0. Outdoor Stability of 2D Perovskite Photovoltaics: A Systematic Review

1. Introduction: Context and Significance of 2D Perovskite Photovoltaics

This introductory section establishes the foundational context for 2D perovskite photovoltaics, outlining their evolution from traditional 3D architectures and emphasizing the critical need for enhanced outdoor stability to achieve commercial viability. The subsequent subsections will delve into the remarkable progress of perovskite solar cells (PSCs), detail the limitations of early-generation 3D perovskites, and highlight the strategic shift towards 2D and quasi-2D architectures as a solution to stability challenges. Furthermore, it will underscore the paramount importance of long-term outdoor stability for the practical application and widespread adoption of PSCs, moving beyond laboratory efficiencies to address real-world performance under environmental stressors.

The initial subsection, "Overview of Perovskite Solar Cells and the Evolution to 2D Architectures," will commence by recognizing the impressive power conversion efficiencies (PCEs) of PSCs, which now rival silicon-based solar cells, attributed to their tunable band gaps, strong light absorption, and defect tolerance [1,5,7,10]. It will then transition to the inherent instability of 3D perovskites, such as $\text{MAPbI}\_3$ and $\text{FAPbI}\_3$, which degrade upon exposure to moisture, high temperatures, and oxygen, forming decomposition products like $\text{PbI}\_2$ [3,5,7,10]. This fundamental limitation drove the innovation towards 2D and quasi-2D architectures, which integrate large organic cations to create a hydrophobic barrier, significantly enhancing stability [1,2,3,4,6,7,8]. While 2D perovskites offer superior stability, they often exhibit a trade-off with efficiency due to broader optical bandgaps and less efficient charge transport, leading to the development of 2D/3D hybrid structures that aim to combine the high efficiency of 3D materials with the improved stability of 2D counterparts [2,4,5,6,8]. However, even 2D/3D hybrids face challenges, as degradation mechanisms like cation diffusion and interstitial defect formation can still compromise performance, particularly under harsh photothermal aging conditions [11].

The subsequent subsection, "The Imperative of Outdoor Stability for Commercial Viability," will emphasize that the commercial success of PSCs hinges on their ability to maintain performance under real-world outdoor conditions, addressing the significant gap between laboratory and practical stability [7,9,10]. It will detail key environmental stressors—moisture, oxygen, elevated temperatures, and UV light—and their resulting degradation pathways, such as ion migration and photodegradation, which can occur even in 2D and 2D/3D structures [5,8]. The commercial imperative for long-term reliability will be discussed, highlighting the industry's demand for decades-long operational lifespans and the limitations of current laboratory testing methods that often fail to predict real-world behavior due to meta-stability effects [9]. This subsection will underscore the critical need for standardized accelerated aging tests and robust encapsulation techniques to bridge the gap between impressive laboratory efficiencies and reliable outdoor performance, ultimately driving the widespread adoption of 2D perovskite photovoltaics [4,7,10].

1.1 Overview of Perovskite Solar Cells and the Evolution to 2D Architectures

Perovskite solar cells (PSCs) have rapidly emerged as a highly promising photovoltaic technology, distinguished by their exceptional power conversion efficiencies (PCEs), which have surpassed 25% and even reached over 26%, rivaling established silicon-based solar cells [1,5,10]. Their appeal stems from a confluence of attractive properties, including tunable band gaps, strong light absorption capabilities, and a notable tolerance to defects, which collectively contribute to their impressive performance and low-cost production potential [1,7].

Despite their impressive photovoltaic performance, the practical application and commercialization of early-generation 3D perovskites, such as methylammonium lead iodide ($\text{MAPbI}\_3$) and formamidinium lead iodide ($\text{FAPbI}\_3$), have been significantly hampered by their inherent instability [1,2,3,7]. These materials are highly susceptible to degradation when exposed to common environmental stressors, including moisture, elevated temperatures, and oxygen, leading to the formation of decomposition products like lead iodide ($\text{PbI}\_2$) [3,5,7,10]. A striking visual manifestation of this degradation is the transformation of the photoactive black phase of $\text{FAPbI}\_3$ into a photo-inactive yellow phase upon exposure to humid conditions, signifying structural degradation and a decline in device performance [3]. This fundamental limitation of 3D perovskites underscored the critical need for alternative architectures with enhanced long-term stability for real-world outdoor applications [10].

In response to these stability challenges, the field has witnessed a significant evolution towards 2D and quasi-2D perovskite architectures as a promising solution [1,2,3,4,6,7,8]. These structures incorporate large organic cations that act as a barrier against moisture, significantly improving thermal, chemical, and environmental stability compared to their 3D counterparts [1,2,6].

2D perovskites are characterized by their layered structure, where inorganic perovskite sheets of corner-sharing metal halide octahedra are separated by larger organic spacer cations [6,8]. Quasi-2D perovskites, a related class, feature multiple (n > 1) octahedral layers separated by these same cations. Common (100)-oriented 2D perovskite structures include Ruddlesden-Popper (RP), Dion-Jacobson (DJ), and Alternating Cations in Interlayer space (ACI) structures, with RP materials being the most extensively studied [4,8]. The chemical versatility of 2D perovskites allows for extensive chemical engineering, enabling precise adjustments to their crystal and thin-film structures, bandgaps, and charge transport properties through variations in organic ligands and halide compositions [2].

Despite their superior stability, 2D perovskites, particularly low-n phases, often exhibit a broader optical bandgap and less efficient out-of-plane charge transport, which can limit their efficiency when used as the sole light-absorbing layer in solar cells [2,8]. This trade-off between PCE and stability is a recurring theme in the field: 2D PSCs are inherently more stable but less efficient, whereas 2D/3D hybrid PSCs tend to offer higher efficiency but with a compromise in relative stability [1].

To leverage the best of both worlds—high efficiency from 3D perovskites and enhanced stability from 2D counterparts—a key innovation has been the development of hybrid 2D/3D perovskite structures [2,4,5,6,8]. In these architectures, 2D perovskites function either as a capping layer atop a 3D perovskite absorber or are integrated into heterojunctions. This configuration aims to combine the stability advantages of 2D perovskites, such as their layered structure, hydrophobicity, and suppressed ion migration, with the superior optoelectronic properties of 3D perovskites, including high absorption and efficient charge transport [4,5,8]. Studies have shown that 2D/3D heterojunctions can lead to reduced interfacial recombination and better-aligned energy levels, contributing to improved device performance under mild aging conditions [4].

However, critical comparisons of underlying experimental evidence and theoretical models reveal nuanced challenges. While 2D perovskites generally offer improved stability, their integration into 3D devices does not completely eliminate degradation, especially under harsh conditions [5]. For instance, mono-ammonium-based 2D perovskites (2D-mono) exhibit instability under photothermal aging, decomposing into $\text{PbI}\_2$ and metallic lead ($\text{Pb}^0$) due to vacancy formation and iodide migration. This structural collapse can significantly increase series resistance by reducing the mobility and doping of transport layers [11]. Conversely, di-ammonium-2D (2D-di) based interfaces demonstrate superior structural stability and effectively block iodide migration [11]. However, this blocking mechanism can paradoxically lead to uneven iodide distribution and interstitial defect formation within the 3D layer, exacerbating non-radiative recombination and highlighting a complex interplay of degradation mechanisms that require further understanding [11].

This systematic review aims to address the persistent PCE/stability trade-off and other challenges by delving into the strategies employed to enhance the outdoor stability of 2D perovskite photovoltaics [1]. It emphasizes that achieving long-term outdoor stability for practical applications necessitates both advanced device architectures and effective encapsulation techniques, with rigorous, standardized testing conditions extending beyond basic ambient storage tests to reflect real-world performance accurately [10]. By synthesizing the consensus on the foundational advantages of 2D structures and critically examining the divergences in their performance under varied conditions, this review will provide a comprehensive understanding of the current state and future directions for 2D perovskite photovoltaics.

1.2 The Imperative of Outdoor Stability for Commercial Viability

The commercial viability and widespread adoption of perovskite solar cells (PSCs) are fundamentally contingent upon their long-term outdoor stability [7,9,10]. Despite remarkable advancements in laboratory efficiencies, a persistent and significant gap exists between these controlled conditions and real-world outdoor performance, which necessitates substantial improvements in device durability under environmental stressors [1,2,3,7,9,10].

The primary environmental stressors challenging the longevity of PSCs include moisture, oxygen, elevated temperatures, and ultraviolet (UV) light [5,8]. These factors lead to various degradation mechanisms, such as ion migration and photodegradation, particularly in 2D/3D heterojunctions where elevated temperatures and light can promote cation diffusion between layers, compromising device integrity [5]. For instance, while 2D perovskites initially promised enhanced moisture and thermal stability due to their larger organic cations, which act as a hydrophobic barrier and stabilize the inorganic framework, they are not entirely immune to degradation. The paper on vacuum-induced degradation of 2D perovskites implicitly highlights that even these structures face fundamental degradation pathways [6]. Furthermore, all organic-inorganic halide perovskites inherently exhibit some degree of instability under ambient atmosphere, elevated temperature, illumination, and/or bias voltage [8].

The promise of 2D perovskites as a solution to the stability issues of their 3D counterparts is a recurring theme [2,3]. For example, the incorporation of bulky organic cations in 2D perovskites was anticipated to provide improved resistance to moisture ingress and thermal degradation [3]. However, despite these architectural advantages, persistent challenges remain, particularly regarding outdoor performance. The inherent moisture and thermal stability issues continue to significantly hinder the commercialization of perovskite photovoltaics [3]. The trade-off between high laboratory efficiencies and practical applications, often compromised by instability, underscores the imperative for robust outdoor stability for widespread adoption [1,2].

A critical analysis reveals a quantitative gap between lab efficiencies and real-world performance, directly linking to the commercial imperative for robust 2D perovskite performance. While specific quantitative data on efficiency retention or estimated lifespan requirements for commercial solar technologies are not uniformly provided across all reviewed digests, the consensus is clear: laboratory testing, particularly in inert environments, often overestimates real-world stability. The absence of standardized encapsulation approaches and comprehensive outdoor testing leads to variations in stability reports [10]. For instance, light cycling studies highlight that common indoor light-soaking ageing tests often fail to correlate with outdoor experiments due to substantial meta-stability effects—reversible changes in performance over day-night cycles [9]. This necessitates a focus on understanding real-world behavior through methods like light cycling, which mimic diurnal variations, rather than static light soaking.

The commercial imperative is driven by the industry's need for devices that can reliably operate for decades, typically requiring a 20-25 year lifespan with minimal degradation, often defined by less than 20% power loss over the period. While the reviewed papers emphasize the critical need for improved outdoor stability for commercial viability [1,2,3,7,9,10], few directly provide specific quantitative targets for commercial lifespan. The emphasis shifts to the need for effective encapsulation and the ability to pass standard performance tests such as damp heat testing, which are proxies for long-term outdoor durability [10].

Comparing the underlying experimental evidence and theoretical models, there is a strong consensus that accelerated ageing tests under harsh conditions are crucial for evaluating long-term stability and predicting commercial viability [4,7]. The scientific basis for this approach lies in simulating extreme environmental conditions (e.g., high temperature, humidity, and continuous illumination) to rapidly induce degradation mechanisms observed over longer periods in real-world scenarios. For instance, the degradation mechanisms in 2D/3D perovskite solar cells under photothermal accelerated ageing conditions have been studied, showing how elevated temperatures and light can promote cation diffusion between 2D and 3D layers, compromising device integrity [5]. While the specific degradation rates are not universally quantified across all digests, the general observation of performance drops under stress underscores the importance of understanding and mitigating degradation for practical applications [11].

There is also a convergent view on the need for standardized accelerated testing protocols to reliably predict long-term performance, though consensus on such standards remains emerging [7]. The divergence, however, lies in the specific methodologies and theoretical models employed to bridge the gap between lab and real-world performance. Some studies implicitly underscore the importance of outdoor stability by focusing on degradation mechanisms, without providing explicit quantitative data on the gap or lifespan requirements [6,11]. Others highlight the necessity of methods like light cycling to capture meta-stability effects that indoor tests miss [9]. This divergence reflects the complexity of predicting real-world behavior, which involves dynamic and interacting environmental factors not fully captured by static laboratory tests.

Ultimately, achieving practical application lifetimes for 2D and quasi-2D perovskite solar cells will necessitate multifaceted approaches, including the strategic use of additives, grain boundary passivation, improved interfacial layers, and optimized device architectures, in addition to the inherent advantages offered by 2D or 3D/2D perovskite compositions [8]. The transfer of robust characterization methods and reporting practices from 3D perovskite studies to 2D perovskite research is also deemed essential to enhance the understanding of degradation processes and accelerate progress toward commercially viable devices [8].

2. Understanding the Role of 2D/Quasi-2D Structures in Stability

The distinct structural characteristics of two-dimensional (2D) and quasi-2D perovskites fundamentally differentiate their stability profiles from traditional three-dimensional (3D) counterparts. This section provides a comprehensive exploration of how these unique 2D/quasi-2D architectures inherently confer stability advantages, particularly against prevalent degradation pathways such as moisture ingress and ion migration [2,7,8]. It also delves into the specific degradation mechanisms that, while mitigated by the 2D structure, still present challenges, and evaluates the current methodologies for accelerated aging, critically assessing their correlation with real-world outdoor performance [4,6,9].

The first subsection, "Inherent Stability Advantages of 2D/Quasi-2D Perovskites," establishes the foundational understanding of how the layered arrangement and the incorporation of bulky organic spacer cations significantly enhance stability [1,3]. It will detail the barrier function provided by these cations against moisture and their role in restricting ion migration, critically assessing the varying stability across different 2D perovskite structural types (Ruddlesden-Popper, Dion-Jacobson, and A-site deficient) and the profound influence of spacer cation type (e.g., di-ammonium vs. mono-ammonium) and n-value on structural integrity and resistance to degradation [4,5,8].

Following this, "Comparative Degradation Mechanisms in 3D and 2D Perovskites" offers a granular analysis of degradation pathways. This section will begin by outlining general degradation factors common to both 3D and 2D perovskites, such as moisture, temperature, illumination, and intrinsic ion migration, emphasizing the interplay between photothermal stress and ionic defect formation [5,10]. It will then progress to "Degradation Pathways Specific to 2D and Quasi-2D Perovskites," which is further segmented to explore environmental factors (humidity, oxygen, temperature), light-induced degradation and cycling effects, and intrinsic chemical instabilities (vacuum, ion migration) that uniquely manifest in 2D systems [6,8,9]. This subsection will highlight how the layered structure, while beneficial, can also introduce specific vulnerabilities, such as intra-layer interstitial defects due to ion confinement [5].

The concluding subsection, "Accelerated Ageing and Real-World Conditions," will critically assess the efficacy of current accelerated testing methodologies, such as photothermal tests, in predicting actual long-term outdoor performance [4,9]. It will address the limitations of conventional constant-stress protocols and advocate for more sophisticated, standardized methods, particularly those incorporating light cycling and maximum power point tracking, to bridge the gap between laboratory results and real-world outdoor stability [7,10]. This integrated approach aims to provide a comprehensive understanding of the intricate relationship between 2D/quasi-2D structures and the long-term outdoor stability of perovskite photovoltaics, guiding future research toward robust material and device design.

2.1 Inherent Stability Advantages of 2D/Quasi-2D Perovskites

Two-dimensional (2D) and quasi-2D perovskites inherently offer significant stability advantages over their three-dimensional (3D) counterparts, primarily due to their distinctive layered structures and the strategic incorporation of bulky organic spacer cations [1,2,3,6,7,8]. These structural modifications are instrumental in mitigating prevalent degradation pathways such as moisture ingress and ion migration, which are critical limitations in 3D perovskite photovoltaics [2,7,8].

The fundamental mechanism behind this enhanced stability lies in the barrier function provided by the bulky organic cations. These cations, situated between the inorganic octahedral layers, create a hydrophobic environment that significantly impedes water molecule penetration into the active material [3,7]. This increased hydrophobicity is crucial for preventing hydrate formation, a common moisture-induced degradation pathway in 3D perovskites [7]. Furthermore, the layered arrangement physically restricts the movement of ions, particularly iodide ions, across the material, thereby suppressing detrimental ion migration phenomena [2,8]. The inclusion of larger, less volatile organic cations also contributes to improved thermal and chemical stability [1].

A critical assessment of the extent of these improvements reveals varying degrees of stability across different 2D perovskite structural types, namely Ruddlesden-Popper (RP), Dion-Jacobson (DJ), and A-site deficient (ACI) phases [8]. DJ and ACI materials generally exhibit superior stability compared to RP materials [8]. This superiority is attributed to specific structural characteristics: DJ and ACI phases lack or significantly reduce the weak van der Waals forces typically present between spacer cation bilayers in RP structures, leading to shorter interlayer distances and enhanced intrinsic stabilization through hydrogen bonding between the spacer cations and the inorganic slabs [8]. These stronger interactions contribute to a more robust and less permeable structure, effectively bolstering resistance against moisture ingress and ion migration.

The type of organic spacer cation also profoundly influences stability. Di-ammonium based 2D perovskites (2D-di) have demonstrated superior intrinsic stability under harsh conditions, including photothermal aging, compared to mono-ammonium based 2D perovskites (2D-mono) [4,5]. This enhanced stability of 2D-di structures is attributed to the bidentate hydrogen bonding formed between the di-cation and the $PbI\_6$ octahedron [4,5]. This bidentate bonding strengthens the interaction between adjacent inorganic layers and increases the overall structural rigidity, thereby suppressing critical degradation pathways such as deprotonation of organic cations and decomposition of the $PbI\_6$ octahedron [4,5]. Experimental evidence supports this, showing that only 2D-di structures could withstand photothermal aging, while 2D-mono structures rapidly decomposed [4]. The superior structural stability of 2D-di based interfaces is also effective in preventing iodide migration into the transporting layer [11].

While the layered structure of 2D perovskites offers substantial advantages, certain structural configurations can introduce new vulnerabilities. For instance, while the di-ammonium based 2D perovskites effectively block iodide migration, this robust blocking can paradoxically lead to detrimental effects such as uneven iodide distribution and the formation of interstitial defects within the perovskite lattice [11]. This highlights a trade-off where enhanced barrier properties, while beneficial for overall stability, can introduce localized issues that may impact device performance or long-term integrity under specific conditions. Furthermore, while the general principle of bulky organic cations preventing ion migration is widely accepted, the molecular-level details of how the layered structure precisely prevents ion intercalation are not always deeply elaborated in all discussions, suggesting an area for more detailed theoretical and experimental investigation [1].

The "n-value," representing the number of inorganic layers between spacer cations in quasi-2D perovskites, also plays a role in stability. Increasing the n-value, meaning a thicker inorganic slab, can further improve stability by enhancing the structural integrity and reducing the relative influence of the interface on the bulk properties [5]. This parameter allows for fine-tuning the balance between stability and charge transport characteristics, as higher n-values lead to more 3D-like charge transport properties.

The consensus across multiple studies is that the increased hydrophobicity and suppressed ion migration, stemming from the bulky organic spacer cations and the layered structure, are the primary mechanisms for enhanced stability in 2D/quasi-2D perovskites compared to 3D counterparts [1,2,3,7,8]. Divergence exists in the specific emphasis on certain structural features and the molecular-level explanations, with some studies providing detailed experimental evidence for specific cation types (e.g., di-ammonium vs. mono-ammonium), while others focus on broader structural categories (RP, DJ, ACI). The scientific basis for these differences lies in the specific chemical interactions and structural packing efficiencies, with bidentate hydrogen bonding and shorter interlayer distances in DJ/ACI phases providing more robust protection. While 2D perovskites offer significant stability improvements, particularly against moisture and ion migration, the field continues to explore the optimal balance of structural configurations to maximize stability without introducing new vulnerabilities related to charge transport or defect formation [11].

2.2 Comparative Degradation Mechanisms in 3D and 2D Perovskites

This section provides a comprehensive overview of the degradation mechanisms affecting both 3D and 2D perovskite photovoltaics, with a particular focus on delineating the unique pathways pertinent to 2D and quasi-2D architectures. While 3D perovskites suffer from well-documented instabilities, 2D perovskites, owing to their layered structure and the presence of bulky organic cations, generally exhibit enhanced resistance to environmental stressors [1,2,3,7]. However, their unique structural characteristics also introduce distinct degradation challenges that warrant detailed investigation [7,8].

The initial subsection, "General Degradation Pathways in Perovskite Photovoltaics," will establish the foundational understanding of degradation in perovskite materials, covering common extrinsic stressors such as moisture, elevated temperature, illumination, and oxygen, as well as intrinsic factors like ion migration and phase decomposition [5,7,8,10]. It will highlight how these factors lead to material decomposition, structural changes, and performance loss, with a particular emphasis on the interplay between photothermal stress and ionic defect formation [4]. The subsection will also address the ongoing discrepancies in identifying dominant stressors, emphasizing the need for multi-stressor accelerated aging tests to fully understand synergistic degradation effects [9].

Subsequently, "Degradation Pathways Specific to 2D and Quasi-2D Perovskites" will delve into the nuances of degradation in 2D systems. This section will be structured into three sub-sections. "Environmental Factors: Humidity, Oxygen, and Temperature" will examine how the unique composition of 2D perovskites, particularly the role of large organic cations, enhances resistance to moisture and oxygen, while also addressing the specific vulnerabilities to thermal stress and photo-oxidation that persist [1,2,3,7,8]. It will highlight the importance of spacer cation type (e.g., mono-ammonium vs. di-ammonium) on stability under photothermal conditions [4].

The next sub-section, "Light-Induced Degradation and Cycling Effects," will differentiate between continuous illumination and the more realistic light cycling conditions, emphasizing that the latter can unveil distinct degradation pathways not captured by conventional testing, such as fatigue and accumulation of irreversible changes [4,9]. It will discuss light-induced halide and cation segregation, and how their dynamics are altered by intermittent light exposure, underscoring the necessity for standardized light cycling protocols [7].

Finally, "Intrinsic Chemical Instabilities (e.g., Vacuum, Ion Migration)" will focus on degradation pathways originating from the inherent chemical properties of 2D perovskites. This includes vacuum-induced degradation, characterized by metallic lead formation and organic cation loss, where the choice of spacer cation is critical [6,8]. A significant part will be dedicated to ion migration, illustrating how the layered structure can confine migration, potentially reducing its extent compared to 3D systems, but also how this confinement can paradoxically lead to intra-layer interstitial defects and non-radiative recombination [5,8].

The concluding subsection, "Accelerated Ageing and Real-World Conditions," will critically assess the current methodologies for accelerated aging, such as photothermal tests, and their correlation with actual long-term outdoor performance [4,5]. It will highlight the limitations of current protocols in capturing dynamic outdoor phenomena and meta-stability effects, advocating for more sophisticated and standardized accelerated testing methods, particularly those incorporating light cycling and maximum power point tracking, to achieve more reliable predictions of outdoor stability [7,9]. This integrated approach aims to provide a comprehensive understanding of degradation in 2D perovskite photovoltaics and guide future research towards robust material and device design for outdoor applications.

2.2.1 General Degradation Pathways in Perovskite Photovoltaics

Perovskite solar cells (PSCs), particularly organic-inorganic hybrid perovskites, are susceptible to various degradation mechanisms that significantly limit their long-term outdoor stability. Key environmental stressors include moisture, elevated temperature, illumination, and oxygen [8,10]. Beyond these external factors, intrinsic mechanisms such as ion migration and phase decomposition also contribute substantially to performance loss [5,7].

Moisture sensitivity is a widely acknowledged degradation pathway, leading to the formation of hydrated phases and the decomposition of $PbI\_3$ into $PbI\_2$, which subsequently alters the structural integrity and electronic properties of the perovskite material [3]. This process renders the device unsuitable for practical applications. Thermal instability similarly contributes to the breakdown of perovskite materials, with studies indicating that exposure to elevated temperatures can induce phase segregation and decomposition [3].

Photodegradation, another critical pathway, involves light-induced changes within the perovskite layer. Ion migration, particularly of mobile iodide ions, is a significant contributor to degradation under illumination and thermal stress. These ions can migrate to and react with charge transport layers and electrodes, causing interfacial degradation and an increase in series resistance [4,5]. Specifically, iodide migration can lead to screening of the electric field, halide phase segregation, and the formation of detrimental defects. Cation migration can also induce phase segregation within the 3D perovskite layer, further compromising device stability [5].

A representative study by Min et al. systematically investigated degradation mechanisms in mono-ammonium-based 2D perovskites (2D-mono) under photothermal accelerated ageing, noting decomposition into $PbI\_2$ and metallic lead ($Pb^0$) [4,11]. This structural collapse was linked to vacancy formation and subsequent iodide migration to the anode. The migrating iodide ions trigger a redox reaction, which reduces the mobility and doping concentration of the transport layer, resulting in increased series resistance. For 3D devices, degradation was further linked to increased $Pb^0$ acting as deep traps and the generation of $I\_2$, which exacerbates ion migration [4]. This study provides crucial experimental evidence for the interplay between photothermal stress, chemical decomposition, and ionic defect formation.

While 3D perovskites are known to suffer from severe instability, implying common degradation mechanisms, 2D perovskites generally demonstrate improved stability due to their layered nature and the presence of larger organic cations that mitigate the ingress of moisture and oxygen [1,2,7]. However, 2D perovskites are not immune, and specific degradation pathways persist [7]. For instance, 3D perovskites degrade even under vacuum conditions, exhibiting methylammonium iodide sublimation and metallic lead formation, providing a baseline understanding for similar mechanisms in 2D systems [6].

A key area of divergence in the literature pertains to identifying the dominant environmental stressor for specific perovskite compositions. While some studies emphasize the pervasive influence of moisture and heat [3], others highlight the critical role of light-induced degradation and ion migration under photothermal conditions [4]. The "light cycling" study underscores that constant light indoor tests are often insufficient predictors of real-world outdoor performance, suggesting that the dynamics of light exposure significantly influence degradation patterns and vary across different device architectures [9]. This discrepancy indicates that the relative importance of stressors is likely composition- and architecture-dependent, and that the interaction between multiple stressors (e.g., photothermal stress) can lead to accelerated degradation pathways that are not evident when stressors are studied in isolation.

To resolve these discrepancies and establish a more comprehensive understanding, future research should focus on multi-stressor accelerated aging tests, explicitly designed to decouple and quantify the synergistic effects of light, heat, moisture, and bias. This would involve controlled experiments under simulated outdoor conditions, varying individual parameters while monitoring the degradation rates and specific chemical/structural changes. Advanced in-situ characterization techniques, such as operando X-ray diffraction, transient absorption spectroscopy, and electrochemical impedance spectroscopy, could provide real-time insights into the evolution of ionic migration, phase segregation, and defect formation under combined stress conditions. Quantitative data on degradation rates (e.g., power conversion efficiency retention, short-circuit current density (Jsc) decrease, or fill factor (FF) degradation) under varying relative humidity, temperature cycles, and light intensities would be invaluable. For instance, comparing the half-life of devices under 85°C/85% RH versus continuous illumination would help delineate the dominant pathways for specific 2D perovskite compositions, providing clear, actionable data for material and device design improvements.

2.2.2 Degradation Pathways Specific to 2D and Quasi-2D Perovskites

This section delves into the intricate degradation mechanisms inherent to 2D and quasi-2D perovskite photovoltaics, contrasting them with those observed in their 3D counterparts. While 2D architectures generally offer enhanced stability against certain environmental factors, their unique structural features also introduce novel degradation pathways and challenges [8]. The subsequent subsections systematically categorize and analyze these pathways, providing a comprehensive overview of how environmental stressors (humidity, oxygen, temperature), light-induced effects, and intrinsic chemical instabilities (vacuum, ion migration) contribute to device degradation.

The initial subsection, "Environmental Factors: Humidity, Oxygen, and Temperature," will critically examine the role of external environmental elements. It will discuss how the incorporation of large organic cations in 2D perovskites provides a hydrophobic barrier, significantly enhancing moisture resistance compared to 3D perovskites [1,2,3]. However, it will also highlight that despite improved resistance, moisture ingress can still induce disproportionation or hydration in specific 2D structures, and how thermal stress can lead to spacer cation diffusion, increasing moisture sensitivity [8]. The discussion will extend to the impact of oxygen, particularly in photo-oxidation, and temperature, noting that while 2D perovskites show improved thermal stability, this is often dependent on spacer cation and layer thickness, with contradictions existing in the literature regarding optimal structural configurations for stability [8].

Following this, "Light-Induced Degradation and Cycling Effects" will explore the complex interplay of light with 2D perovskite stability. This subsection will emphasize that traditional continuous illumination tests may not fully capture the degradation behaviors observed under real-world light cycling conditions [9,10]. It will cover light-induced phenomena such as halide segregation and cation migration, explaining how these processes can be influenced by intermittent light exposure and how they contribute to degradation, particularly at 3D/2D interfaces under photothermal accelerated aging [4,5]. The section will also address the varying photostability among different 2D compositions and the potential for contradictory findings regarding their resilience under illumination [8].

Finally, "Intrinsic Chemical Instabilities (e.g., Vacuum, Ion Migration)" will detail degradation pathways originating from the inherent chemical properties of 2D perovskites. This will include vacuum-induced degradation, characterized by the formation of metallic lead and loss of organic cations, with the extent of degradation being highly dependent on the choice of organic spacer cation [6,8]. A significant focus will be placed on ion migration, illustrating how the layered structure of 2D perovskites can spatially confine ion migration, thereby reducing its extent compared to 3D systems [8]. However, it will also discuss the paradoxical effect where blocking inter-layer migration can lead to intra-layer interstitial defects, exacerbating non-radiative recombination [4,5]. The influence of spacer cation type on ion migration rates and the broader implications for device stability will be thoroughly examined, synthesizing experimental evidence and identifying areas where a deeper scientific understanding is still required for optimizing outdoor performance.

2.2.2.1 Environmental Factors: Humidity, Oxygen, and Temperature

The outdoor stability of 2D perovskite photovoltaics is profoundly influenced by environmental factors such as humidity, oxygen, and temperature, with varying degrees of sensitivity reported across studies. Generally, 2D perovskites demonstrate enhanced resilience to these stressors compared to their 3D counterparts, primarily due to their unique structural and compositional attributes [1,2,3]. However, the specific mechanisms and quantitative degradation rates under isolated stress conditions remain areas requiring further detailed investigation.

Humidity is consistently identified as a primary degradation factor across multiple studies, causing perovskites to form hydrates that weaken chemical bonds and can lead to decomposition into PbI$\_2$ via volatile HI [7,8]. Moisture ingress can induce disproportionation in quasi-2D/3D-2D films or trigger phase changes, with certain structures, like Dion-Jacobson (DJ) perovskites, forming a 1D hydrated form upon exposure, which is partially reversible through annealing [8]. The incorporation of large organic cations in 2D perovskites, as highlighted by [1,2], acts as a hydrophobic barrier, significantly improving moisture resistance. For instance, hot-cast 2D perovskite devices have demonstrated remarkable stability, maintaining performance even under 72% relative humidity without encapsulation, achieving dark half-lives of 90 hours, which is a substantial improvement over the 10-hour half-lives of 3D devices under similar conditions [3]. This suggests a consensus on the role of large organic cations and the inherent structural advantages of 2D perovskites in mitigating humidity-induced degradation.

Oxygen, particularly in conjunction with UV light (photo-oxidation), is another critical environmental stressor that can lead to perovskite decomposition [7]. Perovskites with less acidic cations, such as formamidinium (FA) or cesium (Cs), are reported to be more stable against photo-oxidation compared to methylammonium (MA)-based ones [7]. While the general consensus indicates that oxygen contributes to degradation, quantitative data specifically isolating oxygen's effect on 2D perovskite degradation rates is less detailed in the provided digests compared to humidity or combined stress conditions. The necessity for very low oxygen transmission rates (OTR) in encapsulation materials for perovskite solar cells underscores the sensitivity of these materials to oxygen exposure [10].

Elevated temperature also plays a significant role in perovskite degradation. While 2D perovskites generally exhibit improved thermal stability compared to 3D counterparts, this depends heavily on the spacer cation and the number of layers [8]. High temperatures can induce phase transitions, accelerate decomposition pathways, and potentially cause spacer cation diffusion [7,8]. MA-based perovskites are notably less resistant to decomposition at moderate temperatures compared to Cs or FA-based compositions or when passivated by 2D layers [7].

A notable area of investigation involves combined stress conditions, particularly photothermal accelerated aging, which combines elevated temperature and light. Studies employing such conditions, for example, 85 °C under 1-sun or 2-sun illumination, reveal that the combination significantly accelerates the decomposition of 2D-mono structures and promotes iodide migration [5,11]. Specifically, mono-ammonium based 2D perovskite passivation layers demonstrated rapid decomposition and significant power conversion efficiency (PCE) loss, primarily due to a strong drop in current density (JSC), under harsh conditions of 85 °C and 2-sun illumination. In contrast, di-ammonium based 2D perovskite passivation layers exhibited superior stability, performing similarly to control 3D devices under these conditions [4]. This highlights that the type of organic cation (mono-ammonium vs. di-ammonium) in the 2D layer profoundly impacts thermal and photothermal stability.

Despite the general agreement on the negative impact of humidity, oxygen, and temperature, specific quantitative comparisons across different 2D perovskite compositions under isolated stress conditions are often lacking or inconsistently reported. For instance, while some papers generally state that 2D perovskites are more resilient to humidity and temperature due to hydrophobic organic cations [1,2], they often do not provide specific details on the chemical changes induced by these factors or precise quantitative degradation rates and experimental parameters for various formulations. This makes direct, quantitative comparisons challenging.

A notable inconsistency lies in identifying the "dominant" environmental stressor. While humidity is frequently cited as a major concern due to its direct chemical interaction and hydrate formation, the severity of degradation under photothermal conditions suggests that high temperatures combined with illumination can be equally or more detrimental for certain 2D compositions, particularly mono-ammonium based structures [4]. The interplay of these factors means that a single dominant stressor may not exist universally across all 2D perovskite compositions; rather, the primary degradation pathway can be context-dependent. For example, while 2D perovskite hot-cast devices showed robustness against 72% relative humidity, the same stability may not hold true under extreme photothermal stress. To resolve these discrepancies, future research should focus on systematic, multi-factorial degradation studies. This would involve controlled experiments that isolate the effects of humidity, oxygen, and temperature, and then systematically combine them, utilizing standardized testing protocols (e.g., IEC 61215 for PV modules). Such studies should quantify degradation rates, monitor material transformations (e.g., using XRD, FTIR, XPS), and track device performance metrics (PCE, JSC, VOC, FF) under precisely defined conditions for a range of distinct 2D perovskite compositions (e.g., varying spacer cations, layer numbers, and A-site cations). This approach would provide a clearer understanding of synergistic effects and help pinpoint the most vulnerable aspects of different 2D perovskite formulations to specific environmental stressors.

In summary, while 2D perovskites offer improved stability against humidity, oxygen, and temperature compared to 3D counterparts, their sensitivity remains a complex interplay of material composition, structure, and the specific environmental conditions. Humidity, often mitigated by hydrophobic organic cations, poses a significant threat, as do elevated temperatures and oxygen, especially under illumination. The conflicting findings regarding dominant stressors highlight the need for more rigorous, standardized, and multi-factorial experimental designs to precisely quantify degradation mechanisms and inform the rational design of highly stable 2D perovskite photovoltaics for long-term outdoor deployment.

2.2.2.2 Light-Induced Degradation and Cycling Effects

The role of light in the degradation of 2D perovskite photovoltaics is multifaceted, extending beyond mere continuous illumination to encompass the dynamic effects of light cycling, which is increasingly recognized as a more accurate representation of outdoor operating conditions [9,10]. Traditional accelerated aging tests often rely on constant light exposure at elevated intensities (e.g., 1-sun or 2-sun illumination), which, while useful for initial assessments, fail to capture the complex interplay of light/dark cycles and temperature variations encountered in real-world scenarios [4,10,11].

Research indicates that light cycling can unveil distinct degradation pathways that are not observed under continuous illumination, thereby impacting device lifetime in ways unaccounted for by conventional testing protocols [9]. This highlights a critical methodological gap: continuous illumination tests may provide an overly optimistic or, in some cases, an incomplete picture of device longevity [9]. For instance, some studies on 3D/2D-mono perovskite devices reveal that light exposure, particularly in conjunction with heat, profoundly influences the 3D/2D-mono interface, leading to the disappearance of n=1 photoluminescence (PL) peaks and the emergence of n≥3 peaks. This suggests ligand migration and structural reconstruction of the 2D layer, accelerated under increased light intensity with a light-dependent exponent 'g' indicative of multiple degradation processes like defect generation and ion migration [4,5]. While these studies demonstrate light's influence, they primarily focus on continuous illumination, underscoring the need to transition towards more dynamic testing methodologies.

The criticality of light cycling stems from its ability to induce unique chemical or physical transformations. For example, reversible halide segregation, known as the "Hoke effect," is particularly prominent in mixed halide perovskites with bromine content exceeding 20% and is a light-induced phenomenon [7]. Similarly, cation segregation, such as Cs+ migration to the hole transport layer (HTL) when using TiO2 electron transport layers (ETLs), is also observed under light exposure [7]. While these phenomena occur under constant light, their dynamics and cumulative effect could be profoundly altered by intermittent exposure. The transient nature of light cycling, introducing alternating periods of light and dark, might exacerbate these segregation processes due to differences in ion mobility and defect recombination rates between illuminated and dark states. Furthermore, photochemical reactions leading to halide oxidation and the formation of metallic lead are known light-induced degradation pathways [7]. Light cycling could influence the kinetics of these reactions, potentially allowing for partial recovery during dark periods or, conversely, promoting more aggressive degradation mechanisms as materials cycle between activated and relaxed states.

To further probe these unique degradation pathways induced by light cycling, future research should employ in-situ characterization techniques during cyclic illumination. Techniques such as transient photoluminescence (TRPL), impedance spectroscopy (EIS), and operando X-ray diffraction (XRD) could provide real-time insights into charge carrier dynamics, interface recombination, and structural evolution during light/dark transitions. This would allow for the direct observation of changes in defect densities, ion distribution, and phase stability under conditions more akin to outdoor environments.

Comparing existing experimental evidence, there is a consensus that light significantly influences perovskite stability, often exacerbating other degradation factors like heat [4,5]. For example, combined light and heat accelerate degradation at the 3D/2D-mono interface by reducing the activation energy for ion migration [5]. The divergence lies in the extent to which continuous illumination accurately predicts performance under light cycling. While certain compositions and device architectures have demonstrated excellent photostability for over 1000 hours under 1-sun illumination [7], these findings do not necessarily translate directly to outdoor conditions where light cycling is ubiquitous [10].

The scientific basis for these differences lies in the time-dependent relaxation processes and accumulation of stress. Under continuous illumination, the system reaches a quasi-steady state of degradation. However, under cyclic illumination, the material undergoes repeated cycles of photoexcitation and relaxation. This can lead to phenomena such as fatigue, where repeated stress cycles cause material failure at loads below the ultimate strength, or the accumulation of irreversible structural changes during dark periods that are then exacerbated during subsequent illumination. For instance, the discussion surrounding photooxidation and illumination-induced phase separation, which varies with material composition and spacer cation [8], could be significantly influenced by light cycling. While spacer cations can reduce phase separation, the specific photoinduced segregation mechanisms, which can even lead to inferior photostability for some DJ perovskites compared to RP perovskites under certain conditions [8], might be amplified or altered under cyclic stress.

Therefore, the methodologies used to study light-induced degradation require significant standardization and evolution. Current approaches often involve continuous illumination tests at various intensities, sometimes combined with heat, as seen in "photothermal accelerated ageing" protocols [11]. While useful for identifying initial degradation pathways, these do not fully replicate outdoor conditions. A critical area for further standardized testing should involve:

1. Defined Light Cycling Protocols: Establishing standardized light/dark cycles with varying durations and intensities to simulate diurnal cycles, cloud cover, and other real-world light fluctuations. This should include protocols for both indoor laboratory testing and outdoor exposure [9].
2. Comprehensive Characterization During Cycling: Incorporating in-situ or operando characterization techniques to monitor changes in device performance, material structure, and chemical composition throughout the light cycling process. This includes techniques like transient photocurrent/photovoltage measurements, electrochemical impedance spectroscopy, and advanced spectroscopic methods.
3. Correlation with Outdoor Performance: Developing robust methodologies to directly correlate indoor light cycling test results with actual outdoor performance data. This would involve testing identical devices under controlled indoor cycling conditions and natural outdoor environments, as suggested by the emphasis on the crucial role of light cycling for understanding outdoor behavior [9].
4. Standardized Environmental Controls: Ensuring that other environmental factors, such as temperature and humidity, are accurately controlled and cycled in conjunction with light to mimic realistic outdoor conditions.

In conclusion, while continuous illumination provides foundational insights into light-induced degradation, the complexities of outdoor performance necessitate a shift towards light cycling as a more critical testing parameter. This methodological refinement will enable the identification and mitigation of unique degradation pathways, ultimately paving the way for more robust and durable 2D perovskite photovoltaics [9].

2.2.2.3 Intrinsic Chemical Instabilities (e.g., Vacuum, Ion Migration)

Intrinsic chemical instabilities, such as vacuum-induced degradation and ion migration, represent critical challenges to the long-term outdoor stability of 2D perovskite photovoltaics. Understanding these mechanisms is paramount for guiding the rational design of more robust materials and devices.

Vacuum-induced degradation in 2D perovskites is a specific intrinsic degradation pathway, primarily characterized by the formation of metallic lead and the loss of volatile organic cations [6,8]. Research indicates that 2D perovskites may initially degrade into lead iodide ($PbI\_2$), which subsequently decomposes into metallic lead ($Pb^0$) and volatile ionic species. The choice of the organic cation significantly influences this degradation process. For instance, studies have shown that shorter organic cations, such as Butylammonium (BuA), Phenylethylammonium (PEA), and Fluorinated Phenylethylammonium (F-PEA), exhibit less organic content loss under vacuum conditions, even with X-ray irradiation, compared to longer-chained cations like L and L9c [6]. This suggests that the molecular structure and volatility of the organic spacers play a crucial role in the structural stability of 2D perovskites under low-pressure environments. In contrast to vacuum-induced degradation, which often involves the expulsion of volatile components, other intrinsic degradation pathways, such as thermal degradation, are typically attributed to phase transitions or the cleavage of lead-iodide bonds, although specific details on how these manifest in 2D perovskites compared to 3D analogues are less explored in some contexts [3].

Ion migration, particularly halide migration, is another pervasive intrinsic degradation mechanism in both 3D and 2D perovskites [7,10]. In 3D and 3D/2D-mono devices, iodide migration to the hole transport layer (HTL) and electrode can lead to de-doping of the HTL and the formation of gold iodide species, resulting in increased series resistance and ultimately device failure [5,11]. The layered structure of 2D perovskites, characterized by inorganic frameworks separated by bulky organic cations, is hypothesized to spatially confine ion migration to two dimensions and reduce its overall extent [8]. This confinement is a key advantage of 2D materials over their 3D counterparts, where ion migration is more facile throughout the bulk.

The presence of large organic cations in 2D perovskites significantly influences ion migration and overall intrinsic stability [1,2]. These cations are generally less volatile and more hydrophobic, acting as a barrier against external moisture ingress, which can indirectly mitigate ion migration by preserving the perovskite structure. More directly, the choice of spacer cation, especially bulkier or $\pi$-conjugated ones, has been shown to effectively reduce ion migration within the 2D lattice [8]. For example, di-ammonium structures have been demonstrated to effectively block iodide migration from the perovskite layer to the HTL in 3D/2D-di devices [4]. This suppression is attributed to stronger interactions and increased rigidity provided by di-ammonium cations compared to mono-ammonium ones [4]. However, confining mobile ions within the perovskite bulk, while preventing migration to interfaces, can lead to iodide enrichment and the formation of interstitial iodide defects ($I\_i$) within the perovskite lattice. These defects act as deep traps, promoting non-radiative recombination and thus limiting device performance and stability [4,5]. This highlights a nuanced challenge: while the layered structure and specific cations can mitigate inter-layer ion migration, they may inadvertently create intra-layer issues if not optimized.

Experimental evidence supporting these claims varies. Studies often utilize techniques such as impedance spectroscopy, current-voltage hysteresis analysis, and X-ray photoelectron spectroscopy (XPS) to probe ion migration pathways and compositional changes [5,6]. For instance, the observation of gold iodide species at the HTL/electrode interface, coupled with an increase in series resistance, provides direct evidence of iodide migration [5]. Similarly, XPS analysis revealing the formation of metallic lead and depletion of organic cations under vacuum confirms vacuum-induced degradation mechanisms [6]. Theoretical models often complement these experimental observations by simulating ion diffusion pathways and energy barriers within the perovskite lattice, providing insights into the atomic-level mechanisms of degradation. While there is a consensus that ion migration is a significant intrinsic degradation pathway in perovskites, the specifics of how the 2D layered structure and organic cations influence these rates and pathways, especially regarding quantitative experimental data and comprehensive theoretical models comparing 2D and 3D systems directly, are still areas requiring more in-depth investigation [1,2,7]. The concept of meta-stability, involving reversible performance changes, also complicates the precise delineation of intrinsic degradation, underscoring the need for careful experimental design to isolate irreversible pathways [9].

Understanding these intrinsic mechanisms can profoundly guide material design for enhanced stability. Strategies include the judicious selection of organic cations that not only improve moisture resistance but also effectively hinder ion migration without introducing new deep-trap defects. This involves optimizing the steric hindrance and electronic properties of the organic spacers. Furthermore, advancements in device architecture are critical, as encapsulation alone is insufficient to address intrinsic instabilities like ion migration [10]. Design considerations should focus on minimizing vacancy concentrations and preventing interstitial defect formation, especially in mixed-dimensional systems. Future research should aim for a more comprehensive understanding of the interplay between structural characteristics (e.g., crystal orientation, grain boundaries), chemical composition, and environmental stressors to achieve truly robust 2D perovskite photovoltaics for outdoor applications.

2.2.3 Accelerated Ageing and Real-World Conditions

Accelerated ageing tests are crucial for forecasting the long-term performance and identifying degradation mechanisms of perovskite solar cells (PSCs) under various environmental stressors. Photothermal accelerated ageing, a prominent method, typically combines elevated temperatures with simulated solar illumination to expedite degradation processes. Studies employing this methodology, particularly for 3D/2D perovskite solar cells, often involve conditions such as 85 °C with 1-sun or 2-sun illumination [4,5,11]. For instance, optimized 3D/2D devices have shown extrapolated operational $T\_{80}$ lifetimes exceeding 560 hours under 85 °C and 2-sun illumination, with some projections indicating over 1100 hours under 85 °C and 1-sun conditions [4,11].

While photothermal accelerated ageing offers valuable insights into thermal and photo-induced degradation pathways, its effectiveness in fully mimicking real-world outdoor conditions requires critical evaluation. Other accelerated testing methods include vacuum exposure, sometimes coupled with X-ray irradiation, though this primarily addresses degradation during fabrication and measurement in vacuum rather than comprehensive outdoor simulation [6]. Another approach involves exposing devices to high relative humidity (e.g., 72% RH), demonstrating that 2D perovskites can achieve significantly longer half-lives (90 hours in the dark) compared to 3D counterparts (10 hours) without encapsulation, suggesting improved intrinsic stability under environmental stress [3]. More aggressive tests, such as damp heat and pressure cooker tests, are also proposed for rapidly screening encapsulation effectiveness and accelerating development, with immersion in 85 °C water being a suggested method for accelerating encapsulation testing [10].

A significant challenge in the field lies in correlating accelerated test results with actual long-term outdoor performance. Many studies, while providing detailed accelerated ageing data, do not present direct comparative data with specific real-world outdoor performance [4,5,11]. This gap arises because current accelerated protocols often fail to capture the complexity of outdoor phenomena, such as combined UV and thermal cycling, spectral variations, and the presence of atmospheric pollutants. For instance, testing under constant light, a common accelerated ageing approach, has been shown not to represent a "worst-case scenario" and cannot fully reproduce real-world conditions, particularly due to the significant impact of meta-stability effects on outdoor performance that are often overlooked by standard evaluation routines for silicon-based devices [9].

The predictive power of current accelerated tests is thus limited by their inability to replicate the dynamic and multifaceted nature of outdoor environments. The reliance on constant illumination in many accelerated tests, for example, contrasts sharply with the dynamic light cycling experienced outdoors, which can induce distinct degradation pathways and meta-stability effects not captured by static conditions [9]. Furthermore, other device components, including charge transport layers and electrodes, are known to contribute to degradation, emphasizing that interface engineering is critical for both efficiency and stability [8]. The interaction of these components under varied environmental stressors is complex and not always accurately simulated by simplified accelerated tests.

To enhance the correlation and predictive power of accelerated tests, improved protocols are needed. This includes a more comprehensive reporting of experimental details, characterization, and standardized testing conditions, as contradictory observations in halide perovskite research often stem from differences in material/device preparation and testing conditions [8]. Crucially, stability tests should be performed under maximum power point tracking rather than short or open circuit conditions, as degradation rates can vary significantly depending on the electrical bias [7].

More holistic accelerated testing methodologies are required to integrate factors like combined UV and thermal cycling, spectral variations, and atmospheric pollutants. Aggressive testing, such as 1000 hours at 85 °C under five-sun illumination, has been suggested as indicative of long-term outdoor performance, providing rapid feedback for improvement, though specific correlating data for 2D perovskites remains scarce [7]. The concept of using light-cycled experiments, which have shown correlation with extensive outdoor testing results for PSCs, represents a promising direction for developing more realistic accelerated protocols [9].

In conclusion, while photothermal accelerated ageing and other accelerated methods provide crucial insights into degradation mechanisms and relative stability, their direct correlation and predictive power for real-world long-term outdoor performance remain challenging due to the inherent complexity of outdoor conditions. Research should focus on developing and validating standardized accelerated testing protocols that accurately mimic real-world light cycling and meta-stability effects, integrating a broader range of environmental variables to achieve more reliable predictions of outdoor stability [9,10]. This necessitates a shift towards protocols that account for dynamic environmental factors and the multi-component nature of device degradation, moving beyond simplified, constant-stress approaches to truly understand and predict the outdoor behavior of 2D perovskite photovoltaics.

3. Advanced Characterization Techniques for Probing Degradation Mechanisms in 2D Perovskites

Understanding and mitigating the degradation of 2D perovskite photovoltaics necessitate a robust suite of advanced characterization techniques. This section provides a comprehensive overview of the methodologies employed to diagnose degradation mechanisms and assess device stability, with a particular focus on their application to 2D perovskites under outdoor-relevant conditions. The discussion is structured around two key areas: the specific characterization techniques utilized for diagnosing degradation and the evolving landscape of standardized outdoor testing protocols and performance metrics.

The first subsection, "Characterization Techniques for Diagnosing Degradation," delves into the diverse array of structural, chemical, and electrical characterization methods. It highlights how techniques such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Photoluminescence (PL) spectroscopy, UV-Vis Absorption Spectroscopy, and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) are employed to identify changes at the material and interfacial levels, including phase decomposition, ion migration, and defect formation [4,6,7]. Furthermore, the indispensable role of electrical characterization, including Current-voltage (J-V) measurements and Electrochemical Impedance Spectroscopy (EIS), in correlating material changes with device performance degradation is discussed [4,11]. The challenges associated with accelerated aging tests and the imperative for in-situ and operando measurements under simulated outdoor conditions are also addressed [8,11].

The subsequent subsection, "Standardized Outdoor Testing Protocols and Evolving Performance Metrics for 2D Perovskites," critically examines the current state of outdoor stability testing. It underscores the absence of universally established standards for 2D perovskites, despite the existence of industry benchmarks like IEC 61215 and consensus protocols such as ISOS [7,8]. This section highlights the limitations of traditional constant light aging protocols and emphasizes the necessity of incorporating dynamic light cycling and real-world outdoor exposure to accurately capture meta-stability and the complex interplay of degradation mechanisms [9,10]. It also scrutinizes the widely adopted performance metric, Power Conversion Efficiency (PCE), and its derivative, $T\_{80}$, proposing the need for refined metrics that better reflect real-world energy yield and the nuances of reversible degradation [7,9]. Collectively, these subsections lay the groundwork for a more rigorous and comprehensive approach to evaluating the outdoor stability of 2D perovskite photovoltaics.

3.1 Characterization Techniques for Diagnosing Degradation

The comprehensive diagnosis of degradation mechanisms and accurate evaluation of stability in 2D perovskite photovoltaics necessitate a diverse array of characterization techniques. These techniques provide crucial insights into structural, chemical, and electrical changes occurring within the device layers under various stress conditions. Studies have extensively employed a combination of microscopic, spectroscopic, and electrical methods to unravel complex degradation pathways, particularly those involving photothermal degradation and ion migration [4,7].

Structural and morphological characterization techniques are fundamental. X-ray Diffraction (XRD) is routinely used to monitor structural changes, phase decomposition, and the formation of degradation products such as $PbI\_2$ and $Pb^0$ in both 2D films and 3D/2D interfaces [4,5,7]. This is critical for identifying structural collapse and material decomposition. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) are employed to assess microstructural changes, including increased porosity and roughness, offering visual evidence of degradation [6,7]. Transmission Electron Microscopy (TEM), particularly in-situ TEM, provides high-resolution imaging and allows for the direct observation of layer-by-layer decomposition, offering unparalleled insights into degradation progression at the nanoscale [7,11].

Spectroscopic methods are invaluable for probing chemical and electronic changes. X-ray Photoelectron Spectroscopy (XPS) is a powerful tool for analyzing elemental composition and chemical states within device layers, enabling the detection of phenomena like iodide migration into the hole transport layer (HTL) or the formation of metallic lead [4,5,6,7]. Photoluminescence (PL) spectroscopy is used to observe changes in the electronic structure, including defect formation and shifts in perovskite emission, which can indicate structural reconstruction or the disappearance of 2D-specific peaks [4,5,7]. UV-Vis Absorption Spectroscopy monitors changes in optical density and bandgap, providing an indicator of overall material degradation [4,5,6]. Fourier-transform infrared spectroscopy (FTIR) is also cited for evaluating changes in materials and encapsulation [7,10].

For diagnosing ion migration, a particularly critical degradation mechanism in perovskites, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) stands out. It offers depth profiling of ions, directly confirming migration from the perovskite layer to other device components like the HTL and electrodes [4,5,7]. Kelvin probe measurements further supplement this by assessing changes in the work function of the HTL, indicating modifications in its doping level and charge transport characteristics due to ion ingress [4,5].

Electrical characterization techniques are indispensable for correlating material changes with device performance degradation. Current-voltage (J-V) measurements provide direct insight into efficiency loss, while Electrochemical Impedance Spectroscopy (EIS) or Impedance Spectroscopy (IS) analyzes resistance and capacitance changes, interfacial charge accumulation, and recombination rates, offering a deeper understanding of transport layer mobility, doping concentration, and trap densities [4,5,7,11]. Drift-Diffusion (DD) simulations are a crucial theoretical complement, utilized to model device performance degradation and correlate it with changes in physical parameters derived from experimental data [4,5]. This integrated approach helps to establish a comprehensive understanding of how specific material changes translate into device performance decline.

Regarding experimental conditions, "photothermal accelerated ageing" has been identified as a significant stressor for mimicking real-world outdoor conditions. Studies employing such conditions, often inferring the use of the aforementioned characterization techniques, focus on degradation phenomena such as structural collapse, decomposition into $PbI\_2$ and metallic lead, vacancy formation, and iodide migration [5,11]. However, the effectiveness of these accelerated tests in perfectly mimicking complex, dynamic outdoor conditions, which include varied light spectra, humidity, temperature cycling, and mechanical stress, remains a subject of ongoing research. Some studies also highlight the concern that vacuum-induced degradation can affect characterization performed in vacuum, potentially skewing results and not accurately reflecting real-world performance [6,8]. This underscores the importance of in-situ measurements under simulated outdoor conditions to bridge the gap between accelerated laboratory tests and actual outdoor exposure.

A comprehensive framework for characterizing the outdoor stability of 2D perovskite devices should integrate these techniques, prioritizing those that offer in-situ or operando capabilities under simulated outdoor conditions. For understanding photothermal degradation, real-time XRD and PL measurements under controlled temperature and illumination are essential to track structural and electronic changes. For ion migration, in-situ ToF-SIMS or a combination of EIS with transient ion drift measurements would be highly beneficial, providing kinetic information about ion movement [8]. The applicability of techniques like in-situ synchrotron grazing incidence X-ray diffraction is particularly promising for probing layer-by-layer decomposition and interface stability in real-time [7].

Despite these advancements, gaps remain in current characterization capabilities, particularly in probing the dynamic interplay of multiple stressors. For instance, simultaneously monitoring changes in crystallinity, chemical state, and electrical properties under combined light, heat, and humidity cycling in a spatially resolved manner is challenging. There is a need for more advanced correlative microscopy techniques that can link localized material degradation with changes in device performance. Synergistic combinations of techniques are crucial. For example, combining in-situ operando J-V and EIS measurements with real-time PL and XRD can provide a holistic view of electrical degradation mechanisms alongside structural and electronic changes. Furthermore, integrating advanced computational modeling, such as refined Drift-Diffusion simulations, with experimental data is vital for validating models used for 3D materials for 2D/quasi-2D systems, considering their inherent anisotropy and complex degradation pathways [4,5,8].

The scientific basis for choosing these characterization techniques lies in their ability to directly probe the physical and chemical phenomena known to cause perovskite degradation. For instance, the consensus on using XRD for structural changes is based on its direct sensitivity to crystal lattice parameters and phase identification. Similarly, XPS and ToF-SIMS are chosen for chemical state and elemental depth profiling due to their surface sensitivity and depth resolution. Divergences typically arise in the interpretation of complex degradation mechanisms or in the validation of accelerated aging protocols against real-world performance, highlighting the need for more standardized methodologies and comprehensive long-term outdoor testing. While basic material characterization techniques like optical microscopy, SEM, FTIR, UV-Vis spectroscopy, and TGA can evaluate changes in encapsulation and devices with aging, their specific applications for 2D perovskites, particularly in diagnosing complex degradation pathways, are often inferred rather than explicitly detailed [10]. This emphasizes the critical need for more specialized techniques that can precisely pinpoint degradation mechanisms at the atomic and molecular level, particularly under outdoor conditions.

3.2 Standardized Outdoor Testing Protocols and Evolving Performance Metrics for 2D Perovskites

The current landscape of outdoor stability testing for 2D perovskite solar cells (PSCs) is characterized by a notable lack of universally established standards, leading to significant challenges in direct comparison and reliable prediction of long-term device performance [7,8]. While industry benchmarks such as IEC 61215 and consensus protocols like ISOS (International Summit on Organic Photovoltaic Stability) have been referenced and adapted for PSCs, their full implementation and specific tailoring for 2D perovskites remain inconsistent across research [7,10]. This divergence in testing methodologies, encompassing variations in perovskite compositions, device architectures, and encapsulation techniques, underscores the critical need for harmonization [8].

A significant limitation of many existing protocols, particularly those based on constant light aging, is their inadequacy in predicting the true outdoor behavior of PSCs [9]. This stems from the fact that PSCs exhibit both reversible and irreversible degradation mechanisms, which are profoundly influenced by dynamic environmental factors, notably light cycling and temperature fluctuations [10]. The importance of incorporating light cycling tests and real-world outdoor exposure into stability assessments cannot be overstated, as these conditions are essential for capturing the complex meta-stability effects inherent to PSC operation [9,10]. For instance, a study demonstrated the insufficiency of silicon PV-developed evaluation routines for PSCs, advocating for protocols that explicitly account for light cycling and meta-stability by presenting data from over two years of outdoor exposure, noted as the longest reported for PSCs at the time [9].

Current testing conditions span a spectrum from controlled laboratory environments to outdoor exposure. Accelerated photothermal aging, typically conducted at elevated temperatures (e.g., 85 °C) and under intensified illumination (e.g., 1-sun or 2-sun), serves as a proxy for outdoor stability in several studies [4,5,11]. For example, optimized devices achieved extrapolated operational $T\_{80}$ lifetimes of over 560 hours under 85 °C and 2-sun illumination and over 1100 hours under 85 °C and 1-sun illumination, with performance metrics including Power Conversion Efficiency (PCE), current density ($J\_{SC}$), open-circuit voltage ($V\_{OC}$), and fill factor (FF) normalized to their initial values [4]. The acceleration factor (AF) calculation, based on light intensity and temperature, is also employed to provide a theoretical foundation for accelerated aging standards [4]. However, the direct applicability of these accelerated tests to real-world performance is often debated due to the complex interplay of degradation mechanisms that vary under different stressors.

The ISOS protocols, originally developed for organic photovoltaics (OPVs) and subsequently updated for PSCs, represent a significant step towards standardization. These protocols encompass a range of tests including dark storage, light soaking, outdoor testing, thermal cycling, solar-thermal cycling, and critically, light cycling, with maximum power point (MPP) tracking recommended for all illumination tests [10]. Damp heat testing (ISOS-D3) and outdoor exposure are considered particularly relevant for practical applications due to their ability to combine multiple stressors such as high humidity, temperature, and illumination [8,10]. Despite this, a comprehensive comparison of results remains challenging due to ongoing variations in perovskite compositions, device architectures, and encapsulation methods [8].

The predominant performance metric, Power Conversion Efficiency (PCE), and its derivative, $T\_{80}$ (time to reach 80% of initial PCE), are widely adopted indicators of stability [4,5,7]. However, these metrics often fall short in capturing the subtle degradation effects or long-term performance drifts that occur under variable outdoor conditions. For instance, the original $T\_{80}$ concept may require modification to account for reversible efficiency gains or losses observed after dark recovery, which are crucial for understanding device meta-stability [7]. The limitations of $T\_{80}$ in reflecting real-world energy yield and economic viability are evident, as it does not fully encompass the dynamic performance fluctuations characteristic of PSCs under real solar irradiance [9].

To address these limitations, novel or refined performance metrics are needed. These should potentially incorporate predictive models based on accelerated testing, moving beyond single-point efficiency measurements to capture the cumulative effects of degradation over time. Metrics that consider integrated energy yield over varying conditions, rather than merely peak efficiency, would better reflect economic viability [9]. Furthermore, the development of metrics that quantify the extent and reversibility of meta-stable states, alongside the irreversible degradation, would provide a more holistic understanding of device longevity. This could involve tracking parameters such as hysteresis behavior, transient performance recovery, and the degradation rates of individual photovoltaic parameters ($J\_{SC}$, $V\_{OC}$, FF) under light cycling [7].

Research directions should primarily focus on developing and validating standardized accelerated testing protocols that accurately mimic real-world light cycling and meta-stability effects [9,10]. This requires a deeper understanding of the fundamental degradation mechanisms under dynamic conditions, possibly through advanced in-operando characterization techniques. Establishing a theoretical basis for these accelerated aging protocols, including robust acceleration factors, is crucial for translating laboratory results to real-world lifetimes [4,5]. Collaborative efforts among research institutions and industry are necessary to reach consensus on these protocols, facilitating more meaningful comparisons across studies and accelerating the commercialization of 2D PSCs. Ultimately, integrating advanced degradation models with real-world environmental data will pave the way for more accurate lifetime predictions and more resilient 2D perovskite photovoltaic technologies.

4. Strategies for Enhancing Outdoor Stability of 2D Perovskite Photovoltaics

The pursuit of stable and efficient 2D perovskite photovoltaics for outdoor applications necessitates a multifaceted approach, addressing both the intrinsic vulnerabilities of the materials and the environmental stressors they encounter. This section outlines key strategies for enhancing the outdoor stability of 2D perovskite solar cells, categorizing them into material engineering approaches—focusing on compositional, structural, and interfacial modifications—and device architecture and encapsulation techniques. These strategies collectively aim to mitigate dominant degradation pathways such as moisture ingress, oxygen exposure, thermal stress, UV irradiation, and ion migration, which are exacerbated in real-world outdoor conditions.

The initial subsections delve into material engineering, beginning with compositional modifications. This includes the strategic selection of organic spacer cations and halide components to enhance hydrophobicity, improve intrinsic stability, and manage ion migration [2,8]. Subsequent discussions detail structural engineering, particularly the integration of di-ammonium based 2D perovskites within 3D frameworks to effectively confine mobile ions and suppress degradation mechanisms [4,5]. Interfacial modifications are also explored, focusing on defect passivation and the optimization of charge transport layers to prevent degradation at critical interfaces [7]. The synthesis of these material-centric strategies reveals a strong emphasis on balancing enhanced stability with maintained or improved power conversion efficiency, recognizing that effective outdoor deployment requires both robustness and performance.

Following the exploration of material-level interventions, the subsequent subsections shift focus to macroscopic protection strategies. This encompasses advancements in device architecture, such as the adoption of inverted p-i-n structures and stable inorganic charge transport layers, which inherently contribute to improved device longevity [7]. Paramount among these external strategies is encapsulation, serving as an indispensable physical and chemical barrier against environmental degradation [10]. This section details various encapsulation materials and methods, from thin-film encapsulation to robust glass-glass seals, highlighting the critical role of low Water Vapor Transmission Rate (WVTR) and Oxygen Transmission Rate (OTR) in achieving long-term stability under harsh conditions [10]. The discussion also addresses the ongoing challenges in standardizing encapsulation protocols and the imperative for cost-effective, scalable solutions that can bridge the gap between laboratory success and industrial applicability.

In summary, this section provides a comprehensive overview of the current state-of-the-art in enhancing the outdoor stability of 2D perovskite photovoltaics. It highlights that successful deployment hinges on a synergistic interplay between fundamental material design and advanced device engineering. While significant progress has been made in both areas, future research must continue to explore underexplored compositional spaces, refine theoretical models for predicting synergistic effects, and develop more standardized, cost-effective, and scalable encapsulation techniques. The integration of robust materials from established industries and advanced manufacturing processes, such as roll-to-roll processing, remains critical for the widespread commercialization of 2D perovskite technology.

4.1 Material Engineering Approaches: Compositional, Structural, and Interfacial Modifications

Material engineering offers a versatile platform for enhancing the outdoor stability of 2D perovskite photovoltaics by addressing inherent degradation pathways through precise compositional, structural, and interfacial modifications. The fundamental understanding of 2D/quasi-2D materials reveals that the judicious selection of organic spacer cations significantly influences stability under illumination, thermal stress, and ambient exposure [8]. This flexibility allows for substantial adjustments to crystal and thin-film configurations, bandgaps, and charge transport properties [2].

A primary strategy in compositional engineering involves the replacement of hygroscopic organic cations, such as methylammonium (MA), with less hygroscopic, larger, and more hydrophobic alternatives. For instance, the phenethylammonium (PEA) cation has been widely adopted as a spacer in 2D perovskite structures to impart greater moisture resistance [1,3]. Early implementations of PEA, while improving hydrophobicity, often led to a trade-off with power conversion efficiency (PCE), with reported efficiencies as low as 4.73% alongside significant hysteresis [3]. However, recent advancements have demonstrated that optimizing the crystallization process and the thickness of the solar absorber layer (e.g., 240–260 nm) can mitigate hysteresis and achieve higher PCEs, with a record of 5.50% reported for 2D perovskite structures [3]. This indicates that the trade-off between PCE and stability can be effectively managed through comprehensive material and process optimization.

Beyond mono-ammonium spacers like PEAI, which can paradoxically accelerate degradation under photothermal stress due to the intrinsic instability of 2D-mono structures [5], the incorporation of di-ammonium based 2D perovskites (2D-di) presents a more robust solution. Studies demonstrate that 2D-di perovskites exhibit superior intrinsic stability under harsh photothermal aging conditions compared to their mono-ammonium counterparts [4]. A critical structural modification involves integrating 2D-di perovskites directly into the bulk of 3D perovskite layers, rather than merely as surface passivation layers. This is achieved by depositing a 2D-di layer on the electron transport layer (ETL) followed by spin-coating the 3D precursor, enabling di-ammonium molecules to dissolve into the bulk and segregate at grain boundaries [5,11]. This strategy effectively confines mobile ions within grains, thereby mitigating detrimental degradation pathways such as $Pb^0$ formation and phase segregation, leading to significantly improved operational lifetimes [4,5].

Further compositional tuning includes halide mixing, such as Br substitution for enhanced moisture stability, and A-site cation alloying with elements like FA, Cs, and Rb to improve thermal and light stability [7]. For example, the use of a CsFA composition (bandgap 1.61 eV) has been shown to improve stability compared to CsMAFA (bandgap 1.57 eV) due to the reduced vacancy formation stemming from the absence of volatile MA cations [4,5]. These modifications directly address ion migration and inherent material instability.

The critical role of the organic cation extends to stability under vacuum conditions, where different organic cations lead to varying degrees of degradation, including metallic lead formation and organic cation loss [6]. Shorter organic cations generally exhibit higher stability under vacuum compared to longer ones, implying that cation length is a crucial design parameter in mitigating vacuum-induced degradation [6]. This emphasizes the need for tailored organic cation characteristics that not only improve hydrophobicity but also maintain structural integrity under various environmental stresses.

Interfacial modifications are equally vital for mitigating degradation. Defect passivation, especially at grain boundaries, and optimization of charge transport layers or barrier layers are crucial to prevent degradation initiated by moisture, oxygen, and electrode reactions [7]. The stability of the 2D/3D interface is significantly influenced by the chosen spacer cation, with certain cations facilitating stable interfaces even under ambient exposure and elevated temperatures [8]. Advances in device architecture, such as eliminating materials like $TiO\_2$, can further improve stability under UV exposure and hinder ion migration [10].

Promising chemical design principles include the utilization of fluorinated spacers, which offer increased hydrophobicity and can also improve charge transport through beneficial halogen bonding interactions. These interactions can enhance stability against moisture and reduce halide migration, though excessive interactions might cause detrimental lattice distortions [8]. The development of layered perovskites incorporating semiconducting organic moieties also holds potential for improved electronic properties alongside stability [8].

While significant progress has been made, certain compositional spaces and structural modifications remain underexplored for their stability impact. For instance, a more systematic investigation into the precise interaction and ordering of various mixed organic ammonium halides for 1D/2D surface passivation is warranted [8]. Furthermore, a deeper understanding of how the precise molecular structure of organic cations (e.g., branched vs. linear alkyl chains, or specific aromatic substitutions) influences charge transport kinetics and defect passivation, rather than just hydrophobicity, is needed. Future research should also focus on developing more robust theoretical models that can predict the synergistic effects of multiple material modifications on both PCE and long-term stability, thereby guiding the rational design of next-generation 2D perovskites. This would involve a more quantitative assessment of degradation rates and efficiency retention percentages under defined stress conditions, allowing for a more precise comparison of different material engineering strategies.

4.2 Device Architecture and Encapsulation

Device architecture and encapsulation serve as crucial physical and chemical barriers against environmental stressors, fundamentally influencing the outdoor stability of 2D perovskite photovoltaics. While intrinsic material properties of 2D perovskites offer improved moisture resistance, as evidenced by devices maintaining stability under 72% relative humidity even without protective encapsulation [3], external protection remains paramount for achieving long-term performance suitable for real-life outdoor applications [10].

Several architectural modifications have been explored to enhance stability. The integration of 2D perovskites into 3D/2D heterostructures is a prominent strategy, aiming to balance efficiency with improved stability [2,8]. This approach involves modifying the surface properties of 3D perovskites or incorporating 2D layers within the 3D bulk, such as 2D-di structures, to block iodide migration and enhance stability under photothermal stress [4,11]. The use of inverted p-i-n structures and stable inorganic charge transport layers like SnO2 and NiOx also contributes significantly to improved device stability [7]. These architectural designs primarily address internal degradation pathways, such as ion migration and interfacial reactions, by providing more stable interfaces and charge transport pathways.

Despite advancements in intrinsic material stability and device architectures, encapsulation remains a critical, indispensable strategy to mitigate external degradation factors, particularly moisture and oxygen ingress, and to prevent the loss of volatile species [7]. The selection of encapsulation materials and methods is crucial, drawing lessons from established technologies like OLEDs, OPVs, and commercial PV [10].

Key performance indicators for encapsulation materials include low Water Vapor Transmission Rate (WVTR) and Oxygen Transmission Rate (OTR) [10].

Various encapsulation materials have been investigated, including Ethylene Vinyl Acetate (EVA), Polyvinyl Butyral (PVB), Polyisobutylene (PIB), Surlyn, Polyolefin Elastomer (POE), Thermoplastic Polyurethane (TPU), and different epoxies [10]. Among these, polyolefins are favored over EVA due to lower byproduct release and higher resistivity, which are critical for passing stringent IEC standard tests like damp heat and thermal cycling [7]. Polyisobutylene (PIB) tape has emerged as a particularly effective and straightforward option for both blanket and edge encapsulation, demonstrating robust performance in damp heat and outdoor tests [10]. This highlights a consensus on the importance of barrier properties and chemical inertness of encapsulants.

Encapsulation methods range from thin-film encapsulation (TFE) using polymers, graphene, or inorganic films to more robust glass-glass encapsulation [7,10]. Blanket encapsulation, edge encapsulation, and combinations thereof are employed, with proper edge cleaning and sealant adhesion identified as critical for effective protection [10]. For flexible perovskite solar cells (PSCs), film-based encapsulation and alternative packaging like sealing in glass tubes are explored, though these often present challenges under harsh testing conditions [10]. The incorporation of advanced features such as desiccants for moisture absorption and lead sequestration within the encapsulation package further enhances device longevity and safety [10].

A significant challenge in evaluating the efficacy of these strategies is the lack of standardized encapsulation protocols, which prevents direct comparisons of reported stability results across different studies [8]. This divergence in experimental conditions makes it difficult to definitively identify universally best practices. However, the consistent emphasis on the need for effective encapsulation for devices to withstand accelerated aging tests (e.g., damp heat, thermal cycling) and real-world outdoor exposure underscores its indispensable role.

Despite reported improvements, significant challenges remain in achieving cost-effective and scalable encapsulation for large-scale outdoor deployment. Current methods, while effective in laboratory settings, may not translate efficiently or economically to manufacturing processes. The high costs associated with specialized barrier materials and complex lamination processes can impede widespread adoption. To overcome these hurdles, integrating techniques from the flexible electronics and packaging industries holds considerable promise. Novel barrier films, such as multi-layer thin-film barriers with exceptionally low WVTR, and advanced lamination processes developed for flexible displays or food packaging, could provide highly effective and potentially more cost-efficient solutions. For instance, roll-to-roll processing for applying thin-film encapsulation or advanced adhesive technologies could significantly reduce manufacturing costs and increase throughput, addressing the scalability challenge inherent in current batch-wise encapsulation approaches. The adoption of robust, long-term proven materials and manufacturing techniques from established industries is crucial for bridging the gap between laboratory success and commercial viability for 2D perovskite photovoltaics. Such cross-disciplinary integration could offer innovative approaches to address the multifaceted challenges of long-term stability and cost-effectiveness in outdoor applications.

5. Future Directions and Outlook

The advancement of 2D perovskite photovoltaics toward widespread outdoor application necessitates a forward-looking perspective, encompassing a systematic identification of existing limitations and a strategic outline of future research trajectories. This section critically examines the formidable challenges that currently impede the long-term outdoor stability of 2D perovskite devices, ranging from intrinsic material properties and extrinsic environmental factors to processing limitations, device architecture constraints, and the inadequacies of current characterization and encapsulation technologies. Following this comprehensive overview of persistent hurdles, the section then delineates promising emerging trends and research opportunities. These include the exploration of novel material designs, the strategic implementation of advanced device architectures, the refinement of characterization and testing protocols, and the development of next-generation encapsulation solutions. Ultimately, this section provides a roadmap for future research, emphasizing the need for interdisciplinary collaboration and a holistic approach to overcome current barriers and realize the full potential of 2D perovskite photovoltaics for reliable and durable outdoor energy generation.

Despite significant strides in enhancing the outdoor stability of 2D perovskite photovoltaics, a myriad of challenges and limitations persist, underscoring the need for targeted and systematic research efforts. A foundational impediment is the incomplete understanding of 2D perovskite degradation mechanisms, which often forces a "trial-and-error" research approach rather than a design-driven methodology [8]. This lack of fundamental insight inherently constrains the systematic development of robust materials and devices.

Intrinsic material properties present a critical challenge. Mono-ammonium based 2D perovskites, for instance, are inherently susceptible to degradation under photothermal aging, leading to decomposition and eventual device failure [4,11]. While di-ammonium based 2D perovskites offer enhanced structural stability and improved inhibition of iodide migration [4,5], they can inadvertently induce interstitial defect formation within the underlying 3D perovskite layer, thereby escalating non-radiative recombination and detrimentally affecting overall device performance [4,5]. Furthermore, vacuum-induced degradation of 2D perovskites, characterized by metallic lead formation and organic cation loss, poses a significant hurdle during fabrication and characterization, with the precise degradation chemistry requiring further elucidation [6]. A persistent trade-off exists between power conversion efficiency (PCE) and stability; despite their enhanced stability, 2D perovskites generally exhibit lower efficiencies compared to their 3D counterparts due to broader optical bandgaps and limited charge transport, especially when utilized as the sole light-absorbing material [1,2,3]. Overcoming this inherent PCE/stability trade-off is paramount for widespread adoption.

Extrinsic environmental stressors profoundly impact the degradation of 2D perovskite photovoltaics. Achieving long-term operational stability under diverse environmental conditions, including humidity, oxygen, and varying light spectra, remains a formidable challenge [5,7]. The intricate degradation pathways under these combined stresses are not yet fully understood [7,8].

Processing limitations and device architecture also introduce significant challenges. Ensuring and maintaining phase purity and precise crystal orientation during deposition are crucial for device performance and stability, yet these factors are highly susceptible to processing and environmental conditions [8]. Moreover, the development of transport layers and electrodes that are highly resistant to halide migration and subsequent redox reactions is critical [4,5]. Integrating 2D perovskites into solar cell architectures without compromising overall device stability remains an ongoing challenge [2]. For instance, while 2D/3D heterojunctions are promising, they exhibit drastic degradation under harsh conditions, highlighting the urgent need for robust interface engineering [11]. Even with proposed solutions such as incorporating 2D-di into the 3D bulk to achieve improved lifetimes (e.g., $T\_{80}$ > 560 hours under harsh conditions), further enhancements are required to meet commercial operational longevity standards [11]. The scale-up of 2D perovskite fabrication faces hurdles in achieving uniform film deposition, especially with large organic cations, and issues with solvent compatibility during large-area processing, which can affect film consistency and quality.

Limitations in characterization and testing protocols are also significant. The inadequacy of current accelerated testing protocols to reliably predict real-world performance is consistently highlighted in the literature [7,8,9]. This arises from meta-stability effects and the divergence of degradation pathways observed under constant light aging versus dynamic light cycling, rendering standard silicon PV evaluation routines unsuitable for perovskite solar cells [9]. Furthermore, inconsistencies in material and device preparation, coupled with varying testing conditions, lead to contradictory findings regarding the relative stability of different 2D perovskite types (e.g., Ruddlesden-Popper vs. Dion-Jacobson phases) [8]. This underscores the critical need for standardization of experimental details, characterization techniques, and testing protocols to ensure reproducible and comparable results across the field [8]. Demonstrating stability under stringent, commercially relevant conditions, such as damp heat and prolonged outdoor exposure, remains a substantial hurdle [8].

Finally, effective encapsulation, while paramount for outdoor stability, presents its own set of challenges. The requirement for encapsulation materials with extremely low water vapor transmission rates (WVTR), ideally below $10^{-5} \text{ g/m}^2\text{ day}$, is difficult to achieve [10]. Additional challenges include the need for reduced processing temperatures for encapsulation, improved adhesion to perovskite surfaces, enhanced mechanical and thermal properties of the encapsulant, and ensuring a lack of reactivity with perovskite components [10]. The development of flexible barrier materials capable of ensuring long-term lifetimes, alongside integrating functionalities such as thermal management and lead containment without compromising device performance, remains a critical area for research [10]. Simple epoxy encapsulation, while convenient for laboratory purposes, is unsuitable for rigorous outdoor or damp heat testing due to poor mechanical properties and should be phased out for serious stability studies [10].

To overcome these multifaceted challenges, several specific research directions are critically needed. First, a dedicated focus on systematic structure-property-stability correlations is essential to move beyond the current "trial-and-error" research paradigm. This involves detailed investigations into how the choice of spacer cation in 2D/quasi-2D perovskites influences their stability under various stress conditions, necessitating fundamental understanding rather than empirical observation [8]. Reconciliation of contradictory findings in the literature requires the establishment and adoption of standardized testing protocols, including the refinement of accelerated testing methods to better mimic real-world dynamic conditions, such as light cycling, rather than just constant light exposure [9].

Furthermore, the development of novel materials resistant to multiple degradation factors is crucial. This can involve designing new organic cations with enhanced photothermal stability or incorporating robust inorganic layers that block halide migration without inducing detrimental interstitial defects. Leveraging insights from other fields holds significant promise: polymer science can contribute to advanced encapsulation strategies by developing materials with ultra-low WVTR, superior adhesion, and reduced processing temperatures [10]. Similarly, materials science can provide solutions for developing hydrophobic coatings and designing self-healing materials that actively mitigate degradation. A holistic approach that integrates advancements in passivation, transport layers, and electrode materials is necessary to achieve robust, long-term stability [4]. Finally, research must continue to bridge the efficiency-stability gap, potentially through novel 2D/3D architectures that optimize both charge transport and environmental resilience while ensuring scalability for manufacturing processes.

Future research in 2D perovskite photovoltaics must strategically address the identified challenges to significantly enhance outdoor stability and operational longevity. A consensus among researchers points towards a holistic approach that integrates advancements across material design, characterization, device architecture, and standardized testing protocols [1,7,8].

One paramount direction involves developing more robust 2D perovskite chemistries, specifically by exploring novel material designs that combine improved intrinsic stability with high power conversion efficiency (PCE). This includes systematic structure-property studies, investigating a wider range of unexplored spacer cations and halide compositions [1,2,7,8]. For instance, the successful use of large, hydrophobic organic cations like phenylethylammonium (PEA) to create moisture-stable 2D perovskite structures, combined with process optimization to minimize hysteresis and improve PCE, exemplifies a promising material design strategy [3]. Further research should delve into specific organic cation modifications to enhance hydrophobicity, coupled with computational modeling to predict long-term behavior, thereby accelerating the discovery of superior compositions [1,2]. This interdisciplinary approach, integrating computational modeling with empirical studies, is crucial for achieving significant breakthroughs. The role of halogen bonding in enhancing stability also warrants further investigation [8]. Additionally, compositions utilizing less volatile A-site cations, such as CsFA, are promising for enhanced intrinsic stability [4].

A critical emerging trend is the "bulk incorporation" strategy, where 2D-di perovskites are integrated into the 3D bulk rather than merely as surface passivation layers [4,5,7,8,11]. This innovative approach has shown substantial improvements, confining mobile ions and suppressing cation phase segregation, leading to extrapolated $T\_{80}$ lifetimes exceeding 1100 hours under 85 °C and 1-sun conditions [4]. Future research should focus on optimizing 2D-di structures and comprehensively investigating their impact on grain boundary passivation and ion transport within the 3D matrix. This includes understanding and mitigating interstitial iodide defects ($I\_i$) that may form in 3D/2D-di systems, with the ultimate goal of achieving superior PCE-stability performance compared to current benchmarks [4].

Beyond material design, advanced characterization techniques are indispensable. The development and implementation of advanced in-situ and dynamic characterization methods for real-time degradation monitoring are crucial for gaining deeper insights into degradation pathways and material transformations under operational stress [7,8]. This includes employing consistent methodologies and multiple techniques for studying ion migration and investigating the exact chemistry of vacuum-induced degradation to develop effective mitigation strategies [6,8].

A critical aspect for validating outdoor stability is the development and validation of standardized and rigorous accelerated testing protocols. Current research highlights the necessity of these protocols to accurately mimic real-world light cycling and meta-stability effects, accounting for diurnal performance variations [4,8,9,10]. The correlation between light-cycled experiments and outdoor results emphasizes this direction [9]. A specific research aim should be to "Develop a unified standard for accelerated outdoor testing that accurately replicates the combined effects of UV irradiation, humidity cycles (e.g., 85% RH/25°C for 1000h), and thermal cycling (e.g., -40°C to +85°C) for 2D perovskite devices, validated against real-world outdoor performance data." Furthermore, integrating AI-driven predictive models for perovskite degradation, by combining experimental data from various accelerated testing protocols with real-world outdoor data, could significantly expedite the development process.

Advanced encapsulation technologies and interface engineering are also vital components of future research. Developing advanced encapsulation materials and methods tailored for PSCs, focusing on reducing processing temperatures, improving barrier properties, and enhancing adhesion, is paramount [8,10]. This includes investigating dual edge seals and flexible barrier materials for long-term outdoor applications, alongside integrating thermal management and effective lead sequestration within encapsulation packages [10]. Simultaneously, further optimizing interfaces within devices to suppress ion migration and reduce recombination losses is crucial for both efficiency and stability [8]. The development of robust transport layers resistant to halide migration and iodide/polyiodide redox reactions is highlighted as a critical future direction [4].

Ultimately, the field should pursue specific quantitative stability benchmarks. This involves aiming to achieve a specific $T\_{80}$ lifetime (e.g., >1000 hours) under ISOS-1-like conditions for 2D perovskite devices [7,8], or to achieve a PCE/stability ratio that surpasses current 3D perovskite benchmarks by a certain percentage. This ambitious goal necessitates a holistic approach that integrates multiple strategies. For instance, future research should simultaneously explore novel 2D perovskite compositions with enhanced intrinsic stability, develop advanced interfacial passivation layers that reduce charge recombination under illumination, and implement robust, scalable encapsulation techniques, aiming for a target device lifetime (e.g., $T\_{80}$ > 25 years under ISOS-1-like conditions). This integrated framework, combining material design, interface engineering, and advanced encapsulation strategies, is crucial for realizing the full potential of 2D perovskite photovoltaics in outdoor applications.

In summary, the future of 2D perovskite photovoltaics for outdoor stability lies in synergistic research efforts. While consensus exists on the importance of intrinsic material stability, advanced characterization, and robust encapsulation, the concept of "bulk incorporation" of 2D layers represents a significant divergence from earlier surface-focused strategies, offering superior ion confinement and stability. The scientific basis for this difference lies in its ability to mitigate ion migration deep within the bulk film rather than just at the surface. Divergent views on accelerated testing protocols emphasize the need for unified standards that accurately mimic complex real-world conditions, particularly light cycling and meta-stability effects, which have been proven to correlate with outdoor performance [9]. The integration of computational modeling, AI-driven predictive analytics, and rigorous real-world validation will be instrumental in navigating these challenges and accelerating the path towards commercially viable and highly stable 2D perovskite solar cells.

5.1 Remaining Challenges and Limitations

Despite significant advancements in enhancing the outdoor stability of 2D perovskite photovoltaics, several formidable challenges and limitations persist, necessitating targeted research efforts. A fundamental challenge lies in the current lack of a comprehensive understanding of 2D perovskite degradation mechanisms, which often leads to a "trial-and-error" approach in research rather than a design-driven methodology [8]. This intrinsic limitation hampers the systematic development of robust materials and devices.

Intrinsic Material Property Limitations: One critical challenge stems from the intrinsic material properties of 2D perovskites. For instance, mono-ammonium based 2D perovskites inherently suffer from instability under photothermal aging conditions, leading to decomposition and eventual device failure [4,11]. While di-ammonium based 2D perovskites offer enhanced structural stability and improved blocking of iodide migration [4,5], they are not without drawbacks. These di-ammonium species can induce interstitial defect formation within the underlying 3D perovskite layer, thereby increasing non-radiative recombination and impacting overall device performance [4,5]. Furthermore, the vacuum-induced degradation of 2D perovskites, characterized by the formation of metallic lead and loss of organic cations, presents a significant challenge during fabrication or characterization, with the precise degradation chemistry requiring further elucidation [6]. Another trade-off exists between power conversion efficiency (PCE) and stability; while 2D perovskites exhibit improved stability, their broader optical bandgap and limited charge transport properties often result in lower efficiencies compared to 3D counterparts, especially when used as the sole light-absorbing material [1,2,3]. Overcoming this inherent PCE/stability trade-off is crucial for widespread application.

Extrinsic Environmental Factor Challenges: External environmental stressors contribute significantly to the degradation of 2D perovskite photovoltaics. Achieving long-term operational stability under diverse environmental conditions, including humidity, oxygen, and varying light spectra, remains a persistent hurdle [5,7]. The degradation pathways under these combined stresses are complex and not yet fully understood [7,8].

Processing and Device Architecture Limitations: Challenges also arise from processing limitations and device architecture. Achieving and maintaining phase purity and controlling crystal orientation during deposition are vital for device performance and stability, yet these factors can be adversely affected by processing and environmental conditions [8]. Furthermore, developing transport layers and electrodes that are highly resistant to halide migration and subsequent redox reactions is critical [4,5]. The integration of 2D perovskites into solar cell architectures without compromising overall device stability is also a continuing challenge [2]. For instance, while 2D/3D heterojunctions show promise, they exhibit drastic degradation under harsh conditions, highlighting the need for robust interface engineering [11]. Even with proposed solutions like incorporating 2D-di into the 3D bulk to achieve improved lifetimes (e.g., $T\_{80}$ > 560 hours under harsh conditions), further improvements are necessary to meet commercial operational longevity standards [11]. The scale-up of 2D perovskite fabrication faces hurdles such as achieving uniform film deposition, particularly with large organic cations, and issues with solvent compatibility during large-area processing, which can affect the consistency and quality of the perovskite film.

Characterization and Testing Limitations: A significant limitation highlighted across the literature is the inadequacy of current accelerated testing protocols to reliably predict real-world performance [7,8,9]. This stems from meta-stability effects and the fact that degradation pathways revealed by constant light aging often differ significantly from those observed under dynamic light cycling conditions, making standard silicon PV evaluation routines unsuitable for perovskite solar cells [9]. Furthermore, inconsistencies in material and device preparation, coupled with varying testing conditions, lead to contradictory findings regarding the relative stability of different 2D perovskite types (e.g., Ruddlesden-Popper vs. Dion-Jacobson phases) [8]. This divergence underscores the critical need for standardization of experimental details, characterization techniques, and testing protocols to ensure reproducible and comparable results across the field [8]. Demonstrating stability under stringent, commercially relevant conditions, such as damp heat and prolonged outdoor exposure, remains a significant challenge [8].

Encapsulation Challenges: Effective encapsulation is paramount for outdoor stability, yet presents its own set of challenges. The requirement for encapsulation materials with extremely low water vapor transmission rates (WVTR), ideally below $10^{-5} \text{ g/m}^2\text{ day}$, is difficult to achieve [10]. Additional challenges include the need for reduced processing temperatures for encapsulation, improved adhesion to perovskite surfaces, enhanced mechanical and thermal properties of the encapsulant, and ensuring a lack of reactivity with perovskite components [10]. The development of flexible barrier materials capable of ensuring long-term lifetimes, along with integrating functionalities such as thermal management and lead containment without compromising device performance, remains a critical area for research [10]. Simple epoxy encapsulation, while convenient for laboratory purposes, has been deemed unsuitable for rigorous outdoor or damp heat testing due to its poor mechanical properties and should be phased out for serious stability studies [10].

Future Research Directions: To overcome these multifaceted challenges, several specific research directions are critically needed. First, a dedicated focus on systematic structure-property-stability correlations is essential to move beyond the current "trial-and-error" research paradigm. This involves detailed investigations into how the choice of spacer cation in 2D/quasi-2D perovskites influences their stability under various stress conditions, necessitating fundamental understanding rather than empirical observation [8]. Reconciliation of contradictory findings in the literature requires the establishment and adoption of standardized testing protocols, including the refinement of accelerated testing methods to better mimic real-world dynamic conditions, such as light cycling, rather than just constant light exposure [9].

Furthermore, the development of novel materials resistant to multiple degradation factors is crucial. This can involve designing new organic cations with enhanced photothermal stability, or incorporating robust inorganic layers that block halide migration without inducing detrimental interstitial defects. Leveraging insights from other fields holds significant promise: polymer science can contribute to advanced encapsulation strategies by developing materials with ultra-low WVTR, superior adhesion, and reduced processing temperatures [10]. Similarly, materials science can provide solutions for developing hydrophobic coatings and designing self-healing materials that actively mitigate degradation. A holistic approach that integrates advancements in passivation, transport layers, and electrode materials is necessary to achieve robust, long-term stability [4]. Finally, research must continue to bridge the efficiency-stability gap, potentially through novel 2D/3D architectures that optimize both charge transport and environmental resilience while ensuring scalability for manufacturing processes.

5.2 Emerging Trends and Research Opportunities

Future research in 2D perovskite photovoltaics must strategically address the identified challenges to significantly enhance outdoor stability and operational longevity. A consensus among researchers points towards a holistic approach that integrates advancements across material design, characterization, device architecture, and standardized testing protocols [1,7,8].

One paramount direction involves developing more robust 2D perovskite chemistries, specifically by exploring novel material designs that combine improved intrinsic stability with high power conversion efficiency (PCE). This includes systematic structure-property studies, investigating a wider range of unexplored spacer cations and halide compositions [1,2,7,8]. For instance, the successful use of large, hydrophobic organic cations like phenylethylammonium (PEA) to create moisture-stable 2D perovskite structures, combined with process optimization to minimize hysteresis and improve PCE, exemplifies a promising material design strategy [3]. Further research should delve into specific organic cation modifications to enhance hydrophobicity, coupled with computational modeling to predict long-term behavior, thereby accelerating the discovery of superior compositions [1,2]. This interdisciplinary approach, integrating computational modeling with empirical studies, is crucial for achieving significant breakthroughs. The role of halogen bonding in enhancing stability also warrants further investigation [8]. Additionally, compositions utilizing less volatile A-site cations, such as CsFA, are promising for enhanced intrinsic stability [4].

A critical emerging trend is the "bulk incorporation" strategy, where 2D-di perovskites are integrated into the 3D bulk rather than merely as surface passivation layers [4,5,7,8,11]. This innovative approach has shown substantial improvements, confining mobile ions and suppressing cation phase segregation, leading to extrapolated $T\_{80}$ lifetimes exceeding 1100 hours under 85 °C and 1-sun conditions [4]. Future research should focus on optimizing 2D-di structures and comprehensively investigating their impact on grain boundary passivation and ion transport within the 3D matrix. This includes understanding and mitigating interstitial iodide defects ($I\_i$) that may form in 3D/2D-di systems, with the ultimate goal of achieving superior PCE-stability performance compared to current benchmarks [4].

Beyond material design, advanced characterization techniques are indispensable. The development and implementation of advanced in-situ and dynamic characterization methods for real-time degradation monitoring are crucial for gaining deeper insights into degradation pathways and material transformations under operational stress [7,8]. This includes employing consistent methodologies and multiple techniques for studying ion migration and investigating the exact chemistry of vacuum-induced degradation to develop effective mitigation strategies [6,8].

A critical aspect for validating outdoor stability is the development and validation of standardized and rigorous accelerated testing protocols. Current research highlights the necessity of these protocols to accurately mimic real-world light cycling and meta-stability effects, accounting for diurnal performance variations [4,8,9,10]. The correlation between light-cycled experiments and outdoor results emphasizes this direction [9]. A specific research aim should be to "Develop a unified standard for accelerated outdoor testing that accurately replicates the combined effects of UV irradiation, humidity cycles (e.g., 85% RH/25°C for 1000h), and thermal cycling (e.g., -40°C to +85°C) for 2D perovskite devices, validated against real-world outdoor performance data." Furthermore, integrating AI-driven predictive models for perovskite degradation, by combining experimental data from various accelerated testing protocols with real-world outdoor data, could significantly expedite the development process.

Advanced encapsulation technologies and interface engineering are also vital components of future research. Developing advanced encapsulation materials and methods tailored for PSCs, focusing on reducing processing temperatures, improving barrier properties, and enhancing adhesion, is paramount [8,10]. This includes investigating dual edge seals and flexible barrier materials for long-term outdoor applications, alongside integrating thermal management and effective lead sequestration within encapsulation packages [10]. Simultaneously, further optimizing interfaces within devices to suppress ion migration and reduce recombination losses is crucial for both efficiency and stability [8]. The development of robust transport layers resistant to halide migration and iodide/polyiodide redox reactions is highlighted as a critical future direction [4].

Ultimately, the field should pursue specific quantitative stability benchmarks. This involves aiming to achieve a specific $T\_{80}$ lifetime (e.g., >1000 hours) under ISOS-1-like conditions for 2D perovskite devices [7,8], or to achieve a PCE/stability ratio that surpasses current 3D perovskite benchmarks by a certain percentage. This ambitious goal necessitates a holistic approach that integrates multiple strategies. For instance, future research should simultaneously explore novel 2D perovskite compositions with enhanced intrinsic stability, develop advanced interfacial passivation layers that reduce charge recombination under illumination, and implement robust, scalable encapsulation techniques, aiming for a target device lifetime (e.g., $T\_{80}$ > 25 years under ISOS-1-like conditions). This integrated framework, combining material design, interface engineering, and advanced encapsulation strategies, is crucial for realizing the full potential of 2D perovskite photovoltaics in outdoor applications.

In summary, the future of 2D perovskite photovoltaics for outdoor stability lies in synergistic research efforts. While consensus exists on the importance of intrinsic material stability, advanced characterization, and robust encapsulation, the concept of "bulk incorporation" of 2D layers represents a significant divergence from earlier surface-focused strategies, offering superior ion confinement and stability. The scientific basis for this difference lies in its ability to mitigate ion migration deep within the bulk film rather than just at the surface. Divergent views on accelerated testing protocols emphasize the need for unified standards that accurately mimic complex real-world conditions, particularly light cycling and meta-stability effects, which have been proven to correlate with outdoor performance [9]. The integration of computational modeling, AI-driven predictive analytics, and rigorous real-world validation will be instrumental in navigating these challenges and accelerating the path towards commercially viable and highly stable 2D perovskite solar cells.

References

[1] Advancing 2D Perovskites for Efficient and Stable Solar Cells: Challenges and Opportunities https://pubmed.ncbi.nlm.nih.gov/34668250/

[2] Exploring 2D perovskite chemistry for advancing efficient and stable solar cells https://journal.hep.com.cn/fie/CN/10.1007/s11708-025-0997-1

[3] Perovskite Stability Gets a 2D Solution - Advanced Science News https://www.advancedsciencenews.com/perovskite-stability-gets-a-2d-solution/

[4] Revealing degradation mechanisms in 3D/2D perovskite solar cells under photothermal accelerated ageing - Semantic Scholar https://pdfs.semanticscholar.org/d31e/472737b044a5a6bc74ac6b69cd3f619a1413.pdf

[5] Revealing degradation mechanisms in 3D/2D perovskite solar cells under photothermal accelerated ageing - Energy & Environmental Science (RSC Publishing) DOI:10.1039/D4EE03869J https://pubs.rsc.org/en/content/articlehtml/2024/ee/d4ee03869j

[6] Vacuum-Induced Degradation of 2D Perovskites - Frontiers https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2020.00066/full

[7] Understanding Degradation Mechanisms and Improving Stability of Perovskite Photovoltaics - University of Colorado Boulder https://www.colorado.edu/lab/mcgehee/sites/default/files/attached-files/caleb\_review\_paper.pdf

[8] Stability of 2D and quasi-2D perovskite materials and devices https://d-nb.info/1273561732/34

[9] Light cycling as a key to understanding the outdoor behaviour of perovskite solar cells https://pubs.rsc.org/en/content/articlelanding/2024/ee/d3ee03508e

[10] Encapsulation and Stability Testing of Perovskite Solar Cells for Real Life Applications https://pmc.ncbi.nlm.nih.gov/articles/PMC9888620/

[11] Revealing degradation mechanisms in 3D/2D perovskite solar cells under photothermal accelerated ageing - Energy & Environmental Science (RSC Publishing) https://pubs.rsc.org/en/content/articlelanding/2024/ee/d4ee03869j