Triple-junction solar cells enter the mainstream

Triple-junction solar cells enter the mainstream
Optics & Laser Europe ^ | 8/17/2006 | Michael Hatcher




Triple-junction solar cells enter the mainstream
Two commercial deals reveal how triple-junction solar cells based on compound semiconductors are now challenging silicon-based photovoltaics for large-scale terrestrial applications.


Triple-junction solar cells, which were originally developed to provide high-efficiency conversion of light into electricity for space applications, are extending their reach into ground-based applications.


Evidence comes from two recent commercial deals: US start-up SolFocus has won $25 m in funding to start production of GaAs-based multi-junction cells, while Spectrolab has secured a multi-million pound deal to deliver half a million solar-cell assemblies to the Australian firm Solar Systems. Concentrator power Concentrator power


The power stations being built by Solar Systems are expected to generate more than 11 MW of electricity in total -- enough to power 3500 homes deep in the Australian Outback. According to Solar Systems' managing director Dave Holland, the latest supply deal could be just the start of what may become a more extensive agreement.


"The breakthrough shows the potential for concentrating photovoltaics to dramatically change the economics of solar power," said Holland. "We expect this to be the first commercial phase of a very large and valuable relationship."


Spectrolab, a Boeing subsidiary that specializes in multi-junction cells based on compound semiconductors and germanium, has partnered with Solar Systems for some time, and in April this year the two companies developed a 35 kW solar generator. In May, one of Spectrolab?s concentrator systems began operating in Hermannsburg, deep in Australia?s Northern Territory.


These concentrators generate up to 30 kW of power by using a set of curved mirrors to direct a concentrated beam of sunlight onto just 0.25 m2 of the triple-junction material. A tracker system follows the path of the Sun throughout the day, maximizing the collection of direct sunlight, and diesel generator takes over at night. SolFocus's wafers SolFocus's wafers
Meanwhile, SolFocus -- a start-up company based at the Palo Alto Research Center (PARC) near San Francisco -- will use its $25 m to build a 10 MW pilot production line for multi-junction solar cells based on GaAs alloys and germanium substrates. The deal with New Enterprise Associates (NEA) and seed investors NGEN Partners and Yellowstone Capital is part of a series A financing round that SolFocus hopes to close at $32 m.
The SolFocus team has also gained technical expertise from Bell Laboratories legend and Nobel laureate Arno Penzias, who joins as part of the company?s technical advisory board. Penzias was a key member of the research team that discovered the cosmic microwave background -- a hugely significant scientific breakthrough that confirmed the Big Bang theory of the Universe.


These two commercial developments suggest that the photovoltaics business is looking away from silicon and towards compound semiconductors as a key technology for utility-scale, high-efficiency solar power. Although silicon-based photovoltaics has dominated the terrestrial market, it is much less efficient than the compound semiconductor approach.


Another problem with silicon is that it is not compatible with concentrator systems, which means that a far greater area of semiconductor die is required for large-scale solar "farm" applications. The relative shortage of crystalline silicon has also pushed up prices in recent years, which has strengthened the economic case for concentrator systems that need on a much smaller area of triple-junction die based on compound semiconductors. About the author


Michael Hatcher is editor of Compound Semiconductor magazine.