Open access peer-reviewed chapter

Encapsulation of Perovskite Solar Cells with Thin Barrier Films

Written By

Katherine Lochhead, Eric Johlin and Dongfang Yang

Submitted: 23 June 2022 Reviewed: 18 August 2022 Published: 25 September 2022

DOI: 10.5772/intechopen.107189

From the Edited Volume

Thin Films - Deposition Methods and Applications

Edited by Dongfang Yang

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Abstract

Long-term stability is a requisite for the widespread adoption and commercialization of perovskite solar cells (PSCs). Encapsulation constitutes one of the most promising ways to extend devices for lifetime without noticeably sacrificing the high power conversion efficiencies that make this technology attractive. Among encapsulation strategies, the most investigated methods are as follows: (1) glass-to-glass encapsulation, (2) polymer encapsulation, and (3) inorganic thin film encapsulation (TFE). In particular, the use of UV-, heat-, water-, and/or oxygen-resistant thin films to encapsulate PSCs is a new and promising strategy for extending devices for lifetime. Thin films can be deposited directly onto the PSC, as in TFE, or can be used in conjunction with glass-to-glass and polymer encapsulation to effectively prevent the photo-, thermal-, oxygen-, and moisture-induced degradation of the perovskite. This chapter will outline perovskite degradation mechanisms and provide a summary of the progress made to-date in the encapsulation of PSCs, with a particular focus on the most recent and promising advances that employ thin films. Additionally, the strengths and limitations of TFE approaches will be identified and contrasted against existing encapsulation strategies. Finally, possible directions for future research that can further enhance encapsulation effectiveness and extend PSC for lifetimes towards the 25-year target will be proposed.

Keywords

  • perovskite solar cells
  • thin film encapsulation
  • perovskite degradation
  • thin films
  • photovoltaics

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.

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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 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 photo- and 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.

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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 14. 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.

Table 1.

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

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.’

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.’

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 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 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, 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 vacuum-based 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 cost-prohibitive 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.

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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.

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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.

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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.”

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Written By

Katherine Lochhead, Eric Johlin and Dongfang Yang

Submitted: 23 June 2022 Reviewed: 18 August 2022 Published: 25 September 2022