While bulk semiconductors tend to be brittle and rigid, growing the GaN LEDs as vertical nanowires (NW) only 20µm long and 1 to 3µm in diameter not only provides them with inherent flexibility, their individual footprints are much smaller than the typical curvature radius of LEDs. Once embedded into a flexible transparent polymer membrane (filling the gaps between the nanowires), properly connected arrays of vertically aligned nanowire LEDs could be wrapped flexibly around any shape.
In a paper titled "Nitride-nanowire-based flexible LEDs" posted on the SPIE Newsroom, the researchers describe a novel fabrication procedure for such flexible NW-LEDs. First using a catalyst-free metal-organic chemical vapour deposition (MOCVD), they grow self-assembled gallium nitride (GaN) NWs on c-plane sapphire substrates. The nanowires are designed as core/shell n–p junctions into which are incorporated multiple radial InGaN/GaN quantum wells (the emission colour being controlled by changing the indium concentration of the InGaN emitting layer).
The NW-LED array is then embedded into polydimethylsiloxane (PDMS) to be peeled-off from the sapphire host substrate. Once flipped onto any arbitrary substrate, a Ti/Au back electrode is created through metal deposition. Flipped again, the embedded array is mounted on a flexible substrate (a metal or plastic foil) before a front transparent electrode is deposited. In their experiment, the researchers chose a mesh of silver nanowires, combining a good electrical conductivity with optical transparency and an excellent flexibility.
Testing a set of blue and green polymer-embedded flexible NW LEDs, they found that the devices had the typical behaviour of nitride NW LEDs, with a turn-on voltage of 3V, and they could be bent to a curvature radius of 3mm without any degradation of their electrical or luminescent properties (unlike conventional OLEDs). And unlike OLEDs, the non-encapsulated devices could be stored in ambient conditions for several months without degradation (typically only a few hours for OLEDs).