solar junction interview
There is a point at which if we sell our device at a certain price, [makers of 39.2 percent efficient cells] have to give it away, or pay for the customer to take
it.At some point, cell efficiency trumps everything.
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 intended for. But in future Solar Junction is likely to use sample testing, an approach that the company says it can justify, thanks to the high yield of its production process. However, if customers prefer, each cell can be tested prior to shipment.
The costs associated with the production of the company’s cells are similar to those of traditional triple-junction devices, according to Jeff Allen, Solar Junction’s Director of Business Development. “And as the incumbents adopt a metamorphic cell, which requires thicker graded-buffer layers and lift-off, we become cost-advantageous.” What’s more, by employing a dilute nitride platform, the addition of a fourth and fifth junction does not lead to a significant change in overall device thickness. “So our advantage from a cost standpoint increases, because we get a substantial improvement in power output and efficiency while costs are very flat,” adds Allen.
Today, Solar Junction is sampling cells to all the major CPV system manufacturers, plus many of their smaller rivals. And according to Yuen, feedback is excellent: “It is the most power output they have seen from all the cells they have.” The Stanford start- up is now entering qualification with some of these companies, and also taking orders.
Solar Junction’s highly efficient cells seem to be destined to have a profound impact on the CPV cell market. Modelling by the Stanford spin-off has compared the performance of CPV systems employing cells with 41 or 42 percent efficiency, and those delivering 39.2 percent, the value quoted for Spectrolab’s devices that it is shipping today. “There
is a point at which if we sell our device at a certain price, [makers of 39.2 percent efficient cells] have to give it away free, or pay for the customer to take it,” claims Wiemer. “At some point, cell efficiency trumps everything.”
Arguably, the only significant obstacle in the path of Solar Junction’s future success is the raising of cash needed to expand manufacturing capacity from today’s level, 7 MW, to 250 MW (capacities assume cell operation at 1000 suns). Urgency in this capacity build-out is critical, says Sabnis, because the likes of Soitec, Ammonix and Sol Focus are experiencing a tremendous ramp in orders for their CPV systems. “Our biggest focus right now is getting designed into their systems, going through the various qualification procedures necessary to do that, and developing a reliability data path on our cells.”
If Solar Junction can raise the capital to expand its manufacturing capacity, it will surely have every opportunity to become one of the biggest makers of CPV cells in the world. And the company is giving itself a great chance of executing on this by not getting distracted by thoughts of becoming a vertically-integrated supplier of CPV modules and systems, and not diluting its resources by also battling to break into the market for solar cells for powering satellites. Any by focusing on what it does best – making really efficient cells – it to be promises to become a major force in helping CPV to establish its own space within the CPV market.
© 2011 Angel Business Communications. Permission required.
Dies and integrated assemblies
Customers can choose to either buy bare die from Solar Junction, or integrated solar assemblies. The San Jose start-up never intended to offer the latter on a commercial basis – it planned to just use the assemblies for testing. However, the company received several requests for this product from customers, who explained that known good-die shipped by the big solar cell manufacturers were failing after contract manufacturers had inserted them in solar assemblies. “[Our customers] were basically pleading with us: ‘We just wish the cell manufacturers would do this for us to prevent this problem’,” says Jeff Allen, Solar Junction’s Director of Business Development. The company was well placed to address this issue, because it had hired engineers with tremendous expertise in packaging and reliability.
July 2011
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