Hydroxylammonium formate-assisted crystal regulation of α-FAPbI3

Additives play a significant role not only in regulating crystal growth of formamidinium lead iodide (FAPbI3 ) by interacting with precursors but also in determining the phase stability by coordinating to the surface defect. While alkyl ammonium-based additives are widely utilized in a form of alkylammonium halide for the production of high-performing FAPbI3 , the simplest structure of hydroxylammonium (HA) is barely studied. In this study, hydroxylammonium formate (HAHCOO) is incorporated in a precursor solution of FAPbI 3 , coupled with methylammonium chloride. Fourier transform-infrared spectroscopy reveals that lone pairs in HA + and HCOO¯ function as Lewis base and interact with Pb 2+ as Lewis acid in the precursor solution, facilitating the crystal growth and resulting in an increased crystallinity with a large grain size, as evidenced by X-ray diffraction patterns and scanning electron spectroscopy. In the meantime, HA+ and HCOO-, being expelled to grain boundaries during the crystal growth, coordinate to the crystal surface, effectively retarding the α-to-δ phase transition of FAPbI3 .The inclusion of a new additive, HAHCOO, therefore, successfully leads to a slight PCE increase with an improved operational stability under maximum power point tracking.