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Emcore beats efficiency mark for space-based solar

The US company says that its development of multi-junction solar cells with a conversion efficiency of 31 percent will lead to a new class of flexible, lightweight, high-efficiency solar arrays for space applications.

A record-breaking multi-junction cell from Emcore promises to usher in a new class of lightweight, high-efficiency solar arrays to power next-generation spacecraft and satellites, says the company.

Emcore s Albuquerque-based photovoltaics operation has come up with the so-called "inverted metamorphic" (IMM) cell design, which operates at a record conversion efficiency (for space applications) of 30.9 percent, and is more than an order of magnitude thinner than conventional cells.

Describing the latest developments at last week s CS Mantech conference in Austin, Texas, Emcore s VP and general manager of photovoltaics David Danzilio said that the new cell architecture would cut down on the number of lattice defects seen in metamorphic structures.

In metamorphic solar cell designs, junctions based on different materials are fabricated within the same structure. A composition-graded buffer layer is deposited between the different layers, so that the lattice mismatch can be overcome.

However, the mismatch between germanium - the material used in the bottom layer of the metamorphic structure - and the other compounds means that lattice defects still mount up, restricting the cell s overall efficiency.

In Emcore s IMM design, the junctions are deposited in a different order to normal, with InGaAs sitting on top of InGaAlP and germanium junctions in the final structure. "It s like growing the top cell first," explained Danzilio.

Design flexibility
Because the inverted design absorbs sunlight more effectively, cells based on the structure are less than one-tenth the thickness of conventional structures, thereby saving on materials costs. Arrays based on the thinner cells would also be lighter and more flexible than current designs.

"The successful demonstration of this IMM cell represents the most significant improvement in terms of watts per kilogram and dollars per watt in the past decade," added Danzilio. "[It] will enable never-before-envisioned space power applications."

The IMM cells could also be used in terrestrial applications, where Emcore s current production devices already have a peak conversion efficiency of 37 percent under concentration.

Emcore is busy scaling up production of terrestrial cells to meet an expected boom in demand over the next several years. Speaking to compoundsemiconductor.net at the CS Mantech conference, Danzilio said that he expected the global market for solar power systems to be worth between $7 billion and $8 billion by 2010.

Around 20 percent of that market could be served by systems based on multi-junction cells, equating to some $1.5 billion, he added. The photovoltaic element comprises anywhere between 10 and 20 percent of the system cost for multi-junction systems, suggesting that the market for terrestrial applications of III-V solar cells should be worth around $200 million in 2010.

"We are at the leading edge of this at the moment, and have an [order] backlog of 20-30 MW," said Danzilio, adding that, on average, about one thousand 4-inch germanium wafers are needed to produce cells capable of 1 MW power generation.

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