Methylammonium lead halide

Methylammonium lead halides (MALHs) are solid compounds with perovskite structure and a chemical formula of [CH₃NH₃]⁺Pb²⁺(X⁻)₃, where X = Cl, Br or I. They have potential applications in solar cells,^[2] lasers, light-emitting diodes, photodetectors, radiation detectors,^[3][4] scintillator,^[5] magneto-optical data storage^[6] and hydrogen production.^[7]

[CH₃NH₃]PbX₃ crystal structure.^[1]

Properties and synthesis

The first MALHs to be synthesized were the methylammonium derivatives [CH₃NH₃]SnX₃ and [CH₃NH₃]PbX₃. Their potential in the area of energy conversion wasn't realized until decades later.^[8] In the [CH₃NH₃]PbX₃ cubic crystal structure the methylammonium cation ([CH₃NH₃]⁺) is surrounded by PbX₆ octahedra. The X ions are not fixed and can migrate through the crystal with an activation energy of 0.6 eV; the migration is vacancy assisted.^[1] The methylammonium cations can rotate within their cages. At room temperature the ions have the CN axis aligned towards the face directions of the unit cells and the molecules randomly change to another of the six face directions on a 3 ps time scale.^[9]

Growth of a [CH₃NH₃]PbI₃ single crystal in gamma-butyrolactone at 110 °C. The yellow color originates from the lead(II) iodide precursor.^[7]

Growth of a [CH₃NH₃]PbBr₃ single crystal in dimethylformamide at 80 °C.^[7]

The solubility of MALHs strongly decreases with increased temperature: from 0.8 g/mL at 20 °C to 0.3 g/mL at 80 °C for [CH₃NH₃]PbBr₃ in dimethylformamide. This property is used in the growth of MALH single crystals and films from solution, using a mixture of [CH₃NH₃]X and PbX₂ powders as the precursor. The growth rates are 3–20 mm³/hour for [CH₃NH₃]PbI₃ and reach 38 mm³/hour for [CH₃NH₃]PbBr₃ crystals.^[7]

The resulting crystals are metastable and dissolve in the growth solution when cooled to room temperature. They have bandgaps of 2.18 eV for [CH₃NH₃]PbBr₃ and 1.51 eV for [CH₃NH₃]PbI₃, while their respective carrier mobilities are 24 and 67 cm²/(V·s).^[7] Their thermal conductivity is exceptionally low, ~0.5 W/(K·m) at room temperature for [CH₃NH₃]PbI₃.^[10]

Thermal decomposition of [CH₃NH₃]PbI₃ gives methyl iodide (CH₃I) and ammonia (NH₃).^{[11] [12]}

[CH₃NH₃]PbI₃ → PbI₂ + CH₃I + NH₃

Applications

MALHs have potential applications in solar cells, lasers,^[13] light-emitting diodes, photodetectors, radiation detectors,^[4] scintillator^[5] and hydrogen production.^[7] The power conversion efficiency of MALH solar cells exceeds 19%.^[14][15]

Historic references

• Weber, Dieter (1978). "CH3NH3SnBrxI3-x (x = 0-3), ein Sn(II)-System mit kubischer Perowskitstruktur / CH3NH3SnBrxI3-x (x = 0-3), a Sn(II)-System with Cubic Perovskite Structure". Zeitschrift für Naturforschung B. 33 (8): 862–865. doi:10.1515/znb-1978-0809. ISSN 1865-7117.
• Weber, Dieter (1978-12-01). "CH3NH3PbX3, ein Pb(II)-System mit kubischer Perowskitstruktur / CH3NH3PbX3 , a Pb(II)-System with Cubic Perovskite Structure". Zeitschrift für Naturforschung B. 33 (12): 1443–1445. doi:10.1515/znb-1978-1214. ISSN 1865-7117. S2CID 93597007.*Grätzel, Michael (2014). "The light and shade of perovskite solar cells". Nature Materials. 13 (9): 838–842. Bibcode:2014NatMa..13..838G. doi:10.1038/nmat4065. ISSN 1476-1122. PMID 25141800.

See also

• Perovskite solar cell
• Methylammonium halide