Nanowire solar cells made efficient

At first glance, an array of nanowires makes for a lousy solar cell. The nanowires’ surface-to-volume ratio is huge. So when an absorbed photon produces an electron–hole pair, the charges may separate only briefly before recombining at surface defects. It’s no surprise, then, that power-conversion efficiencies for such devices have remained low, about 3–5%. Still, nanowire-based solar cells require far less material than thin-film planar versions and thus can reduce costs. Researchers led by Magnus Borgström (Lund University in Sweden) have now shown that a judicious choice of material and refinements to its growth can dramatically improve the solar cells’ efficiency. The researchers used indium phosphide because it exhibits slower electron–hole surface recombination. And through modeling, they optimized two key design parameters: the nanowires’ diameter and the length of the n-doped segment at the top of each wire. A proof-of-concept device converts about 14% of the incoming light into electric power—a new record for nanowire-based solar cells. What’s more, that amount of electricity is nearly as much as the electricity produced by more conventional InP thin-film cells, despite the fact that the nanowires’ packing fraction is just 12% of the same area. A few of the 4 million nanowires—each 180 nm in diameter and 1.5 µm tall—in the team’s 1-mm² cell are pictured here from above and on angle. Though not yet efficient enough for commercialization, the device is readily scalable to wafer-sized cells. (J. Wallentin et al., Science, in press, doi:10.1126/science.1230969.)—R. Mark Wilson