Photovoltaic technology has taken another step forward as researchers at the National Renewable Energy Laboratory (NREL) have demonstrated a photocell with an external quantum efficiency over 100 percent using quantum dots. The new cell uses a process called Multiple Exciton Generation (MEG) that produces more than one electron-hole pair per absorbed photon, and reached a level of 114 percent.

This development offers the possibility of increased efficiency in solar panels, and the technology is able to be manufactured using high-throughput roll-to-roll manufacturing. With the use of quantum dots, photocells could theoretically see as much as a 35 percent increase in power conversion efficiency above contemporary cells. The research cell was constructed as a "layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode."

Note that this does not mean that the entire panel would have a total efficiency above 100% (which would be thermodynamically impossible). The quantum efficiency means only that the number of electron-hole pairs created in the cell is greater than the number of photons that are absorbed. Nonetheless, the advance provided by MEG could lead to the next generation of even more efficient solar energy collectors.

image: Lawrence Berkeley Lab and CC-BY-SA 3.0 by Opticks3

Google has really been cranking out the solar energy investments this year.  Just a few months ago, the company put up $75 million toward rooftop solar installations out West and in the spring they made headlines by investing $168 million in Brightsource Energy's huge Mojave Desert project.

Now, in time to make us feel all warm and fuzzy during the holiday season, the tech giant has announced that they're investing $94 million in a group of four solar projects by Recurrent Energy.  This latest investment brings the total of the company's renewable energy investments to almost $1 billion.

The four solar photovoltaic projects will have a combined capacity of 88 MW and will be located near Sacramento, California.  The projects will provide enough power for 13,000 homes.  A power purchase agreement has already been signed by the Sacramento Municipal Utility District for 20 years.

via Treehugger