Wadia synthesizes the nanocrystals of copper sulfide by injecting copper and sulfide salts and a surfactant into a three-neck flask over a hot plate; as a magnetic stir bar spins inside, nanoparticles of the compound form. After removing the surfactant and resuspending the nanoparticles in chloroform, he transfers them into a glove box. Inside is a glass chip, about 2.5 centimeters square, that has been coated with a thin layer of indium tin oxide, which acts as an electrical contact. Wadia places the glass chip on a small disc and pipettes the inky black suspension of pyrite nanocrystals onto it. He starts the disc spinning rapidly for a minute to spread the nanocrystals in an even layer. Then he sets the chip on a hot plate and heats it for 10 to 15 minutes to fix the particles to its surface.
After Wadia repeats the process with the copper sulfide solution, the bottom electrical contact is covered by the nanoparticle layers. He gives the chip a quick swipe with a plain cotton swab to reëxpose a strip of the indium tin oxide that acts as the bottom electrical contact for the cell. He then covers the chip with a mask, or stencil, that outlines two sets of four squares with rectangular tails. Wadia places the chip and a small piece of solid aluminum inside a thermal evaporator that looks like a metal bell jar. After he seals the jar, he heats it; the aluminum evaporates, and as it cools, it settles on the exposed parts of the chip. This creates eight square electrical contacts with tails that lead to the edge of the chip.
Pyrite Sees the Light
The chip is now ready for testing. Wadia unscrews a solar-cell tester, places the chip inside, and screws it back together. He then illuminates it with light that mimics the distribution of wavelengths found in sunlight. When the light hits the chip, the system measures the current, the voltage across the chip, and other properties. A screen displays a plot of the current running through the cell against the voltage running across it. So far, the pyrite-based cells have proved disappointing in their performance, though the Berkeley researchers have used copper sulfide in combination with cadmium sulfide to make cells that have a 1.6 percent efficiency. That’s not good enough for practical use, but the results are promising enough to justify continuing work on the technology.
Cells incorporating pyrite would be preferable because the material is less toxic and cheaper to recover than cadmium compounds. When the pyrite nanoparticles are spun onto the chip, however, nanoscale pinholes tend to form. To electrons, such minuscule gaps look like the Grand Canyon–they cannot cross and migrate into the external electrical circuit. Instead, the electrons tunnel down to the bottom electrode, causing the cell to short-circuit.
It’s difficult to make good pyrite films because the nanocrystals tend to sink to the bottom of any liquid. The better a particle is suspended, the smoother the film it will form. Wadia believes that smaller particles might lead to better suspensions: the pyrite particles are 20 to 100 times the size of the copper sulfide particles, which are about five nanometers across. Wadia is trying everything he can to make them smaller, including mechanically pressing or grinding them and tinkering with reaction conditions. He’s also collaborating with bioengineers at the Lawrence Berkeley National Laboratory to genetically engineer viruses so that they accumulate pyrite nanoparticles on their coats; the next step would be to get the viruses to line up into uniform films.
Wadia acknowledges that he’s still many years away from making an efficient solar cell with pyrite nanocrystals. Ultimately, though, his goal is to produce a cell that’s cheap enough to make solar energy the dominant power source. He says, “I just need the science to work.”