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Two men in white lab coats are shown in a scientific laboratory operating equipment that deposits layers of a solar cell.

Mark Wanlass (left) and Jeffrey Carapella with the metal-organic vapor phase epitaxy deposition system used to grow IMM solar cells.
Photo by Patrick Corkery, NREL

IMM Solar Cell Shows Its Versatility

The IMM cell is both highly efficient and adaptable to many applications.

Inventing a new type of solar cell is one thing. Setting efficiency records with it and winning major awards add to the achievement. But when one of the world's leading manufacturers of compound semiconductor devices likes the technology enough to nurture and develop it for commercial use, that's a line drive straight out of the park.

All of the above applies to NREL's invention of the inverted metamorphic multijunction (IMM) solar cell and the path to commercializing it by RF Micro Devices (RFMD).

The IMM cell is engineered to capture energy from a major portion of the solar spectrum and be both highly efficient and extremely versatile. These capabilities all stem from how the cell is grown—a process that reverses the usual sequence for triple-junction cells because the top layer is deposited first and the bottom layer is deposited last. Then the cell is flipped over, mounted to a "handle" material, such as a thin metal foil, and the substrate that the cell was grown on is removed. One advantage of this approach is that the expensive substrate can potentially be reused, which results in significant cost savings. Another is that because the handle material doesn't need to be crystalline or even a semiconductor, it can be chosen to meet the needs of a particular application.

Triple-junction solar cells such as the IMM are typically used in concentrating photovoltaic devices (CPV), in which lenses and mirrors concentrate the sun's energy to hundreds or thousands of times its normal strength. One problem facing such devices is the buildup of heat. The inverted growth process of the IMM cell makes it easy to add an effective back-surface reflector, which directs any remaining absorbable light energy back into the active subcell layers, where it can be captured and used to boost the cell's overall efficiency. In addition to increasing efficiency, the back-surface reflector rejects the lower-energy infrared radiation, which keeps the cell cooler and allows it to operate more efficiently.

These and other benefits attracted the interest of RFMD, a leading semiconductor manufacturer and telecommunications giant headquartered in Greensboro, North Carolina. Looking to take its expertise in mass-producing semiconductor devices and apply it to the solar energy field, RFMD signed a cooperative research and development agreement (CRADA) with NREL in 2009. NREL scientist John Geisz, one of the key developers of the IMM cell, is the CRADA's principal investigator.

"This entry of the telecom industry into the solar arena is key," said Daniel Friedman, manager of NREL's III-V multijunction device research. "Partnering with the private sector to find new applications for our technologies is important to accelerate the pace of moving energy solutions to the marketplace."

Illustrating this flexibility, in March 2011 RFMD announced the fabrication of dual-junction PV cells that integrate gallium arsenide and indium gallium phosphide PV junctions using the company's existing 6-inch semiconductor equipment. The successful fabrication of the dual-junction PV cells clears the way for RFMD to develop triple-junction structures, with the ultimate goal of developing a commercially viable and high volume-capable compound semiconductor-based process for high-performance PV cells. The conversion efficiency achieved across RFMD's 6-inch wafers was exceptionally uniform, which allows high device yields and tight distributions in CPV product performance.

"RFMD is very pleased with the world-class performance of our dual-junction cells," said Bob Bruggeworth, president and CEO of RFMD. "With this achievement, RFMD is demonstrating we possess the critical technologies to produce a low-cost PV product with competitive solar cell conversion efficiency, supported by the quality, reliability, and volumes that characterize the cellular handset market."

The IMM technology won an R&D 100 Award in 2008 and a Federal Laboratory Consortium Technology Transfer Award in 2009. It has demonstrated one of the world's highest reported solar cell conversion efficiencies at 40.8%, and research shows that it is likely to continue to substantially improve cell efficiency.

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Photovoltaics Research

Deliberate Science

Winter 2012 / Issue 2

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