# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 5 Other Strategies to Improve Thermal Stability
The effect of thermal stress was investigated on an encapsulated device at 45 \({}^{\circ}\)C under 10 mW/cm\({}^{2}\) illumination. The CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\)-ABPA modified devices maintained 90% of their initial efficiency after one week. A heat stress test was performed on CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\)-ABPA device at 85 \({}^{\circ}\)C in the dark for 350 h to confirm if the modified device could endure the exposure to high temperature and the modified device maintained 80% of its initial PCE after 350 h of continuous annealing. Figure 11: (**a**) A schematic of 2D/3D stacking structure obtained from BA-treated perovskite film. (**b**,**c**) 2D/3D molecular junctions on the surface and at grain boundaries of 3D perovskite films with BA and BAI treatments, respectively. SEM images of (d) MAPbI\({}_{3}\) films, (**e**) BA-treated MAPbI\({}_{3}\) films, and (**f**) BAI-treated MAPbI\({}_{3}\) films. Capacitance\(-\)frequency spectra (**g**) before and (**h**) after thermal stress at 85 \({}^{\circ}\)C for 20 h of the control, BA-treated, and BAI-treated devices; (**i**) Time evolution of normalized PCE under thermal stress. Reprinted with permission from [74]. Copyright (2018) American Chemical Society. Perovskite thin films are very sensitive to oxygen and moisture, even though different cations and mixed halides have been employed in perovskite structures. Moisture ingress is known to accelerate the thermal degradation of perovskite solar cells, so encapsulation is used to protect perovskites from moisture. [PERSON] et al. (2018) compared five different glass-glass sealing procedures and found an optimized procedure using Kapton and glue bonded glass. They also showed that an additional edge sealing increased moisture resistance, leading to improved thermal stability with the ability to maintain initial PCE for over 1300 h of shelf-life. Furthermore, they found that the optimized sealing procedure prevents the intrinsic degradation of CH3 NH3 PbI3, although a reduction in PCE of 0.1%/h at 60 \({}^{\circ}\)C and 0.21% at 85 \({}^{\circ}\)C was also observed. They suggested that this reduction was attributed to the degradation of Spiro-OMeTAD HTL. A different sealing procedure approach can include the use of polymers such as poly(methyl methacrylate) (PMMA), polycarbonate (PC), or polystyrene to protect perovskite films from oxygen and moisture ([PERSON] et al., 2019). Along with protection from air, polymers act as an insulating tunneling contact and passivate defects in perovskite film by increasing the efficiency by more than 18% ([PERSON] et al., 2019). [PERSON] et al. proposed a device architecture consisting of single-walled carbon nanotubes (SWNTs) and a polymer matrix of PMMA or PC. The SWNTs in such architecture facilitate selective charge extraction, while the polymer matrix serves as an encapsulating layer to protect the device from moisture penetration ([PERSON] et al., 2019). The proposed devices showed enhanced thermal stability at high temperatures of 80 \({}^{\circ}\)C in ambient humidity conditions, compared to the control devices. Another approach to improve thermal stability is substituting the methylammonium (MA) cation of the perovskite material, with mixed cation-based perovskites having been extensively studied by researchers. Enhanced crystallinity and structural stability with improved power conversion efficiency were obtained by replacing MA with formamidinium (FA) ([PERSON] et al., 2016; [PERSON] et al., 2017; [PERSON] et al., 2018), cesium (Cs) cations, ([PERSON] et al., 2017; [PERSON] et al., 2018; [PERSON] et al., 2019; [PERSON] et al., 2020) or a mixture of the two ([PERSON] et al., 2018; [PERSON] et al., 2019; [PERSON] et al., 2020; [PERSON] et al., 2021). [PERSON] et al. reported a relatively slower degradation for FAPbI3 compared to MAPbI3 when annealed at 150 \({}^{\circ}\)C ([PERSON] et al., 2018).