Methylammonium lead halides (MALHs) are solid compounds with

perovskite structure A perovskite is any material with a crystal structure following the formula ABX3, which was first discovered as the mineral called perovskite, which consists of calcium titanium oxide (CaTiO3). The mineral was first discovered in the Ural mou ...

and a chemical formula of CH₃NH₃PbX₃, where X = I, Br or Cl. They have potential applications in solar cells, lasers,

light-emitting diodes A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...

photodetectors Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or by ...

, radiation detectors,

scintillator A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed ...

, magneto-optical data storage and hydrogen production.

Properties and synthesis

In the CH₃NH₃PbX₃

cubic crystal structure In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties of ...

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. 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. 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 CH₃NH₃PbBr₃ in dimethylformamide. This property is used in the growth of MALH single crystals and films from solution, using a mixture of CH₃NH₃X and PbX₂ powders as the precursor. The growth rates are 3–20 mm³/hour for CH₃NH₃PbI₃ and reach 38 mm³/hour for CH₃NH₃PbBr₃ crystals. The resulting crystals are metastable and dissolve in the growth solution when cooled to room temperature. They have bandgaps of 2.18 eV for CH₃NH₃PbBr₃ and 1.51 eV for CH₃NH₃PbI₃, while their respective carrier mobilities are 24 and 67 cm²/(V·s). Their thermal conductivity is exceptionally low, ~0.5 W/(K·m) at room temperature for CH₃NH₃PbI₃.

Photodecomposition and thermal decomposition of CH₃NH₃PbX₃

Initially, a proposed decomposition pathway mechanism for CH₃NH₃PbI₃ in presence of water releasing CH₃NH₂ and HI gases was broadly adopted by researchers in perovskite solar cell. Later, it was found that the major gases released during high temperature (> 360 °C) thermal degradation of CH₃NH₃PbI₃ are

methyl iodide Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one h ...

(CH₃I) and ammonia (NH₃). : ->

Delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, at a river mouth * D (NATO phonetic alphabet: "Delta") * Delta Air Lines, US * Delta variant of SARS-CoV-2 that causes COVID-19 Delta may also re ...

+ + In 2017, it has been inferred using in situ XPS measurements that in the presence of water vapour, CH₃NH₃I salt can not be a product of the degradation of CH₃NH₃PbI₃ perovskite. Similar high temperature degradation reaction has been confirmed for CH₃NH₃PbBr₃ : ->

+ + Furthermore, high resolution

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

measurements at low temperature conditions (< 100 °C) compatible with photovoltaic operation found that CH₃NH₃PbI₃ undergoes reversible, : <=> Delta,h\nu + + + + and irreversible chemical decomposition reactions under vacuum when illumination or heat pulses are applied. : -> Delta,h\nu + + + Recently, a method to quantify the intrinsic chemical stability of arbitrarily mixed hybrid halide perovskites has been proposed.

Applications

MALHs have potential applications in solar cells, lasers,

, radiation detectors,

and hydrogen production. The power conversion efficiency of MALH solar cells exceeds 19%.

References

{{DEFAULTSORT:Methylammonium lead halide Lead(II) compounds Perovskites Articles containing video clips Methylammonium compounds

Properties and synthesis

Photodecomposition and thermal decomposition of CH3NH3PbX3

Applications

See also

References

Photodecomposition and thermal decomposition of CH₃NH₃PbX₃