Metal Oxide Thin-Film Transistors as a Key Enabler for AMOLED Displays

Friday 10 August 2012, Amsterdam

After years of rapid growth, the flat-panel display (FPDs) industry is slowing down. This is due to sluggish economies worldwide, in part. But another force is at work too; market saturation. When LCDs first reached the market at affordable prices – some 15 years ago or so now – there was a huge installed base of CRT monitors and televisions just ripe for the picking.

Today, the old CRTs are mostly gone. The display industry is looking for new strategies to restore growth. One of these is to bring to market new display technologies that offer something that LCD cannot. The hope is that with the next big display technology hitting the market, consumers will be willing to swap out their old LCDs for “what’s next.” This might revive the display industry’s flagging fortunes.

A successful new display technology must offer a sufficient improvement over LCD and at an attractive enough price to incent consumers to behave as the display makers would like them to. This is not an easy goal to achieve when one considers that LCD can now boast a couple of decades of manufacturing and product engineering experience to its credit.

The Problem with AMOLEDs

A key example of what we mean here is active matrix OLEDs (AMOLEDs). Because they promise more vibrant color than LCD, as well as flexibility and dramatic thinness, they have long been touted as a successor to LCD. They are already present in the market place in small area applications like smartphones, but are expected to make a concerted push into the market segments where the display area is significantly larger, such as TVs.

But AMOLED displays have had trouble making it to commercialization. This has been especially true in large area displays, where production issues have been plaguing the technology. Additionally, AMOLED displays need high mobility backplanes because of the high current levels needed to drive light emission from the organic diodes.

Amorphous silicon (a-Si) backplanes, which are commonly used in LCD displays, have not worked well in the AMOLED environment and until recently, the only other high mobility backplane material available to the AMOLED market low temperature poly-silicon (LTPS).

LTPS is feasible in small area applications, and is in fact the backplane used in the AMOLEDs displays in the smartphone market at present. But it is prohibitively expensive to manufacture for large area screens. It has potential issues with uniformity and stability in such applications.

Enter Metal Oxides

If AMOLEDs cannot be deployed for large-area applications, then, by definition, AMOLEDs cannot replace LCDs as a dominant display technology. Worse, if AMOLEDs are restricted to small mobile displays then economies of scale for both OLED material manufacture and the production of AMOLEDs themselves cannot kick in, again thwarting high hopes for AMOLED technology. This report believes that the technology that will cut through this Gordian knot are backplanes that are based on metal oxide thin-film transistors (TFTs). Such TFTs will also be sold into the conventional LCD sector and will generate more revenues from LCD applications than for AMOLED applications. But in the AMOLED sector, they will be more essential and will prove a key enabling technology for AMOLEDs. Here’s what metal oxide TFTs can offer the AMOLED business.

• A high mobility, with big enough currents to drive AMOLED displays and respond to the higher refresh rates of next generation displays;

• Relatively cheap large-scale production, that is easily scalable to large substrates;

• Small pixel sizes and hence, high-resolution displays; and

• Larger aperture ratios compared to a-Si, allowing for higher transmission through the backplane which can increase the brightness of the display without an increase in power requirements.

Growing Industry Interest in Oxide Backplanes for AMOLED TVs

We note that hardly a display industry conference goes by these days without multiple papers being read on oxide TFTs. And this report believes that oxide TFT-based backplanes may well be the technology that propels AMOLED displays into the display mainstream. There can be no doubt that – after many years in the R&D wilderness – oxide TFTs are attracting considerable attention from big name firms such as Sharp and Samsung, the latter being especially important because it dominates OLED display manufacturing.

The primary focus of much of this activity is television sets. As we have already noted, this is where the backplane challenge is the greatest. It is also where the willingness of the consumer to switch technologies would have the most impact; because TV displays are so much larger and more expensive than mobile displays. And while all the focus on AMOLED TVs in the large OLED display space at the present time, can OLED computer displays be far behind?

However, this report is not suggesting that in either the computer sector nor in the TV sector will change come quickly. Both computer displays and TVs are products with relatively long lifetimes and, in any case, there are other problems with large OLED displays that need to be overcome.

On the other hand, the intrinsic economics of oxide backplanes will be pushing display manufacturers to adopt such backplanes quite quickly. What oxide backplanes bring to the table are an attractive combination of low capital expenditures and low-cost manufacturing process:

• From the capital expenditure point of view, the dollar requirement for refitting a production line for large area LTPS production can run into the low billions of dollars. By contrast, oxide TFTs plants are about a sixth to eighth of that capital expenditure.

• While AMOLED prototypes were demonstrated early in 2012 with both LTPS and oxide TFTs, it is the opinion of NanoMarkets that in going forward, AMOLED TVs will favor oxide TFTs, as their low cost and production scalability are enabling factors in the AMOLED TV reaching a reasonable level of penetration.

And Smartphones Too

The role of oxide TFT backplanes in the smart phone sector is a little different than in the TV sector. On the one hand, we are talking about a sector that is much more of a reality than the AMOLED TV sector. Millions of smartphones using AMOLEDs are now shipped every year and they seem to be doing quite well in the marketplace.

In addition, metal oxides backplanes are ideal for satisfying important requirements of smartphone (and tablet) displays including:

• Low power consumption. Obviously important in a handheld device with limited battery life and where the display is a major contributor to the drain on the battery.

• Screen resolution. This has been of growing importance as consumers expect better image, photo and video quality out of their handheld devices. This, in turn, has been due to the evolution of handheld devices into entertainment products, with cameras, games and applications becoming strong selling points

• Sunlight readability. Unlike in indoor large area screens, is an important consideration for handheld displays and implies a requirement for a high degree of brightness.

This report believes that Oxide TFT backplanes can help in all these regards; at least to some extent. This because of the inherent high mobility of the material, combined with the small pixel sizes that it can be used to create.

However, all these positive factors for the use of oxide TFT backplanes must be balanced against the fact that – unlike in the large display sector – oxide backplanes can expect to face significant competition from LTPS.

This is not to ignore the disadvantages of LTPS that we have already mentioned. However, as we have also indicated, these are less of an issue where LTPS is used in small displays, such as those found in smartphones.

But what might deter the use of oxide backplanes for smartphones is that:

• LTPS is already common in AMOLED displays for smartphones and displays makers in this sector may be quite reluctant to shift to oxides, since many of them have only recently established LTPS facilities.

• While making such a switch in the long run may be cost advantageous, equipment will have to be amortized first. Since oxide TFTs do meet the mobility requirements of AMOLED displays could still place oxide TFTs in a strongly competitive position with TFTs in the smartphone sector over the course of the next decade.

• In addition, while oxide TFTs are superior to a-Si TFTs along a number of dimensions, they are superior to LTPS TFTs only on cost. Metal oxide backplanes cannot match LTPS on mobility and driving currents. So any future switch from LTPS to oxides would have to be justified from the perspective of cost efficient production alone.

This report believes that high mobility backplanes have a very important role to play in the AMOLED display industry over the course of this forecast period and beyond. As should be clear from the analysis above metal oxide TFTs represent a technology that offers value propositions for the AMOLED sector that are hard to match with any other available technology.