Nature of the cubic to tetragonal phase transition in methylammonium lead iodide perovskite

Abstract

Hybrid organic-inorganic perovskites, as well as the perovskites in general, are known for their phase complexity evidenced by the stabilization of different polymorphs, and thus an understanding of their regions of stability and transitions can be important for their photovoltaic and optoelectronic technologies. Here we use a multiscale approach based on first-principles calculations with van der Waals corrections and classical force-field molecular dynamics to determine the finite-temperature properties of the tetragonal and cubic phases of CHNHPbI. Temperature effects are implicitly included using the quasi-harmonic approximation that can describe anharmonic behavior due to thermal expansion through the dependence of the harmonic frequencies on structural parameters. Our finite-temperature free-energy surfaces predict the lattice and elastic moduli evolution with temperature, and show in particular that the calculated lattice parameters of the cubic and tetragonal phases are to within 1% of experimental values. Further, our results show that the phonons are the major contributing factor for stabilizing the cubic phase at high temperatures mainly due to the low-energy phonon modes that are associated with the inorganic lattice. On the other hand, the configurational entropy due to CHNH rotational degrees of freedom is slightly more favored in the cubic phase and amounts to less than 0.2% of the T = 0 K free-energy difference between the two phases.

Effect of Thermal Cycling on the Structural Evolution of Methylammonium Lead Iodide Monitored around the Phase Transition Temperatures

“…First Cycle: On the first heating stage, in Figure 8a, the intensity of the strongest satellite decreases to zero only above 346 K, indicating that at this temperature only the cubic phase is expected. [30] However, in conformity with Tyson et al, [32] we observed a pseudocubic unit cell in the whole 310-350 K range. Another degenerate case, when α ¼ γ ¼ 0.5, was observed below 315 K during the first thermal cycle heating stage.…”

“…The high‐temperature transition results in the reduction of the unit cell in the c ‐direction and could be driven by an instability of the crystal with respect to the emergence of a particular phonon mode . This hypothesis is supported by the theoretical work of Saidi and Choi, in which the stabilization of the cubic phase is explained through vibrational energy and entropy considerations.…”

Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination

“…1. These low-frequency phonon modes are the optical phonons contributed by the inorganic Pb-I lattice (35,36). On the other hand, the absence of appreciable peaks in the 150 to 3500 cm −1 range indicates that MA molecular vibrations are not coupled with the CBM, the VBM, or defect states.…”

Stability Issue of Perovskite Solar Cells under Real‐World Operating Conditions

“…Thermal stability is a very important criterion for the commercialization of OIHP‐based solar cells. The OIHP usually undergoes degradation or phase transformation under long‐term high‐temperature stress . The thermal instability of PSCs can be mainly ascribed to the low thermal conductivity of the OIHP layer.…”

“…www.advancedsciencenews.com www.entechnol.de vibrational, and thermodynamic properties of MAPbI 3 , confirming the tetragonal to cubic at T ¼ 327 K driven largely by phonon modes associated with the inorganic lattice. [26,37] The configurational disorder associated with the organic molecule has very little effect on this transition. Table 1 describes the phase transition parameters of a CH 3 NH 3 SnCl 3 compound.…”