Nanowire, usually made of inorganic compounds, is of great interest for piezoelectric and photovoltaic energy harvesting.
Nanowire solar cells (left) by Canadian researchers and (right) by Konarka in the USA
Sources McMaster University, Konarka
While thin film technologies are receiving worldwide attention with their potential to lower the cost of solar energy, there are researchers looking into different approaches that will result in cost reductions for photovoltaic technologies. One comes from the collaboration of the Department of Engineering Physics at McMaster University, Cleanfield Energy and the Ontario Centers of Excellence OCE in Canada, which have formed a partnership to pursue the commercialization of nanowire technology in the production of solar cells.
"One of the biggest obstacles to widespread use of solar cells as a clean source of energy is cost," said Ray LaPierre, assistant professor of engineering physics at McMaster University and project leader for the collaboration. "Our work with nanowire fabrication at this stage shows the potential for greater energy efficiency with less costly materials."
Semiconducting nanowires (e.g., Si, InP, GaN, etc) exhibit aspect ratios (length-to-width ratio) of 1000 or more. As such they are often referred to as one-dimensional structures with controlled lengths of one to five microns and diameters of 10 to 100 nanometres (thousands of times thinner than a human hair).
Some of the advantages they offer over thin film and crystalline silicon technologies (both currently used in solar cell production) include:
- low material utilization
- use of low-cost substrates
- defect-free materials with high conversion efficiency
- strong light trapping and absorption
The exceptional properties of nanowires that are not seen in bulk or 3-D materials are due to the lateral quantum confinement of electrons. Nanowires are excellent at trapping light, very efficient at absorbing the sun's energy and allow for greater electrical output per unit surface area.
How to grow nanowires
A common technique for creating a nanowire is the Vapor-Liquid-Solid (VLS) synthesis method. Microscopic balls of gold or aluminum are deposited on a surface that is exposed to a feed gas (such as silane / gallium arsenide). The gas atoms are absorbed by the gold to form a layer and as each layer is added, the nanowire is created. The process is repeated until a desired length and thickness is reached.
The researchers are now exploring growth of nanowires on a variety of substrates including silicon, glass, flexible metal foils and even fabric made of carbon nanotubes. They are also exploring nanowires grown on material, removed then embedded in flexible plastics.
Professor Ray LaPierre says his aim is to achieve 20 per cent efficiency in the next five years. He reported on the synthesis of coaxial compound semiconductor III-V nanowires and fabrication of nanowire-based solar cells as well as their enhanced carrier extraction, light trapping effects, and light absorption at the IDTechEx conference Photovoltaics Beyond Conventional Silicon USA 2008 in Denver, Colorado, on June 17-18, 2008.
Antenna resonance from EM radiation can be used as a collector and disseminator of electrical energy. The antenna approach is a transformational technology and it may become a better and much more efficient energy source. Past difficulties surrounding this approach have been due to the size of the antenna, which has to be relative to the wavelength of incident light to achieve resonance. The challenge of scale-up for this technology has also been a barrier to low cost manufacturing. However, Idaho National Laboratory (INL) team has made some significant advances toward this end and has created a large scale demonstration product with trillions of antenna structures.
There are already advances in large-scale production and roll-to-roll manufacturing. The difficulty has been creating nantennas of the proper size to capture the wavelengths emitted by the sun. The ability to tune the antenna to the size of the wavelength is very important. Already, Dr Steve Nowak of Idaho National Laboratory claims, "We're very good at tuning antennas to one specific frequency and very good at making them broadband." Near-term (2-4 years) and long-term (3-6 years) applications are being pursued, including:
- Passive, energy-neutral cooling by converting infrared radiation into radiation that we do not feel as heat (eg radio waves)
- Passive heating by turning radiation we do not feel as heat into infrared radiation
- Extremely efficient lighting by broadcasting photons from the nantennas. As it is basically the solar process in reverse (photons from electrons, instead of electrons from photons), this is just as feasible as the solar applications
- Passive heating or cooling within clothing
INL nantennas on film
The INL is capable of producing antennas that can absorb electromagnetic radiation at the near and mid infrared part of the spectrum. Drawing from roll to roll processing expertise at Microcontinuum, a Cambridge based start up that spun out of Polaroid, INL is able to print (using e-beam UV lithography printing) millions of nano-sized antennas per square inch. INL claims that the ability to harvest infrared energy means that these antennas can convert residual heat into electricity even after the sun has gone down with a collection that is four times more efficient than in conventional photovoltaics. There is a need for further work on the rectification of that signal though, because at IR wavelengths, frequencies are in the range of 30 THz. Costs are uncertain as yet.