
Methylammonium lead halide

Methylammonium lead halides (MALHs) are solid compounds with perovskite structure and a chemical formula of CH3NH3PbX3, where X = I, Br or Cl. They have potential applications in solar cells, lasers, light-emitting diodes, 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 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 CH3NH3PbI3 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 (CH3I) and ammonia (NH3).
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, 
and irreversible chemical decomposition reactions under vacuum when illumination or heat pulses are applied.
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, light-emitting diodes, photodetectors, radiation detectors, scintillator and hydrogen production. The power conversion efficiency of MALH solar cells exceeds 19%.