Two-for-One Solar

Jun 09 - Mechanical Engineering

Proponents of solar energy like to point out that on a sunny day, a typical square foot of ground is bathed in 100 watts of radiation. That figure is a bit misleading, though. Photovoltaic cells that capture this free energy are very inefficient, producing at most one electron for every photon absorbed. That leaves 80 to 95 percent of the photon's energy to be dissipated as waste heat.

But in April, physicists at Los Alamos National Laboratory in New Mexico reported in the journal Physical Review Letters that they had found a way to get semiconducting crystals to produce multiple electrons from a single photon. Although still far from commercial application, photovoltaic cells made up of such crystals could be much more efficient than conventional solar cells.

The key to this advance is the use of nanocrystals made of lead and selenium. These crystals are less than 10 nanometers in diameter and absorb blue-green light. Thanks to an effect called carrier multiplication, electrons generated through the photovoltaic effect in these nanocrystals are imbued with enough energy that they essentially knock free other electrons.

This so-called impact ionization increases overall efficiency by as much as 35 percent, the researchers say. But it remains to be seen whether nanocrystal solar cells can be made cheaply enough for them to be cost-effective.

JEFFREY WINTERS

Copyright American Society of Mechanical Engineers Jun 2004