Wafer thin light-emitting film pushes flexible electronics to the limits

Researchers in Europe and Japan say that they have produced the first polymer organic light-emitting diodes (PLEDs) that can be stretched while lit. At 2µm thick, Matthew White from the Linz Institute for Organic Solar Cells in Austria and his teammates call them ‘the thinnest and most flexible electroluminescent devices’ yet.

Thinner PLEDs can be more flexible. And though polymer materials just a few hundred nanometers thick can emit light, existing commercial PLEDs surround them with comparatively bulky layers. They must be assembled on a robust base substrate and use a comparatively stiff indium-tin oxide (ITO) electrode. Also, today’s commercial materials are air- and water-sensitive and therefore need a protective encapsulant layer on top. That makes final devices over 100µm thick.

White and colleagues produced their PLEDs on a 1.4µm thick PET foil substrate. ‘This is the same foil as in helium balloons, only a lot thinner,’ White says. It’s hard to spin-coat PLED materials onto such thin films, but working together with researchers from the University of Tokyo, his team did just this. They stuck films onto silicone-coated rigid glass, which holds them in place using van der Waals forces alone, allowing easy post-fabrication removal. ‘It’s like a screen cover on a new iPhone,’ White says. ‘You just peel it off.’ The scientists also swap ITO for a polymer electrode, and synthesised an air-stable red light-emitting polymer so they could dispense with the protective layer. However, as the devices’ electrode materials weren’t air stable, they only worked for a few hours

Being so thin gives their devices uniquely small bending radii, which allows them to be crumpled. Exploiting this, the team made stretchable PLEDs by attaching their films to extended elastomeric tape. When the tape contracts the PLEDs fold randomly, ready to be pulled flat when stretched. ‘This can add functionality without adding weight,’ White says. ‘Obvious applications would be police tape at a traffic accident that glows bright red, or balloons where the wrinkles would break a non-flexible chip.’ He adds that his team hopes to make foils combining entire PLED displays with a power source.

Andy Monkman, head of the organic electroactive materials research group at Durham University, UK, is impressed by the devices’ bend radii. But he thinks their durability and current low performance are problems. ‘Going so thin gives quite remarkable properties, but you can’t do anything until you encapsulate it.’