INTERVIEWSOLAR JUCTION
funding, working out what to do next,” reminisces Sabnis, the company’s Vice President of Technology.
As they sat together in an executive office building, the team spent many hours trying to figure out what their business model should be. They key question that they chewed over was this: should they adopt an outsourcing model, following in the footsteps of Cyrium and Quantasol; or should they be an integrated device-making company? While the founders mulled over this question, they were able to continue developing their material technology by booking time at the public fabrication facility at Stanford University. In addition, they worked with epifoundries. “That wasn’t surrounding our proprietary IP – we were basically getting our feet wet in multi-junction technology,” explains Sabnis. In addition, the team learnt how to model multi-cell devices.
After the Stanford spin-off had been going for a few months, its founders started trying to secure investment. And by April 2008 they had won funding from three VCs: NEA, Advanced Technology Ventures and Draper Fisher Jurvetson. The team had also come to a conclusion about the company’s future. “We decided that, at least at the early stage of our existence, we would do everything internally,” says Sabnis. “We were not going to rely on outsourced vendors. The technology was too complicated to use an epifoundry, or even a wafer-processing foundry.”
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 buying 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, respectively.
Simple structures
The device behind these record-breaking figures is anything but esoteric, and 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 multi-junction cell is free from quantum wells, quantum dots and metamorphic layers, structures that are interesting from a research perspective but difficult to grow. Instead, the device 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.
“One of the exciting things about our approach is that the dilute nitride is not a one-technology-node solution,” enthuses Yuen. He points out that this layer 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 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
An engineer at Solar Junction performs on-wafer testing of the solar cells
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www.solar-pv-management.com Issue VI 2011
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