Phase-Pure γ-CsPbI 3 for Efficient Inorganic Perovskite Solar Cells

Cesium lead triiodide (CsPbI 3 ) has attracted enormous attention due to its outstanding thermal stability and ideal bandgap (∼1.75 eV) for tandem applications. However, it is still challenging to prepare high-efficiency fully inorganic CsPbI 3 perovskite solar cells. In this work, we developed a formamidine acetate (FAAc)-assisted strategy to prepare phase-pure high-quality γ-CsPbI 3 perovskite film on a planar substrate. Mechanistic studies reveal that the incorporation of FAAc into perovskite precursor solution alters the phase transition process with an additional intermediate phase formation, which leads to an improved crystallinity of γ-CsPbI 3 film and better interfacial contact. As a result, the FAAc-assisted γ-CsPbI 3 solar cells exhibit a power conversion efficiency (PCE) of over 18% and retain 95% of the initial efficiency after 30 days of aging.