Silicon nanocrystals promise 40% solar-cell efficiency

A research team at the NREL (National Renewable Energy Laboratory) claims to have observed production of two to three electrons from a single photon in a silicon nanocrystal. The result is potentially highly significant for the photocell business. Scientists had previously thought that silicon could produce only one excited electron per photon—limiting the conversion efficiency of silicon photocells without the aid of light-concentrating hardware to approximately 20 to 30%.

Previously, other researchers had observed multiple-electron production in exotic materials, but these substances offered little hope of production in the volumes necessary to impact the world’s growing energy needs. The NREL work, in contrast, used ordinary silicon. The trick is that these researchers confined the silicon into nanocrystals. The fact that the size of the crystal approaches atomic scale means that the behavior of subatomic particles, such as electrons, in the material can fundamentally differ from their behavior in the same material in bulk.

Some questions about the methodology remain, because researchers can only indirectly observe electron production in photovoltaics of this type. And no one has contemplated mass production of large arrays of quantum dots. But nanocrystals of semiconductor material are relatively easy to produce through solution chemistry, so such problems may not turn out to be showstoppers. The payoff could be commercial photovoltaic panels with as much as 40% unaided efficiency.

National Renewable Energy Laboratory, www.nrel.gov.