Researchers at Berkeley Lab are developing a novel technology that allows the fabrication of low-cost, high efficiency solar cells from any semiconductor material. The new technology allows scientist to sidestep the difficulty in chemically tailoring many earth-abundant, non-toxic semiconductors such as metal oxides, sulfides and phosphides.
The paper describing this research was published in Nano Letters with the title “Screening-Engineered Field-Effect Solar Cells.” The authors were Alex Zettl, William Regan, Steven Byrnes, Will Gannett, Onur Ergen, Oscar Vazquez-Mena and Feng Wang. The research was a joint collaboration of the US Department of Energy’s Lawrence Berkeley National Laboratory and the University of California Berkeley.
Solar cells convert sunlight into electricity using semiconductor materials that absorb photons and release electrons that can be channeled into an electrical circuit. They are the ultimate source of clean, green and renewable energy. However, these cells use relatively scarce and expensive semiconductors such as large crystals of silicon and thin films of cadmium telluride or copper indium gallium selenide.
According to Zettl, solar technologies at present face a cost-to-efficiency trade-off that has hampered its widespread implementation. He adds that their new technology reduces the cost and complexity of fabricating solar cells, and has the potential to accelerate the usage of solar energy.
The new technology is called “screening-engineered field-effect photovoltaics,” or SFPV. It alters the concentration of charge-carriers in a semiconductor by the use of an applied electric field.
In the SFPV technology, a carefully designed partially screening top electrode lets the gate electric field sufficiently penetrate the underlying semiconductor. This modulates the semiconductor carrier concentration and type to induce a p-n junction. This allows the creation of high quality p-n junctions in semiconductors that are otherwise difficult or impossible to dope by conventional chemical methods.
Regan says the SFPV technology only requires electrode and gate deposition. It eliminates the need for high-temperature chemical doping, ion implantation or other complex and expensive processes. The key to the technique is the minimal screening of the gate field, which is achieved through geometric structuring of the top electrode. This makes it possible to simultaneously perform carrier modulation and electrical contact to the semiconductor.
The Berkeley researchers experimented with two configurations for the top electrode. In one configuration using copper oxide, the electrode contact was shaped into narrow fingers. This allowed the gate field to create a low electrical resistance inversion layer between the fingers and a potential difference beneath them.
In another configuration using silicon, the electrode contact was ultra-thin (single layer graphene) across the surface. This allowed the gate field to penetrate and deplete/invert the underlying semiconductor.
The researchers reported that both configuration designs yielded high quality p-n junctions. Feng Wang further asserted that a stable, electrically contacted p-n junction could be achieved with nearly all semiconductor and electrode material through the application of a gate field.
The researchers also demonstrated a self-gating configuration of the SFPV system. In this configuration, the gate was internally powered by electrical activity within the cell. This could simplify the practical implementation of the device since no external gate power source is needed.
The US Department of Energy and the National Science Foundation funded the research.