Another Step Towards Better, Cheaper 8K OLED Displays
At the SID Display Week, scientists from the Semiconductor Energy Laboratory Co Ltd (SEL) in Japan showed a remarkably good looking 8K OLED display with just an 8.3″ diagonal. The details of the research that led to the demo are available online as the paper was recognized as a Distinguished Paper by the SID. SEL also presented a number of other papers at SID.
SEL are a well established research organization in the field of transistor substrates for displays and has often worked in the past with Sharp and Japan Display Inc, so the research is quite likely to move at some point to a commercial application.Â
Backplanes, the controlling transistors for many flat panel displays, are a key part of making good displays, and expertize in the field is a source of real competitive advantage. The author remembers, some years ago, when OLED was not yet established as a commercial product, that the VP of Marketing for LG.Philips Display (later LG Display), Bruce Berkoff, said at a conference,
“I don’t care what kind of display you build on my transistors”.
Berkoff understood that the expertise in the backplane was the key advantage for display makers. That has not changed.
Initially, the thin film transistors (TFTs) on the back of active matrix LCDs (the TFTs form the matrix) were made of amorphous silicon (a-si). The low quality of a-si TFTs meant that speeds were low and the transistors were (relatively) big. An improvement was to use lasers to melt and re-crystallize the silicon to become polisilicon (LTPS). Although described as ‘Low temperature’, LTPS is typically produced using temperatures of >450 degrees C. This has been effective but difficult to do for displays over around 20-30″ and was expensive as the lasers had a limited life. LTPS has never scaled beyond G6 substrates. To get around this, researchers, including some from SEL, developed oxides such as IGZO. (for a more detailed look at this topic see this article)Â
Further developments were made by combining LTPS and oxides to create LTPO transistors which have been used to enable variable frame rates in smartphones to reduce power consumption. The author wrote an explanation of LTPO here. The technology dominates the OLEDs used in premium smartphones these days.
Challenges to make 8K at 8.3″
To make an 8K full color display at 8.3″, you need to create over 1,000 pixels per inch and each of those pixels represents three sub-pixels, so you have to place over 3,000 transistors per inch. That’s if you were making an LCD with just one transistor per pixel. OLEDs need at least two, so that’s a lot of transistors in a dense array. The transistors are small and so have to have high mobility – the metric that tells you how efficiently the electrons can move through them. The lower the mobility, the harder it is for the electrons to move around.
At the short course for display engineers at Display Week, the mobilities for existing backplane materials were given as:
- A-si – around 1 cm2/Vs
- Oxide TFT – 10 – 50 1 cm2/Vs
- LTPS TFT – around 100 1 cm2/Vs
So LTPS is without doubt the best in this respect, but the Crystal InOx developed by the SEL researchers is said to exceed 90 1 cm2/Vs but with the huge advantage that the material can be used for large displays. However, the Crystal InOx also has another big benefit – it has a very low off-state current, which is not true for LTPS and that’s the key reason for the LTPO architecture. The Oxide transistor in LTPO has low off-state current, or leakage.Â
The Crystal InOx also has advantages in stability and in carrying higher current and that makes them better for displays where the backplane material is also used to create the driver circuits. The lack of hysteresis in the switching of the transistor also means that the new transistors can be used either as switching or driving transistors.
Of course, to make the display, SEL had also to pattern the OLED materials very precisely, and we’ll come back to that.
The Critical Step
The key step in the process of making the new Crystal InOx material is annealing – that is to say, heat treatment. The researchers investigated a number of different temperatures but created the demonstration display at around 400 degrees C. The lower temperature than polysilicon is important because higher temperatures can damage the glass substrate.
So the new Crystal InOx material appears to have all the advantages of LTPO but with much less processing and that means lower costs. The other big advantage is that the process can be scaled to larger substrates, bringing real advantages to TVs and monitors that want higher resolutions and higher frame rates.
A number of other papers looking at research into the general area of polycrystalline Oxide TFTs were presented at Display Week, especially from Korean researchers, but SEL was the only demonstration that we spotted.
Patterning
SEL also had to pattern the materials very accurately to achieve 1000 ppi and the group said that it had used its own ‘Metal maskless lithography’ or MML, which us an alternative approach to that used for most OLEDs of ‘metal mask’ deposition. SEL reported on this technology at Display Week in 2023. In broad principle, this is not dissimilar to the Applied Materials MAXOLED technology that we have written about and which continues to look very promising for the high resolution display industry.
While SEL is a research laboratory, Applied Materials is the leading supplier of equipment to the display industry, so the organization has a lot of industrial power.
What Does This Mean for 8K?
As we have written before, it is really hard to make great OLEDs, and even harder to make 8K OLEDs, so anything that reduces the technology barriers or makes displays more efficient could be a real help to panel makers that want to make large 8K OLEDs and compete better with LCDs.
Of course, as the Crystal InOx materials are in the backplane, the technology could also significantly improve LCDs as well. Boosting the mobility of the backplane helps to reduce the size of transistors needed, so less of the pixel is blocked leading to lower power consumption and higher brightness.
