Recently I’ve been searching the literature for some good references on the roll-to-roll printing of organic electronics for the introduction of my MRes report, which is due at the start of September.
The performance of organic semiconductors is much lower than that of the conventional inorganic semiconductors. One reason why researchers are interested in organic semiconductors is that they can be produced using printing presses (literally the same technology to print things like magazines, newspapers and T-shirts) so cheaply compared to inorganic semiconductors that it no longer matters that their performance isn’t as good. The biggest application is probably solar cells and it’s really important that costs are kept as low as possible for them to be economocially viable.
Despite being part of the Centre for Plastic Electronics at Imperial, I don’t know of anyone who completely prints solar cells, let alone using roll-to-roll processes. There’s a lot of reasons why (e.g. printing presses take up a lot of space, use a lot of material) but it does my head in that people are use techniques like spin coating and vacuum deposition, anneal at high temperatures in inert atmospheres for long times and use environmentally-unfriendly cholorinated solvents. Papers describing devices made using these techniques frequently laud the scalable and low cost nature of plastic electronics. But are these techniques scalable to large area, high throughput, continuous printing processes? No, not without spending a lot of money, but then it’s not economically viable.
Today I came across a good review[^ref] in Materials Today (open access!) on the roll-to-roll fabrication of polymer solar cells. The first paragraph sums this issue up nicely (emphasis added):
In order to reach its full potential, the imminent realization of the 10 %-10 yr target[^target] within the laboratory must transcend into a realistic industrial process. While this may seem trivial to many and even obvious to some, there are challenges that have perhaps been taken too lightly in laboratory reports. Often tiny spin coated devices prepared on rigid glass through toxic solvent processing and metal evaporation is said to be roll-to-roll and industry compatible. The view held here is that claiming to be roll-to-roll and industrially compatible without such instruments is similar to claiming that one can learn how to swim on a floor.
I love that last sentence so much I’m tempted to quote it at the start of my report.
To summarise: I think researchers need to stop simply writing in the introductions of papers about scalabilty and low fabrication costs and actually start considering it in the lab or, even better, dropping these unscalable techniques and compounds from the lab altogether.
[^ref]: R. Søndergaard, M. Hösel, D. Angmo, T. T. Larsen-Olsen, F. C. Krebs, Materials Today, 15 (1-2), 36-49. DOI: 10.1016/S1369-7021(12)70019-6
[^target]: 10% device efficiency from devices that last >10 years.