A technology that improves the photovoltaic qualities of virtually any semiconductor could lead to the use of plentiful, relatively inexpensive materials that once were considered unsuitable for solar cells because their properties were so difficult to tailor by chemical means. Semiconductor materials used in solar devices display what is called the photovoltaic effect — they absorb photons of light and release electrons that can be channeled into an electrical current. Not all semiconductor materials exhibit this effect, and those that do — such as silicon, cadmium telluride or copper indium gallium selenide — are rare and expensive to fabricate. Researchers with the US Department of Energy’s Lawrence Berkeley National Laboratory and the University of California, Berkeley, thus sought to use cheaper, easier-to-obtain semiconductor materials — such as metal oxides, sulfides and phosphides — to develop photovoltaic devices. "It's time we put bad materials to good use," said physicist Alex Zettl, who led this research along with colleague Feng Wang. "Our technology allows us to sidestep the difficulty in chemically tailoring many earth-abundant, nontoxic semiconductors and instead tailor these materials simply by applying an electric field." The new technology, called screening-engineered field-effect photovoltaics (SFPV), utilizes the electric field effect, a phenomenon in which the concentration of charge-carriers in a semiconductor is altered by the application of an electric field. The SFPV technology was tested for two top electrode architectures: (a) the top electrode is shaped into narrow fingers; (b) top electrode is uniformly ultrathin. (Image: Berkeley Lab) With the SFPV method, a carefully designed partially screening top electrode lets the gate electric field sufficiently penetrate the electrode and more uniformly modulate the semiconductor carrier concentration and type to induce a p-n junction. In solar devices, a p-n junction is the active site where electrons move to generate current. By creating or inducing this junction, the team created a material capable of the photovoltaic effect. "Our demonstrations show that a stable, electrically contacted p-n junction can be achieved with nearly any semiconductor and any electrode material through the application of a gate field provided that the electrode is appropriately geometrically structured," Wang said. Alex Zettl (left) and Will Regan can make low-cost, high-efficiency solar cells from virtually any semiconductor material. (Image: Roy Kaltschmidt) The scientists also demonstrated the SFPV effect in a self-gating configuration in which the gate was powered internally by the electrical activity of the cell itself. "The self-gating configuration eliminates the need for an external gate power source, which will simplify the practical implementation of SFPV devices," said lead author William Regan. "Additionally, the gate can serve a dual role as an antireflection coating, a feature already common and necessary for high-efficiency photovoltaics." The work, which appeared in Nano Letters, was supported by the DoE’s Office of Science and the National Science Foundation. For more information, visit: www.lbl.gov