Mixed halide perovskite compounds with the formula AMX 3 (A þ ¼ CH 3 NH 3 (MA)/HC(NH 2) 2 (FA)/Cs/Rb; M 2þ ¼ Pb, Sn; X À ¼ Cl, Br, I) have emerged as a potential material for fabrication of next-generation solar cells with high photovoltaic performance. [1] A recent study has shown that perovskite solar cells (PSCs) can be fabricated with a high power conversion efficiency (PCE) of 25.5% with Pb as the divalent metal in the compound. [2] Unfortunately, the presence of toxic Pb remains questionable for largescale production. Fortunately, Sn, which is located above Pb in group IV of the periodic table of elements, is a promising candidate for harmless photovoltaic applications. Compared to Pb, Sn is less toxic and can form perovskite compounds with various cations. Sn-based perovskite compounds possess fascinating properties for solar cell applications, such as a narrower bandgap, a lower exciton binding energy, a high charge carrier mobility, and a slightly smaller radius than Pb 2þ , which allows for the replacement of Pb by Sn while retaining the perovskite structure. Notably, the Sn-perovskite materials possess an optimal bandgap for solar cell applications between 1.1 and 1.4 eV and can be decomposed to nontoxic materials such as Sn II oxide (SnO 2). [3] However, fabricating high-quality Sn-based perovskite films is still unmanageable due to the lack of a basic understanding of the Sn II compounds. Such lacking hinders the potential realization of the corresponding film properties, such as surface morphology, pinhole-free layer formation, and low crystallinity. Until now, the most successful of the Sn-PSCs have been fabricated with FASnI 3 as the absorber layer.