# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 3 Impact of Thermal Stress on Perovskites
### Morphological Degradation  
Several reports have investigated the effect of temperature on the morphology of perovskites. As mentioned above, [PERSON] and co-workers [38] studied the thermal stability of perovskite solar cells with an ITO/TiO\({}_{2}\)/MAPbI\({}_{3}\) structure when subjected to heat treatment at 85 \({}^{\circ}\)C for 24 h under various environmental conditions. High-angle annular dark field (HAADF) TEM was used to understand the effect of temperature on devices in different environmental conditions. Cross-sectional views of the HAADF images are shown in Figure 4a,b. The pristine perovskite device exhibited well-defined layers stacked evenly on top of each other, while the degraded samples exhibited structural variations. Interestingly, a large PbI\({}_{2}\) grain, highlighted in red, is formed in the O\({}_{2}\) environment (see Figure 4c). The calculated ratio of Pb/I determined by EDX revealed the highlighted oblong structure to be PbI\({}_{2}\). When exposed to ambient atmosphere, more severe device degradation occurred with spot-like structures in the perovskite and delamination from the TiO\({}_{2}\) layer, as shown in Figure 4d. Additionally, topographical AFM images (Figure 4e-h) showed significant structural changes for samples measured under ambient conditions. Significant dark areas appearing in AFM images were associated with PbI\({}_{2}\) regions, which indicates a photocurrent reduction in those regions upon degradation.  
[PERSON] et al. [17] tested the stability of perovskite solar devices in an environmental chamber, where the temperature was controlled in the range of \(-\)20 \({}^{\circ}\)C to 100 \({}^{\circ}\)C. The actual cell temperature inside the chamber was found to be approximately 30 \({}^{\circ}\)C higher than the environmental temperature, so the temperatures were denoted as \"environmental temperature (actual cell temperature)\", e.g., 55 \({}^{\circ}\)C (85 \({}^{\circ}\)C) [17]. The degradation mechanism of CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) solar cells was analyzed for 500 h at 55 \({}^{\circ}\)C (85 \({}^{\circ}\)C) using cross-sectional focused ion beam-scanning electron microscopy (FIB-SEM). The cross-sectional FIB-SEM images shown in Figure 5 reveal the degradation mechanism of an encapsulated device. The direct exposure equivalent to one sun illumination clearly damaged the entire device, with the most degradation observed in the silver layer. A number of degradation features were observed in the degraded cells; including degradation of the silver layer, formation of voids in spiro-OMeTAD and the perovskite layer, and delamination of the perovskite layer from the TiO\({}_{2}\) layer, as shown in Figure 5a-c. Particularly, the formation of a PbI\({}_{2}\) layer was found, as indicated by a bright contrast in the SEM image (see Figure 5d). It is anticipated that the degradation was initiated by the reaction of HI gas and Ag in an encapsulated device structure. Therefore, the authors suggested replacing the silver contact and using highly heat-resistant encapsulating materials.  
Figure 4: (**a-d**) Cross-sectional views of perovskite solar cells heated at 85 \({}^{\circ}\)C for 24 h in different atmospheric conditions. The area highlighted in red in the degraded device in O\({}_{2}\) environment indicates the presence of PbI\({}_{2}\) grain. (**e-h**) Topographical AFM images of perovskite films that were heated at 85 \({}^{\circ}\)C for 24 h in different atmospheric conditions. Reprinted from [38] with permission.