Hydroxylammonium formate-assisted crystal regulation of α -FAPbI ₃

Abstract Additives play a significant role not only in regulating the crystal growth of formamidinium lead iodide (FAPbI ₃ ) by interacting with precursors but also in determining the phase stability by coordinating with the surface defect. Although alkyl ammonium-based additives are widely used in the form of alkylammonium halide for the production of high-performing FAPbI ₃ , the simplest structure of hydroxylammonium (HA) has barely been studied. In this study, hydroxylammonium formate (HAHCOO) was incorporated in a precursor solution of FAPbI ₃ , coupled with methylammonium chloride. Fourier transform-infrared spectroscopy revealed that the 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 large grain size, as evidenced by x-ray diffraction patterns and scanning electron microscopy. Meanwhile, HA ⁺ and HCOO ⁻ , expelled to grain boundaries during crystal growth, coordinate with the crystal surface, effectively retarding the α -to- δ phase transition of FAPbI ₃ . The inclusion of a new additive, HAHCOO, therefore, successfully leads to a slight power conversion efficiency increase with an improved operational stability under maximum power point tracking.