Touch in flexible displays: which technologies will prevail?

The technologies seeking to unseat ITO (indium tin oxide) are by now themselves old, at least in the sense that they have been around for a decade and a half. They seem to have been forever in the waiting mode, anticipating the arrival of their major breakthrough.

A key differentiator has long been higher flexibility and foldability over ITO. The question now is: how will different technology options fare given that the paradigm shift towards flexible displays is finally upon us, despite the recent product launch delays?

Analysts at IDTechEx have been researching the technology and market landscape for the past eight years. The results of their ongoing research can be found in the report ‘Transparent Conductive Films and Materials 2019–2029: Forecasts, Technologies, Players’, which provides a detailed analysis of all the key players and technologies and offers market forecasts segmented by more than 10 technologies and 20 application areas.

Film vs on-cell: a story of trade-offs

In general, two approaches have emerged: film and on-cell types. In the former case, a flexible film is bonded atop the OLED device. In the latter, the touch layers are deposited and patterned directly on, or perhaps later as part of, the thin film encapsulation (TFE).

The table below provides a comparison of the two approaches. Film-type is the simplest to implement. Crucially, it decouples the production yield of the touch layer from that of the display layer. Furthermore, the films could be manufactured using lower-cost assets. The scalability to larger areas will also be simpler and production speeds likely higher as R2R film production techniques can be deployed. Therefore, the film-based approach is a more accessible technology that offers an easier and lower-cost roadmap towards larger-area flexible displays.

However, this approach falls short on performance. Material choices are now available which offer high flexibility and foldability. Metal mesh films are likely to be good enough for medium levels of folding. The picture for higher bending degrees becomes hazier; however, technology options such as silver nanowires have already demonstrated that they can sustain the technology roadmap towards high bending levels. Clearly all technology choices require further improvement; however, current issues are unlikely to be a fundamental showstopper. As such, transparent conductive layer technology choice itself is not a performance bottleneck.

The real limitation of the film-based approach is that it requires an additional substrate plus an additional bonding layer. This increases overall thickness, which in turn lowers flexibility. In future, combining the touch with other layer functionalities may partially alleviate this shortcoming. This trend towards combinations will accelerate perhaps with wide availability of CPI films. Candidates for function combination include the hard coat layer, the polariser, the barrier film (if film is used) and so on. The evolution of this trend can have important implications for the long-term viability of film-based solutions.

The other approach is on-cell. Here, the touch layers are deposited and photolithographically patterned inline directly on top of the OLED-TFT stack. In the future, it might be possible to weave the patterned touch electrodes as part of the TFT structure, although that can require a difficult manufacturing challenge.

The key advantage of this approach is that it eliminates the additional substrate, thus resulting in a thin and flexible solution. The challenge, however, is that it dramatically increases the cost of production defects in the touch layer as the entire stack — including the OLED and the TFT — will be thrown away. Success here, therefore, requires outstanding production know-how and optimisation. Furthermore, this process ties up more expensive production assets on a usually low-cost item, which is the touch layer. It is also likely to require slow and controlled deposition within the narrow parameter space limited by the already-deposited materials and layers. Importantly, it is not clear whether this approach can be readily scaled to larger areas or not. This is because one would require the inline TFE as well as the patterned touch electrode deposition processes to be scaled up without compromising quality or cost.

Parameter	Film type	On (or in) cell
Cost of defect (importance of yield)	Low	High
Production complexity	Low	High
Ability to do R2R	High	Low
Ease of technology access/implementation	High	Low
Overall thinness (thus flexibility)	Low (unless multifunction substrates are developed)	High
Ease of scale-up to larger areas	High	Low
IP risk	Low	High

This is a comparative table; therefore, ‘low’ means lower than the other in this comparison. The same applies to ‘high’. Another key parameter is the ability to sustain ever tightening foldability requirements. A thinner solution will offer higher foldability but this may need to be balanced by mechanical robustness and lifetime. Further note that the material of choice could be different in each case.

In general, IDTechEx considers all display makers will build some know-how for both just in case, but that no single approach will totally prevail in production soon. Those with sufficient technology know-how and IP protection will pursue the on-cell approach for the current display sizes. They will also continue to make progress on translating this technology to ever larger areas, often chasing a high-priced, high-performance premium display positioning. Others will pursue a film-based strategy, giving them easier and low-cost access to touch technology for flexible displays. In the medium term, IDTechEx envisages that a more diverse range of flexible display products will emerge, each offering a different performance. This will further sustain the existence of multiple approaches in the market. Note that the battle between film type vs on (or in)-cell type is also in force in rigid displays. Here, too, both approaches are used although trends can shift the balance one way or the other.

To learn more about these trends and the dynamics of the flexible touch industry visit www.IDTechEx.com/Research/PE.

Image credit: ©stock.adobe.com/au/frog

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Originally published here.