# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 5 Other Strategies to Improve Thermal Stability  
Several techniques can be used to achieve thermally stable perovskite solar cells. Manipulating the electron and hole transport layers and metal contacts as discussed extensively in the previous section is one approach. Here, we describe complementary technologies that researchers have adopted to improve the thermal stability of perovskite solar cells. [PERSON] et al. [74] showed that the thermal stability of perovskite solar cells can be improved by constructing 2D/3D stacking structures. 2D perovskite materials exhibit better moisture stability than 3D perovskite materials [75; 76; 77; 78; 79], but the highest efficiency of 2D perovskite solar cells reported in literature is relatively low [80]. [PERSON] et al. fabricated a thin layer of 2D perovskite on top of 3D perovskites in order to cover the surface and grain boundaries. Since the grain boundaries of the perovskite are defective and degradation occurs at the grain boundaries [81], covering the crystal boundaries of the 3D structure by the 2D stacking structure improves overall device stability and maintains high efficiency. They constructed 2D/3D stacking structures by reacting n-butylamine (BA) with the surface of 3D perovskites (see Figure 11a-c). Two types of solutions were used to investigate the thermal stability: BA dissolved in chlorobenzene and n-butylamine iodide (BAI) dissolved in isopropyl alcohol. The SEM images (Figure 11d-f) revealed that the BA solution spun-coated on top of the 3D perovskite layers enhanced the surface smoothness due to the gradual diffusion of BA molecules into the perovskite crystal structure, passivating surface and grain boundaries. The BAI treated film exhibited a relatively rougher surface morphology (Figure 11f), which was attributed to 3D to 2D phase transition [82]. The thermal stress studies using capacitance versus frequency measurements showed that when the device was annealed at 85 \({}^{\circ}\)C for 20 h, the device based on the 3D structure had a capacitance greater than that of the fresh perovskite in the low-frequency regime. This high capacitance in the low-frequency regime was attributed to ionic migration at moderately high temperature [8; 83; 84]. The modified perovskite film also showed better thermal stability under continuous annealing for 1000 h compared to the fresh perovskite, with the modified devices maintaining 96.5% and 88.2% of their initial efficiencies for devices with BA and BAI treatments, respectively, while the fresh device degraded to 69.8% of its initial efficiency under thermal stress [74]. Figure 10: (**a**) A proposed mechanism for Ag-electrode-induced thermal degradation of the inverted perovskite solar cells (PSCs). Reprinted from [71], with permission. (**b**) A schematic of fabricated perovskite solar cells (PSCs) with carbon electrodes and (**c**) Photovoltaic parameters of the encapsulated perovskite solar cell plotted versus time during outdoor aging in Jeddah, Saudi Arabia. Reprinted from [16], with permission. Diagram of a three-layer perovskite device: (**d**) over-sealed and (**e**) side-sealed cell. (**f**) photo-conversion efficiency (PCE) of PSCs fabricated using the side-sealing configuration during thermal stability testing at different temperatures in the dark. Reprinted from [73], with permission. [PERSON] et al. investigated the thermal stability of MAPbI\({}_{3}\) by modifying its surface. In that study, an additive butylphosphonic acid 4-ammoniumchloride (4-ABPACI) was spun-coated onto the surface [16]. Structural, morphological, and elemental examinations then showed that the additive acted as a crosslink between perovskite grains. The phosphonic acid ammonium additive facilitated obtaining a smooth surface layer because perovskite was incorporated into the mesoporous TiO\({}_{2}\) scaffold. The 4-ABPACI additive increased the absorption and almost doubled the efficiency of the devices, compared to the control device.