Flexible LED Fashioned with Inorganic Nanowires

Michael A. Greenwood

An inorganic LED that is flexible and potentially stretchable has been developed with ZnO nanowires serving as the optically active component.

In a continuation of their previous work, where an emission at 393 nm was obtained from ZnO nanowires (see “ZnO Nanowire LEDs Have UV Output,” Photonics Spectra, January 2006, page 135), the researchers are reporting that they recently created a flexible hybrid LED that generates a broad emission spectrum, covering the visible range and reaching into the near-infrared (from 500 to 1100 nm).

Figure 1. A flexible LED structure consists of vertically oriented single-crystalline nanowires as the optically active component. Reprinted with permission of Nano Letters.

Investigators led by Rolf Könenkamp, a professor at Portland State University in Oregon, reported that the inorganic LED could become a viable alternative to the all-organic electronic and photonic devices now in use.

The team constructed the flexible nanowire-based LEDs (Figure 1) on a substrate of transparent polyethylene terephtalate foil that was sputtered with an indium tin oxide layer. The single-crystalline nanowires were then grown via electrodeposition carried out at 80 °C on the indium tin oxide layer.

After about one hour of deposition, the researchers achieved a homogeneous array of vertically oriented ZnO nanowires. The wires typically measured ∼2 μm in length and from 70 to 120 nm in diameter. The wires were embedded in an insulating polystyrene film (Figure 2) that was applied by spin coating. The technique filled in the gaps between the wires and thinly covered the tips of the wires with ∼10 nm of film. A top contact consisting of a thin poly(3,4-ethylene-dioxythiophene) poly(styrenesulfonate) layer and an evaporated gold film also was added to serve as the hole injection anode in the LED.

Figure 2. Nanowires are embedded in an insulating polystyrene film, producing a coverage that is ∼10 nm thick. Courtesy of Rolf Könenkamp.

The researchers found that the nanowires strongly adhered to the substrate, even when it was bent over a curvature radius of <10 μm.

In terms of emission spectra, the flexible LED exhibited slightly less intensity than the LED on a solid glass substrate that was used for comparison. Both LEDs exhibited similar spectral distributions, but the peak of the flexible LED was red-shifted in comparison with the spectra obtained from the one on the glass substrate.

Although the flexible LED did not exhibit UV emission, the researchers said that they would expect this to occur with the use of a more robust polymer substrate, such as kapton, which can stand anneal temperatures of ~300 °C.

Nano Letters, February 2008, pp. 534-537.