Materials for Flexible Substrates: Plastics, Metals and Beyond
That displays, PV, R2R, and a few “nichier” markets offer the main opportunities for flexible substrates somewhat begs the question of what these substrates will consist of. Flexible substrates have so far fallen into two general categories - metal foils and polymer films:
• Metal foils like aluminum and stainless steel have taken the lead in flexible PV because they are generally more heat resistant and less easy to deform than polymers while still offering good flexibility. They also offer a higher level of barrier protection for the back side of the PV cell versus polymer films. Due to their durability, metal films are the substrate of choice for aerospace applications of PV. Metal foils are a mature product that also has a lot of potential for thinning—and thus cost reduction.
• But polymer films are also a strong possibility as substrates for flexible PV, although they require either low-temperature processing or a high-temperature polymer like polyimide. So far, manufacturers are taking the polyimide route to produce flexible PV cells on polymer substrates while still processing the cells at relatively high temperatures. In any case, efforts to use flexible substrates for thin-film PV have often touted the advantages of roll-to-roll processing. Polymer films can be made extremely thin and should become a much smaller proportion of total device cost over time, especially if process development allows cheaper, less temperature-tolerant plastics to be used.
It remains to be seen whether other substrate materials will succeed for flexible PV:
• Perhaps the most likely contender is the new type of flexible glass that companies including Corning and Schott have developed. Presumably, these ultrathin sheets would remain almost as impermeable and heat tolerant as rigid glass panes while allowing a level of flexibility. But these materials are not yet viable options for thin-film PV.
• Other new materials for flexible substrates are likely to be applied in more specialized markets, such as sensors of various kinds. Here we find the need for such novel flexible substrates as paper and textiles. But mostly in this area we find exciting prototypes and proofs-of-concept, but not so many actual market opportunities. Our analysis seeks to identify the most promising niches for flexible substrates of these kinds.
As we have seen, different flexible electronics/flexible PV applications have different needs and these different requirements impact the choice of flexible electronics:
• Plastics and metal foils are less expensive than crystalline silicon or glass. These cost advantages would accrue even for displays or PV panels destined for installation in rigid frames.
• In other situations, the flexibility of the substrate improves the device performance in some way, for example by reducing transportation and installation costs.
• Finally, in some applications, the flexibility is an enabling attribute of the device. Electronic textiles are often used as an example of this last category, while a less fanciful example might be a sensor network able to conform to the surface of a structure being monitored.
These different demands, in turn, place different constraints on the substrate:
• At one end of the spectrum, we find thick foils with a limited bending radius along only one axis at a time. At the other, there are materials that crumple or stretch in three dimensions.
• Some applications require only that the devices be able to withstand the rigors of manufacturing and shipping on the way to a rigid, final installation. In others, the device must be able to tolerate repeated cycles of bending, stretching, or crumpling.
• Suitability for a particular application will depend on the mechanical and electrical qualities of the substrate, but also on optical characteristics, permeability to air and water vapor, and cost.