From the Labs: Materials

Saving Silicon in Solar Cells
A microwire composite could help reduce the cost of solar power

Source: “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications”
Harry Atwater et al.
Nature Materials 9: 239-244

Results: Sparse arrays of silicon microwires embedded in a rubbery polymer absorb as much light as the active layer in a high-performance solar cell but use just 1 percent as much silicon. The key to the material’s efficiency is that highly reflective alumina nanoparticles in the polymer direct incoming sunlight toward the microwires. Computational models predict that the composite material could be made into solar cells that convert 15 to 20 percent of the energy in sunlight into electricity. This performance is close to that of the best cells on the market today.

Why it matters: Because it uses much less of the expensive silicon in high-­performance solar cells without sacrificing the amount of light absorbed, the new composite has the potential to dramatically reduce the cost of solar power.

Methods: Microwires are grown from silane gas, at high pressure and temperature, on the surface of a reusable template that Caltech researchers designed after testing various geometric arrangements and calculating how thick and how far apart the wires should be. Next, the wires are treated with an antireflective coating, and a polymer mixed with alumina nanoparticles is poured over them. When it solidifies, the polymer can be peeled off the template, taking the array of silicon wires with it. Incoming light bounces off the nanoparticles until it can be absorbed by a wire. If any light makes it through the material without being absorbed, a silver backing reflects it back in.

Next steps: So far, the researchers have made small arrays–on the order of several square centimeters. Once they’re able to make the arrays hundreds of times bigger, they will build solar cells by adding layers such as the electrodes needed to extract electrons and generate power.

Ultrafast Graphene
IBM creates large arrays of transistors that are ­speedier than silicon

Source: “100-GHz Transistors from Wafer-Scale Epitaxial Graphene”
Yu-Ming Lin, Phaedon Avouris, et al.
Science 327: 662

Results: Researchers at IBM have made graphene transistors that operate at 100 gigahertz, 10 times as fast as the speediest silicon transistors of the same size and faster than previous graphene transistors. They produced large arrays of these transistors, using methods that are compatible with semiconductor manufacturing.

Why it matters: Although graphene transistors made in earlier research have shown promising speed, the techniques used for making them haven’t lent themselves to mass production. Because the new transistors are easy to make, they could be a practical substitute for conventional transistors in military communications, radar, medical and security imaging, and a variety of other applications that require high-frequency analog transistors.

Methods: The researchers heated a five-centimeter-wide wafer of silicon carbide until the silicon at the surface evaporated, leaving behind graphene, an atom-thick sheet of carbon. Then they used conventional lithographic techniques to lay down source and drain electrodes and a dielectric (or insulating) layer, resulting in a pattern of numerous transistors. The addition of a thin polymeric layer between the graphene and the dielectric layer kept electrical charges in the insulator from compromising the graphene’s electrical properties, thus dramatically improving the transistors’ performance.

Next steps: The researchers plan to improve upon their current work by building integrated circuits like those used in computer processors. They will also attempt to speed up the transistors further by making them smaller. So far, the graphene transistors they’ve made are 10 times the size of the best silicon transistors.