Theoretical calculations reveal the microscopic processes of CsPbI3 kinetic phase transitions

On August 25, 2022, the team of Professor Yin Wanjian of Soochow University and Shanghai Zhizhi Research Institute, together with Academician Gong Xingao of Fudan University and Shanghai Zhizhi Research Institute, published a new study entitled “Kinetic pathway of γ-to-δ phase transition in CsPbI3” in the journal Chem.

The research group adopted the solid-solid phase variable path search method based on “stochastic potential energy surface walking search” developed by Professor Liu Zhipan of Fudan University, and gave the phase transition path of the inorganic halide perovskite CsPbI3 atomic level, and the transformation of the γ phase of CsPbI3 to its δ phase needs to undergo a multi-step transformation of three intermediate states.

The corresponding author of the paper is Yin Wanjian; The first author is Chen Gaoyuan.

CsPbI3’s perovskite phases (α, β, and γ) have excellent photoelectric properties, making it ideal for the preparation of solar cells, light-emitting diodes, and photodetectors. However, the perovskite phase of CsPbI3 is thermodynamically unstable in the atmospheric environment and will spontaneously transform into a non-perovskite phase (i.e., δ phase), δ-CsPbI3 band gap value is too wide, about 2.8 eV, and is not suitable for use as a photoelectric conversion device. Researchers have made a lot of efforts to inhibit the transition of CsPbI3’s perovskites to non-perovskites, thereby dynamically stabilizing CsPbI3 in the metastable perovskite phase. However, the mechanism of transition from CsPbI3 perovskites at the atomic level to non-perovskite phases is still unknown. The structure of the perovskite phases of the three CsPbI3 is similar, but the difference between the perovskites of CsPbI3 (all below using γ-CsPbI3) and non-perovskites (δ-CsPbI3) is very large, and there is no simple lattice and atomic correspondence. Unlike the simple martensitic phase transition mechanism, the transition from γ-CsPbI3 to δ-CsPbI3 is a typical diffusion phase transition. Due to the rapid reorganization of local atoms during phase transitions, atomic paths, intermediate states, and transition barriers are difficult to characterize experimentally.

Recently, the team of Professor Yin Wanjian of Soochow University and Shanghai Zhizhi Research Institute, together with Academician Gong Xingao of Fudan University and Shanghai Zhizhi Research Institute, used the solid-solid phase change path search method of “random potential energy surface walking search” to obtain the atomic-level phase change path of γ-CsPbI3 to δ-CsPbI3 minimum energy barrier for the first time. γ-CsPbI3 is not directly transformed into δ-CsPbI3 in one step, but through three intermediate states (MS1, MS2 and MS3) multi-step transformation, the transformation process has been in the direction of the destruction of the PbI6 octahedral co-vertex connection and the formation of the PbI6 octahedral common vertex connection, the corresponding CsPbI3 crystal structure framework from the PbI6 co-vertex 3D mesh, through the 2D layer to the 1D chain. The minimum barrier transition path is γ (3D) →Pm (3D)→Cmcm (2D) → Pmcn (1D)→δ (1D).

Figure 1: The full path of γ→δ phase transition of CsPbI3

Identify the path of the lowest transition barrier:Using γ-CsPbI3 as the initial state (IS), the potential energy surface (PES) around it was searched in detail, and nearly 2000 local steady-state structures were obtained. Taking the total energy as the vertical axis and the one-step transformation energy barrier as the horizontal axis, 2000 structures are displayed in Figure 2(A), and within the energy range of CNPb=6 and Etot< Etot(γ)+25meV/atom, 8 representative crystal structures are obtained, represented in Figure 2(B). According to Table 1, CsPbI3 of the γ and δ phases have a full PbI6 co-vertex (ηc = 1) and a partial commonality (ηe = 2/3, ηi = 1/3) connected structures, respectively. Furthermore, phase transitions occur gradually according to the shared category of I anions (PbI6 octahedral connectivity mode), corresponding to γ (ηc = 1, ηe = 0, ηi = 0) →str2 (ηc = 2/3, ηe = 1/3, ηi = 0) →str6 (ηc = 1/3, ηe = 2/3, ηi = 0) →δ phase (ηc = 0, ηe = 2/3, ηi = 1/3). The distance-weighted ordinal parameter (OP) of the Steinhardt type further proves such a transition path, as shown in Figure 2(B), which yields a transition path of γ (0.326) →str2 (0.319) → str6 (0.272) →δ (0.261). If str2 is denoted as MS1 and str6 is denoted as MS2, the corresponding transition path is denoted as γ→ MS1→ MS2 →δ. For the transition path, further extrapolation verification from TS to IS and FS must be done, γ→ the path extrapolation verification of MS1, MS1→ MS2 can be successful, but the path extrapolation of MS2 →δ cannot be successfully extrapolated, but extrapolated to get a new structure different from MS2 and δ, which we define as MS3, which is an intermediate structure that MS2 needs to undergo to transition to δ. So far, the only phase transition path of perovskite CsPbI3 has been determined: γ→ MS1→ MS2→ MS3 →δ. (γ)+25mev>

fig2: (A) Total energy (with γ-CsPbI3 as the reference zero) and a one-step transition energy barrier from γ-CsPbI3 to nearly 2,000 local steady-state (FS), with different color circles representing the average coordination number of Pb atoms CNPb. (B) 8 representative local metastable structures, with the horizontal axis representing the order parameter (OP).

Table 1: Representative crystal structures and their lattice parameters, band gap values, and I ion ratios for different classes

γ→δ the dynamic path of the phase transition:According to Figure 3, through the analysis of the transition path at the atomic level, the decisive step of the γ→δ phase transition is MS2→ MS3, and the transition energy barrier is 31 meV/atom, which is much lower than the 124 meV/atom reported in the previous literature. γ→ MS1 is a critical step in determining its optoelectronic performance, as the band gap calculated by PBE is significantly increased from 1.84 eV (γ) to 2.25 eV (MS1), thus destroying the excellent photoelectric performance of CSPbI3. γ→ MS1→ MS2, PbI6 octahedron[001]Axis rotation, accompanied by the transformation of the PbI6 frame from a 3D mesh to a 2D layer; MS2→ MS3→δ, PbI6 octahedron around[110]The shaft rotates as the PbI6 frame changes from a 2D layer to a 1D chain. In addition, all transitions cause significant changes in the lattice, suggesting that strain may be an effective means of increasing the transition barrier and thus stabilizing CsPbI3 in its metastable perovskite phase.

Figure 3: (A) the full path and energy barrier of the γ→δ, (B) and (C) give the corresponding PbI6 octahedral angles and gaps and volumes, respectively. (D-L)CsPbI3 γ→δ critical state and transition state structure during transformation.

Figure 4: (A) Dopant elements in the periodic table of the B site of CsPbI3. (A) CsPbI3 doped γ→ MS1 transition energy barrier volcano map in CsPbI3 with ion radius.

Figure 5: Γ→ the transition energy barrier of MS1 with (A) hydrostatic pressure and (B) (001), (010) and (100) crystal faces epitaxial stress sum[100]、[010]、[001]Changes in uniaxial strain.

Doping and strain effects:According to Figure 4 and Figure 5, with the increase of the ion radius from small to large, the transformation energy barrier shows a volcanic distribution, and the metal ions in the high valence state and medium ion size have obvious lifting effects, which is very conducive to the stability of the CsPbI3 perovskite phase. This volcanic map distribution can be understood through Pauling rules and cohesion energy. In addition, the lattice vector b-axis ([010]Crystal direction, including (001)/(100) epitaxial sum[010]Uniaxial) strain, improve the transformation barrier effect is obvious, but also about to involve the lattice vector b-axis ([010]Crystal direction) or axial contact with the substrate is conducive to the growth of a more stable CsPbI3 perovskite phase.

This work is supported by the National Key Research and Development Program (approval number: 2020YFB1506400), the National Natural Science Foundation of China (approval number 11974257), the Jiangsu Outstanding Young Talents Fund (approval number: BK20200003) and the Key Science and Technology Program of Yunnan Province (approval number: 202002AB08001-1). (Source: Science Network)

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