Berkeley Lab technology could lead to photovoltaics from any semiconductor
By Anthony Capkun
July 26, 2012 – A technology that would enable low-cost, high-efficiency solar cells to be made from virtually any semiconductor material has been developed by researchers with the U.S. Department of Energy’s (DoE) Lawrence Berkeley National Laboratory and the University of California (UC) Berkeley. This opens the door, say researchers, to the use of “plentiful, relatively inexpensive semiconductors, such as the promising metal oxides, sulfides and phosphides, that have been considered unsuitable for solar cells because it is so difficult to tailor their properties by chemical means”.
“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, non-toxic semiconductors and instead tailor these materials simply by applying an electric field.”
Today’s photovoltaic technologies utilize relatively scarce and expensive semiconductors, say researchers, such as large crystals of silicon, or thin films of cadmium telluride or copper indium gallium selenide, that are tricky or expensive to fabricate into devices.
“Solar technologies today face a cost-to-efficiency trade-off that has slowed widespread implementation,” Zettl said. “Our technology reduces the cost and complexity of fabricating solar cells and thereby provides what could be an important cost-effective and environmentally friendly alternative that would accelerate the usage of solar energy.”
This new technology is called ‘screening-engineered field-effect photovoltaics’ (SFPV) because it utilizes the electric field effect, a phenomenon by which the concentration of charge-carriers in a semiconductor is altered by the application of an electric field. With the SFPV technology, 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. This enables the creation of high-quality p-n junctions in semiconductors that are difficult—if not impossible—to dope by conventional chemical methods.
“Our technology requires only electrode and gate deposition, without the need for high-temperature chemical doping, ion implantation, or other expensive or damaging processes,” said lead paper author William Regan. “The key to our success is the minimal screening of the gate field which is achieved through geometric structuring of the top electrode. This makes it possible for electrical contact to and carrier modulation of the semiconductor to be performed simultaneously.”
Under the SFPV system, the architecture of the top electrode is structured so that at least one of the electrode’s dimensions is confined. In one configuration, working with copper oxide, the Berkeley researchers shaped the electrode contact into narrow fingers; in another configuration, working with silicon, they made the top contact ultra-thin (single layer graphene) across the surface. With sufficiently narrow fingers, the gate field creates a low electrical resistance inversion layer between the fingers and a potential barrier beneath them. A uniformly thin top contact allows gate fields to penetrate and deplete/invert the underlying semiconductor. The results in both configurations are high-quality p-n junctions.
Photo 1: Alex Zettl (left) and Will Regan can make low-cost, high efficiency solar cells from virtually any semiconductor material.
Photo 2: The SFPV technology was tested for two top electrode architectures: (A) the top electrode is shaped into narrow fingers; (B) top electrode is uniformly ultra-thin.