Printing OLED displays: has the time finally come?
Solution-processed light-emitting organic materials were demonstrated in 1989 with a tiny 0.1 EQE (external quantum efficiency). Since then there has been tremendous progress on material synthesis/production as well as on display processing. However, thus far, commercialisation has remained elusive.
In this article, we will argue that the page may be about to turn. This could have important consequences for the large-area display markets, especially if printing can enable a significant reduction in the production cost. This could reconfigure the landscape.
The limitations of current techniques
There are two main approaches today to OLED display production. Both utilise evaporated organics. In one, the emissive layers are evaporated through a fine metal mask (FMM) to achieve patterned side-by-side R, G and B emitters. This approach works well for small up to medium-sized displays. Indeed, today, it fully dominates the market for such products.
The technology, however, has proven difficult to scale up to large areas. The FMM is often made very thin to minimise shadowing effects during the evaporation. The downside of this thinness is that wide-area masks become flimsy, easily sagging or warping under their own weight. This distorts the mask pattern. The FMM also, by definition, blocks and thus wastes two-thirds of the material. The blocked materials further add weight to the mask film, thus further complicating the situation.
Numerous approaches have been proposed to overcome these shortcomings, eg, scanning the scanner over the substrate. None have, however, fully resolved the issues. Therefore, FMM has failed to transition to large area displays.
The other approach is based on white OLED (WOLED). Here, the organic layers are not patterned. Instead, they are blanket deposited. This way multiple OLED stacks are formed on top of one another to achieve white colour emission. In this approach, it is often common to have two blue layers to divide the drive voltage across two stacks while maintaining the brightness levels.
In general, WOLED involves depositing many layers. The RGB colour effect comes from colour filters. As such, the system has low efficiency (filters waste two-thirds of light). In turn, this demands higher brightness, leading to harsher drive conditions and shorter lifetimes. The primary advantage of this approach is that it can be scaled to large areas. This is why it is the technology of choice for OLED TVs. Reducing production cost, however, thus far has proven complicated although steady progress is being made.
Both these approaches reply on evaporated materials. An immediate question then is why is printing not being used. This way direct RGB patterning over large areas with high material utilisation could be achieved.
Printed OLED displays: are the materials good enough yet?
Developing solution-processible OLED materials has not been easy. At first, only polymeric materials could be solubilised. In the early generations, this would give very low EQEs and lifetimes. With time, polymer/small molecule and even solution-processible small molecule OLED materials were developed. The polymeric-only products also improved tremendously.
The upshot is that today, after three decades of global research and development effort, solution-processible OLEDs are becoming a viable proposition. Finally, the performance gap in Cd/A between leading printable OLEDs and commercial evaporated ones has been nearly bridged. This is true for green and red materials, and it is also the case for the blue with appropriate colour coordinates commonly used in displays.
Previously, any transition from evaporation to solution processing would impose a significant performance penalty. This no longer is necessarily the case. Therefore, the performance progress on the material side has eliminated, or at least strongly weakened, a major traditional reason against the adoption of printing.
The lifetime of solution OLEDs need to be further extended. An exact benchmarking is difficult because suppliers do not always speak the same language and mean different things by ‘lifetime’. Each can use a different brightness level or lifetime definition (T90, T50, etc). Nonetheless, it is still the case that the lifetime of solution OLED materials lags behind evaporated ones. This gap will likely be closed with further development.
Printed OLED displays: can they be commercially manufactured?
The processing of solution OLED materials over large areas and in a commercial production is not, and has not been, easy. However, the community now has extensive accumulated know-how. This is because it has been working on printing OLEDs for over 15 years. Indeed, the first commercial product with a printed OLED display appeared as early as 2004. Granted, this was a small passive matrix display.
Since then there have been numerous prototypes. These prototypes often targeted large-area displays, eg, TVs, since the benefits of printing are stronger in that size range. Furthermore, large-area displays can do with low PPI levels, thus relaxing fine feature printing requirements. Of course there have also been small-sized, high-PPI demonstrators, but there are far fewer since the commercial motivation behind them is weaker.
Today, inkjet-printed mid-size, eg, 21.6″, 204 PPI OLEDs displays are transitioning into production by a Japanese consortium. This consortium brings together Japanese material suppliers, equipment makers and display manufacturers. We feel that the objective of the consortium is to pool the early risks involved in the learning curve of printing OLED displays. But once the technical hurdles are sufficiently overcome and the technology is mature enough, there will likely be a tech transfer from the consortium to a display maker.
Interest is not limited to Japanese and Chinese makers though. In fact, even today’s leading OLED manufacturers are, and have been, actively developing printing technology. Indeed, there are many strong strategic motivations that keep printing firmly on the agenda. First, those with an FMM technology want to urgently obtain or establish a technology towards large area OLED displays. Second, incumbents are concerned that printing can one day bring about a step change in the production cost, thus reconfiguring the competitiveness map to leave them behind. Third, they see the mastery of display printing as being essential for quantum dot (QD) displays.
In the latter case, printing is/will be heavily used. It will be used in QD colour filter displays both on LCDs and OLEDs. More importantly, in the long term, it will be used to develop the ultimate QLED (quantum dot light emitting diode). We say ultimate because this display technology, despite its current immaturity and technology challenges, offers complete contrast, extremely wide colour gamut, thinness, flexibility, etc.
In general, printing can no longer be dismissed as immature or futuristic. Material-side progress has eliminated or substantially narrowed the EQE penalty that traditionally would have to be paid when adopting printed materials. Steady, long-term progress on the processing side is also enabling commercial production.
Of course, there is much further progress to accomplish. The process yield and TACT will need to be further improved. The dimensions of the commercially printed OLEDs displays will need to be enlarged from the monitor to the TV range. The lifetime of solution OLEDs will need to be further extended, especially if the end product is to have an extend life cycle. The printing resolution will need to become finer.
These ongoing development trends are, however, often incremental in nature and are present in almost all display technologies. The important point is that printed OLEDs have already broken through enough technical barriers to demonstrate their viability. As such, printed OLEDs have becoming an exciting topic. We think this is a space to closely watch in the coming years.
More information on the OLED industry can be found in the report ‘OLED Display Forecasts and Technologies 2019–2029: The Global Rise of Flexible and Foldable Displays’, available at www.IDTechEx.com/display.
Originally published here.
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