**3. Perovskite solar cell encapsulation**

To promote commercial viability, a PSC encapsulation should: (1) be impermeable to water and oxygen; (2) prevent organic and halide materials volatized by illumination and/or heat as well as toxic lead-based degradation products from escaping into the environment; (3) have very high visible light transparency so as not to compromise device efficiencies, (4) be chemically inert, and; (4) have sufficient mechanical durability and abrasion-resistance to tolerate the stress, wear and weathering introduced in normal installation and operation. Important performance metrics for an encapsulant are the water vapor transmission rate (WVTR) and the oxygen transmission rate (OTR). These measurements quantify the amount of water vapor or oxygen that permeate through the encapsulation material per unit time. Since water and oxygen are two of the most pervasive sources of degradation in perovskites, these metrics give a good indication as to the overall quality of the encapsulation. An adequate seal is achieved when the WVTR and OTR are in the range of or less than 10−3–10−6 g∙m−2 ∙ day−1 and 10−4–10−6 cm3 ∙m−2∙day−1∙atm−1, respectively [20, 21]. Additionally, though not typically a focus, the ideal encapsulation system should also provide protection against UV irradiation and act as a thermal barrier to prevent UV-induced and thermal degradation.

Generally, encapsulation strategies have involved either the deposition of a transparent thin film encapsulant or the use of an edge sealant material to encapsulate the device between sheets of glass or polymers [22]. This precedent provides a framework for diving the encapsulation techniques into the following categories: (1) glass-to-glass (Section 3.1); (2) polymer (Section 3.2), and; (3) inorganic thin film encapsulation (Section 3.3). In glass-to-glass encapsulation, a glass cover is used in conjunction with a sealant to form the protective packaging. Conversely, polymer encapsulation encompasses the strategies that employ polymeric barriers – either as cover sheets or thin films. Finally, in the third category, thin inorganic barrier films form the encapsulation. A fourth category – hybrid encapsulations – will also be introduced in Section 3.4, and consists of any combination of the aforementioned three encapsulation strategies.

Thin films (organic or inorganic) in particular are uniquely suitable for encapsulation because they can serve as dense, pin-hole-free barriers to oxygen and water, yet remain lightweight and thin enough to not adversely affect the mechanical flexibility of the solar stack and can thus be compatible with roll-to-roll processing. This work will briefly contextualize the progress made to-date in PSC encapsulation, with an emphasis on techniques that incorporate thin barrier films. The most noteworthy encapsulation examples from the literature are summarized in **Tables 1**–**4**. Therein, the PCE of the encapsulated PSC and a schematic of the encapsulation are provided. Additionally, the WVTR of the encapsulant and outcomes of stability testing (% PCE retained) are given to provide a framework for comparing encapsulation strategies.

#### **3.1 Glass-to-glass encapsulation**

Derived from the standard encapsulation technique of the silicon solar technology, glass-to-glass encapsulation sandwiches the PSC between two sheets of glass which are sealed together by means of a sealant. Since the WVTR and OTR of glass are near zero, glass-to-glass encapsulation provides excellent protection from water- and oxygen-induced degradation, while maintaining high light transparency. Furthermore, since glass is easy to clean, has very good mechanical durability and is currently more cost-effective than alternative encapsulating systems, it is considered a highly efficient and industrially attractive encapsulant material [8]. However, moisture and oxygen ingress through the sealant at the edges of glass-to-glass encapsulated devices is significant enough to cause degradation [35]. As a result, recent efforts have been placed on optimizing sealant materials such that the WVTR and OTR are minimized. For example, butyl rubber edge sealants, such as polyisobutylene (PIB), have attracted


#### **Table 1.**

*Notable glass-to-glass perovskite solar cell encapsulations from the literature. WVTR is reported in g∙m−2∙day−1.*

considerable attention for their low WVTR (10−2–10−3 g∙m−2∙day−1) [23]. Like PIB, many encapsulant adhesives and edge sealants are thermo-curable. However, curing at high temperature can degrade thermally unstable perovskites and reduce power conversion efficiencies even before aging tests begin [36]. While UV-curable epoxies are more costly, they are advantageous in that heat need not be applied to form the seal [35]. But, UV light, particularly in the presence of water and/or oxygen can also cause perovskite degradation. Nevertheless, Dong et al. observed a significant improvement in the PCE of devices encapsulated with a UV-curable epoxy (14.8%) compared to a thermally-curable one (8.9%) [32]. To eliminate the need for a sealant altogether, hermetic glass frit encapsulation has also been proposed [24]. **Table 1** compares PIB and glass frit sealed glass-to-glass encapsulations, demonstrating extremely low WVTR and high corresponding retained PCEs after aging. However, in both cases, the PSC has low initial PCE as a result of degradation caused by the encapsulation process and/or substitutions to internally stabilize the solar stack.

Many researchers believe that the competitiveness of PSCs lies almost exclusively in their efficiencies. Others are willing to incorporate more inexpensive materials and processes to reduce costs, even if it means sacrificing some efficiency. In order to keep the price per Watt (\$/W) of a perovskite solar module low, these researchers are keen on retaining device flexibility, such that the solar cells can be made at the large-scale by low-cost roll-to-roll (R2R) processing. Recent work on ultra-thin glass encapsulation [37] has produced PSCs with retained flexibility, but further studies are required to properly assess their long-term stabilities.

#### **3.2 Polymer encapsulation**

Since glass-to-glass encapsulation is not inherently compatible with R2R processing, recent attention has been placed on polymer cover encapsulation. Herein, polymer sheets sealed with thermally-/UV-curable epoxies or pressure sensitive adhesives are used to


#### **Table 2.**

*Notable polymer perovskite solar cell encapsulations from the literature. WVTR is reported at ambient conditions in g∙m−2∙day−1. 'N.R.' indicates that a value was 'not reported.'*


#### **Table 3.**

*Notable thin film perovskite solar cell encapsulations from the literature. WVTR is reported in g∙m−2∙day−1. 'N.R.' indicates that a value was 'not reported.'*

