Low‐Temperature Phase‐Transition for Compositional‐Pure α‐FAPbI3 Solar Cells with Low Residual‐Stress and High Crystal‐Orientation

Abstract Transition of δ‐phase formamidinium lead triiodide (δ‐FAPbI 3 ) to pure α‐phase FAPbI 3 (α‐FAPbI 3 ) typically requires high processing temperature (150 °C), which often results in unavoidable residual stress. Besides, using methylammonium chloride (MACl) as additive in fabrication will cause MA residue in the film, compromising the compositional purity. Here, a stress‐released and compositional‐pure α‐FAPbI 3 thin‐film is fabricated using 3‐chloropropylammonium chloride (Cl‐PACl) by two‐step annealing. The 2D template of n = 2 can preferentially form in perovskite with the introduction of Cl‐PACl at a temperature as low as 80 °C. Such a 2D template can guide the free components to form ordered α‐FAPbI 3 and promote the transition of the formed δ‐FAPbI 3 to α‐FAPbI 3 by reducing the phase transition energy. As a result, the obtained perovskite films via low‐temperature phase‐transition have a high degree of crystal orientation and reduced residual stress. More importantly, most of the Cl‐PACl is volatilized during the subsequent high‐temperature annealing process accompanied by the disintegration of the 2D templates. The residual trace of Cl‐PA + is mainly concentrated at the grain boundary near the perovskite surface layer, stabilizing α‐FAPbI 3 and passivating defects. Perovskite solar cell based on pure α‐FAPbI 3 achieves a power conversion efficiency of 23.03% with excellent phase stability and photo‐stability.