What are Methylammonium and Solvent Fates upon Halide Perovskite Thin-Film Preparation and Thermal Aging?

Abstract

The recent remarkable achievements in terms of efficiency performance and stability of hybrid organic–inorganic perovskite solar cells (PSCs) are notably obtained by optimizing the A-site cation composition of formamidinium-based 3D perovskites. As methylammonium chloride is ubiquitously employed in precursor solution for very high efficiency formamidinium-based PSC, the purpose of the present paper is to unveil the exact role of methylammonium (MA+), chloride, and solvent on the film growth and their fate upon the layer thermal annealing process. Methylammonium is shown to react with formamidinium to form two methyl compounds while its excess is eliminated along with chloride. The final perovskite layer A-sites are mainly occupied by formamidinium, whereas MA+ content is only 2–3 mol%, and below the 3-N-methyl formamidinium one. 1-N-methyl formamidinium is detected as traces. Meanwhile, the solvent is homogenously eliminated throughout the layer thickness. Upon thermal aging stress, the layer is degraded from its top with the formation of PbI2. MA+ is rapidly fully eliminated while the stable methyl formamidinium compounds remain in the perovskite layer.