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
Solar cells made from an inexpensive and increasingly popular material called perovskite can more efficiently turn sunlight into electricity using a new technique to sandwich 2 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. A new design achieves an average steady-state efficiency of 18.4%, with a high of 21.7% and a peak efficiency of 26%.
“This has a great potential to be the cheapest photovoltaic on the market, plugging into any home solar system,” said Onur Ergen, 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 2 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 as the materials degrade one another’s electronic performance.
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, eg – 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.”
The key to mating the two materials into a tandem solar cell is a single-atom layer of hexagonal boron nitride, 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 1eV – infrared, while 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.
The perovskite/boron nitride sandwich is placed atop 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. The whole thing is capped at the bottom with a gold electrode and at the top by a gallium nitride layer that collects the electrons generated within the cell. The active layer of the thin-film solar cell is ~400nm thick.
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 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-toroll mass production; it’s really almost like spray painting.”
http://news.berkeley.edu/2016/11/07/major-advance-in-solar-cells-made-of-cheap-easy-to-use-perovskite/
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