Graded Bandgap Perovskite Solar Cells Set Efficiency Record
Solar cells made from perovskite can more efficiently turn sunlight into electricity employing a new method to sandwich two types of perovskite into a single photovoltaic cell.
Perovskite solar cells are comprised of a blend of organic molecules and inorganic elements that together capture light and convert it into electricity, just like today’s more common silicon-based solar cells. Perovskite photovoltaic devices, however, can be made more easily and cheaply than silicon and on a flexible rather than rigid substrate.
The first perovskite solar cells could go on the market next year, and some have been reported to capture 20 percent of the sun’s energy.
26% Peak Efficiency
University of California, Berkeley, and Lawrence Berkeley National Laboratory scientists report a new design that already achieves an average steady-state efficiency of 18.4 percent, with a high of 21.7 percent and a peak efficiency of 26 percent.
Senior author Alex Zettl, a UC Berkeley professor of physics, said:
“We have set the record now for different parameters of perovskite solar cells, including the efficiency. The efficiency is higher than any other perovskite cell – 21.7 percent – which is a phenomenal number, considering we are at the beginning of optimizing this.”
This first version of a new layered perovskite solar cell already achieves an efficiency of more than 20 percent, rivaling many commercial solar cells. Flexible and easy to make, it can produce more than half a volt of electricity. Credit: Onur Ergen, UC Berkeley
The efficiency is also better than the 10-20 percent efficiency of polycrystalline silicon solar cells used to power most electronic devices and homes. Even the purest silicon solar cells, which are extremely expensive to produce, topped out at about 25 percent efficiency more than a decade ago.
The achievement is thanks to a new way to combine two perovskite solar cell materials, each tuned to absorb a different wavelength or color of sunlight, into one “graded bandgap” solar cell that absorbs nearly the entire spectrum of visible light. Previous attempts to merge two perovskite materials have failed because the materials degrade one another’s electronic performance.
“This is realizing a graded bandgap solar cell in a relatively easy-to-control and easy-to-manipulate system,” Zettl said. “The nice thing about this is that it combines two very valuable features – the graded bandgap, a known approach, with perovskite, a relatively new but known material with surprisingly high efficiencies – to get the best of both worlds.”
Hexagonal Boron Nitride
Essential to mating the two materials into a tandem solar cell is a single-atom thick layer of hexagonal boron nitride, which looks like a layer of chicken wire separating the perovskite layers from one other.
In this case, the perovskite materials are made of the organic molecules methyl and ammonia, but one contains the metals tin and iodine, while the other contains lead and iodine doped with bromine. The former is tuned to preferentially absorb light with an energy of 1 electron volt (eV) – infrared, or heat energy – while the latter absorbs photons of energy 2 eV, or an amber color.
Materials like silicon and perovskite are semiconductors, which means they conduct electricity only if the electrons can absorb enough energy, from a photon of light, for example, to kick them over a forbidden energy gap or bandgap. These materials preferentially absorb light at specific energies or wavelengths, the bandgap energy, but inefficiently at other wavelengths.
“Our architecture is a bit like building a quality automobile roadway,” Zettl said. “The graphene aerogel acts like the firm, crushed rock bottom layer or foundation, the two perovskite layers are like finer gravel and sand layers deposited on top of that, with the hexagonal boron nitride layer acting like a thin-sheet membrane between the gravel and sand that keeps the sand from diffusing into or mixing too much with the finer gravel. The gallium nitride layer serves as the top asphalt layer.”
It is possible to add even more layers of perovskite separated by hexagonal boron nitride, though this may not be necessary, given the broad-spectrum efficiency they’ve already obtained, the researchers remark.