Two things hold back the mass adoption of solar energy as a source of sustainable energy. One is the need to store and transmit excess power, a problem people like Danielle Fong are working on solving by developing innovative new ways to store power. The other is the high cost of solar panels. One of the reasons solar panels are so expensive is that it’s tricky to extract electric currents from semiconductors, the materials used to convert solar radiation into electrical energy.

Up til now, this could only be done with a few materials – usually silicon. But a new breakthrough will enable manufacturers to make efficient photovoltaics using almost any semiconductor, including cheap and abundant materials like metal oxides, sulfides, and phosphides.

A typical photovoltaic cell is built with silicon and treated with chemicals. This treatment is called "doping," and it creates the driving force needed to extract power from the cell. Photovoltaics can also be built with cheaper materials but many of these can't be doped chemically. But a method developed by Professor Alex Zettl’s research group at Lawrence Berkeley National Laboratory and University of California at Berkeley makes it possible to dope nearly any semiconductor by applying an electric field instead of chemicals. The method is described in a paper published in the journal Nano Letters.

According to Will Regan, lead author of the paper, it's long been known in the transistor industry that applying an electric field could be used for doping, but existing electrode designs were incompatible with photovoltaic cells. What the researchers discovered is a new way of designing electrodes to allow an electric field to pass through and dope the semiconductor.

"Graphene was the inspiration," Regan explains. Graphene is a highly conductive, one-atom-thick sheet of carbon. The team at the Zettl Research Group began experimenting with graphene as a transparent electrode for silicon photovoltaics and realized they could directly influence the semiconductor with an applied electric field. Once they'd realized that a very thin conductor could be used, they realized a very narrow one would be suitable as well. The paper describes two ways of building the electrodes: one with graphene, the other with extremely narrow nanowires.

While there’s a fair amount of inertia in the solar manufacturing industry, Regan is optimistic that this new method will be adopted, noting that these cells could be made using simple and cost-effective tweaks to existing fabrication processes.

Photo courtesy of Paul Takizawa, the Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley.