## 1 Introduction
### Encapsulation of the Perovskites  
The parameters that need to be controlled for any encapsulation material are the elastic modulus, the impermeability against water, the volume resistivity and the protection against UV irradiation.  
In mechanical terms, the perovskite layer has a thermal expansion coefficient ten times higher than the glass substrate or than the transparent conductive oxide it sits upon, so the mechanical stresses caused by temperature changes results in delamination processes between these layers, accelerating their decomposition [27; 28]. Therefore, it is important to choose an encapsulation material that has a low elastic module. Furthermore, as has been described previously, moisture quickly attacks the perovskite structure, so the encapsulation material of choice must have a low rate of water vapor transmission (WVTR) [4]. Additionally, PCSs have high voltages. These voltages increase considerably when they are used to produce modules. High voltage also activates degradation processes on the PCSs, producing chemical reactions that generate harmful by-products. Therefore, it is important to select an encapsulation material that has a high volume resistivity [4]. Finally, as has been described previously, UV irradiation starts degradation processes on the PCSs, which is triggered by photo-degradation [29]. Therefore, it is important to select an encapsulation material that prevents the passage of UV irradiation.  
The first encapsulates tried were based on the use of thin layers, formed by either Al\({}_{2}\)O\({}_{3}\) or inorganic/organic multilayers, applied together with an adhesive. This kind of encapsulation presented many advantages, such as having low deposition temperature requirements that prevented the evaporation of organic volatile compounds, and its low WVTR, below 10-5 (g/m\({}^{2}\)/day) [30; 31]. However, the high elastic modulus caused delamination after several thermal cycles. Currently, these encapsulants are still in use with flexible devices [32]. These encapsulates must take special care in sealing the edges, since the humidity that penetrates through the sides of the encapsulation is sufficient to initiate degradation [33; 34]. Some of these encapsulants are Teflon [35], Adamantane Nanocomposite [22], UV curable fluoropolymers [36], Al\({}_{2}\)O\({}_{3}\)/pv\({}_{3}\)D\({}_{3}\)[32] or organosilicates [37].  
Nowadays, the encapsulation of PSCs is based on designs that are used in CIGS and CdTe photovoltaic cells, since these cells have stability issues similar to those of PSCs, mainly in terms of humidity [38]. To solve these issues, glass is used as a cover material together with polymer layers and sealant materials applied at the edges [39]. The most common materials used for encapsulation are epoxy resin [40; 41], ethylene-vinyl acetate (EVA) [42; 43], surlyn [41; 42; 43; 44; 45], ethylene-vinyl-alcohol (EVOH) [46], poly (ethylene terephthalate) (PET) [41; 47], poluisobutylene (PBI) [48], poly methyl methacrylate (PMMA) [41], UV-curable glue [49] and polyolefilm [43]. From all of these materials, EVA is the most widely used encapsulant in the photovoltaic industry, due to its low cost, high light transmittance (93%; freestanding EVA) and low curing temperature (140 \({}^{\circ}\)C) [4].