As solar electricity continues its progress toward widespread commercial viability, researchers are on the lookout for ways to make photovoltaic cells cheaper, more efficient, or both. Increasing efficiency will eventually require exceeding the Shockley–Queisser limit—the maximum power-conversion efficiency of a semiconductor solar cell with a single bandgap, one electron–hole pair excited per photon, and no multiphoton processes.

In such a cell, photons with energies less than the bandgap have no effect, and their energy is lost. More energetic photons excite electrons from the valence band to the conduction band, but the charge-carrier pairs quickly relax to the energy of the bandgap, and any excess energy is lost as heat. (See the article by George Crabtree and Nathan Lewis in PHYSICS TODAY, March 2007, page 37.) The power-conversion efficiency, averaged over the solar spectrum, is a function of the bandgap energy, and it cannot exceed 33%.

There are several ways...

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