**1. Introduction**

In the history of all photovoltaic technologies, the swift evolution of perovskite solar cells (PSCs) remains completely unprecedented. With the achievement of efficiencies that have increased from 14% to as high as 25.7% in less than 10 years [1], PSCs are on the verge of disrupting the incumbent crystalline silicon technology. These efficiencies are a result of high optical absorption, long carrier diffusion lengths and excellent charge transport, and lead to the generation of exceptional open-circuit voltages (Voc) as high as 1.2 V [2]. According to the Shockley-Queisser (SQ ) limit, the photo-conversion efficiency (PCE) of PSCs with absorber band gaps of 1.6 eV can reach 30.14%, corresponding to a short circuit current density (Jsc) of 25.47 mA/cm2 , a Voc of 1.309 V and a fill factor (FF) of 90.5% [3]. Fundamentally, a perovskite is any material which has a crystal structure that can be described by the general chemical

formula ABX3. Herein, A represents a cation, B represents a metal cation with two valence electrons, and X represents an anion [4]. Since a variety of elements can be chosen to fill the A, B or X locations in the crystal structure, perovskites can easily be tuned for their physical, optical, and electrical properties. The highest efficiencies reported to-date are for the organic–inorganic lead halide perovskite, CH3NH3PbX3 (X = I, Br, Cl), for which the shorthand notation, MAPbX3, is commonly used. Organic–inorganic lead halide perovskites will be the focus of this work, and, unless otherwise stated, the subject of any reference to the term perovskite.

Recent life cycle assessments and techno-economic analyses [5, 6], have indicated that delaying degradation and extending the lifetime of PSCs is essential for sustainability and commercial viability. Since competitive efficiencies have already been demonstrated, the success of PSCs relies now on the improvement of their stabilities. To ensure that this technology will be profitable, lifetimes of at least 15 years [6], but ideally 25 years should be realized [7]. The solution, however, is not so straightforward. Perovskites degrade readily upon exposure to oxygen and moisture, therefore necessitating strategies for degradation mitigation or prevention. Additionally, the perovskite crystals are thermally unstable and have low decomposition temperatures as a result of their ionic nature and the use of organic meythlammonium (CH3NH3 + , MA) cations. Photo-induced degradation of perovskites constitutes another major issue.

### **2. Perovskite degradation**

Moisture is one of the most prevalent causes of degradation in PSCs. Water molecules that are able to permeate through the solar cell stack will react with the A-site organic cation in the perovskite and form hydrogen bonds. This weakens the bonds to the B- and X-site halogenated lead, rendering the perovskite more susceptible to thermal- and UV-induced degradation [8]. Additionally, water will react with X-site iodide ions to decompose the perovskite into hydroiodic acid (HI) and lead iodide (PbI2) [8]. Therefore, to improve the intrinsic moisture stability of the perovskite, X-site and A-site substitutions have been suggested. For example, substituting X-site iodine with bromine increases the strength of cation-lead halide bonds, thereby reducing the susceptibility of the perovskite to moisture-induced degradation [9]. Further, since grain boundary defects act as a host for these detrimental reactions with water, passivating perovskite grain boundaries and increasing grain sizes has been found to extend perovskite lifetimes in humid environments [10, 11].

Oxygen is another significant contributor to degradation in PSCs. Oxidative degradation occurs significantly in both the charge transport and perovskite layers. Oxidation of organic charge transport layers results in compromised carrier mobility and solar conversion efficiencies [8]. Conversely, metal oxide charge transport layers (e.g., TiO2, etc.) are not sensitive to oxidation, themselves. However, they can absorb oxygen, and when combined with UV light, photo-excitation yields reactive superoxide (O2 − ), which then catalyzes the rapid oxidative degradation of the adjacent perovskite layer [8]. Termed 'photo-oxidation,' this is an accelerated form of oxidative degradation, which occurs upon simultaneous exposure to UV light and oxygen. The perovskite crystal itself is also highly susceptible to photo-oxidation. Photo-excitation of the perovskite increases the density of halide vacancies, which serve as gateways for diffusion of oxygen into the perovskite lattice [12]. Again, superoxide species initiate the degradation, resulting in the formation of decomposition products such

#### *Encapsulation of Perovskite Solar Cells with Thin Barrier Films DOI: http://dx.doi.org/10.5772/intechopen.107189*

as yellow-colored PbI2 [8]. Strategies such as doping the perovskite with cadmium (Cd) have been used to decrease the density of halide vacancies and increase intrinsic resistance to photo-oxidation [13]. Similarly, Gong et al. report a PSC with 10.4% efficiency that employs doping the perovskite with Se2− to strengthen the interaction between the MA cation and its inorganic framework, thereby improving stability by 140 times compared to the undoped film, and achieving 70% PCE retention after 700 hours of exposure to air [14]. Further, since oxidative degradation is most harmful when catalyzed with UV light, filtering out energetic UV photons constitutes another promising strategy for extending device lifetimes.

While intrinsic stability improvements remain necessary to prevent water- and oxygen-induced degradation during manufacturing and assembly, encapsulation provides the most effective barrier against moisture and oxygen. Even so, since package leakage and small amounts of water and oxygen permeation are inevitable, enhancing intrinsic stability and encapsulating devices will likely need to be applied in synergy to provide sufficient protection from all catalysts of degradation. However, even when a hermetic encapsulation is achieved, (i.e., permeation of water and oxygen is considered negligible) PSCs still suffer from UV-induced degradation. For instance, illumination can result in the reversible segregation of halide and cation species, which can hinder the performance of devices [15].

Heat constitutes a final extrinsic stressor which can accelerate the reactions responsible for degradation in PSCs [8]. For example, the PbI2 decomposition product has been observed from prolonged exposure of MAPbI3 perovskites to temperatures as low as 85°C [16]. This can be detrimental since many manufacturing steps, including the annealing and encapsulating stages, occur at high temperature. Since organic materials are relatively volatile and are most sensitive to thermal degradation, the use of the common MA A-site organic cation can be problematic. Therefore, A-site cation substitution and mixing, with more thermally-stable materials such as formamidium (FA), cesium (Cs) and rubidium (Rb), is a popular strategy to improve thermal stability in perovskites [17]. All-inorganic PSCs represent another promising avenue towards stability, by eliminating issues associated with the thermal degradation of the organic cation. Liu et al. devised a CsPbI2Br-based PSC with an efficiency of 13.3%, which exhibited 80% PCE retention after thermal treatment at 85°C for 360 hours [18]. Another stabilizing strategy was demonstrated by Yun et al., who to reduced photoand thermal degradation by incorporating LiF passivators in organic–inorganic lead halide perovskites with efficiencies up to 20%. Remarkably, they observed 90% PCE retention after 1000 hours of exposure to 1 sun illumination or 85°C temperatures [19].

Significant progress has been on the intrinsic stabilization of PSCs. While the results are promising, no solution has been reported to-date that has demonstrated the long-term operation of PSCs in outdoor conditions. Therefore, it is clear that a *combination* of intrinsic stabilization and encapsulation strategies will be necessary to produce a PSC that can appropriately withstand the breadth of illumination, heat, moisture and oxygen conditions encountered during manufacturing and operation. Hereafter, this work will focus on reviewing the recent progress in PSC encapsulation and on introducing novel directions for further improvement.
