Stabilization of Perovskite Lattice and Suppression of Sn2+/Sn4+ Oxidation via Formamidine Acetate for High Efficiency Tin Perovskite Solar Cells

Tin halide lead-free perovskite solar cells (TPSCs) have received tremendous research interest recently due to their nearly ideal bandgap, broad light absorption, non-toxicity, and environmental friendliness. However, the uncontrollable crystallization process and the facile oxidation of Sn²⁺ limit the further increase of power conversion efficiency (PCE). To solve these problems, a series of acetates are introduced into the perovskite precursor solution to regulate the crystallization process. It is revealed that formamidine acetate (FAAc) has strong CO Sn coordination with Sn²⁺ compared with acetic acid (HAc) and methylammonium acetate (MAAc), which can stabilize the lattice structure, minimize defect states and suppress the oxidation of Sn²⁺. Meanwhile, benefiting from this coordination ability, it not only leads to large-size colloidal clusters in precursor but also slows down the crystallization process and improves the crystallinity of tin halide perovskite films. The device with FAAc achieved an increased PCE from initially 9.84% to 12.43%, and it could maintain 94% of its initial value for 2000 h in N₂ atmosphere. This work provides a feasible strategy for depositing high-quality tin perovskite films with low defect density and lattice distortion, which will be crucial for related photovoltaics and other optoelectronic devices.