06 May 2011
A new device created by Rice University researchers can act as an optical antenna by collecting and focusing light, and as a photodiode by converting light into a current of electrons. These nanoantennas have the potential to dramatically improve the spectral range of silicon photovoltaics.
Artist’s rendering of a nanoantenna array under laser illumination. The nanorods (gold) are covered with a transparent ITO layer (purple), and surrounded by an insulating layer of silica (blue).
"We're merging the optics of nanoscale antennas with the electronics of semiconductors," said lead researcher Naomi Halas
, Stanley C. Moore Professor in Electrical and Computer Engineering. "There's no practical way to directly detect infrared light with silicon, but we've shown that it is possible if you marry the semiconductor to a nanoantenna. We expect this technique will be used in new scientific instruments for infrared-light detection and for higher-efficiency solar cells."
The Rice team extended silicon’s frequency range for electricity generation into the infrared by growing arrays of gold nanoantennas directly on a silicon surface. Infrared light hitting the antennas excites oscillating waves of electrons, or surface plasmons, which travel near the surface of the metal. Each nanoantenna is a nanorod formed of 30 nm of gold and 1 nm of Ti covered with a transparent conductive indium tin oxide layer, and surrounded by an insulating layer of silica. The bottom of the nanorod is in intimate contact with silicon forming a metal-semiconductor, or Schottky, barrier. Following optical excitation a plasmon either decays through scattering or by generating a single nonequilibrium (‘hot’) electron that can cross the Schottky barrier as a photocurrent.
A representation of a single Au resonant antenna on an n-type silicon substrate. Images © Science/AAASRice graduate student Mark Knight, lead author on the paper, together with Rice theoretical physicist Peter Nordlander, his graduate student Heidar Sobhani, and Halas were able to directly detect the hot electrons resulting from plasmon decay. "The nanoantenna-diodes we created to detect plasmon-generated hot electrons are already pretty good at harvesting infrared light and turning it directly into electricity," Knight said. "We are eager to see whether this expansion of light-harvesting to infrared frequencies will directly result in higher-efficiency solar cells."
The researchers say that the photodetection mechanism could be combined with existing, above–band gap photodetectors to greatly extend the spectral range of silicon light-harvesting devices, including silicon-based solar cells, into the infrared region of the spectrum.
Research Paper: Photodetection with Active Optical Antennas, Science, Vol. 332 no. 6030 pp. 702-704, DOI: 10.1126/science.1203056.
Written by Nancy Lamontagne, Contributing Editor - US