Solar crystals get 2-for-1

By Kimberly Patch, Technology Research News

Researchers from Los Alamos National Laboratory have tapped the efficiencies of nanotechnology to increase solar cells' potential energy production by as much as 37 percent.

Solar cells generate electricity by absorbing photons and directing the resulting energy to move an electron from the low-energy valence band in a material to a higher-energy conduction band where it is free to flow.

Researchers working to squeeze more energy from sunlight are generally aiming for solar cells that can absorb and use a higher percentage of the wavelengths of light in the sun's spectrum. Today's commercial solar cells can use anywhere from 10 percent to 35 percent.

The Los Alamos researchers have found that it is possible to increase a cell's energy production by making each photon move two electrons. "Carrier-multiplication-enhanced solar cells can, in principle, produce twice as large a current as conventional solar cells," said Victor Klimov, a team leader at Los Alamos National Laboratory.

The method could increase what has been thought of as the maximum power conversion of solar cells by as much as 37 percent, depending on the materials used, resulting in a solar cell with a potential efficiency of over 60 percent. The method could also be used to increase the efficiency of other optical components, including amplifiers, lasers, switches and light absorbers, according to Klimov.

Key to the method is the use of lead selenium nanocrystals. The nanocrystals measure about ten nanometers in diameter, which is the span of 100 hydrogen atoms, or about 7,500 times narrower than human hair.

In today's solar cells a photon moves one electron and produces some waste heat. Carrier multiplication, a phenomenon discovered in the 1950s, happens when a photon moves more than one electron at a time.

This happens via impact ionization. "In this effect, the conduction-band electron [excess] energy is transferred to the valence-band electron and excites it across the energy gap," said Klimov. "As a result, instead of one conduction-band electron we have two electrons that can contribute to electrical current," he said. "Normally, without impact ionization, the... energy is lost as heat."

In traditional semiconductor materials, carrier multiplication can be used to increase energy production by about 1 percent. In nanocrystals, however, carrier multiplication occurs much more efficiently. "Carrier multiplication occurs with extremely high-efficiency -- up to 100 percent -- at photonenergies that are relevant to solar power generation," said Klimov.

The breakthrough that enabled the discovery was a method for detecting impact ionization, said Klimov. Detecting impact ionization involves measuring the time difference between a single electron and double electron reaction. The single electron reactions happen more slowly than the double electron reactions -- in under one microsecond, or millionth, of a second versus less than 100 picoseconds, or trillionths of a second.

The researchers measured the difference in lead selenium nanocrystals by hitting the crystals with pulses of light that were only 50 femtoseconds, or million billions of a second, long. The researchers were surprised to find that the ionization effect, "which is almost nonexistent in bulk semiconductors, turned out to be 100 percent efficient and semiconductor nanocrystals," said Klimov.

Solar cells that use the researchers method could become practical in two to three years, said Klimov. Klimov's research colleague was R. D. Schaller. The work is scheduled to appear in Physical Review Letters. The research was funded by the U.S. Department of Energy.