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Future Solar Cells Made of Bismuth Ferrite Could Yield Higher Voltages

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Domain walls - two-dimensional sheets that run through a multiferroic and serve as transition zones, separating regions of different ferromagnetic or ferroelectric properties.

Scientists from the Lawrence Berkeley National Laboratory found a way to use a ceramic material made from bismuth, iron and oxygen (bismuth ferrite) to fabricate solar cells in a fashion that nobody ever tried to do. They even have results yielding high voltages out of their material.

The researchers found out that the photovoltaic effect can manifest itself at nanoscale levels, because of the ceramic material’s rhombohedrally-distorted crystalline structure, also demonstrating that by applying an electric field, they can manipulate the crystalline structure and control its light-harvesting capacities.

“We’re excited to find functionality that has not been seen before at the nanoscale in a multiferroic material,” said Jan Seidel, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the UC Berkeley Physics Department. “We’re now working on transferring this concept to higher efficiency energy-research related devices.”

Regular solar cells have a p-n junction, as a semiconductor. If you remember from high-school physics classes, a semiconductor has “holes” – on the positive part and negatively charged electrons, on the other. When photons hit the semiconductor surface, 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, however, 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.

They discovered that by applying white light to bismuth ferrite (material that is both ferroelectric and antiferromagnetic), they could generate voltages within submicroscopic areas between one and two nanometers across. These photovoltages were significantly higher than bismuth ferrite’s electronic bandgap.

“The bandgap energy of the bismuth ferrite is equivalent to 2.7 volts. From our measurements we know that with our mechanism we can get approximately 16 volts over a distance of 200 microns. Furthermore, this voltage is in principle linear scalable, which means that larger distances should lead to higher voltages.”

“While we have not yet demonstrated these possible new applications and devices, we believe that our research will stimulate concepts and thoughts that are based on this new direction for the photovoltaic effect,” Seidel says.

Bismuth ferrite can play a good piezoelectric material, being able to harvest road vibrations, a fact that UC Berkeley was also interested in, last year.

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