Inhibition of Sn²⁺ Oxidation in FASnI₃ Perovskite Precursor Solution and Enhanced Stability of Perovskite Solar Cells by Reductive Additive

Tin-based halide perovskite solar cells (Sn-PSCs) have attracted a progressive amount of attention as a potential alternative to lead-based PSCs (Pb-PSCs). Sn-perovskite films are fabricated by a solution process spin-coating technique. However, the efficiency of these devices varies significantly with the different batches of precursor solution due to the poor chemical stability of SnI₂-DMSO and the oxidation of Sn²⁺ to Sn⁴⁺. This study investigated the origin of Sn²⁺ oxidation before film formation, and it was identified that the ionization of SnI₂ in dimethyl sulfoxide (DMSO) causes the oxidation of free Sn²⁺ and I⁻ ions. To address these issues, this study introduces the reductive additive 4-fluorophenylhydrazine hydrochloride (4F-PHCl) in the FASnI₃ perovskite precursor solution. The hydrazine functional (-NH-NH₂) group converted detrimental Sn⁴⁺ and I₂ defects back to Sn²⁺ and I⁻ in precursor solution while retaining the properties of the perovskite solution. Furthermore, the addition of 4F-PHCl in the precursor solution effectively slows the crystallization process, enhancing the crystallinity of FASnI₃ perovskite films and guaranteeing the Sn²⁺/I⁻ stoichiometric ratio, ultimately leading to a power conversion efficiency (PCE) of 10.86%. The hydrophobic fluorinated benzene ring in 4F-PHCl ensures moisture stability in perovskite films, allowing unencapsulated PSCs to retain over 92% of their initial PCE in an N₂-filled glovebox for 130 days. Moreover, the 4F-PHCl-modified encapsulated PSCs showed superior operational stability for 420 h and maintained 95% of their initial PCE for 300 h under maximum power point tracking at 1 sun continuous illumination. This study's findings provide a promising pathway to create a controlled Sn-based perovskite precursor solution for highly reproducible and stable Pb-free Sn-PSCs.