Properties and applications
Perovskite is a mineral chemically referred to as calcium titanate. It has the chemical formula CaTiO3. Because of its characteristic crystal structure, the mineral perovskite was also used as the name for an entire class of materials, the perovskites, because there are many compounds with the general formula ABC3 that form comparable crystal lattices to the mineral perovskite. Strictly speaking, the mineral perovskite, i.e. calcium titanate, does not crystallise in a perovskite structure at all, but only in a distorted perovskite structure. Perovskites found in nature are mostly composed of divalent and tetravalent ions (e.g. in calcium titanate, CaTiO3 the composition is A = Ca2+; B = Ti4+, C = O2-, Ca = calcium, Ti = titanium, O = oxygen). One can compare the structure with a long-term parking garage with permanently allocated parking spaces, in which the arrangement of the parking spaces is fixed. Which vehicles (= atoms, molecules or ions) are then parked in the car park is initially irrelevant, so in the original perovskite mentioned above, Ca, Ti and O are "parked". Crystallographically, perovskites consist of octahedra linked on all sides, cf. right image. The left-hand image shows that the B ion (yellow) is located in the centre of the octahedron, which is surrounded by C ions (blue) (shown in the front of the left-hand image). Between the octahedra there are the A ions (green), which shift the linked octahedra depending on the size of A and thus disturb the crystal lattice.
Linked octahedra in perovskite © julia_faranchuk-adobestock
Position of the A, B, and C atoms or molecules in the perovskite - Image source: Lonxos@de.wikipedia.org
This article focuses on the perovskites used in new, innovative solar cells. Such perovskites for solar cells, deviating from the composition described above, are often synthesised from mono- and dications as well as monoanions in the laboratory e.g. (CH3NH3)[PbBr3] i.e. A= CH3NH3+, B= Pb2+, C=Br-, (or if one writes out the molecular formulas: methylammonium tribromoplumbate from A = methylammonium ions, B = lead(II) ions and C = bromide ions).
Since perovskites are a large class of materials, there are a variety of possible applications.
The perovskite materials for solar cells are currently research materials, i.e. these materials developed in research projects are not yet on the market.
Perovskites in solar cells and energy-converting devices
At the beginning of 2022, solar cells made of perovskite materials were the main topic of discussion in the press, because the industry hopes to achieve inexpensive and easy-to-manufacture photovoltaic units here and to be able to dispense with larger quantities of expensive silicon. For the three components A, B and C in the typical ABC3 perovskite, the following ions or compounds are usually mentioned in the literature :
- A: Cs+, CH3NH3+or HC(NH2)2)+ (caesium, methylammonium or formamidinium cations)
- B: Pb2+ or Sn2+ (lead(II)- or tin(II) Ions)
- C: Cl-, Br-or I- (chloride, bromide or iodide ions)
However, materials containing lead may release this toxic element into the environment, which should be prevented urgently . Therefore, the focus of some research has been on tin containing materials . However, these can release tin ions, which can also be toxic.
Perovskite solar cells convert a high proportion of incoming light directly into usable electricity. ( Picture:Fabian Ruf/Scilight)
In addition to the low price of perovskite solar cells, their property of being semi-transparent in a thin layer is particularly interesting. This means that another solar cell can be placed under a perovskite solar cell, which can also convert the remaining transmitted light into electricity. Such solar cells are called tandem solar cells because two energy-converting layers are present.
In addition, in the field of energy conversion, thermoelectrics are also powered by perovskites, which means heat is converted into electricity. If you combine conventional solar cells with thermoelectrics, you get tandem solar modules with two energy-converting layers again. The two individual components, solar cell/photovoltaics and thermoelectrics, then contribute to the efficiency of the solar cells, hence the electricity yield.
If you coat a display with a transparent perovskite solar cell, you could create a power-supplying display.
Other applications of perovskites
Much less visible is the use of perovskites such as barium titanate (BaTiO3, Ba = barium, Ti = titanium, O = oxygen) as dielectrics in capacitors or as ferroelectrics. The charge separation capability in perovskites is also used for these applications.
In the field of energy transport, the YBCO (chemical formula YBa2Cu3O7-x, Y = yttrium, Ba = barium, Cu = copper, O = oxygen) superconductor, which crystallises in a distorted perovskite structure, has achieved a certain prominence. The abbreviation YBCO is a simplified description of the most popular and relatively easy to produce so-called high-temperature superconductor (production only under the supervision of experts!), which already becomes superconducting when cooled with liquid nitrogen (-196°C).. The YBCO was a further development of the superconductor with the formula La2-xBaxCuO4, (La = lanthanum) which was awarded a Nobel Prize in Physics in 1987 and describes the first representative of the class of ceramic high-temperature superconductors.
These applications show that perovskites can be innovative materials. Nanoscale perovskites are also used in the field of thermoelectrics.
Furthermore, perovskites are discussed as coating materials for implants, e.g. in dental prostheses, to improve their biocompatibility . Other applications of perovskites are described in the literature for wastewater treatment, adsorbents or catalysts, and bioimaging .
Origin and production
Perovskite mineral © vvoe - stock.adobe.com
Perovskites occur naturally in various rocks. It is remarkable, for example, that a large part of the Earth's mantle could consist of a silicate perovskite, which would make it the most common mineral on Earth. It is formed under immensely high pressure at a depth of several kilometres in the Earth's mantle .
All perovskites used in solar cells, thermoelectrics, dielectrics or superconductors do not occur naturally. They are exclusively produced in the laboratory. An exception are the perovskites used for coating materials, e.g. CaTiO3 (calcium titanate): although this occurs naturally, laboratory-produced CaTiO3 is used for implant coating. It can thus be shaped directly into the form needed for the dental treatment. Moreover, synthetically produced CaTiO3 is usually much purer than naturally obtained CaTiO3.
Since perovskites are not a single material but a class of materials, there is also no uniform method of production .