Perovskite solar cells (PSCs) are a potential game-changer in the photovoltaic (PV) industry due to their high power conversion efficiencies (PCEs) and cost-effective manufacturing. However, their long-term stability remains a major obstacle to achieving a Levelized Cost of Energy (LCOE) competitive with crystalline silicon (c-Si) solar cells. PSC lifetime is affected by intrinsic factors (polymorphism, defects, lattice strains, ion migration) and extrinsic factors (moisture, oxygen, heat, UV light, reverse bias). Degradation pathways include structural transitions, phase segregation, and morphological alterations, often initiated by ion migration and outgassing. These are exacerbated by lattice defects/strains and interfacial stresses from material mismatch. Various strategies to improve intrinsic stability have been proposed, including compositional and dimensional engineering, defect passivation, grain boundary modification, and material interface engineering. Extrinsic factors like moisture and oxygen react with perovskite absorbers, causing decomposition. High temperatures, temperature variations, illumination, and reverse bias worsen intrinsic degradation. Encapsulation is crucial for achieving the 20-year outdoor lifespan needed for commercial viability. Existing encapsulation strategies for c-Si are insufficient for PSCs, requiring encapsulants that are chemically inert and compatible with PSC materials, possess low WVTR and OTR, resist UV degradation, are thermally stable, optically transparent, electrically insulating, and mechanically flexible and adhesive. While small-area PSCs have shown success in accelerated aging tests according to IEC 61215 and ISOS protocols, these results haven't been consistently replicated in perovskite solar modules (PSMs), highlighting a critical gap between scientific advancements and technological readiness for commercialization. Challenges in PSM encapsulation include the need for high-throughput, cost-effective processes, thermal management functionalities due to perovskite's low thermal conductivity, Pb-sequestering abilities to prevent environmental contamination, and minimizing edge seal area to avoid cell-to-module losses. Most stability results involve encapsulant areas larger than the photoactive area, necessitating validation on practical device configurations.

The literature extensively discusses the instability of perovskite solar cells and various approaches to enhance their stability. Studies have highlighted the critical role of intrinsic and extrinsic factors in degradation pathways, such as ion migration, phase segregation, and the effects of moisture and oxygen. Numerous strategies for improving the intrinsic stability of perovskite materials have been explored, including compositional engineering, defect passivation, and interface engineering. The importance of encapsulation in protecting PSCs from environmental factors has also been widely acknowledged, with various encapsulant materials and techniques being investigated. However, challenges remain in developing encapsulation strategies suitable for large-area modules and in achieving consistent results in accelerated aging tests that fully reflect real-world conditions. The existing literature emphasizes the need for industrially scalable, low-cost, and highly effective encapsulation methods capable of addressing the unique requirements of perovskite solar cells and modules.

This study proposes an industrially compatible, solvent- and strain-free encapsulation strategy using a viscoelastic (semi-solid)/highly viscous (liquid) polyolefin, specifically homopolymer polyisobutylene (PIB). The molecular weight of the PIB is selected to enable a semi-solid to liquid transition within the temperature range of typical PV device aging tests (-40°C to 85°C). Unlike commercially available PIB-based encapsulants that contain various additives, this work uses low-molecular-weight homopolymer PIB, which is transparent and processable as laminable films. The adhesion, barrier, and thermal management properties of the PIB encapsulant are further enhanced by adding two-dimensional (2D) hexagonal boron nitride (h-BN) flakes produced via a wet-jet milling (WJM) exfoliation process. The encapsulants' properties are characterized through electrochemical methods (ASTM G5-14 standard), water vapor transmission rate (WVTR) measurements using a calcium corrosion test, and thermal imaging. Mesoscopic n-i-p PSCs (1 cm² active area) with different architectures (PTAA and spiro-OMeTAD HTLs) and planar n-i-p PSCs (SnO₂ ETL) and inverted p-i-n PSCs are fabricated and encapsulated using a high-throughput lamination protocol (duration <45 min). The encapsulated PSCs undergo ISOS-D-2 (85°C, >1000 h), ISOS-L-1 (>1000 h), customized thermal shock tests (200 cycles between -40°C and 85°C), and modified humidity freeze tests (10 cycles). Mesoscopic n-i-p PSMs (10 cm² total active area) are also fabricated and encapsulated, subjected to the same accelerated ageing tests. The Pb leakage of encapsulated and unencapsulated PSMs is evaluated using inductively coupled plasma optical emission spectroscopy (ICP-OES). Finally, the PIB encapsulant's suitability for semi-transparent PSCs is investigated using FAPbBr₃ perovskite, assessing PCE and bifaciality factor before and after encapsulation. Detailed descriptions of material sourcing, encapsulant preparation, characterization techniques, device fabrication, encapsulation procedure, and device characterization are provided in the Methods section.

The study demonstrates that homopolymer PIB and PIB:h-BN encapsulants provide excellent barrier properties, significantly reducing corrosion rates compared to uncoated steel. The WVTR of both encapsulants is measured at approximately 2 × 10⁻⁵ g m⁻² d⁻¹. The addition of h-BN enhances the thermal management capabilities, reducing the time to reach 30°C during cooling by 11.2% compared to PIB alone. The encapsulation process minimally impacts the initial performance of the PSCs and PSMs. Unencapsulated PSCs degrade rapidly during ISOS-D-2 and ISOS-L-1 tests, while encapsulated PSCs show T₅₀ > 1000 h regardless of the encapsulant type. Encapsulation is similarly effective for PSCs with spiro-OMeTAD HTLs and planar and inverted configurations, significantly increasing their T₈₀ values. Unencapsulated PSMs degrade quickly during ISOS-D-2 and ISOS-L-1 tests (T₈₀ < 100 h and <3 h respectively), whereas encapsulated PSMs exhibit T₈₀ > 1000 h. Furthermore, the encapsulated PSMs successfully withstand 200 thermal shock cycles and 10 humidity freeze cycles, retaining more than 80% of their initial PCE. Importantly, these results are achieved without using edge sealants. The PIB:h-BN encapsulant effectively inhibits Pb leakage from the PSMs, preventing the release of harmful PbI₂ into the environment. Finally, semi-transparent PSCs encapsulated with homopolymer PIB retain their performance, exhibiting a PCE of 6.8% and a high bifaciality factor (89%).

The findings demonstrate the effectiveness of a novel, industrially scalable encapsulation strategy for PSCs and PSMs. The use of viscoelastic semi-solid/highly viscous liquid PIB, with or without h-BN addition, effectively mitigates thermomechanical stresses during lamination and operation, providing superior long-term stability under rigorous accelerated aging tests. The elimination of edge sealants further simplifies the process and reduces manufacturing costs. The results address a significant gap in the field by demonstrating long-term stability in PSMs, a critical step toward commercialization. The success of the encapsulation with different PSC architectures highlights its versatility. The inhibition of Pb leakage underscores the environmental benefits of this approach. The use of PIB for semi-transparent cells expands its application scope. This research significantly advances the prospects of perovskite solar technology by providing a reliable and cost-effective encapsulation solution.

This study presents a significant advancement in perovskite solar cell and module encapsulation. The low-temperature, strain-free lamination of PIB-based encapsulants, with or without h-BN, provides excellent long-term stability without edge sealants. This industrially compatible method works across various cell configurations, including semi-transparent devices, paving the way for wider adoption of perovskite solar technology. Future work could explore further optimization of encapsulant composition and investigation of long-term outdoor performance.

While this study demonstrates excellent stability under accelerated aging tests, long-term outdoor testing is needed to fully validate the encapsulant's performance in real-world conditions. The specific perovskite chemistry used (MA-based) might limit the generalizability of findings. Further improvements in PSC efficiency could further enhance the overall performance of encapsulated modules. The current study focuses on a specific type of PIB; investigating alternative polymers could potentially lead to further improvements in stability and other properties.