**4. Encapsulation materials and by-products**

The previous section mentioned the encapsulation structures that have been used for perovskite-type solar cells (PSCs). For the manufacture of these structures, methods similar to those used for the encapsulation of silicon panels have been adopted, such as vacuum or roll-to-roll lamination processes [25]. These methods may include UV curing, high-pressure lamination, and temperatures between 80°C and 140°C to ensure good adhesion and avoid thermal degradation of the PSC [26]. The encapsulant materials must be chemically inert to the PSC layers and also serve as a barrier blocking extrinsic factors such as H2O and O2. In addition, the encapsulant materials must be stable under temperature, humidity, and illumination conditions, ensuring adequate electrical, optical, and mechanical properties of the resulting device (**Figure 3**). For instance, the materials used for FE (film encapsulant) and ES (edge sealant) must have a high volumetric resistance to offer electrical insulation, among these materials, EVA, Surlyn (ethylene methacrylic acid copolymer), and polyolefin (POE) have volume resistivities of 1 × 1014, 6 × 1015 years 3 × 1016 Ω cm, respectively [27]. The main properties of the most studied materials used for FE and ES are presented below.

#### **4.1 Optical properties**

Mainly for FE, the materials used must present high optical transmittance since they are deposited on the active area of the solar cell and must guarantee that the light passes through. On the other hand, ES materials have fewer optical requirements since they are located in the non-active area of the cell.

EVA, Surlyn, polyvinyl butyral (PVB), UV-cured epoxy resins, polyurethane (PU), and POE have optical transmittance values around 90% [27]. However, the encapsulant EVA (the most common encapsulant used in commercial silicon cells) turns yellowish/brownish after a few years of operation leading to a decreased transmittance [28]. This is due to thermal stress of the copolymer, degradation by UV radiation, or a combination of both factors [28]. This outcome indicates that even for polymers that show high optical transmittance after the lamination process, it is necessary to perform accelerated aging testing under thermal and UV radiation conditions to confirm their durability. If after this kind of tests, the materials show a significant decay in transmittance, a possible solution is the incorporation of antioxidants, UV absorbing materials, or the use of Ce-doped glasses to absorb UV and prevent the aging of the encapsulant [29].

**Figure 3.** *Scheme of complete encapsulation system.*

*Encapsulation against Extrinsic Degradation Factors and Stability Testing of Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.106055*

#### **4.2 Mechanical properties**

The materials used in FE and ES must have a low elastic modulus in order to relieve strain and avoid delamination processes. The copolymers EVA and POE, mainly used in FE, have an elastic modulus between 10 and 80 MPa [27]. On the other hand, Surlyn copolymer has an elastic modulus around 400 MPa. For this reason, it has been reported that due to the brittleness of this material, delamination occurs after several measurement cycles [30]. These polymeric materials with low elastic modulus that minimize the presence of cracks under applied stresses are promising candidates for encapsulating flexible perovskite-based devices.

Among the materials that have shown the best results as edge sealing against moisture and oxygen are PIB and glass frits [25, 31]. PIB is a more versatile material with a low modulus of elasticity (9 MPa), which makes it a candidate for rigid and flexible encapsulation systems, while glass frits and epoxy resins are an option for encapsulation in rigid substrates, due to their mechanical rigidity making them prone to cracking. In addition, as further limitations, glass frits require temperatures >100°C for curing and currently are costly [25].

Solar cells must operate in ambient conditions, and it is necessary to anticipate the mechanical behavior of the encapsulating materials in different temperatures. For this reason, they are usually subjected to a thermal cycling test between −40°C and 85°C [27]. The glass transition temperature (*T*g) of the encapsulant should be low enough to prevent the encapsulant from embrittlement in low-temperature conditions and failure, resulting in water and oxygen ingress and subsequent degradation. Polymeric materials that have a lower *T*g usually have a lower cross-link density, a more flexible structure, and a higher free volume. For instance, the nonpolar chains in Surlyn are grouped together, and the polar ionic groups attract each other. This condition allows the polymer to behave similar to cross-linked polymers being more rigid and less permeable (**Figure 4a** and **b**), whereas EVA has a polymer structure that results in a more flexible but more permeable structure (**Figure 4c**).
