BERKELEY Lab scientists say solar cell boltages could be boosted with a new discovery relating to ferroelectric materials.
Scientists know that ferroelectrics (materials which have overall electrical polarisation) can develop very high photovoltages under illumination. But until now, no one knew how this process occurs.
The team of researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley has resolved the high-voltage mystery for one ferroelectric material and determined that the same principle should be at work in all similar materials.
The scientists worked with very thin films of bismuth ferrit (BFO). These thin films have domains where the electrical polarisation points in different directions. They were grown with control over the domain structure.
The BFO films studied by the researchers have a unique periodic domain pattern extending over distances of hundreds of micrometers. The domains form in stripes, each measuring 50 to 300 nanometers across, separated by domain walls two nanometers thick. In each of these stripes the electrical polarization is opposite from that of its neighbors.
The researchers were able to gain full microscopic understanding of what went on within each separate domain, and across many domains.
When light was shone on the BFO thin films, large voltages resulted, many times the band gap voltage of the material itself. The incoming photons free electrons and create corresponding holes, and a current begins to flow perpendicular to the domain walls – even though there's no junction, as there would be in a solar cell with negatively and positively doped semiconductors.
In an open circuit the current flows at right angles to the domain walls, and to measure it the researchers attached platinum electrical contacts to the BFO film.
It was clear that the domain walls between the regions of opposite electrical polarization were playing a key role in the increasing voltage. The scientists then constructed a detailed charge-transport model of BFO. This model indicated that the opposite charges on each side of the domain wall create an electric field that drives the charge carriers apart. On one side of the wall, electrons accumulate and holes are repelled. On the other side of the wall, holes accumulate and electrons are repelled.
While a solar cell loses efficiency if electrons and holes immediately recombine, that can't happen here because of the strong fields at the domain walls created by the oppositely polarised charges of the domains.
The efficiency of BFO's response to light – the ratio of charge carriers per incoming photons – is best near the domain walls. While very high voltages can be produced, the other necessary element of a powerful solar cell, high current, is lacking.