Low-Temperature Phase-Transition for Compositional-Pure α-FAPbI₃ Solar Cells with Low Residual-Stress and High...

Transition of δ-phase formamidinium lead triiodide (δ-FAPbI₃ ) to pure α-phase FAPbI₃ (α-FAPbI₃ ) 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-FAPbI₃ 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-FAPbI₃ and promote the transition of the formed δ-FAPbI₃ to α-FAPbI₃ 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-FAPbI₃ and passivating defects. Perovskite solar cell based on pure α-FAPbI₃ achieves a power conversion efficiency of 23.03% with excellent phase stability and photo-stability.