On December 14, the National Renewable Energy Laboratory, or NREL, announced that it had confirmed an efficiency rating greater than 41 percent for solar cells made by Durham, North Carolina-based Semprius.
This efficiency rating is calculated on the ability of the low-cost lenses incorporated Semprius’s gallium-arsenide (GaAs) cells to concentrate sunlight (to an intensity of 1,000 suns) on solar cells with a sufficiently wide optical angle to maximize solar insulation and reduce tracker costs.
The company is using GaAs, but notes on its website that other semiconductors are also suitable, including silicon, amorphous silicon, gallium nitride, indium phosphide and even diamond, since the original substrate can be used repeatedly.
The result is concentrating solar photovoltaics, or CPV, from triple-junction GaAs cells a mere 600 microns in diameter. The cornea in the human eye is about 560 microns thick.
Perhaps the most important part of Semprius’s manufacturing process, and the innovation that earned them $500,000 in stimulus seed money in 2009 and a spot, in 2010, with the U.S. Department of Energy’s (DOE’s) SunShot Initiative program, is their massively parallel patented micro-transfer printing.
This process allows fabricators to use standard semiconductor processes to print a solar cell and then transfer the “print” to another substrate so that the original substrate can be used again and again. The process is also applicable to other forms of manufacturing as well.
SunShot is run out of NREL, one of 12 DOE labs vested in energy efficiency and renewable energy technologies. The intent of the program is to make solar energy ubiquitous both in terms of penetration (into traditional energy markets) and cost – this latter goal known as “achieving grid parity.”
The Semprius process means that, after preproduction using standard printing processes, and once the electrical elements are defined, etch chemistry cuts these elements free and a transfer stamp removes them and places them on a destination substrate, rather like picking up ink on a traditional stamp and depositing that ink on a separate sheet of paper. When the transfer is complete, the stamp returns to the next “ink pad” (or die), one up or over in the source material, and repeats the process.
Perhaps as important as efficiency is the cell’s tiny size, taking up only one-one thousandth of the entire solar module surface area. This reduced size also helps prevent the heat buildup that can adversely impact solar cell efficiency – think of too many bodies in a crowded room.
The 41-percent efficiency rating is one of the highest recorded at this (light) concentration, and is challenged only by Stanford University spinoff Solar Junction’s multijunction GaAs cells, confirmed at 43.5 percent efficiency. The company is hoping for a DOE loan guarantee to expand production.
Apparently Semprius’s inexpensive solar cell manufacturing technology was so tempting that global electronics giant Siemens AG (NYSE: SI) took a 16-percent stake in the company in June. The company will begin manufacture from its North Carolina plant in 2012, producing solar panels in a volume that will reportedly be cost competitive with fossil-fuel technology.