# Developments on Perovskite Solar Cells (PSCs): A Critical Review
## 6 Device Challenges
### Perovskite Solar Cell Device Stability
#### 6.1.2 Environmental Stability  
The perovskite ABX\({}_{3}\) materials are chemically active in environmental conditions. A series of chemical reactions occur which slowly break down the ABX\({}_{3}\) material into its constituents and additional byproducts. The installation of PSCs exposes the device to environmental contaminants. Oxygen, moisture, and UV radiation constantly wear off the PV perovskite active layer. The chemical reactions below suggest possible perovskite degradation. Moisture breaks down the perovskite layer by dissolving the organic cation (MA\({}^{+}\)) to yield PbI\({}_{2}\) and HI. This degradation mechanism is an irreversible process as the presence of oxygen and UV radiation further breaks down the hydroiodic acid (HI) producing water (H\({}_{2}\)O), hydrogen (H\({}_{2}\)) and iodine (I\({}_{2}\)) gases as byproducts (see chemical reactions (2)-(5)) [83]. The chemical reactions (2)-(5) and (6)-(9) are proposed degradation reactions initiated by the presence of UV radiation and moisture. In a study to investigate the degradation of MAPbI\({}_{3}\) material exposed to a humid environment, [PERSON] (2016) reported a rapid decomposition in the perovskite material at a low relative humidity of 40% caused by either or both of the following: the formation of PbI\({}_{2}\) phase from the desorption of the volatile organic MAI cation, or the formation of a perovskite hydrate phase, resulting from the interaction of H\({}_{2}\)O molecules and the absorber N-H bond [84]. In a similar study, [PERSON] (2015) suggested that proper selection of a hydrophobic HTM stabilizes the perovskite layer deterioration; however, the effort only shifts the perovskite lifespan to a few more hours before it begins degrading [85]. \[\text{CH}_{3}\text{NH}_{3}\text{PbI}_{3}(\text{s})\leftrightarrow\text{PbI}_{ 2}(\text{s})+\text{CH}_{3}\text{NH}_{3}\text{I}(\text{aq}) \tag{2}\]  
\[\text{CH}_{3}\text{NH}_{3}\text{I}(\text{aq})\leftrightarrow\text{CH}_{3} \text{NH}_{2}(\text{aq})+\text{HI}(\text{aq}) \tag{3}\]  
\[4\text{HI}(\text{aq})+\text{O}_{2}\leftrightarrow 2\text{I}_{2}+2\text{H}_{2} \text{O}(\text{I}) \tag{4}\]  
\[2\text{HI}(\text{aq})\leftrightarrow\text{H}_{2}(\text{g})+\text{I}_{2}(\text{ s}) \tag{5}\]  
\[\text{CH}_{3}\text{NH}_{3}\text{PbI}_{3}(\text{s})\leftrightarrow\text{PbI}_{ 2}(\text{s})+\text{CH}_{3}\text{NH}_{2}\uparrow+\text{HI}\uparrow \tag{6}\]  
\[2\text{I}^{-}\leftrightarrow\text{I}_{2}+2\text{e}^{-} \tag{7}\]  
\[3\text{CH}_{3}\text{NH}_{3}\text{NH}_{2}\uparrow+3\text{H}^{+} \tag{8}\]  
\(\text{I}^{-}+\text{I}_{2}+3\text{H}^{+}+2\text{e}^{-}\leftrightarrow 3\text{HI}\uparrow\). In Figure 15a\(\sim\)c, SEM surface images reveal perovskite film morphology exposed to 40% relative humidity (RH) at various exposure times. Figure 15a reports a smooth surface at 20 min exposure with a high level of surface coverage; Figure 15b reports SEM surface morphology at 24 h of exposure, showing the formation of evenly distributed holes reducing surface homogeneity (as indicated by arrow 1). The generation of holes results from the coalescence of the inorganic PbI\({}_{2}\) phase structure as the organic MAI cation dissolves in moisture; Figure 15c demonstrates the SEM image at 48 h of exposure, showing well-defined rod-like PbI\({}_{2}\) structures (as indicated arrow 2) and the extent\(\sim\)of hole formation in the film morphology [85]. Many attempts focus on eliminating the effect of moisture on PSCs by designing new PSC architectures, encapsulating the device using polymers, proposing new ETM and HTM materials, among others, modifying the ABX\({}_{3}\) materials. In order to eliminate the effect of moisture and investigate the effect on UV radiation alone, [PERSON] (2016) studied the degradation of PSCs in an inert atmosphere (glove box) by exposing the PSC devices to UV light. The effect of UV exposure on the device was characterized through XRD and I-V techniques, as shown in Figure 15d,e, respectively.