Thiocyanate-Stabilized Pseudo-cubic Perovskite CH(NH₂)₂PbI₃ from Coincident Columnar Defect Lattices

Abstract

α-FAPbI 3 (FA + = CH(NH 2 ) 2 + ) with a cubic perovskite structure is promising for photophysical applications. However, α-FAPbI 3 is metastable at room temperature, and it transforms to the δ-phase at a certain period of time at room temperature. Herein, we report a thiocyanate-stabilized pseudo-cubic perovskite FAPbI 3 with ordered columnar defects (α′-phase). This compound has a √5a p × √5a p × a p tetragonal unit cell (a p : cell parameter of primitive perovskite cell) with a band gap of 1.91 eV. It is stable at room temperature in a dry atmosphere. Furthermore, the presence of the α′-phase in a mixed sample with the δ-phase drastically reduces the δto-α transition temperature measured on heating, suggesting the reduction of the nucleation energy of the α-phase or thermodynamic stabilization of the α-phase through epitaxy. The defect-ordered pattern in the α′-phase forms a coincidence-site lattice at the twinned boundary of the single crystals, thus hinting at an epitaxy-or strain-based mechanism of α-phase formation and/or stabilization. In this study, we developed a new strategy to control defects in halide perovskites and provided new insight into the stabilization of α-FAPbI 3 by pseudo-halide and grain boundary engineering.

FA4Pb2I7.5(SCN)0.5: n = 3 Member of Perovskite Homologous Series FAn+1Pbn−1I3n−1.5(SCN)0.5 with Columnar Defects

“…All of the diffraction peaks in x = 0 can be assigned to a cubic perovskite structure of α -FAPbI 3 . In x = 0.2, we observed small satellite peaks in addition to the original perovskite peaks, which correspond to √5 a p × √5 a p × a p superlattice reflections, as reported previously . Surprisingly, we also found a different pattern of superlattice reflections at x = 0.33.…”

“…The obtained crystal is rectangularly shaped with a microscale and yellow-orange color (Figure c, Figure S2b). The crystal is sensitive to moisture in the air as also reported in FA 6 Pb 4 I 13.5 (SCN) 0.5 ( x = 0.2 or n = 5) . It changes to a dark-red rod-shaped crystal and then degrades to yellow δ -FAPbI 3 even in the Fluorolube oil (Figure S2c,d).…”

“…The Tauc plot of the absorption edge indicates a band gap energy ( E g ) of 2.3 eV, consistent with the red-orange color of the powder (Figure S7). This value is larger than that of the α -FAPbI 3 ( x = 0 or n = ∞: E g = 1.45 eV) and the FA 6 Pb 4 I 13.5 (SCN) 0.5 ( x = 0.2 or n = 5: E g = 1.91 eV) . This means that E g can be tuned by defect concentration (Figure ).…”

“…We recently reported the columnar defect ordering in pseudocubic perovskite lattice by introducing thiocyanate ion (SCN − ) into FAPbI 3 (Figure a,b) . The columnar defects are ordered one-fifth in the lattice, forming √5 a p × √5 a p × a p supercell ( a p : cell parameter of primitive perovskite cell).…”

“…The atomic ratio of I/S = 0.857(7)/0.143(7) was obtained from single crystal analysis. This value is similar to the case of x = 0.2 ( n = 5), where one-eighth of I − is substituted by SCN − at the eight vertices of the box formed by the PbI defect. This indicates that the SCN ion directs inside the box to remove two Pb ions on the top and bottom of the box (I ions are also removed).…”