Thermodynamic Analyses on Nanoarchitectonics of Perovskite from Lead Iodide: Arrhenius Activation Energy.

Amongst the different perovskites being investigated for application in solar cells, one of the most frequently scrutinized is methylammonium lead iodide CH3NH3PbI3 (or MAPbI3), which is usually obtained by the reaction of lead iodide (PbI2) with methylammonium iodide (MAI). Although this perovskite has been extensively studied and utilized in the manufacture of high-efficiency solar cells, its formation chemistry is still not well understood. Reliable experimental determination of the activation energy between PbI2 and MAI has been difficult due to the rapid reaction at room temperature. In this work, we determined the activation energy by adopting the Arrhenius equation. This was possible by controlling the reaction using MAI vapor, instead of liquid solution. This procedure allowed the reaction to be carried out at temperatures of up to 150 °C. The formation of MAPbI3 films was obtained by a two-step process: deposition of thin PbI2 film by thermal evaporation and subsequent conversion into perovskite by exposure to MAI vapor. The conversion of PbI2 to MAPbI3 as a function of temperature was probed by X-ray diffraction. An activation energy of 0.12 ± 0.02 eV was obtained. This low value explains the ease of MAPbI3 formation at low temperatures, and partially explains its instability in environmental conditions. [ABSTRACT FROM AUTHOR]