One way around the environmental problem is to use silicon as the active material. Since it is the main ingredient in dirt, it is abundant and harmless… It also makes a fairly awful solar cell. The natural ability of silicon to absorb light isn’t as good as the exotic materials, and the ultrathin crystals needed for a solar cell just can’t achieve the efficiencies required. Consequently, silicon solar cells need to cover a huge area to produce enough electricity to be feasible, and that brings us back to the problem of cost. To see how Paetzold et al. try to correct this, we need to consider the solar cell in action.
The cell is a tiny, flat plate of silicon exposed to the sunlight. Photons—particles of light—continuously stream through the cell, going in one side and out the other. Once in a while, the silicon catches a photon and converts it into an electrical charge, which gets stored in a battery. A silicon wafer of the usual thickness is able to convert only about one out of every five photons it sees, and that is the absolute state of the art. It seems that we could increase that number by making the plate thicker, but that strategy turns out to be flawed. Not only would using more silicon be expensive, but thicker material makes it harder to carry the electricity to a battery. The not-so-obvious alternative is to make each photon pass through the silicon many times.
Several research groups have proposed that the front or the back surface of the cell could be coated with metal nanoparticles, each about a quarter of the size of a virus. If a photon runs into one of them, it has a good chance of bouncing off, or “scattering,” in some direction. With enough nanoparticles, each photon might scatter many times, passing though the silicon over and over, creating more opportunities for absorption. Early experiments have shown a slight increase in efficiency using this method, but Paetzold et al. think we can do better.
It is possible that if we could control the exact size, shape, material, and placement of the nanoparticles, we could maximize the time that a photon spends in the silicon, resulting in the best possible conversion efficiency. The authors use a computer program to simulate many different kinds of particles in solar cells. The program uses Maxwell’s equations, which govern the physics of light, to show how light interacts with the particles and to predict the solar cell’s efficiency for each color of visible light. They find that hemispheres of silver spaced by approximately 450 billionths of a meter do a good job of scattering the light back into the material, but many cases are analyzed in detail, including the performance of single nanoparticles. The research group believes that the ability to fabricate arrays of nanoparticles of this kind is honed, their work should find ready application in solar cell design. Assuming the process isn’t too expensive, we can expect to see solar cells like these emerging as a reality in the near future.
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