Trojans That Flip the Black Phase: Impurity-Driven Stabilization and Spontaneous Strain Suppression in γ-CsPbI₃ Perovskite

Abstract

The technological progress and widespread adoption of all-organic CsPbI3 perovskite devices is hampered by its thermodynamic instability at room temperature. Because of its inherent tolerance toward deep trap formation, there has been no shortage to exploring which dopants can improve the phase stability. While the relative size of the dopant is important, an assessment of the literature suggests that its relative size and impact on crystal volume do not always reveal what will beneficially shift the phase transition temperature. In this perspective, we analyze the changes in crystal symmetry of CsPbI3 perovskite as it transforms from a thermodynamically stable high-temperature cubic (α) structure into its distorted low-temperature tetragonal (β) and unstable orthorhombic (γ) perovskite structures. Quantified assessment of the symmetry-adapted strains which are introduced due to changes in temperature and composition show that the stability of γ-CsPbI3 is best rationalized from the point of view of crystal symmetry. In particular, improved thermal-phase stability is directly traced to the suppression of spontaneous strain formation and increased crystal symmetry at room temperature.

The halogen chemistry of halide perovskites

“…Similarly, the Goldschmidt tolerance factor [30], also derived from geometric considerations, identifies limits to A-site-cation radius based on the Bsite-cation and halide radii [31,32]. Compositions that deviate from the tolerance factor for an ideal, undistorted perovskite, with 180°B-X-B angles (Figure 1B), will either adopt a lower-symmetry perovskite structure (e.g., with tilted octahedra) (Figure 1A) or a nonperovskite structure [33].…”

Impact of Bi doping on nonradiative carrier recombination in CsPbI3

“…Assuming a high Bi Pb concentration, e.g. 10 20 cm À3 , which is also the typical experimental value, 6,15,22 we use C total limited by C 0 p (wo) to derive an A value of 10 3 s À1 , corresponding to a carrier lifetime of 1 millisecond. Such a long carrier lifetime indicates that Bi Pb is not an efficient recombination center even with heavy doping, verifying our prediction.…”

“…Second, Na has an ionic radius of 1.02 Å, comparable to that of Bi (1.03 Å), but smaller than that of Pb (1.19 Å); 25 it is expected that Na doping can improve the phase stability of CsPbI 3 as Bi doping does due to the optimized tolerance factor. 15 Fig. 4(a) displays the formation energies of Na Pb , Na Cs and Na i in CsPbI 3 .…”

“…Besides tuning the band gap, Bi inclusion was shown to enhance the phase stability of CsPbI 3 . 6,15 Thus, it is highly significant to investigate this system. Our first-principles study shows that Bi itself is benign in CsPbI 3 , but it facilitates the formation of interstitial iodine (I i ), which is detrimental to carrier collection.…”

Self‐Straining Nanocrystals Strategy: Temperature and Pressure Co‐Induced Phase Transitions of CsPbBr3 in Amorphous Matrices

“…Notably, the phase transition processes of halide perovskite NCs are not consistent, [ 10 ] depending on several internal self‐factors, [ 11 ] such as particle size, ion doping, surface modification, etc. Besides, the pressure and other external environmental factors also have a significant influence on the phase transitions of halide perovskite NCs owing to their soft structural characteristics.…”