In the context of modern solar cells,
perovskite is a synthetic material that has the same crystalline structure as the naturally occurring mineral perovskite, first found in the Ural Mountains of Russia in 1839. The mineral perovskite is a calcium titanium oxide with formula CaTiO3. The more
general perovskite structure is ABO3 where A and B are cations, i.e., positively charged ions, and O is oxygen. In the most symmetric form of the material, a cubic configuration, BO6 octahedra (a B cation surrounded by 6 oxygen atoms) are connected at the corners indefinitely in 3 dimensions. An A cation fills every hole created by 8 octahedra. (See Figure 1 for the example SrTiO3.)
The first indication that perovskites had potential as solar cell material was published in 2009 by
Tsutomu Miyasaka from the University of Tokyo in the Journal of the American Chemical Society. Miyasaka and his coworkers found that two perovskite nanocrystals sensitized a titanium dioxide (TiO2) layer and converted visible light conversion to electricity. Although this arrangement resulted in only a 3.8% power conversion efficiency and was only stable for a few minutes, it opened the door on a new area of reasearch and led the way for rapid technological advancements. See video for more details on perovskite solar cells.
Perovskite solar cells have a number of advantages over silicon cells. While both consist of abundant and inexpensive materials, silicon cells require expensive manufacturing processes which include vacuum chambers and high temperatures. Perovskite cells can be manufactured using chemical processes at relatively moderate temperatures. One technique creates a liquid substance which can be painted on a surface and left to dry or even printed. It is also possible to tune the perovskite material to different light frequencies by slightly modifying its chemical composition. Thus perovskites can be manufactured into solar cells that are flexible, semi-transparent, or colored.
A major problem with synthesized perovskites has been their tendency to fall apart when exposed to air, especially humid air. In many previous studies aimed at making perovskite more durable, researchers experimented with adding protective layers to the material.
Work at UCLA (published in 2015) demonstrated that cells consisting of a perovskite layer sandwiched between metal oxide layers lasted 60 days in open-air storage at room temperature, retaining 90 percent of their original solar conversion efficiency. This was a significant improvement but not sufficient for commercial use.
More recent work at UCLA (published in 2022) has developed a new perovskite compound with metallic neodymium ions which have a positive charge and prevent negatively charged ions from exiting the perovskite compound. Working at maximum power and exposed to continuous light for more than 1,000 hours, a solar cell using the augmented perovskite retained about 93% of its efficiency in converting light to electricity. In contrast, a solar cell using standard perovskite lost half of its power conversion efficiency after 300 hours under the same conditions.
A number of companies worldwide are continuing to improve the efficiency of perovskite solar cells and pushing ahead to be the first to offer commercial grade products. Oxford PV, a company spun off from Oxford University set a
new world record in 2020 for the amount of the sun's energy that can be converted into electricity by a single solar cell using ordinary silicon cells coated with a thin film of perovskite. The cell was proven to convert 29.52% of solar energy into electricity. In contrast, standard silicon cells have a theoretical maximum efficiency of 29.1% and an average conversion rate of just 15-20%.
Figure 2 shows the most recent
NREL confirmed conversion efficiencies showing the dramatic increase in perovskite efficiency over the last decade (yellow circles). The figure also shows recent increases in efficiency using tandem solar cells (i.e., multiple layers to increase the frequency range of solar energy that can be captured).
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