INTERVIEWSOLAR JUCTION
Makers of CPV systems also want cells with high levels of reliability. Solar Junction has carried out well-known reliability and ageing tests on its cells, which have revealed device lifetimes of many, many decades
adds to production costs. “It is also very difficult to ensure that those cells are going to remain reliable over a twenty-to-thirty year time frame,” claims Yuen, who expects these issues to slow commercial introduction of inverted metamorphic triple-junction cells.
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In his opinion, these delays are very damaging to the commercial prospects of CPV, which faces strong competition with thin-film and silicon technologies that are getting better and better, and cheaper and cheaper. “There’s no time,” argues Yuen. “You can’t wait five years for a metamorphic structure to be available – you need it here and now.” He argues that if CPV is to be successful, cells must deliver rapid, consistent and sustained improvements in efficiency, reliability and tunability. “That’s why Solar Junction’s story resonates so well with our customers.”
These customers can already receive limited quantities of cells with typically 41.5 percent efficiency, and Solar Junction plans to make this product widely available next year. “We have a
roadmap that is going to take up into the forties and even reach fifty percent, using a four or five junction cell that utilises a dilute nitride platform, in addition to using gallium arsenide, aluminium gallium arsenide and indium aluminium gallium phosphide,” says Yuen. This roadmap should enable Solar Junction to deliver big jumps in device efficiency on a biannual basis, thanks to the introduction of additional cells and evolutionary improvements that occur when devices are made in high volumes.
The high efficiency of Solar Junction’s cell, along with its potential to get markedly better over the next few years, should help to enable CPV to grab a significant share of the PV market. “The dream of CPV is for the PV market to see some real segmentation,” says Yuen. He hopes that in locations with intense solar radiation, the financial case for installing CPV will be so strong that it will soon start to dominate this segment of the market.
Cell characteristics
Employing Solar Junction’s devices in CPV systems operating in these environments can cut generating costs associated with the technology, because these cells are capable of operating efficiently at very high concentrations, such as 1000 suns. Magnification always leads to ‘hot spots’ – in this case local concentrations of 1500 or 2000 suns that produce very high current densities. “If your tunnel junction isn’t very good, you kill cell performance,” says Sabnis. He claims that tunnel junction performance typically falls off above 1000 suns. “Using our approach you can have tunnel junctions that approach 8000 [suns] without any failure, so system manufacturers don’t have to worry about hot spots with our cells.”
Makers of CPV systems also want cells with high levels of reliability. Solar Junction has carried out well-known reliability and ageing tests on its cells, which have revealed device lifetimes of many, many decades. The company currently carries out a ‘flash test’ on every cell produced, evaluating performance at the concentration that the cell is
www.solar-pv-management.com Issue VI 2011
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