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Author: Derek W Palmer
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                General Descriptiom

        Perovskites are materials that exist both as natural minerals and as synthesised solids. They are defined as solids that have a chemical composition of the form ABX3

        In the natural mineral and in completely inorganic perovskites, A and B are metallic elements, acting as positive cations, and X, acting as an anion, is very often oxygen. The atom A is typically larger than the atoms B. The chemical compound CaTiO3 was a mineral found in the Ural Mountains of Russia in 1839 and was give the name "Perovskite" to commemorate the Russian mineralogist L.A.Perovski. Another example of an inorganic perovskite is lanthanum ytterbium oxide, LaYbO3, where the lanthanum atom is larger that the ytterbium atom.

        Because A and B can be the atoms of various elements, there is a great variety of perovskites, and their crystal structures and properties depend on the elements that are A, B and X. The simplest crystal structure of perovskites is cubic, but most others are tetragonal, orthorhombic or trigonal.

        Instead of being an atom of a metallic element, the species A in ABX3 can be an organic radical, with B still being an atom of a metallic element. Some of such compounds have valuable electrical and electronic properties, and some are semiconductors that are suitable for the fabrication of efficient photo-voltaic solar cells. For the latter, the element X is usually a halide instead of oxygen. One perovskite of that form that has been much investigated as a solar cell material is CH3.NH3. Pb. X3 where X= I, Br or Cl.

        Present-day research towards perovskite solar cells (PSCs) seeks to find the optimum organic radicals A, metallic atom B and halide X in semiconducting perovskites that have valence-band to conduction band energy gaps that are suitable for absorption of photons of the solar spectrum at the surface of the Earth, together with high mobilities and lifetimes of electron and holes in the perovskite layer that is absorbing the photons. Depending of the natures of A, B and X, that energy gap can be in the range of about 1.2 to 1.5 eV, which includes the value of 1.34 eV that is calculated to be the optimum bandgap to give the largest photo-voltaic efficiency. It is found that the organic-inorganic halide perovskites have very large optical absorption coefficients for solar photons, and therefore, in a solar cell comprising a metallic Schottky contact on the perovskite photon-absorbing layer, the latter can be very thin, no more than 0.5 micron, with the result that there is little loss of the photon-released electrons and holes that produce the output current of the solar cell.

        For the solar spectrum at the Earth, the theoretical maximum efficiency [input (incident solar power) to output (electric power)] is approximately 34 % for a PIC that is a single-junction (ie, having one perovskite layer), and research and development during recent years has raised the efficiency of an actual PSC to more than 25%, which is larger than the corresponding efficiency of a single-crystal silicon solar cell, which is typically about 20%. A perovskite solar cell that comprises one or more additional perovskite layers of different energy band gaps, so as to allow absorption of more of the solar spectrum, can have an efficiency that is higher than that of a single-layer PSC, and efficiency values as large as 37% have been achieved.

        Thus perovskite solar cells can have efficiencies that significantly surpass those of silicon solar cells that are the standard device for solar-energy conversion at the present time. Also, they are simpler and much less expensive to fabricate than silicon cells. It is therefore considered that PSCS may become the solar cells for the future. However, on-going dedicated R&D is required to find the PSC compositions that show long-term high and stable efficiencies and to determine the optimum construction and fabrication process.

                 Reviews Articles

Mineral Perovskites (from Wikipedia)

Synthetic Perovskites (from Wikipedia)

Perovskite Solar Cells (from Wikipedia)

'Perovskite Solar Cells (from the USA Department of Energy : Solar Energy Technologies)

                Some Recent Research Papers

Yongguang Tu et al, Advanced Materials 33 (2021) 2006545
'Perovskite Solar Cells for Space Applications: Progress and Challenges''

Hussain S et al, 2021, Solar Energy 230 (2021) 501-508
''Decorating wide band gap CH33PbBr perovskite with 4AMP for highly efficient and enhanced open circuit voltage perovskite solar cells''

CaoJ and Yan F, 2021, Energy & Environmental Science 14 (2021) 1286-1325
"Recent progress in tin-based perovskite solar cells"

Suresh Kumar N et al, 2020, Solar Energy 198 (2020) 665-688
"A Review on Perovskite Solar Cells - Evolution of architecture, fabrication techniques, commercialization issues and status

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