Structural Transitions and Stability of FAPbI3 and MAPbI3: The Role of Interstitial Water

Abstract

We studied the influence of water on the structural stability and transformations of MAPI and FAPI by anelastic and dielectric spectroscopies under various temperature and H2O partial pressure protocols. Before discussing the new results in terms of interstitial water in MAPI and FAPI, the literature is briefly reviewed, in search of other studies and evidences on interstitial water in hybrid halide perovskites. In hydrated MAPI, the elastic anomaly between the cubic α and tetragonal β phases may be depressed by more than 50%, demonstrating that there are H2O molecules dispersed in the perovskite lattice in interstitial form, that hinder the long range tilting of the PbI6 octahedra. Instead, in FAPI, interstitial water accelerates in both senses the reconstructive transformations between 3D α and 1D δ phases, which is useful during the crystallization of the α phase. On the other hand, the interstitial H2O molecules increase the effective size of the MA and FA cations to which are bonded, shifting the thermodynamic equilibrium from the compact perovskite structure to the open δ and hydrated phases of loosely bonded chains of PbI6 octahedra. For this reason, when fabricating devices based on hybrid metal-organic halide perovskites, it is important to reduce the content of interstitial water as much as possible before encapsulation.

Effects of alkali metals or Cu+ addition to α-FAPbI3 perovskite crystals on electronic structures and photovoltaic properties

“…On the other hand, it has been reported that a phase transition from photoactive α-FAPbI 3 to photoinactive δ-FAPbI 3 at RT, and the phase transition mechanism has been investigated. [1][2][3][4] To improve the phase stability, low-dimensional perovskite phases were introduced in the bulk or at the interface of FA-based perovskite films, and the phase stability and carrier lifetime were improved. [5][6][7][8][9] Other unique attempts such as low-temperature treatment of A-site cationic solutions 10) and the introduction of H 2 O 11) or CsPbI 3 nanocrystals 12) into the perovskite film have been also reported.…”

K6Sn4F12I2•0.5H2O: a zero-dimensional alkali metal tin mixed halide compound exhibiting color change due to crystal water loss

“…For example, by introducing water molecules to BiF 3 , Liu et al [18] successfully regulated the optical birefringence by changing the coordination environment of Bi 3+ . Yang et al [19] changed the luminescence color from orange to blue by recrystallizing Cs 2 InCl 5 in water. Cordero et al [20] investigated the roles of interstitial water in the structural transitions and stability of FAPbI 3 and MaPbI 3 .…”