Antioxidative solution processing yields exceptional Sn(II) stability for sub-1.4 eV bandgap inorganic perovskite solar cells

Owing to the combined features of sub-1.4 eV bandgap and all-inorganic composition, cesium tin–lead (Sn-Pb) triiodide perovskite is promising for approaching the Shockley-Queisser limit of solar cells while avoiding the use of volatile organic cations. But the low Sn(II) stability in this perovskite remains a hurdle for delivering its theoretically attainable device performance. Herein we present a synthesis method of this perovskite based on an acetylhydrazine-incorporated antioxidative solution system. Mechanistic investigation shows that acetylhydrazine effectively reduces the oxidation of solution-phase Sn(II) and meanwhile creates an electron-rich, protective nano-environment for solid-state Sn(II) ions. These lead to high oxidation resistance of the final film as well as effective defect inhibition. The resultant solar cells demonstrate power conversion efficiencies up to 15.04%, the highest reported so far for inorganic perovskite devices with sub-1.4 eV bandgaps. Furthermore, the T90 lifetime of these devices can exceed 1000 hours upon light soaking in a nitrogen atmosphere, demonstrating the potential advantage when lower-bandgap perovskite solar cells go all-inorganic.