**4.3 OTR, WVTR, and prevention of volatilization of internal decomposition products**

Due to the chemical nature of perovskite solar cells, the main purpose of the encapsulation is to mitigate degradation by extrinsic factors H2O and O2. Water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) are the parameters that allow to quantify the water vapor and oxygen that penetrate through an encapsulant film in specific conditions of temperature and relative humidity. Because water molecules are smaller than O2 molecules, WVTR is used frequently to characterize the barrier properties of the encapsulant. It has been reported that the optimal

**Figure 4.** *Polymer molecules: (a) Surlyn, (b) ionic interaction in the Surlyn copolymer, and (c) EVA.* encapsulation materials should have WVTR between 10−3 and 10−6 gm−2 day−1 [24]. However, the WVTR varies greatly and depends on both the structure of the polymer and the polarity of the molecules. The nonpolar groups in the polymers are associated with low water affinity and result in surface contact angles >90 degrees. For instance, the thermoplastic polyurethane (TPU) has nonpolar groups and presents a contact angle around 150 degrees. This hydrophobicity allows the PSC modules encapsulated with this material retain 97.52% of the initial efficiency after 2136 hours under outdoor conditions [26].

In addition, to prevent extrinsic degradation, the materials used for FE can also contribute to reducing intrinsic degradation. Under stress factors, perovskite can decompose into volatile species such as HI, NH3, and CH3I [25]. However, in the low confined volume of FE/PVSC, the partial pressure of the degradation of volatile species starts to be high, up to the equilibrium point where the decomposition reactions are suppressed. For this reason, the materials used as FE are of special interest, but the reactivity with perovskite layers and the possible formation of by-products must be studied [32].

#### **4.4 Chemical properties and by-products**

Materials used in FE must be chemically inert to PSC under UV-Vis radiation, ambient temperature, and high humidity conditions, while those materials used in ES must not release substances that degrade perovskite during thermal, UV, or laser curing. On the other hand, organic encapsulants should have resistance to UV degradation and should not present hydrolysis reactions, for instance, PDMS, POE, PIB, and glass frits have not exhibited any reactions that promote the degradation of perovskite or the material itself in accelerated aging tests. On the other hand, materials such as EVA and Surlyn under prolonged illumination and thermal stress produce acetic acid and acid methacrylate as by-products, respectively [22]. PU has ester bonding (R–NH–COOR'), which in the presence of high humidity leads to hydrolysis and depolymerization [33]. Similarly, it has been reported that PVB, due to its chemical structure, is sensitive to hydrolysis reactions and should be combined with low WVTR edge sealants [34]. On the other hand, the components of UV curable epoxy resins might be inert with PSC but have traces of moisture among them that subsequently degasify and degrade the perovskite layer.

Finally, most of these encapsulation materials are compatible with silicon cells, including EVA with acetic acid as a by-product. However, for PSC encapsulation materials with higher stability are required. Among these, PDMS, polyolefin, and PIB are shown to be the main candidates for encapsulation. Besides, to mitigate intrinsic and extrinsic degradation, the most promising structure is complete encapsulation, with an FE material that has high compatibility with PSC and an edge sealant with the lowest WVRT.

### **5. Stability testing and characterization**

The main parameter to evaluate the stability performance of solar panels is the maximum power (or the panel efficiency), which depends on environmental variables such as solar irradiance and panel temperature [35]. In this context, the failure of an individual device is defined as the time at which the output power drops to 20% below the initial rated power. This parameter corresponds to the standard definition

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

of the lifetime of photovoltaic devices (*T*80) used as a figure of merit and commonly estimated from regressions analysis using the maximum power as a function of time [36]. Moreover, *T*80 depends on different factors such as the materials and procedures used to fabricate the device, weather, and installation conditions, etc. [37]. Nevertheless, to improve the lifetime, the solar panel must include encapsulation/ packaging materials to mitigate degradation processes, increase the electrical insulation, and provide mechanical and thermal support [38].

Delamination or adhesion loss of the encapsulant can be considered the most frequent and severe cause of module degradation, affecting the sunlight absorption and allowing the water-moisture penetration into the device [39]. In addition, most of the recognized failure models on PV silicon modules are related to the packaging materials [40]. **Figure 5** shows the relationship between the device degradation and some failure modes such as discoloration, hydrolysis, corrosion, current leak, encapsulant embrittlement, and delamination. Herein, the degradation occurs when the substrate (backsheet) or encapsulant (typically EVA) is mainly affected by UV, heat, or water ingress. Besides, **Figure 5** correlates the involved tests according to the failure modes. For example, visual inspection and/or thermographic analysis (IF image) for discoloration, delamination or cell crack, chemical degradation for hydrolysis, series resistance (*R*s) for the corrosion process, insulation test for leaking currents, and validating the status of the packaging material as a dielectric, etc. Finally, the overview also highlights the importance of the I-V curve and the extracted parameters from this curve to track the degradation processes, such as fill factor (FF), series resistance (*R*s), shunt resistance (*R*sh), etc. [41–43].

In this context, PV manufacturers widely recognized international standards such as IEC 61215 to identify potential failures in silicon photovoltaic modules [44]. This qualification testing is based on three stress factors: light (irradiance and UV), heat, and moisture. Herein, the initial stabilization (exposing the modules to simulated sunlight) and characterization (visual inspection, performance, insulation, and leakage currents) are essential to verify manufacture label values (datasheet). The final stabilization and wet leakage current test are performed to determine the module degradation and evaluate the pass criterium. It is worth noting that the output power determination is performed after a defined cooling time. Thus, eight randomly selected modules are tested into five groups as shown in **Figure 6**.

#### **Figure 5.**

*Correlation of failure modes with the packaging materials used in PV modules. Adapted from [40].*

## **Figure 6.**

*Flowchart for design qualification of PV modules according to IEC 61215. Adapted from [45].*


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

• *Group 5:* two modules are tested in order to determine the ability to withstand the humidity penetration (damp heat test, 85°C/85%RH). Moreover, mechanical tests are included.

In the case of perovskite solar cells and modules, a broad range of efficiencies have been reported due to the diversity of structures used in the fabrication, highlighting the critical role of the protocols for obtaining reproducible devices [2]. Moreover, several protocols have been designed to evaluate the stability of perovskite devices focused in laboratory-scale cells. From these, the protocol most widely spread is the result from the International Summit on Organic Photovoltaic Stability (ISOS) [46].

Related to the deployment of perovskite technology, several features must be highlighted from the stability tests reported in the literature. Ethylene-vinyl acetate (EVA) as an encapsulant has been successfully tested following the temperature cycles test suggested by IEC 61215 [30]. Polyisobutylene as a barrier layer showed promising results in thermal cycles and damp heat tests [47]. Carbon layer as a barrier increased the long stability of devices up to 12,000 hours of exposure under continuous illumination [48]. A printable mesoscopic solar cell with carbon as the electrode and hot melt polyurethane as encapsulant passed the accelerated tests suggested by IEC 61215 [5]. Epoxy resin was used as encapsulant to evaluate the outdoor performance for minimodules following the international standard IEC 61853-1 [49]. The lifetime of minimodules encapsulated with EVA was estimated in outdoor tests concerning the depicted degradations patterns for the maximum power evolution and ideality factor providing insight concerning the degradation processes [43].

Although a lot of work has been reported on the stability of perovskite technology, the average lifetime (*T*80) is still short and reaches just a few months [50]. This fact remarks the essential role of the packaging materials to protect the solar cells and electrodes from the environment guaranteeing lower degradation rates. In fact, the stability results suggest that there is still room for improvement, particularly outdoor test investigations to provide insights related to failure modes [51]. Nevertheless, it is worth noting that the qualification testing does not test for all failure mechanisms, and for that, it cannot be used to provide a prediction of the device lifetime [52]. Besides, the qualification testing of IEC 61215 is proper for modules (module level); thus, the scaling of the technology and some particularities of the perovskite technology must be considered to adapt or include other requirements for testing the device stability, as occurred with the light-soaking effects (power stabilization) for thin-film modules [44].

### **6. Security**

In addition to the relevance of encapsulation as an important aspect to overcome the extrinsic degradation and improve the operational stability of the PSCs, health and environmental security are also pertinent toward commercialization of the technology. Like other photovoltaic modules, such as silicon, Perovskite modules can be damaged due to several uncontrollable causes that could include hailstones, fire during operation, or some other natural disaster. Therefore, a proper encapsulation can also contribute to prevent environmental issues associated with constituent materials leakage. Particularly, lead represents the most hazardous environmental contaminant among all the constituent materials in a PSC. It has been estimated that for a typical 400–550 nm thick perovskite layer, the unit area concentration of Pb ranges from 0.4

to 0.75 g/m2 [53, 54], which is a high value, when compared with the amount of lead present in an automobile battery that contains 20 pounds (9,000,000 mg) on average.

If a PSC made from a CH3NH3PbI3 is in direct contact with water, it immediately decomposes into PbI2 and CH3NH3I [55]. When a broken device is exposed to simulated rain, it loses up to 72% of lead after 5 min of leaching and 100% after 72 hours. In order to prevent contamination by lead leakage, there are basically two alternatives. The first one is to make a long-lasting encapsulation that can be "indestructible," which is quite difficult to achieve, but somehow possible using a self-healing coating that could heal itself after any kind of scratch. The second strategy is focused on mitigating the leakage after the encapsulation has failed, by means of chemical lead sequestration, using lead adsorbents in the device structure. A schematic representation of these two strategies is shown in **Figure 7**.

In the case of lead leakage prevention using chemical absorbing materials, resins are the preferred choice. Among them, *P*,*P*′-di(2-ethylhexyl)methanediphosphonic acid (DMDP) and *N*,*N*,*N*′,*N*′-ethylenediaminetetrakis(methylenephosphonic acid) (EDTMP) as Pb-chelating agents, or sulfonic acid cation exchange resins, such as Amberlyst15TMH, can be used [56, 57]. **Figure 8** shows two examples of these materials, one chelating resin and one cation exchange resin. In the first case, the two phosphonic acid groups in each DMDP molecule can strongly bind with a Pb2+ with a binding energy of 252 kJ mol−1. In the second case, the sulfonic acid groups act as adsorption sites for Pb2+ ions and have a surface area (~50 m2 g−1) due to its nanoscopically porous structure with nanoparticle sizes of ~40 nm. As the two materials act when lead is in its ionic state, they can be introduced as an external coating of the device, but also in combination with the solid state perovskite film, in some cases without decreasing the device performance [56].

The use of a self-healing coating to prevent lead leakage is shown in **Figure 9**. Resins with low glass transition temperature (*T*g) are attractive for this purpose. Specifically, epoxy resins with *T*g below 50°C can be sandwiched between the

#### **Figure 7.**

*If a solar cell is broken, the use of two strategies can prevent the lead leakage. The first one (top-right) is the use of a self-healing coating that can automatically repair if it is damaged, and the second one (bottom-right) is the use of an adsorbent chemical that can capture leached lead before it reaches the soil.*

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

#### **Figure 8.**

*Chemical structure of (a) DMDP resin, and (b) Amberlyst15TMH cation exchange resin.*

#### **Figure 9.**

*Schematic representation of the self-healing process of a glass (gray) coated substrate that can be damaged and then recover after heating at low temperature (below 50°C).*

perovskite solar module and the top glass cover, and when the glass and the coating are damaged, the heat caused by sunshine can increase the device temperature above *T*g, leading to a softening of the epoxy resin and a structural accommodation that can fill the empty spaces (cracks of cut areas).
