NREL Solar Cell Sets World-Record Conversion Efficiency of 40.8%

August 20, 2008

Researchers at DOE's National Renewable Energy Laboratory (NREL) have developed a solar cell that can convert a record 40.8% of the sunlight that hits it into electricity. The new NREL solar cell is a triple-junction device, which means it uses three layers of photovoltaic material stacked atop one another to catch different parts of the solar spectrum. Sunlight that isn't converting into electricity by the top layer passes into the lower layers of the solar cell, which convert more of the sunlight into electricity, boosting the cell's conversion efficiency. The cell is unique in that it is grown in an inverted order, from the top down, and is then removed from the substrate it is grown on to produce a lightweight, flexible film. The inverted approach also allows the use of a bottom layer material with a different crystalline lattice spacing than the other materials in the cell, which makes it a "mismatched lattice" or "metamorphic" solar cell. NREL's inverted metamorphic triple-junction solar cell also won an R&D 100 Award this year. It achieved its record efficiency while exposed to sunlight concentrated by a factor of 326—in industry parlance, "326 suns." See the NREL press releases on the record efficiency and the R&D 100 Award.

In recent weeks, researchers around the world have unveiled innovative approaches to capturing sunlight. DOE's Idaho National Laboratory (INL), for instance, has developed a method of producing plastic sheets that contain billions of "nanoantennas"—antennas on the scale of a billionth of a meter—to convert the sun's infrared rays into electricity. The nanoantennas consist of tiny gold squares or spirals set in a specially treated layer of polyethylene, a common plastic. While the device is not yet practical, it has the potential to run off either sunlight or waste heat. Engineers at the University of California, San Diego (UCSD) have employed similar "nanowires" to boost the efficiency of organic solar cells, which are made of plastic. See the press releases from INL and UCSD.

Meanwhile, researchers at the Massachusetts Institute of Technology (MIT) have found a way to convert windows into devices that concentrate sunlight for conversion into electricity. MIT developed a mixture of dyes that can be painted onto a pane of glass or plastic. The dyes absorb sunlight and then re-emit it within the glass in a different wavelength of light, which then tends to reflect off the interior surfaces of the glass. As the light reflects within the glass pane, it tends to get channeled along the length of the glass to its edges, where it is emitted. The MIT researchers estimate that sunlight is concentrated by a factor of 40, allowing solar cells that are optimized for such concentrated sunlight to be mounted along the edges of the window. The unique optics of the approach yields a cheap solar concentrator that does not need to be pointed toward the sun, as is needed for lens-based concentrators. MIT estimates that the process will be commercialized by Covalent Solar within the next 3 years. See the MIT press release and fact sheet.