Thermal and Structural Characterization of Methylammonium- and Formamidinium-Halide Salts

Four organic halide salts of interest to alloyed perovskite solar cell fabrication are characterized using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), powder X-ray diffraction (XRD), and thermogravimetric analysis. The chemical and crystal structures of methylammonium iodide (MAI), methylammonium bromide (MABr), and formamidinium iodide (FAI) are confirmed, and the experimental ATR-FTIR spectrum and XRD pattern of formamidinium bromide (FABr) are presented. The enthalpy, Delta H (vap), and entropy, Delta S (vap), of vaporization are quantified for each salt and are used to estimate their vapor pressures in the temperature range of 150-300 degrees C. MAI, MABr, and FAI have similar vapor pressures in this temperature range, whereas FABr has a higher vapor pressure in the temperature range of 150-240 degrees C. These data provide a foundation for achieving effective control of vapor phase concentrations for vapor processing of alloyed perovskite solar cells.