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Emerald Group Publishing Limited
Copyright © 2012, Emerald Group Publishing Limited
Energy harvesting with nanoantennas
Article Type: Nanosensor update From: Sensor Review, Volume 32, Issue 2
Several groups are investigating the application of nanoantennas to solar energy harvesting. Strictly speaking, antenna-based technologies are not PV: antennas rely on resonance and the bandwidth is a function of physical geometries whereas PV cells are quantum devices and limited by material band-gaps. Ideally, nanoantennas would absorb light at wavelengths between 0.4 and 1.6 μm because these have higher energy than longer, far-IR wavelengths and account for about 85 per cent of the solar radiation spectrum (Figure 8). A significant proportion of the sun’s radiant energy is lost by conventional PV technology because the semiconductor cannot absorb photons with energies below its band-gap. Accordingly, the maximum theoretical efficiency of a standard silicon cell in use today is limited to ~31 per cent. Conventional antennas can be used at optical frequencies if they are shrunk to the nanoscale and operate by exploiting “plasmonic modes”.
The US Department of Energy’s Idaho National Laboratory aims to develop techniques to capture mid-IR solar energy using a technique involving nanoantennas. Various different configurations have been investigated, including a prototype consisting of a 1.0 cm2 array of gold loop nanoantennas fabricated on a silicon substrate (Figure 9) and similar devices based on flexible polymer substrates (Figure 10). Nanoantenna elements have been developed which capture electromagnetic energy from naturally occurring solar radiation and the earth’s thermal radiation and were designed to operate as a reflective band-pass filter centred at a wavelength of 6.5 μm. The incident radiation produces a standing-wave electrical current in the antenna structure and absorption occurs at the antenna’s resonant frequency. When an antenna is excited into a resonance mode it induces a cyclic plasma movement of free electrons which flow freely along the antenna, generating an alternating current at the same frequency as the resonance, i.e. at THz frequencies. Commercial grade electronic components cannot operate at this switching rate without significant loss so further research is planned to explore ways to perform high-frequency rectification. This requires embedding a rectifier diode element into the antenna structure and one possible solution is the use of metal-insulator-metal tunnelling diodes. Both modelling and experimental measurements show that the individual nanoantennas can absorb close to 90 per cent of the available in-band energy and optimisation techniques, such as increasing the radial field size, could potentially increase this efficiency to even higher figures.
In 2011, researchers at Rice University reported the use of plasmonic modes to construct the first optical nanoantenna that also acts as a photodiode (Knight et al., Science, 332, pp. 702-4). The device was fabricated by growing rod-like arrays of gold nanoantennas directly onto a silicon substrate, so creating a metal-semiconductor (Schottky) barrier at the antenna-semiconductor interface. When light hits the antenna, it excites the surface plasmons – oscillating waves of electrons. These energetic electrons are then injected into the semiconductor over the Schottky barrier, thus creating a detectable photocurrent without the need for an applied voltage. The antennas have heights and widths of 30 and 50 nm, respectively, and are between 110 and 158 nm long. Each 15 × 20 array consists of 300 devices with a spacing of 250 nm between the antennas. The structure is surrounded by an insulating later of silicon dioxide and is then electrically connected through an indium tin oxide (ITO) electrode. A critical advantage of the device is that the photocurrent is not limited to photons with energies above the band-gap of the semiconductor but instead to photon energies above the height of the Schottky barrier. This result is important because it provides a new way of capturing and detecting IR photons. As the plasmon resonance wavelengths are in the near IR region of the electromagnetic spectrum, with shorter nanorods giving shorter resonant wavelengths, applications could include silicon-based solar cells that operate at IR as well as visible wavelengths. The fact that the devices work in the broad IR region means that they could also be used as low-cost silicon IR imaging detectors that might replace the more costly indium gallium arsenide detectors that operate in the same spectral region.
Robert BogueAssociate Editor, Sensor Review