufacturing reduces the manufacturing yield and increases the cost.
Back to Kateeva
Returning to the original story, Kateeva makes inkjet printers (IJPs) which are used in the QD-OLED manufacturing process. However, they are not used to deposit the OLED materials, but just for the quantum dot (QD) conversion layers to convert the OLED light to green and red pixels.
Figure 4 A simplified diagram of a QD-OLED. The transistors are behind the Blue & Green OLED layer. The IJP printer from Kateeva will be used to deposit the green and red QDs.
The key point is that the QD conversion layer is independent of the backplane and Kateeva’s IJP will not help in the manufacture of that. For that reason, the availability of the IJP to support 8K is a necessary condition of a boost from 4K, but it’s not, as the saying has it, a sufficient one.
Still, the fact that Kateeva is talking about 8K does suggest to the writer that SDC may be thinking about 8K or asking for 8K support. That would not be very surprising as SDC will want to supply the Samsung TV Division with panels for TVs and Samsung continues to promote the advantages of 8K. Having 8K combined with the QD-OLED technology would lead to some spectacular TV sets and that would help Samsung to maintain its very, very long term position as the leading global TV brand.