The researchers from the University of California, Berkeley, and Lawrence Berkeley National Laboratory use two layers of different perovskite materials that can be sprayed onto a flexible surface. Their design achieves an efficiency of 21.7% and a peak efficiency of 26%.
“We have set the record now for different parameters of perovskite solar cells, including the efficiency,” says Alex Zettl, professor of physics at UC Berkeley, senior faculty member at Berkeley Lab and member of the Kavli Energy Nanosciences Institute. “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.”
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,” says Zettl. “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.”
“In this case,” says Onur Ergen, lead author of the paper and a UC Berkeley physics graduate student, “we are swiping the entire solar spectrum from infrared through the entire visible spectrum. Our theoretical efficiency calculations should be much, much higher and easier to reach than for single-bandgap solar cells because we can maximize coverage of the solar spectrum.”
The key to combining the two materials into a tandem solar cell is a single-atom thick layer of hexagonal boron nitride separating organic molecules methyl and ammonia, one with tin and iodine, the other with lead and iodine doped with bromine. The former is tuned to preferentially absorb infrared light with an energy of 1 eV while the latter absorbs 2 eV yellow photons.
The perovskite/boron nitride sandwich is placed on a lightweight aerogel of graphene that promotes the growth of finer-grained perovskite crystals, serves as a moisture barrier and helps stabilize charge transport though the solar cell, says Zettl, and capped at the bottom with a gold electrode and at the top by a gallium nitride layer that collects the electrons that are generated within the cell. The active layer of the thin-film solar cell is about 400 nanometers thick.
“Our architecture is a bit like building a quality automobile roadway,” says Zettl. “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.”
“People have had this idea of easy-to-make, roll-to-roll photovoltaics, where you pull plastic off a roll, spray on the solar material, and roll it back up. With this new material, we are in the regime of roll-to-roll mass production; it’s really almost like spray painting.”
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