Photon Double Whammy: Careening electrons may rev up solar cells

In ordinary photovoltaic cells, lots of sunlight goes to waste as it heats up the cell. New results suggest that solar cells made from nanocrystals can trade this wasteful heating for an electricity-generating boost.

Theoretical calculations indicate that nanocrystal-based solar cells could convert 60 percent of sunlight into electricity, say Richard D. Schaller and Victor I. Klimov of Los Alamos (N.M.) National Laboratory. The best solar cells today operate at an efficiency of about 32 percent.

Schaller and Klimov describe their results, the first observations of a long-sought cue ball effect in nanometer-scale crystals, in the May 7 Physical Review Letters.

The work is “an important scientific advance,” says Arthur J. Nozik of the National Renewable Energy Laboratory in Golden, Colo. He was the first scientist to propose that nanocrystals, sometimes called quantum dots (SN: 3/6/04, p. 157: Available to subscribers at Quantum sentinels), might exhibit the effect, called impact ionization.

Nozik leads a team that has sought the elusive effect for 6 years. Now, it appears that the Los Alamos researchers have reached the goal first. “We’re kind of chagrined,” Nozik admits.

In silicon or other semiconductor materials typically used for solar cells, electrons require a minimum energy to break free from atoms and join an electric current. Most often, electrons get that energy kick from solar photons that pack more than that minimum energy.

The nanocrystal findings show that the outcome of the extra energy depends in part on the size of the crystal that absorbs an incoming photon, Klimov says.

Ordinary solar cells are often made from semiconductors in the size range of coins or playing cards. In these cells, the leftover energy almost always creates heat via vibrations in the semiconductor’s crystal lattice.

Schaller and Klimov worked instead with nanocrystals, about 5 nanometers in diameter, of the semiconductor compound lead selenide. They mixed a liquid with the crystals, each composed of a few thousand atoms, and sealed a drop in a small glass sheath. The researchers then shot laser pulses at a wide range of photon energies through the sheath to examine the nanocrystals’ responses to light.

When those laser photons carried at least three times as much energy as required to knock an electron loose, impact ionization kicked in, the researchers found. The extra energy of each photon propelled a liberated electron like a cue ball so that it knocked one and sometimes two additional electrons free, making them available to join an electric current, Klimov says.

The finding might also open new ways for engineers to improve the performance of lasers and light-emitting diodes made from nanocrystals, comments Paul Mulvaney of the University of Melbourne in Australia.

Because of the large amount of energy needed to trigger impact ionization in lead-selenide particles and concerns about the toxicity of lead and selenium, scientists are now seeking other materials from which to make the nanocrystals.

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