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industry has managed to cut unit costs by over 5% yearly. Thin-film
PVs have the potential to drop PV electricity costs to 6 US-cents/
kWh. But even if thin-film PVs meet their long-term potential, a
quantum leap in cost reduction is indispensable in order to provide
the scale of applications the world will need. This is where nanotechnology
comes into the picture.
Third generation: New nanomaterials
Nanotechnology, or the ability to architect and assemble things at
the atomic level, is on its way to optimizing solar cells both in terms
of increased conversion efficiency and considerably cheaper raw
material. Third-generation PVs include organic solar cells, photoelectrochemical
cells and nanocrystal solar cells that may ultimately
beat down solar electricity prices to below 5 US-cents/kWh. The
Nobel prize-winning invention of conductive polymers has paved
the way to organic solar cells that are made of inexpensive flexible
plastic which can be wrapped around structures or even applied like
paint. Up to now, its degradation upon exposure to ultra violet (UV)
light and relatively low energy efficiency have been a drawback.
However, researchers from New Mexico State University and Wake
Forest University recently achieved a solar energy efficiency level of
5.2%. They believe plastic solar cells with efficiencies beyond 10%
will be a reality for consumers in four to five years. Early promoters
of the technology are BP Solar and Konarka, a private, US-based
start-up company with R & D subsidiaries in Austria and Switzerland,
whose light-activated Power Plastic TM foil was selected as one of
the best products in 2006 by Builder News magazine. Moreover,
researchers from the University of Toronto recently produced the
first PV cell able to harness the infrared portion of the sunlight
spectrum. It is believed that with further advances of this nanoparticle-enhanced
polymer technology, plastic PV cells could achieve
efficiencies up to 30%.
Another promising emerging technology, invented by researchers
at the Swiss Federal Institute of Technology in Lausanne (EPFL), is
the photoelectrochemical or dye-sensitized solar cell (DSC), often
referred to as “artificial photosynthesis.” Here, the incident light transforms
the dye molecules by exciting their electrons, which are then
absorbed by a titanium dioxide (TiO2) layer to become an electric
current, somewhat similar to the photosynthesis process in plants
(Figure 4). The concept has been out there for quite some time; however,
it did not work well until researchers used TiO2 crystals 30 nanometers
in diameter, which greatly increased the sponge-like contact
area with the dye, enhancing the efficiency of electron absorption.
DSC technology has high market potential owing to the cheap raw
material. The Australian Dyesol, a start-up with close ties to EPFL,
is a leading developer and manufacturer of dye-sensitized PV cells.
Figure 3
PV industry sales estimates
The projected annual growth rates used in the forecast continuously ease
off, from 35% in 2005 to 11% in 2025. System costs per watt are assumed
to fall from USD 8 to USD 2.50 in 2025. Source: Credit Suisse and EPIA/Greenpeace
In billion USD
200
180
160
140
120
100
80
60
40
20
0
2005 2009 2013 2017 2021 2025
Light-absorbing nanocrystals and quantum dots
Another strategy for capturing sunlight energy is to use nanocrystal
solar cells. Silicon nanoparticles of diameters ranging from 1 to 4
nanometers sprayed onto a silicon substrate absorb UV light and
convert it into electrical current. With appropriate connections, this
ultrathin film of silicon then acts as a PV cell. The concept is being
developed by the US start-up Octillion, while a number of research
laboratories are working on similar inorganic cells, including Lawrence
Berkeley National Laboratory. With this ultrathin, inexpensive
technology, glass windows could generate electricity from sunlight,
without losing significant transparency, and rooftops or facades
could be laminated, with virtually no impact on appearance.