Highly Stable Tin Iodide Perovskites Synthesized in Ambient Aqueous Phase using Alkali Iodides and Ascorbic Acid

Tin iodide perovskites (ASnI₃) are emerging as photoactive layers in photovoltaics due to their excellent optical and electronic properties. However, their preparation typically requires an inert environment and anhydrous organic solvents due to their vulnerability to oxygen and moisture. These conditions have resulted in complex and environmentally unfriendly processes. Previous studies have reported the aqueous-phase synthesis of ASnI₃ using hydroiodic acid (HI) as a solubilizer and hypophosphorous acid (H₃PO₂) as a reductant. Nevertheless, HI is highly unstable and cannot be used in ambient air, and both HI and H₃PO₂ are hazardous chemicals. Consequently, there is a high demand for more stable solubilizers and safer reductants to enable a facile and green synthesis of ASnI₃ in an ambient aqueous phase. In this study, we present the ambient aqueous-phase synthesis of ASnI₃ perovskites using alkali iodides and ascorbic acid (AA).<br/>Aqueous solutions containing 6.0 mol L^-1 of HI, lithium iodide (LiI), or sodium iodide (NaI) as solubilizers and 1.0 mol L^-1 of H₃PO₂ or AA as reductants were prepared. Subsequently, 0.5 mol L^-1 of tin iodide was dissolved in these solutions to prepare tin precursor aqueous solutions. The valence states and complex structures of tin species in these precursor aqueous solutions were characterized via electrospray ionization time-of-flight mass spectrometry (ESI-TOFMS). 0.5 mol L^-1 of organic cation iodides was added to the precursor aqueous solutions, and the resulting precipitated samples were collected by suction filtration. Structural, compositional, and optical characterizations were carried out for the samples immediately after synthesis and after exposure to ambient air using X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and optical absorption/photoluminescence measurements.<br/>The tin precursor aqueous solutions containing HI turned muddy and degraded rapidly, even though they contained reductants, whereas those containing LiI or NaI remained completely transparent and stable even in ambient air. Tin species formed complexes with the reductant molecules in the solutions containing reductants. ESI-TOFMS analysis revealed that the reducing and chelating abilities of AA efficiently stabilized the Sn²⁺ valence states. The resulting samples were determined to be pure-phase ASnI₃ perovskites. Interestingly, XRD, XPS, and optical measurements clearly indicated that ASnI₃ perovskites synthesized using NaI and AA exhibited excellent stability after exposure to ambient air. Detailed analyses suggested that Na⁺ ions and a trace amount of AA molecules present on the sample surface passivate the surface defects and stabilize the ASnI₃ phases.<br/>In conclusion, we successfully prepared highly stable tin precursor aqueous solutions even in ambient air, using stable alkali iodides and safe AA. Highly stable ASnI₃ perovskites were synthesized from the solutions containing NaI and AA. These findings pave the way for the green fabrication of highly efficient and long-life perovskite photovoltaics.