High Efficiency Solar Cell Wins Federal High Tech Prize

The Inverted Metamorphic Multijunction, IMM, Solar Cell was named a winner of the 2009 Award for Excellence in Technology Transfer by the Federal Laboratory Consortium for Technology Transfer. These multijunction cells consist of multiple thin films in layers that allow the cell to capture more of the solar spectrum and convert it into electricity.

A solar efficiency record of 40.8 percent under 326 suns concentration was set by IMM solar cells in 2008. While this has since been bettered by a 41.4 percent achieved by the Fraunhofer Institute for Solar Energy systems, research shows that "the material and process used in development is far too expensive to make this cell go commercial," commented "Solar Energy Investing" in January.

Commercialized versions of the IMM cell are aimed at the space satellite market. IMM cells could be incorporated into a satellite's skin or, like an awning, unfurl as a solar array, NREL scientists suggest.

This would eliminate the need for conventional wing-shaped solar arrays with heavy metal frames and balky mechanical controls. NREL says those same qualities also will open new commercial uses for solar power on Earth, especially in the solar concentrator market.

On Earth, IMM cells will be arranged in concentrated photovoltaic arrays, which use lenses or mirrors to focus sunlight onto the solar cells.

The award-winning IMM solar cell (Photo by Pat Corkery courtesy NREL)

For decades, conventional solar cells have featured wafers of semiconducting materials with similar crystalline structure. Their performance and cost effectiveness is constrained by growing the cells in an upright configuration. Meanwhile, the cells are rigid, heavy and thick with a bottom layer made of germanium.

In the new method, the cell is grown upside down. These layers use high-energy materials with extremely high quality crystals, especially in the upper layers of the cell where most of the power is produced.

Not all of the layers follow the lattice pattern of even atomic spacing. Instead, the cell includes a full range of atomic spacing, which allows for greater absorption and use of sunlight. The thick, rigid germanium layer is removed, reducing the cell's cost and 94 percent of its weight.

The result is an ultra-light and flexible cell that converts solar energy with record efficiency.

The original IMM cell was invented by Mark Wanlass of NREL’s Concentrating Photovoltaics Group. Last night, Wanlass and Emcore's director of research and development Paul Sharps received the award at the Federal Laboratory Consortium national meeting in Charlotte, North Carolina.

Sharing the award is NREL's solar cell R&D team of Jeff Carapella, Anna Duda, Daniel Friedman, John Gneiss, Sarah Kurtz, Bill McMahon, Tom Moriarty, Andrew Norman, Waldo Olivarez, Jerry Olson, Manuel Romero, Scott Ward, and Michelle Young.

Since 2005, NREL and Wanlass have worked with Emcore Corp of Albuquerque, New Mexico to develop a commercial version of the IMM cell under a Cooperative Research and Development Agreement with the federal government.

The IMM cell also won R&D 100 honors last year in two categories. http://www.nrel.gov/features/20080701_rd100.html.

Known as "the Oscars of Invention," the R&D 100 Award presented annually by Research & Development (R&D) Magazine since 1969 showcases the 100 most significant new technologies commercialized worldwide.

NREL's second R&D 100 award of 2008 recognizes a new technology for manufacturing thin film solar photovoltaic cells that will allow solar panels to be integrated into metal and glass structures to turn entire buildings into clean energy power plants.

Called Hybrid CIGS, the thin film photovoltaic material consists of layers of copper indium gallium diselenide. NREL has developed a method in which the hybrid CIGS cells are manufactured in layers. Using ink-jet and ultrasonic technology metal-organic inks are applied in separate layers directly into common building materials.

NREL's partner, HelioVolt, has developed a proprietary processing system that quickly bonds the film layers under heat and pressure forming large-grain CIGS crystals.

The process takes seconds to complete at lower temperatures than other manufacturing processes that require hours at temperatures 500-700 degrees Celsius as well as vacuum processing, evaporation and other expensive steps to produce solar cells.

NREL says, "This simpler, combined approach could create enough of the flexible PV film to integrate it with windows, roofing, facades and other structural components, turning entire buildings and other into small, self-sustaining power plants."

Thin films may not achieve the outer limits of solar conversion efficiency produced by the IMM crystalline silicon cells but NREL says they can be manufactured quickly and in large volumes using inks that cost a fraction of the cost of silicon.

Copyright Environment News Service (ENS) 2009. All rights reserved.