# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 3 Impact of Thermal Stress on Perovskites
### Chemical and Structural Degradation  
[PERSON], et al. [40] studied the thermal degradation of MAPbI\({}_{3}\) by fabricating MAPbI\({}_{3}\) microplates. Microplates are highly crystalline and thus provide an outstanding opportunity to study the structural degradation mechanisms of perovskites. The authors investigated the crystal structure under thermal stimulation using in-situ high-resolution transmission electron microscopy (HRTEM). They found that almost 75% of the original perovskite tetragonal phase returned to trigonal PbI\({}_{2}\) after 100 s of heating at 85 \({}^{\circ}\)C. Degradation is initiated by breaking the weak Pb-I-Pb bond along the (001) direction [41], after which the PbI\({}_{2}\) relaxes into its energetically favorable trigonal structure (Figure 2). During this process, CH\({}_{3}\)NH\({}_{2}\) and HI sublime into the gas phase. Their studies conducted in dry and inert gaseous environments revealed that no MAPbI\({}_{3}\) hydrates are generated during the degradation process [42; 43; 44]. The authors therefore concluded that the phase transition to PbI\({}_{2}\) is not initiated by the intrinsic hygroscopicity of alkylammonium cations [38; 44], but instead is completely due to the thermally induced degradation. They suggested that a sequential transition occurs from tetragonal MAPbI \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Halides (O)**} & \multirow{2}{*}{**Temperature (K)**} & \multirow{2}{*}{**Crystal Structure**} & \multirow{2}{*}{**Space Group**} & \multicolumn{3}{c}{**Lattice Parameter (Å)**} \\ \cline{6-7}  & & & & **a** & **b** & **c** \\ \hline Cl & \(>\)178.8 & Cubic & _Pm3m_ & 5.675 & & \\  & 172.9–178.8 & Tetragonal & _P4/mmm_ & 5.656 & & 5.630 \\  & \(<\)172.9 & Orthorhombic & _P222\({}_{1}\)_ & 5.673 & 5.628 & 11.182 \\ \hline Br & \(>\)236.9 & Cubic & _Pm3m_ & 5.901 & & \\  & 155.1–236.9 & Tetragonal & _I4/mmm_ & 8.322 & & 11.832 \\  & 149.5–155.1 & Tetragonal & _P4/mmm_ & 5.894 & & 5.861 \\  & \(<\)144.5 & Orthorhombic & _Pma2_ & 7.979 & 8.580 & 11.849 \\ \hline I & \(>\)327.4 & Cubic & _Pm3m_ & 6.329 & & \\  & 162.2–327.4 & Tetragonal & _I4/mmm_ & 8.855 & & 12.659 \\  & \(<\)162.2 & Orthorhombic & _Pma2\({}_{1}\)_ & 8.861 & 8.581 & 12.620 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structural data for various methylammonium lead trihalides [22]. to trigonal PbI\({}_{2}\) based on the time-dependent studies, which showed an overlap of the MAPbI\({}_{3}\) and PbI\({}_{2}\) crystal structures in the electron diffraction patterns under thermal stress. In particular, the diffraction intensity of the perovskite tetragonal phase was correlated with a direct increase in the PbI\({}_{2}\) trigonal phase intensity. This correlation indicates that the structure is a stacked architecture, with PbI\({}_{2}\) formed on top of the underlying MAPbI\({}_{3}\) layers. In addition, the thickness of each layer varies with the annealing time, with the thermal transformation from MAPbI\({}_{3}\) to PbI\({}_{2}\) occurring through bulk or surface reactions. To understand the actual reaction process, they conducted density functional theory (DFT) calculations [44]. The corresponding reaction rate calculated for the minimum energy path from MAPbI\({}_{3}\) to PbI\({}_{2}\) through bulk degradation was found to be approximately 1000 times slower than surface-initiated decomposition [44]. Therefore, they concluded that surface degradation is kinetically preferred. The first layer is degraded, and the underlying layer gets exposed. The process is repeated until the entire material degrades, which supports the theory that it is a surface-dominated reaction [44]. The thermal stability of perovskite solar cells, with a structure of ITO/TiO\({}_{2}\)/MAPbI\({}_{3}\), was studied by [PERSON] and co-workers [38]. In order to separate the perovskite layer degradation from degradation at other interfaces, the hole transfer layer (HTL) and the top electrode were excluded.