Are Wiry Solar Cells an Alternative to Silicon?

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Tiny wires that behave like antennas could be used to make more efficient solar cells.

The new cells reach efficiencies competitive with those made of silicon, and could offer a way to make photovoltaic cells with less material, scientists say. The findings appear in the Jan. 17 issue of the journal Science.

Most solar cells we're familiar with are flat panels of silicon. Light hits the silicon and knocks off electrons, leaving behind a region of positive charge called a hole. Electrons can only move in one direction, so as they move toward the holes, they create current. Put a wire on each end of the cell, connect it to a load (say, a light bulb) and you have electricity.

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But silicon isn't the only material you could use. Indium phosphate, or InP, also generates current when exposed to light. The difference is that it responds to a slightly different part of the solar spectrum than silicon, and uses more of the energy of the incoming photons to make current. In other words, it’s more efficient at converting sunlight to electricity, explained study co-author Magnus Borgström, assistant professor in solid state physics at Sweden’s University of Lund.

The problem is indium phosphate isn't all that good at absorbing light, at least not when it is in a flat sheet like silicon. So the Lund researchers decided to make a cell with micrometer-long wires of indium phosphate that were each about 180 nanometers wide. Nanowire architectures have been tried before, but the efficiencies were only in the 3 to 5 percent range. The new cell reached up to 13.8 percent, which is comparable to typical commercial photovoltaic cells. The researchers found that the efficiency of the cell depends on the length and diameter of the wires.

Each wire is like a small standing tree, with the bottom part made of InP that is doped to give it an excess of positive charges, by adding occasional atoms of another element (in this case zinc). The middle section is undoped, while the top part is doped to give it an excess of negative charges. The wires are all surrounded by an insulator of silicon dioxide and topped with a layer of indium tin oxide, which is transparent.

Silicon is a stiff material and many large sheets must be combined to make an effective light collector. The nanowires of indium phosphate aren't shaped like that — which means solar cells made from them can be flexible.

Previously, the nanowires were difficult to make, but the Lund group found a way to make the wires come out with consistent sizes and shapes.

Borgström said the advantage of this design is that the indium phosphate only covers about 12 percent of the surface, but converts a bigger share of the sun's energy.

This kind of solar cell is more likely to be used in concentrated photovoltaic systems, said Borgström. That's where light is concentrated onto a small area in order to boost the amount of energy that hits the PV cell. The next step will be trying to boost the efficiency of the cell. "If you get to the 30 percent range, that makes it commercially interesting," Borgström said.

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