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 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 percent of the sun’s energy.
The efficiency is also better than the 10-20 percent 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 percent efficiency more than a decade ago.
Full-spectrum solar cells
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, 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.
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 — while the latter absorbs photons of energy 2 eV, or an amber color.
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 that are generated within the cell. The active layer of the thin-film solar cell is about 400 nanometers thick.
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