The team of researchers from Drexel University, the Shubnikov Institute of Crystallography of the Russian Academy of Sciences, the University of Pennsylvania and the US Naval Research Laboratory are using barium titanate crystals to convert sunlight into electric power much more efficiently than the theoretical Shockley-Queisser limit predicts for a bandgap material that absorbs almost no light in the visible spectrum. This could also be a boost for the development of transparent solar cells.
The energy collection mechanism relies on collecting “hot” electrons, those that carry additional energy in a photovoltaic material when excited by sunlight, before they lose their energy. This bulk photovoltaic effect could open up new cell design techniques.
“Barium titanate absorbs less than a tenth of the spectrum of the sun. But our device converts incident power 50 percent more efficiently than the theoretical limit for a conventional solar cell constructed using this material or a material of the same energy gap,” said Dr Jonathan Spanier, a professor of materials science, physics and electrical engineering at Drexel University in the US and one of the principal authors of the study.
The new approach also makes use of the screening effect in ferroelectric materials. The nanoscale electrode used to collect the current in a solar cell enhances this field, boosting impact ionization and carrier multiplication to create a cascade of electrons. This boosts the efficiency past the Shockley-Queisser limit that assumes the excess energy is lost as heat.