Layered Perovskite Solar Cell Hits 20% Efficiency Level
Solar cells made from the inexpensive and increasingly popular material perovskite can more efficiently turn sunlight into electricity using a new technique to sandwich two types of perovskite into a single photovoltaic cell.
Perovskite solar cells are made of a mix 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% of the sun’s energy.
In a paper prepared for publication in Nature Materials, University of California, Berkeley, and Lawrence Berkeley National Laboratory scientists have reported a new design that already achieves an average steady-state efficiency of 18.4%, with a high 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,” 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% – which is a phenomenal number, considering we are at the beginning of optimizing 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 efficiency is also better than the 10-20% 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% efficiency more than a decade ago.
The achievement comes 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.”
Materials like silicon and perovskite are semiconductors, meaning 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.
“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 maximize coverage of the solar spectrum.”
Cross-section of the new solar cell, showing the two perovskite layers (beige and red) separated by a single-atom layer of boron nitride and the thicker graphene aerogel (dark gray), which prevents moisture from destroying the perovskite. Gallium nitride (blue) and gold (yellow) electrodes channel the electrons generated when light hits the solar cell (photo Zettl Lab, UC Berkeley)
The key 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 – and the latter absorbs photons of energy 2 eV, or an amber color.
The monolayer of boron nitride allows the two perovskite materials to work together and make electricity from light across the whole range of colors between 1 and 2 eV.
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