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

#### **Table 4.**

*Notable hybrid 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.'*

encapsulate a PSC, providing a barrier against extrinsic stressors such as moisture and oxygen. Many low-cost, flexible polymers, such as PET, PMMA and PC, have been used as encapsulating materials, however, much like with glass-to-glass encapsulation, moisture and oxygen ingress through sealants and degradation during curing persists [35]. To overcome this challenge, the deposition of solution-processed polymer layers directly on top of devices has been proposed [25, 38]. McKenna et al. [38] deposited 800 nm thin polymer films (i.e., PMMA, PC, EC, and PMP) directly on top of perovskite layers by spin coating and evaluated their ability to inhibit degradation. When exposed to 60°C heat at ambient conditions for 432 hours, the uncoated perovskite film was completely degraded, while the PMMA-encapsulated film remained in pristine condition (no evidence of PbI2 formation). It is unsurprising that, of the polymers tested, PMMA provided the best device longevity because it has the lowest WVTR (55.2 g∙m−2∙day−1) and OTR (4.8 cm3 ∙m−2∙day−1∙atm−1) [38].

Bella et al. [26] also employed an innovative polymeric coating strategy to simultaneously slow water permeation and prevent UV-induced degradation of PSCs. By spin-coating devices with luminescent downshifting fluoropolymers that absorb incident UV light and re-emit it to the perovskite active layer as visible light, UV-induced degradation is effectively eliminated without sacrificing any photocurrent (PCE = 19%). Concurrently, the multifunctional polymeric coating is hydrophobic and provides a strong barrier to water-induced degradation. Notably, devices encapsulated and the top and bottom with ~5 μm films of this fluoropolymer demonstrated a 95% retention in PCE after 3 months of exposure to outdoor elements including heavy rain and temperatures ranging from −3 to +27°C.

Despite these promising findings, the relatively high WVTR (100 –102 g∙m−2∙day−1) and OTR (101 –102 cm3 ∙m−2∙day−1∙atm−1) of standard polymers limit long-term encapsulation effectiveness [35]. To this end, the incorporation of additives in polymer matrices to form polymer composite encapsulants with unique photo-, moisture- and/ or oxygen-interactions has demonstrated potential for improved stability [27, 39, 40]. Jang et al. [27] fabricated a 100 μm-thick film of poly(vinyl alcohol-co-ethylene) (EVOH) copolymer with dispersions of SiO2 and graphene oxide (GO) fillers. The SiO2 inhibited water permeation by rendering the pathway for penetration through the polymer more tortuous, while the hydrophobicity of the GO repelled water molecules. By including these dispersions, the EVOH/SiO2/GO composite polymer has a remarkable WVTR of 3.34 × 10−3 g∙m−2∙day−1, compared to 4.72 × 10−2 g∙m−2∙day−1 for EVOH only. PSCs encapsulated with the EVOH composite by means of a UV-curable adhesive retained 86% of their original PCE after 5 hours of direct exposure to water. **Table 2** summarizes the polymer encapsulation strategies discussed herein, demonstrating higher WVTR, on average, than with glass-to-glass encapsulations. While all stability tests yield high PCE retention, many of the test conditions were not as harsh as those described in **Table 1**, and constitute less accelerated forms of aging.

Inorganic materials such as metal oxides form denser films with substantially lower WVTR and OTR than their polymer counterparts. To take advantage of this, some researchers have combined transparent thin metal oxide films with polymer encapsulation to provide superior resistance to water- and oxygen-induced degradation. For example, Chang et al. [30] deposited 50 nm thin films of Al2O3 by ALD onto PET substrates that they then used to encapsulate PSC devices. The Al2O3 thin film served as an excellent barrier to moisture and oxygen, having WVTR and OTR of 9.0 × 10−4 g∙m−2∙day−1 and 1.9 × 10−3 cm3 ∙m−2∙day−1∙atm−1, respectively. Moderate increases in WVTR and OTR were observed for Al2O3-coated PET substrates subject to bend testing, indicating that while somewhat compatible with flexible devices,

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

further effort may be required to increase the reliability and longevity of the encapsulation and to prevent partial delamination of rigid inorganic coatings from soft polymeric substrates. Nonetheless, encapsulated devices exposed to ambient conditions (30°C, 65% RH) for 42 days demonstrated negligible degradation in PCE.

The sequential combination of organic and inorganic layers to form organic–inorganic hybrid flexible multilayers has also been demonstrated to further reduce water and oxygen permeation through polymer-based encapsulants. The hybrid multilayers are deposited on polymer substrates, such as PET, that serve as the backbone for the encapsulation. Next, a series of organic polymer-based and metal-oxide inorganic thin films are deposited sequentially, wherein the organic layers help to retain flexibility and ductility and passivate interfacial defects, while the inorganic layers provide enhanced fortification against water and oxygen permeation [35]. WVTRs obtained at standard temperature and pressure (STP) for organic–inorganic hybrid multilayers are about three orders of magnitude less than that of uncoated PET; at elevated temperature and humidity (38°C, 90% RH), WVTR remains below 10−3 g∙m−2∙day−1 [41]. Furthermore, deposition of these complex coating structures in roll-to-roll systems using vacuumbased techniques such as magnetron sputtering has established their compatibility with large-scale production. Kim et al. [42] combined the aforementioned benefits of organic– inorganic hybrid flexible multilayer coatings with the antireflective properties of Nb2O5/ SiO2/Nb2O5 thin films to create a protective barrier for PSCs that minimizes undesirable light reflection and enhances PCE (17%). Additional experimentation is required to assess the effect of these types of encapsulations on the long term stability of PSCs.

#### **3.3 Inorganic thin film encapsulation**

Thin film encapsulation (TFE), wherein a thin barrier film is deposited directly on top of the PSC, is considered a next-generation encapsulation strategy since it can overcome many of the issues associated with glass and/or polymer cover encapsulation (e.g., moisture ingress through edge seals). Importantly, TFE is simultaneously compatible with R2R processing and, depending on the deposition technique and material selected, ultra-low WVTR and OTR can be achieved. In fact, the large variety of materials (e.g., organics, inorganics, organic–inorganic composites, etc.) and deposition techniques (e.g., spin coating, CVD, PVD, etc.) available in TFE provides a unique opportunity to tailor the properties of the barrier coating to better suit the requirements of the application. Inorganic TFE is distinct from the polymeric thin film encapsulations discussed in Section 3.2 in that the thin barrier films are inorganic in nature. As previously detailed, these generally have the advantage of reduced WVTR and OTR compared to their polymeric thin film counterparts. However, a major concern of inorganic TFE is whether the deposition of the thin barrier film can be effectively and efficiently performed at a large-scale; many inorganic TFE strategies involve costprohibitive complex vacuum deposition systems with low deposition rates. Adhesion is another concern. Where the thermal expansion coefficient of the thin inorganic encapsulating film is substantially different than that of the solar stack, mechanical stress, stability testing and even normal operation may cause delamination.

Al2O3 has gained the most attention in inorganic TFE as a result of its high transparency, electrical insulation and extremely low WVTR (9.0 × 10−4 g∙m−2∙day−1) and OTR (1.9 × 10−3 cm3 ∙m−2∙day−1∙atm−1) [28, 29, 43]. Atomic layer deposition (ALD) is often used to deposit the Al2O3 in TFE applications because of the high quality and uniformity of films produced [30]. However, as described in **Table 3**, a trade-off exists in selecting the ALD barrier-film deposition temperature. High temperature depositions

yield pinhole/defect-free coatings, but can cause a significant decrease in PCE due to thermal degradation of organic materials during encapsulation. Conversely, low temperature depositions ensure that the thermally sensitive solar-stack retains high PCE after encapsulation, but yield Al2O3 films that are more prone to moisture and oxygen ingress. For example, while spiro-OMeTAD-based PSCs fabricated by Choi et al. [28] and encapsulated with 50 nm of ALD-deposited Al2O3 demonstrated excellent long-term stability in ambient environments (92% retention in PCE after 7500 hours at 25°C, 50% RH), the PCE of encapsulated devices was moderately compromised compared to that of un-encapsulated devices (16% drop in PCE after encapsulation), due to the elevated ALD deposition temperature of 95°C. Further increases in ALD deposition temperature lead to more severely compromised PCEs, particularly when organic hole transport materials (HTMs), such as spiro-OMeTAD, were used [28]. To prevent thermal degradation induced by the encapsulation process, deposition of Al2O3 by low-temperature ALD has been proposed. Ramos et al. [29] encapsulated spiro-OMETAD-based PSCs with 16 nm Al2O3 thin films deposited by ALD at 60°C. As a result of the reduced operating temperature, encapsulated PSCs had outstanding PCEs as high as 17.4%, representing a 93.6% retention of the original PCE, while the same cells encapsulated at 90°C exhibited a 54% loss in PCE. However, a higher defect density was observed in Al2O3 deposited at 60°C, leading to increased water permeation and worse long-term stability outcomes compared to high-temperature Al2O3 encapsulations. After 2250 hours of exposure to the same ambient conditions as in the previous study by Choi et al. (25°C, 50% RH), a more significant 25% drop in PCE was reported for PSCs encapsulated with the 60°C ALD-deposited Al2O3.

New and innovative strategies have been introduced to overcome the challenges associated with the deposition of a pinhole-free, low-temperature inorganic thin film for encapsulation. The inclusion of organic interlayers in the TFE constitutes one such proposition. The purpose of the organic barrier interlayers is to compensate for defects in the inorganic layers by elongating the pathway for water and oxygen permeation, effectively decreasing WVTR and OTR. Additionally, inorganic/organic encapsulations are less prone to delamination than their brittle, all-inorganic counterparts, due to a reduction in residual stresses and an improvement in flexibility [35]. Lee et al. [31] encapsulated PTAA-based PSCs with a 4-dyad multilayer stack of Al2O3 (21.5 nm)/pV3D3 (100 nm) deposited by ALD at 60°C and initiated chemical vapor deposition (iCVD) at 40°C, respectively. Low processing temperatures lead to negligible losses in PCE during encapsulation (<0.3%). Furthermore, the inclusion of pV3D3 organic interlayers produced PSCs with significantly improved stabilities in accelerated aging conditions: after storage for 300 hours at 50°C and 50% RH, PCEs retained 97% of their initial values. Importantly, this constitutes one of the best stabilities reported for a PSC with an encapsulated PCE higher than 18%.

While much has been done to advance TFE, lifetimes must be extended even further and harsher environmental testing is required to better assess their capacity for degradation prevention. Moreover, further optimization of the TFE deposition process is necessary to overcome limitations associated with the slow deposition rates, scalability and large operating costs of ALD.

### **4. Hybrid encapsulation**

Glass-to-glass encapsulation remains one of most commercially-promising due to relatively low processing costs in conjunction with the effective seal produced.

However, moisture and oxygen ingress through epoxies and edge sealants remains a concern, preventing the realization of sufficiently long device lifetimes. Similarly, while polymer encapsulations are compatible with roll-to-roll processing, high WVTR and OTR limit effectiveness. Finally, while some early success has been demonstrated with TFE, much work remains to reduce operating costs and further improve lifetimes. It is therefore likely that a *hybrid* packaging which combines inorganic TFE with glass-to-glass or polymer encapsulation, thereby simultaneously taking advantage of the unique optical, mechanical and electronic properties of thin films materials and the strong barrier supplied by bulk polymers or glass, will provide the most effective and efficient means of preventing perovskite degradation.

Some of the most promising encapsulations that were previously described under other sub-sections actually employed combinations of glass-to-glass, polymer and inorganic thin film encapsulation and, as such, are more appropriately categorized as hybrid encapsulations. For example, the Al2O3-coated PET encapsulation reported by Chang et al. [30] that was first introduction in Section 3.2 on polymer encapsulation actually involves a combination of polymer and inorganic TFE strategies. It produced encapsulated devices that demonstrated negligible degradation in PCE after exposure to ambient conditions (30°C, 65% RH) for 42 days. Similarly, the PCE-enhancing organic– inorganic hybrid flexible multilayer coatings (PET/Nb2O5/SiO2/Nb2O5/PPFC) by Kim et al. [42] were first introduced in Section 3.2 but more aptly constitute a hybrid encapsulation. Finally, Lee et al.'s 4-dyad multilayer stack of Al2O3/pV3D3 [31] described in the previous section combines polymer and thin film encapsulations to achieve remarkable PCE retention (97%) after storage for 300 hours at 50°C and 50% RH.

Also in pursuant with this hybrid strategy, Dong et al. [32] employed an encapsulation strategy wherein a 50 nm thin film of SiO2 was deposited directly onto the device by electron beam deposition, followed by glass-to-glass encapsulation with UV-curable epoxy and a 180 μm piece of desiccant. Encapsulated devices were subject to accelerated aging tests and remarkably retained 80% of their original PCE after 48 hours under illumination at 85°C and 65% RH. Furthermore, almost full retention of PCE was reported after 432 hours of exposure to humid outdoor conditions where the relative humidity varied between 30 and 90%. Similarly, Liu et al. [33] encapsulated intrinsically stabilized PSCs with a 2 μm polymeric thin film of parylene by chemical vapor deposition and a cover glass. Impressively, by combining the principles of polymer, thin film and glass-to-glass encapsulation, the stability of the encapsulated devices under AM1.5 illumination was demonstrated for 2000 hours of continuous operation (PCE > 85% of initial value). Additionally, in one of the longest PSC stability tests published to-date, Fumani et al. [34] obtained 2-year stable PSCs by encapsulating the cathode and anode side of devices with 1.5 mm of polymer resin embedded with poly(ethylene glycol) (PEG) and glass, respectively. The resin provided a thick barrier against the diffusion of oxygen and water, and the PEG additive was used as a phase change material to limit device overheating cause by illumination. Freshly encapsulated devices had PCEs of 10%, which declined only slightly to 7.9% after 830 days (2.3 years) of storage in ambient conditions (25°C, 28% RH).

All of the hybrid encapsulation strategies described throughout this chapter are compared in **Table 4**. The diversity of all these hybrid encapsulations is reflected in the schematics and stability test results. Generally, hybrid encapsulations have allowed for high initial PCE and good PCE retention after aging. It should be noted that large differences in the severity of stability tests make direct comparison of different encapsulations difficult. A standardization of testing protocols would therefore provide a means for more efficient optimization of encapsulation techniques.

### **5. Conclusions & future research directions**

Though stability of PSCs remains a concern, recent improvements to intrinsic stability and hybrid device encapsulation have produced PSCs with lifetimes up to 2 years [33, 34, 44]. Nonetheless, considerable progress remains to be made before the 15-year lifetimes required for economic feasibility are realized. Furthermore, many of the PSCs that have demonstrated long-term stability on the order of years are limited by relatively poor initial efficiencies due to material substitutions/eliminations for intrinsic stabilization.

As a result of the detrimental nature of water- and oxygen-induced degradation of PSCs, the majority of encapsulations focus on inhibiting moisture and oxygen ingress. However, limited work to-date has focused on using encapsulation strategies to target UV-induced degradation. In fact, UV light constitutes a major factor in perovskite degradation, not only because of illumination-induced reversible phase segregation, but because it catalyzes and accelerates moisture- and oxygen-induced degradation. Therefore, in the absence of UV light, it is conceivable to achieve sufficiently long PSC lifetimes, even for encapsulations with slightly higher-than-ideal WVTR and OTR. This opens the door to obtaining substantially better long-term stabilities with glass-to-glass encapsulation, where moisture and oxygen ingress through the edge sealant is somewhat inevitable, and even for polymer encapsulations, which are limited by inherently poor WVTR and OTR. Thus, the use of a thin film encapsulant material with optical properties tuned to screen or convert UV light into less energetic and harmful irradiation is very compelling. Future research should look to combine the benefits of glass-to-glass or polymer encapsulation with thin film UV-barriers to assess whether this constitutes a step towards PSC longevity. Nonetheless, one thing is for certain: the continued development and evolution of innovative encapsulation strategies such as this and all those presented in this work is certainly required to bridge the gap between lab-scale PSC success and large-scale commercialization.

Furthermore, this work has elucidated the difficulties associated with direct comparison of PSC encapsulations fabricated by different research groups due to the lack of consistency in aging and stability tests performed. In an effort towards test standardization, some researchers have performed PSC testing according to the International Electrotechnical Commission (IEC) standards (e.g., IEC61646), originally designed to assess the field performance of silicon photovoltaic modules. However, due to the differences in degradation pathways between silicon and perovskite photovoltaics, many researchers are critical that the IEC standards do not comprehensively appreciate or assess for all sources of degradation in PSCs. The testing standards proposed specifically for PSCs at the 2018 International Summit on Organic Photovoltaic Stability (ISOS) constitute a good starting point for discussions of PSC-specific stability tests [45]. However, future effort and consensus from the research community is still required to establish standardized testing protocols better suited to assess the long term stability of encapsulated PSCs.

### **Conflict of interest**

Dongfang Yang is the Editor and Katherine Lochhead is Assistant to the Editor of this IntechOpen book: "Thin Film Deposition – Fundamentals, Processes and Applications."

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