# Synopsis: Band together

Semiconductor alloys offer one way to improve the efficiency of solar cells.

The sun offers us light for free, but most of the spectrum is missed by semiconductor solar cells, which are typically only efficient at converting light to electricity over a window of wavelengths. Connecting a series of different semiconductors can capture more of the solar spectrum, but these devices are complex and expensive to fabricate.

Now, in a paper appearing in Physical Review Letters, Nair López Martínez and colleagues at Lawrence Berkeley National Laboratory and RoseStreet Labs Energy, both in the US, present a prototype solar cell where they fold the light sensitivity of many semiconductors into a single material.

A semiconductor will absorb light most efficiently at the energy of its band gap—the energy it takes to excite an electron from the valence to the conduction band. To get around the limitation of a single band gap, López et al. engineered a semiconductor alloy, $\text{GaNAs}$, which has both an intermediate and wide band gap and is therefore sensitive to both low- and high-energy light. In their test of the first “proof of concept” solar cell based on an intermediate band semiconductor, the team shows their device has a promising efficiency over a wide spectrum of the sun’s light. – Jessica Thomas

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