Hydroxylammonium formate-assisted crystal regulation of α-FAPbI3

초록

Additives play a significant role not only in regulating the crystal growth of formamidinium lead iodide (FAPbI3) 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 FAPbI3, the simplest structure of hydroxylammonium (HA) has barely been studied. In this study, hydroxylammonium formate (HAHCOO) was incorporated in a precursor solution of FAPbI3, coupled with methylammonium chloride. Fourier transform-infrared spectroscopy revealed that the lone pairs in HA+ and HCOO- function as Lewis base and interact with Pb2+ 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 alpha-to-delta phase transition of FAPbI3. 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.

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