solar junction interview
These groundbreaking efficiencies at very high concentration factors are a mouth-watering prospect for any concentrating photovoltaic system (CPV) manufacturer. If they could get their hands on large numbers of cells delivering this level of performance, they could cut production costs for systems with a given output power, thanks to the combination of fewer devices, fewer focusing elements and smaller tracking systems.What’s more, Solar Junction promises to elevate solar cell performance to new levels over the next few years. The current design can potentially produce efficiencies of 44 percent of more, and future variants that do not require any major technical innovation could hit efficiencies approaching 50 percent.
From lasers to CPV
The trailblazing technology that lies at the heart of Solar Junction can be traced back to James Harris’ group at Stanford University, CA. Research programmes run by Harris include the development of telecom lasers based around the dilute nitride InGaAsNSb. This material can be grown in a lattice-matched fashion on GaAs substrates, which are cheaper and less fragile than their InP cousins commonly used as a platform for making 1.3 µm and 1.55 µm lasers.
In the mid-noughties, many multi-junction cell developers were trying to fabricate efficient devices that incorporated a bottom junction made from dilute nitrides, which are strong absorbers in the infrared. The Stanford team had a brief stab at this problem, and unlike everyone else, they succeeded. They detailed their efforts in a joint paper with NREL.
Harris discussed these promising results at a conference in 2007, and they piqued the interest of Arno Penzias, a Nobel-prize winning physicist famous for his measurements of microwave background radiation. At that point in his career, Penzias was no longer a researcher – he had racked up ten years as a Venture Partner at New Energy Associates, a major investor in clean technology and a financial backer of the CPV system maker Sol Focus. Penzias was convinced that the Stanford team’s dilute nitride technology could play a major role in accelerating the deployment of CPV technology, and it didn’t take him long to convince NEA to fund a venture to try and make this happen.
A handful of alumni from Harris’ group were tremendously enthused by this opportunity to start a solar cell company. Stanford is renowned for fostering a culture of entrepreneurship, and back in early 2007 many of the founders of Solar Junction were working for local start-up Translucent, a developer of epitaxial materials for the electronics, photonics and solar industries.
On July 16, 2007 three former employees of Translucent and graduates of Stanford’s dilute nitride
July 2011
www.compoundsemiconductor.net 21
group – Homan Yuen, Vijit Sabnis and Mike Wiemer – co-founded Solar Junction, in conjunction with Harris, Craig Stauffer and Jim Weldon. The latter is a former COO of Translucent, and has 29 years of experience in managing a technology company.
To begin with, the founders of Solar Junction based themselves in Cupertino, which is about a mile from Apple’s headquarters. “We were basically just a bunch of guys cramped into one room, with a couple of laptops and some seed funding, working out what to do next,” reminisces Sabnis, the company’s Vice President of Technology.
As they sat together, they tried to figure out what their business model should be. And the 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.”
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