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 A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...

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, radiation detectors, scintillator, magneto-optical data storage and hydrogen production.

Properties and synthesis

In the CH₃NH₃PbX₃ 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 In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...

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

bandgap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (i ...

s 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 The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

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 A perovskite solar cell (PSC) is a type of solar cell that includes a perovskite-structured compound, most commonly a hybrid organic–inorganic lead or tin halide-based material as the light-harvesting active layer. Perovskite materials, such a ...

. Later, it was found that the major gases released during high temperature (> 360 °C) thermal degradation of CH₃NH₃PbI₃ are methyl iodide (CH₃I) and

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

(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 ...

+ + 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 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

, photodetectors, radiation detectors, scintillator 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