Methylammonium lead halide
Methylammonium lead halides (MALHs) are solid compounds with perovskite structure and a chemical formula of CHNHPbX, 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 CHNHPbX cubic crystal structure the methylammonium cation (CHNH) is surrounded by PbX 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 CHNHPbBr in dimethylformamide. This property is used in the growth of MALH single crystals and films from solution, using a mixture of CHNHX and PbX powders as the precursor. The growth rates are 3–20 mm/hour for CHNHPbI and reach  38 mm/hour for CHNHPbBr 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 CHNHPbBr and 1.51 eV for CHNHPbI, while their respective carrier mobilities are 24 and 67 cm/(V·s). Their thermal conductivity is exceptionally low, ~0.5 W/(K·m) at room temperature for CHNHPbI. Photodecomposition and thermal decomposition of CHNHPbX. Initially, a proposed decomposition pathway mechanism for CHNHPbI in presence of water releasing CHNH 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 CHNHPbI are methyl iodide (CHI) and ammonia (NH). In 2017, it has been inferred using in situ XPS measurements that in the presence of water vapour, CHNHI salt can not be a product of the degradation of CHNHPbI perovskite. Similar high temperature degradation reaction has been confirmed for CHNHPbBr Furthermore, high resolution mass spectrometry measurements at low temperature conditions (< 100 °C) compatible with photovoltaic operation found that CHNHPbI 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%.