23 August 2012
The U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California Berkeley have discovered a new way to make high-efficiency, low-cost photovoltaics from virtually any semiconductor — even materials previously considered unsuitable for solar cells. The technology, called screening-engineered field-effect photovoltaics (or SFPV), involves doping semiconductors by applying an electric field through a nanostructured electrode.
Until now, the industry has thought many innocuous materials to be lousy semiconductors, because they cannot be stably chemically doped and form poor heterojunctions. Berkley’s process, however, does not require chemical dopants and enables tuning to make better heterojunctions. “We use an electric field, applied with the gate, to locally change the concentration of mobile charges in the semiconductor; we refer to this as ‘electric-field doping’ as it has an analogous effect to chemical doping,” explains William Regan, graduate student researcher at the Zettl Lab in UC Berkeley’s Department of Physics. “For large enough electric fields, we can even change the type of the majority charge carrier from electrons to holes or vice versa. Since our effect only relies on a very fundamental phenomenon — the movement of charges in an electric field — we can apply it to any semiconductor.”
Crystalline silicon, the dominant semiconductor used in today’s solar cells, though earth-abundant, is very energy-intensive and expensive to process. Regan says, “Many oxide, phosphide and sulfide semiconductors, materials that can now be used with our new technique, are also extremely abundant and have the added benefit of low refining costs.”
Referring to the theoretical Shockley-Queisser limit of 33.7% for solar cell conversion efficiencies, Regan says, “Many of the ‘bad materials’ that we can now use are situated near the peak of that curve. So our technology has the potential to create high-efficiency (mid twenties) solar cells with very cheap and abundant materials.”
Regan, who also is the lead author of the paper "Screening-Engineered Field-Effect Solar Cells" in Nano Letters, thinks of the novel technology as “combining a metal-oxide-semiconductor field-effect transistor (MOSFET) and a standard solar cell.” The materials scientist says the challenge was to adapt the field-effect gating to the solar cell architecture. “We overcame it by figuring out a way to structure our solar cell contacts in a way to let the electric field pass through the contacts and affect the semiconductor.” Actually, his team found two solutions to the problem: making the electrode very narrow (“nanofingers”) or very thin (e.g. graphene).
The production of Berkeley’s cell does not require any new or unusual processes, which could facilitate a relatively swift commercialization of the technology. “The only process that is not yet common on the industrial scale is the fabrication of our new top electrode design, and there are several methods out there for making nanostructured electrodes, which are rapidly gaining traction. So we think that adoption by existing manufacturers could happen relatively soon, perhaps within the next few years,” Regan says. “Commercializing some of the newly useful abundant oxides, sulfides, and phosphides could require new fabrication facilities, which would require more time to finance and build, but they represent a great long term solution to making solar energy affordable and plentiful.”
Image: "Rusty photovoltaics" — an artist's rendering of solar panels made from low-cost, earth-abundant, non-toxic metal oxide. Courtesy of Paul Takizawa.
Written by Sandra Henderson, Research Editor, Solar Novus Today