Scientists at the University of California at Berkeley and the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have revealed the details of the photovoltaic process in ferroelectric materials using bismuth ferrite or BFO ultra thin films.
Domains with opposite electrical polarization, averaging about 140 nanometers wide and separated by walls 2 nanometers thick, form a well-aligned array in a thin film of bismuth ferrite.
The researchers have also discovered that the same principle can be applied on all types of similar materials. The BFO films investigated by the researchers have a specific periodic domain pattern spanning distances of several micrometers. The domains create stripes, each of which has a width of 50 to 300 nm, isolated by domain walls of thickness of 2 nm. The electrical polarization in every stripe is in the opposite direction of that of its adjacent stripes.
Joel Ager, one of the researchers, said that the research team accurately knew the position and the level of the built-in electric fields in BFO films. The researchers observed very high voltages, more than the material’s band gap voltage when the BFO thin films were illuminated, he said. The electrons freed by the incoming photons generate corresponding holes, resulting in the flow of current in the perpendicular direction to the domain walls, he added.
The scientists fitted platinum electrical contacts to the BFO ultrathin films to measure the current. The experiment proved that the domain walls between the areas of opposite electrical polarization were augmenting the photovoltaic voltage. The opposite charges on both sides of the domain wall generate an electric field that forces the charge carriers apart. At one side of the wall, electrons repel and holes are accumulated, while on the other side, holes repel and electrons are accumulated.
Solar cells efficiency drops due to the immediate recombination of holes and electrons. However, in the BFO films, the domains’ oppositely polarized charges produce strong electric fields at the domain walls to prevent the recombination. Electrons and holes move in the opposite direction from the domain walls toward the domain’s center that has a weaker electric field. As the number of electrons is more than the holes, the extra electrons are forced from one domain to the other in the same direction, as directed by the overall current. Ager described it as ‘bucket brigade’ with every bucket of electrons pumped from one domain to the other.
The BFO ultrathin films’ efficiency of light responsiveness is highest near the domain walls. Even though they produce ultrahigh voltages, they are short of high current, another key factor for a powerful solar cell. The combination of ferroelectrics’ ‘bucket brigade’ photovoltaic effect with high currents is able to produce solar cell arrays with superior efficiency.