interview Solar Junction
Taking that decision put the founders of Solar Junction in control of their destiny. They were well aware that they had to develop their technology fast to keep hitting the milestones agreed with investors, and felt that the only way to execute on this front was to have their own facility. The VCs supported this decision, even though it required a substantial amount of money to build a fab. In June 2008 the company moved to its current headquarters in San Jose, before bringing up an epi-reactor in a few months. The team hit a major technical milestone that December, upgraded the fabrication facility in early 2009, and made its first triple-junction cell that April. It had an efficiency of 10 percent. “We were surprised,” says Yuen. “ It was a shot in the dark – we thought we were going to get 0.02 percent!” From that day on cell efficiencies climbed fast, hitting 30 percent in August, and making further gains throughout 2010. This January the company reported an NREL-verified efficiency of 40.9 percent, and this February and April it announced efficiencies of 41.4 and 43.5 percent.
Simple structures
The device behind these record-breaking figures is anything but esoteric – Yuen, Solar Junction’s Vice President of Research and Development, goes so far as to describe it as “boring”. It’s a justifiable description, because the cell is free from quantum wells, quantum dots and metamorphic layers. Instead, it is just a stack of bulk layers on a GaAs substrate – a dilute nitride for the base, and GaAs and GaInP for the middle and top cells, respectively. The dilute nitride can take the role of a tuneable infrared absorber, with a bandgap that can be adjusted from 0.8 eV to 1.4 eV by altering alloy composition.
Another great benefit of dilute nitrides is that they can be lattice-matched to GaAs or germanium substrates. This gives Solar Junction a significant edge over
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www.compoundsemiconductor.net July 2011
many of its rivals, including Spectrolab and Emcore. These incumbents have turned to metamorphic buffer layers to alter the lattice constant within the device, so that they can tune the bandgaps of the cells and ultimately reach higher efficiencies. “Those graded [buffer] layers introduce a lot of crystalline defects, such as dislocations, and managing those defects is a difficult thing,” says Yuen. Yields can fall, and the increased thickness of the epilayers in an inverted metamorphic design 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. 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 tuneability. “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 us 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 significant hikes 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.
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
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