Turbocharging the silicon solar cell to 35 percent efficiency

Solar cells are ridiculously wasteful. Any photon carrying less energy than the bandgap of the semiconductor will not be able to excite an electron and therefore will not contribute to solar current. Other photons do contribute, but all their energy in excess of the bandgap is lost as heat. Additional mechanisms lower efficiency as well, but these spectral losses hurt the most.

One solution is stacking solar cells with different bandgaps, allowing each of them to absorb different parts of the solar spectrum. These tandem cells can be quite effective, but they’re also complex and costly to manufacture.