GaN-based nanowire LEDs beat OLEDs on both flexibility and longevity
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).
The researchers went on to create a flexible white LED by blending a mixture of yellow phosphors into the polydimethylsiloxane membrane hosting blue emitting NW-LEDs, down-converting the light to a broad spectrum.
Because a NW-LED array can be peeled-off and connected as a single LED sheet, several such sheets can be stacked (embedding NW-LEDs from different compositions), as the researchers demonstrated. Thus they created a flexible bi-colour LED sheet made of a fully transparent flexible blue LED on top of a green LED, each biased separately so as to produce either blue or green light, or a mix of the two.