# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 3 Impact of Thermal Stress on Perovskites
### Chemical and Structural Degradation
The samples were heated at 85 \({}^{\circ}\)C for 24 h, under four environmental conditions: pristine film (reference), O\({}_{2}\) environment, N\({}_{2}\) environment and ambient conditions. The X-ray diffraction (XRD) patterns revealed the formation of PbI\({}_{2}\) under ambient conditions, as shown in Figure 3a. [PERSON], et al. [45] showed that the peak at 20 = 14.00\({}^{\circ}\) (corresponding to a unique (001) plane of MAPbI\({}_{3}\)) decreased along with an increase of the peak at 20 = 12.61\({}^{\circ}\) (corresponding to a (003) plane of PbI\({}_{2}\)) (Figure 3b,c). Figure 2: (**a**–**c**) Degradation process of MAPbI\({}_{3}\) grain with direct transition from tetragonal to trigonal crystalline structure. The purple dashed lines in (**a**–**c**) showed the shrinking of perovskite and the insets represented Fast Fourier transform (FFT) phase diagrams of the corresponding HRTEM images (Scale bar = 2 nm). (**d**) Transition from MAPbI\({}_{3}\) with a tetragonal configuration to PbI\({}_{2}\) with a trigonal configuration. Reprinted from [40], with permission from Elsevier. Figure 3: (**a**) XRD patterns of ITO/TiO\({}_{2}\)/perovskite samples heated at 85 \({}^{\circ}\)C for 24 h in different atmospheres. Reprinted with permission from [38]. (**b**) XRD patterns of MAPbI\({}_{3}\) film collected at 120 \({}^{\circ}\)C in air at different times and (**c**) the corresponding rocking curves at 2y = 14.11\({}^{\circ}\) and 2y = 12.71\({}^{\circ}\). Reprinted with permission from [45]. (**d**) The atomic ratio of I/Pb (red circle) and N/Pb (black diamond) at elevated temperatures analyzed from photoelectron spectroscopy measurement. Reprinted with permission from [46]. Copyright (2015) American Chemical Society. All peaks related to the perovskite layer disappeared in the diffraction pattern during thermal annealing. On the basis of these findings, the authors concluded that the perovskite layer degraded into PbI\({}_{2}\) in less than 10 h. Similarly, [PERSON] and co-workers investigated the effect of higher temperatures on both MAPbI\({}_{3}\) and MAPbI\({}_{3-x}\)Cl\({}_{x}\) films [46], but used hard X-ray photoelectron spectroscopy (HX-PES) instead of XRD in their study. HX-PES aids the understanding of the film decomposition procedure by determining the chemical composition of the sample regardless of crystallinity. To investigate the decomposition process, the samples were prepared in an oxygen- and water-free environment and were heated in an analysis chamber under ultra-high vacuum. The films were quantitatively analyzed using HX-PES by extracting the I/Pb and N/Pb atomic ratios. The reduction in these ratios represents the conversion of perovskite to PbI\({}_{2}\). The atomic ratio of I/Pb (red circle) and N/Pb (black diamond) plotted at various temperatures are shown in Figure 3d. Heating at 100 \({}^{\circ}\)C for 20 min significantly reduced both ratios. Furthermore, upon heating at 200 \({}^{\circ}\)C, the two ratios decreased to 2 and 0, respectively. This indicates that the film decomposed completely into PbI\({}_{2}\).