New Path to Solar Energy Via Solid-State Photovoltaics
Friday, April 02 2010
Working with bismuth ferrite, a ceramic made from bismuth, iron, and oxygen that is multiferroic (displaying both ferroelectric and ferromagnetic properties), the researchers discovered that the photovoltaic effect can spontaneously arise at the nanoscale as a result of the ceramic's rhombohedrally distorted crystal structure. They demonstrated that the application of an electric field makes it possible to manipulate this crystal structure and thereby control photovoltaic properties.
At the heart of conventional solid-state solar cells is a p-n junction - the interface between a semiconductor layer with an abundance of positively-charged "holes" - and a layer with an abundance of negatively charged electrons. When photons from the sun are absorbed, their energy creates electron-hole pairs that can be separated within a "depletion zone," a microscopic region at the p-n junction measuring only a couple of micrometers across, then collected as electricity. For this process to take place, the photons have to penetrate the material to the depletion zone, and their energy has to precisely match the energy of the semiconductor's electronic bandgap – the gap between its valence and conduction energy bands where no electron states can exist.
"The maximum voltage conventional solid-state photovoltaic devices can produce is equal to the energy of their electronic bandgap," says Jan Seidel, a physicist with Berkeley Lab's Materials Sciences Division and the UC Berkeley Physics Department. "Even for so called tandem-cells, in which several semiconductor p-n junctions are stacked, photovoltages are still limited because of the finite penetration depth of light into the material."