Photon recycling and quantum wells enhance single-junction solar cell efficiencies and will boost tandem cell performance to triple-junction levels, say Quantasol's Kevin Arthur and Keith Barnham.
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Rising concerns over global warming and energy security are fueling a ramp in solar cell manufacture. Global production is increasing at 47% a year in terms of power, and independent market analyst Peter Lynch claims that this industry is now growing faster than any other.
This rapid expansion has led to a temporary shortage of polycrystalline silicon, the dominant material used for making solar cells. Supply of this semiconductor
has struggled to keep up with recent demand and this has opened the door for thin-film technologies, such as amorphous silicon-on-glass. These alternatives offer a lower cost-per-unit-area and a faster production process, but have the drawback of low efficiencies. While traditional silicon solar cells
can typically deliver efficiencies of 15%, thin-film versions can only yield single-digit values.
Martin Green from the Photovoltaics Research Center at the University of New South Wales, Australia, claims that any further reduction in the solar cell's key metric, the dollar-per-peak-watt cost ($/Wp), will be restricted by the large, fixed, balance-of-systems costs of a photovoltaic power system, such as the inverter that converts the electricity from DC to AC form. So, the best way to
reduce the $/Wp ratio is to use more-efficient cells.
The high-efficiency triple-junction solar cells produced by the likes of Emcore and Spectrolab offer one solution. These devices were mainly developed for space applications where high costs are not a concern, but they are now starting to be deployed into high-concentration systems that focus sunlight onto small cells with cheap lenses and mirrors.
The market for these systems is being stimulated by feed-in tariffs for renewable electricity. These have recently been introduced in a number of countries, with Germany and other parts of Europe leading the way. However, the booming market is also attracting other forms of innovative concentrator technologies, such as large dish systems and novel solar combined-heat-and-power systems.
At Quantasol, a UK spin-out of Imperial College London, based in Richmond upon Thames, we are also planning to enter this market with our patented high-efficiency concentrator cells. These devices share the hallmarks of the triple-junction cell - high efficiencies at high concentrations. However, we avoid problems with material dislocations by strain balancing the entire epistructure. As a result, we can offer unique features that boost efficiency, such as photon recycling and hot-electron effects.