Tandem solar cell achieves 26 percent peak efficiency
Scientists at the University of California, Berkeley, and Lawrence Berkeley National Laboratory have reported a new perovskite photovoltaic device design that achieves an average steady-state efficiency of 18.4 percent, with a high of 21.7 percent and a peak efficiency of 26 percent. The results are published in the journal Nature Materials.
"We have set the record now for different parameters of perovskite solar cells, including the efficiency," said senior author Alex Zettl, a UC Berkeley professor of physics, 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 optimising this."
"This has a great potential to be the cheapest photovoltaic on the market, plugging into any home solar system," said Onur Ergen, the lead author of the paper and a UC Berkeley physics graduate student.
The achievement comes thanks to a new way to combine two perovskite solar cell materials - each tuned to absorb a different wavelength or colour 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 realising 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."
Full-spectrum solar cells
Like all semiconductors, perovskite materials only 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.
"In this case, we are swiping the entire solar spectrum from infrared through the entire visible spectrum," Ergen said. "Our theoretical efficiency calculations should be much, much higher and easier to reach than for single-bandgap solar cells because we can maximise coverage of the solar spectrum."
The key to mating the two materials into a tandem solar cell is a single-atom thick layer of hexagonal BN, 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 colour.
The monolayer of BN allows the two perovskite materials to work together and make electricity from light across the whole range of colours between 1 and 2 eV.
The perovskite/BN 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 stabilise charge transport though the solar cell, Zettl said. Moisture makes perovskite fall apart.
The whole thing is capped at the bottom with a gold electrode and at the top by a GaN 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," 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 BN 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 GaN layer serves as the top asphalt layer."
It is possible to add even more layers of perovskite separated by hexagonal BN, though this may not be necessary, given the broad-spectrum efficiency they've already obtained, the researchers said.
"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," Zettl said. "With this new material, we are in the regime of roll-to-roll mass production; it's really almost like spray painting."
