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 °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 °C. These data provide a foundation for achieving effective control of vapor phase concentrations for vapor processing of alloyed perovskite solar cells. (© 2021 Wiley‐VCH GmbH)