What is a perovskite?
Perovskite solar cells are a relatively new but rapidly expanding area of solar technology. The name perovskite comes from their structure, which is shared with a group of naturally occurring minerals. However, the perovskite material used in solar cells does not look much like the classical perovskites at first glance. Natural perovskites are metal oxides, and are considered to be entirely inorganic materials (i.e they do not contain any carbon-hydrogen bonds). Solar cell perovskites however contain a mixture of inorganic and organic ions, which are arranged in the same way as the classical inorganic perovskites. The formula for any perovskite is ABX3, and in solar cells A+ is the organic cation, B2+ is Pb2+ and X- is usually I- or some mixture of the halides I-, Br- or Cl-.
How do perovskite solar cells work?
There are several different ways to arrange the different layers in a perovskite solar cell. In one common example, the perovskite cell is arranged in much the same way as a dye sensitized solar cell (DSSC) – but instead of a dye anchored to a semiconductor surface there is a layer of the perovskite material absorbing light. As in any solar cell, electrons and holes have to be separated and transported to an external circuit to produce electricity. In contrast to DSSCs, perovskite solar cells do not need a thick layer of porous TiO2 to allow hole-electron pairs to separate, as the charges generated in the perovskite structure can move very quickly away from one another. In transporting holes away from the perovskite organic molecules known as hole-transport materials are typically used. These materials are deposited as a thin film on top of the perovskite, making the cell all solid state and avoiding the need for the type of liquid electrolyte used in DSSCs, which can be prone to leaking.
Perovskites produce very high efficiency solar cells, which has led research into perovskite solar cells becoming very popular over the past few years. Currently the record efficiency is around 22%, which is incredible for a field which is still less than 10 years old.
Perovskite cells can be fabricated by printing from solution, an advantage they share with DSSC and organic photovoltaics, but with much higher efficiency than either of those technologies.
As mentioned before, the cells are well suited to solid-state hole-transport materials and so the problem of corrosive electrolytes which can leak out of the cell is avoided.
A major concern in the application of perovskite solar cells is the cell stability. For example, the perovskite material degrades when in contact with moisture, enough so that cells left out in the atmosphere can lose all activity within a matter of days. A lot of current research is therefore aimed at improving stability, as well as improving techniques to print larger-area cells that would enable practical use in electricity generation.