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‐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.

Deciphering the Roles of MA-Based Volatile Additives for α-FAPbI3 to Enable Efficient Inverted Perovskite Solar Cells

“…This phenomenon may originate from the principle like the liquid−solid interface 48−50 and template growth. 51 In other words, NH 4 Cl may coordinate with Pb and be released to activate atomic movement, thereby reducing the internal energy required to achieve phase transition at 130 °C. Route 3 represents the use of MA-based volatile additives, exemplified by MACl.…”

“…The incorporation of NH 4 Cl in the precursor can decrease the δ to α phase-transition temperature to below 130 °C. This phenomenon may originate from the principle like the liquid–solid interface − and template growth . In other words, NH 4 Cl may coordinate with Pb and be released to activate atomic movement, thereby reducing the internal energy required to achieve phase transition at 130 °C.…”

Impact and Role of Epitaxial Growth in Metal Halide Perovskite Solar Cells

“…This is because the 2D perovskite, which serves as an epitaxial template, guides the formation of ordered α-FAPbI 3 , reducing the formation energy of α-FAPbI 3 and promoting the transition from δ-FAPbI 3 to α-FAPbI 3 . The approach resulted in a champion PCE of 23.03%, with 77% of the initial efficiency retained after continuous illumination for 216 h. 130 Recently, Luo et al first proposed the heteroepitaxial growth of perovskite using highly oriented two-dimensional (2D) lead halide perovskite lead iodide (BDA) 2 PbI 4 (BDA is 1,4butanediammonium) as a crystallization seed by introducing it into the PbI 2 precursor solution 131 (Figure 6a). The presence of 2D seeds allowed the 3D perovskite to skip the nucleation stage and directly crystallize, resulting in a better crystalline quality and preferred crystal orientation (00L) (Figure 6b).…”

“…This is because the 2D perovskite, which serves as an epitaxial template, guides the formation of ordered α-FAPbI 3 , reducing the formation energy of α-FAPbI 3 and promoting the transition from δ-FAPbI 3 to α-FAPbI 3 . The approach resulted in a champion PCE of 23.03%, with 77% of the initial efficiency retained after continuous illumination for 216 h …”

Volatile 2-Thiophenemethylammonium and Its Strongly Bonded Condensation Product for Stabilizing α-FAPbI3 in Sequential-Deposited Solar Cells

“…The spacer cations of 2D perovskites, such as phenylethylammonium (PEA) and butylammonium (BA), are widely used to construct 3D/2D heterojunctions or stabilize perovskite films by doping into the film. , More recently, Seok et al . and our group independently find that the spacer cations of 2D perovskites are volatile during annealing and the presence of the spacer cations in the perovskite precursor solution can promote the formation and stabilization of α-formamidinium lead triiodide (α-FAPbI 3 ) films. − We also noticed that part of the spacer cations will react with formamidinium (FA) in the perovskite film to form the nonvolatile and thermally stable condensation product . We are looking for a strategy to simultaneously exploit the properties of the spacer cations of 2D perovskites to prepare high-quality perovskite films and stabilize them by the condensation product.…”