Focus on Grand Challenges in Halide Perovskites: Stability

Graphitic carbon nitride based heterojunction photocatalysts have gained increasing attention in producing the clean energy source of hydrogen. Coupling carbon nitride (g-C₃N₄) with other semiconductor materials or metals as co-catalysts is considered as an effective strategy to overcome the drawbacks of g-C₃N₄ such as the quick recombination of photogenerated charges. In this review, the recent research advancements in the construction of g-C₃N₄-based heterojunctions as well as their different charge separation/transfer mechanisms will be systematically discussed, making special emphasis on the design and fabrication of type-II, Z-scheme, S-scheme and Schottky heterojunctions and their application towards H₂ generation from water splitting. Finally, a summary and some crucial issues, which should be further resolved for developing advanced g-C₃N₄-based heterojunction photocatalysts, are presented.

Astounding progress in achieved power conversion efficiencies of solar cells based on metal-halide perovskite semiconductors has been achieved. Viable assessment of the long-term device performance is, therefore, now the most critical aspect to reliably predict device's long-term performance. Standard testing protocols to enable cross-laboratory comparison need to be established and adopted. Apart from protocols targeting the assessment of device performance and stability, procedures to investigate potential meta-stabilities in devices under different operation conditions are required to describe degradation mechanisms. This understanding will guide further optimization of materials and devices. In this perspective, we emphasize the importance of wide-spread reporting of experimental data in common databases to keep track of the state-of-the-art of perovskite solar cell performance and stability achieved.

Organic-inorganic perovskite materials are attracting increasing attention for their use in high-performance solar cells due to their outstanding properties, such as long diffusion lengths, low recombination rate, and tunable bandgap. Finding an effective method of defect passivation is thought to be a promising route for improvements toward narrowing the distribution of the power conversion efficiency (PCE) values, given by the spread in the PCE over different devices fabricated under identical conditions, for easier commercialization. In this work, we add 2‐(4‐fluoroph-enyl)ethyl ammonium iodide (p-f-PEAI) into the bulk of a mixed cation lead halide perovskite (CH₃NH₃PbBr₃)_0.15(HC(NH₂)₂PbI₃)_0.85 thin film. We investigate the influence of different p-f-PEAI concentrations on the optical properties, morphology, crystal orientation, charge carrier dynamics, and device performance. We observe that introducing the proper amount of p-f-PEAI changes the preferential orientation of the perovskite crystals, promotes the strength of the crystal textures, and suppresses non-radiative charge recombination. Thus, we obtain a narrower distribution of the PCE of perovskite solar cells (PSCs) without sacrificing the PCE values reached. This is an important step toward better reproducibility to realize the commercialization of PSCs.

Organometal halide perovskite solar cells (PSCs) have emerged as promising candidates for next-generation thin-film solar cells. Over the past ten years, the efficiency of PSCs has increased from 3.8% to over 25% through the optimization of the perovskite film formulation and the engineering of suitable fabrication strategies and device architectures. However, the relatively poor long-term device stability, which has not been able to exceed some hundreds of hours until now, represents one of the key aspects still hampering their widespread diffusion to commercial contexts.

After briefly introducing the origin and basic mechanisms behind PSC degradation and performance decline, a systematic outline and classification of the available strategies to improve the long-term stability of this class of photovoltaic devices will be presented, mainly focusing on encapsulation procedures. Indeed, the aim of this review is to offer an in-depth and updated account of the existing encapsulation methods for PSCs according to the present understanding of reliability issues. More specifically, an analysis of currently available encapsulation materials and on their role in limiting the penetration of UV light and external agents, such as water vapour and oxygen, will be proposed. In addition, a thorough discussion on various encapsulation techniques and configurations will be presented, highlighting specific strengths and limitations of the different approaches. Finally, possible routes for future research to enhance the effectiveness of the most performing encapsulation procedures will be suggested and new paths to be explored for further improvements in the field will be proposed.

With a record efficiency above 25%, the main hurdle for the commercialization of perovskite solar cells (PSCs) is their long-term operational stability. Although different strategies have been applied, the stability of PSCs is still far below the 25 year requirement demonstrated by commercial photovoltaic technologies. To advance in the former, a lab-scale stability analysis should resemble real testing conditions, and this is only possible through the interaction of several stress factors. Here, we briefly introduce the reader to the general degradation mechanisms observed on PSCs and the state-of-the-art strategies applied to realize long-term stable devices. Finally, we highlight the imperative need to engineer multiple components of the PSCs simultaneously and propose a rational design of PSC's constituents to obtain long-term operational solar cells. This perspective article will benefit the progression of PSCs as a reliable photovoltaic technology.

The presence of lead in novel hybrid perovskite-based solar cells remains a significant issue regarding commercial applications. Therefore, antimony-based perovskite-like A₃M₂X₉ structures are promising new candidates for low toxicity photovoltaic applications. So far, MA₃Sb₂I₉ was reported to only crystallize in the 'zero-dimensional' (0D) dimer structure with wide indirect bandgap properties. However, the formation of the 2D layered polymorph is more suitable for solar cell applications due to its expected direct and narrow bandgap. Here, we demonstrate the first synthesis of phase pure 2D layered MA₃Sb₂I₉, based on antimony acetate dissolved in alcoholic solvents. Using in situ XRD methods, we confirm the stability of the layered phase towards high temperature, but the exposure to 75% relative humidity for several hours leads to a rearrangement of the phase with partial formation of the 0D structure. We investigated the electronic band structure and confirmed experimentally the presence of a semi-direct bandgap at around 2.1 eV. Our work shows that careful control of nucleation via processing conditions can provide access to promising perovskite-like phases for photovoltaic applications.

Photovoltaic technology is progressing very fast, both in a new installed capacity, now reaching a total of more than 400 GW worldwide, and in a big research effort to develop more efficient and sustainable technologies. Organic and hybrid solar cells have been pointed out as a technological breakthrough due to their potential for low economical cost and low environmental impact; but despite impressive laboratory progress, the market is still beyond reach for these technologies, especially for perovskite-based technology. In this review, the historical evolution and relationship of efficiency and stability is addressed, including Life Cycle Assessment studies which provide a quantitative evaluation of environmental impacts in several categories, such as human health or freshwater ecotoxicity, with special focus on lead toxicity. The main conclusion is that there is no unsurmountable barrier for the massive deployment of photovoltaic systems with perovskite solar modules, if the stability is extended to lifetimes similar to technologies already in the market. The results of this review provide some recommendations mainly focused on the best options for improved stability (avoiding mainly moisture and oxygen degradation) by using metal oxides, ternary or quaternary cations, or the novel 2D/3D approach, and the encapsulation effort which should also take into account the recyclability of the materials and the low environmental impact processes for up-scaled industrial production. Research guidelines should take into account the end-of-life of the devices and cleaner routes for production avoiding toxic solvents.

Power conversion efficiencies of lead halide perovskite solar cells have rapidly increased in the decade since their emergence, reaching 25% this year. However, reliable film uniformity and device stability remain hard to achieve and often require precise compliance with complicated protocols, which hampers upscaling towards industrial applications. Here, we explore the potential of an alternative route towards high-quality perovskite films: The reaction between a pre-existing perovskite film and methylamine (MA) gas has been shown to possess the striking ability to both improve film morphology and increase grain size drastically, boosting device performance. This post-deposition treatment could provide the means to decouple film quality from the initial deposition process, thus promising to facilitate upscaling and lowering production costs. Furthermore, such MA gas treatments show great promise regarding the stability of fabricated devices, as they open up the opportunity to reduce or even eliminate the adverse role of grain boundaries in film degradation.

Perovskite solar cells (PSCs) will eventually operate outdoors, subjected to diurnal cycles with varying irradiance and cell temperature throughout 24 h periods. Hereby, we show the PSC stability results from laboratory accelerated stress tests can not obviously suggest their stability in outdoor-like situations. Thus, to validate PSC outdoor stability, it is necessary to emulate outdoor conditions, for which we propose possible test scenarios.

The stability of perovskite photovoltaics is a very hot topic nowadays because a long lifetime of the devices is one of the main conditions required for the commercialization of perovskite technologies. Although a lot of research on stability has been done on small solar cells, there is not too much information about the stability of the modules. A very important question arises: whether the stability of the modules is different from the stability of the individual cells. With the future goal of scale-up perovskite PV technology, it is important to understand possible the degradation mechanisms which are specific to the modules. This perspective article highlights the stability issues, which are unique to modules but not observed in small cells.

This short review aims at summarizing the current challenges related to poor Perovskite Solar Cells (PSCs) stability which nowadays puts severe constrains on near future device commercialization. As a game changer in the field of photovoltaics (PVs), PSCs are highly efficient and cheap to fabricate. However, they suffer from poor long-term stability upon exposure to heat, moisture, oxygen and light, and combinations thereof. Poor device stability originates from intrinsic instability issues of the perovskite active layer itself, as well as extrinsic factors due to partial degradation of the layers composing the device stack. Here we briefly review the chemical and physical processes responsible for intrinsic material instability, and we highlight possible solutions to overcome it; we then consider the whole device, discussing properties and interactions of the stacked layers. Finally, particular emphasis is placed on the need of shared standards for stability tests, which should include detailed report on experimental conditions over a statistically significant number of samples, allowing for a direct comparison of results across different groups and fostering a rapid advance of our understanding of degradation mechanisms and of the solutions to overcome them.

With the ferroelectric nature of modern perovskite solar cells being more and more accepted by the community, new questions arise. How do the microscopic electric fields within the polar domains affect the device performance, and how must measurement routines be adapted to account for the ferroelectric effect within the light-harvesting layer? This becomes particularly important, if devices are measured constantly for a long time as commonly performed in solar cell ageing tests. In this perspective article, we discuss which effects may arise from creeping poling even under low driving voltages or under illumination, as well as effects from phase transitions when crossing the Curie temperature for accelerated ageing at elevated temperatures. We elucidate why ferroelectric effects must be carefully considered when assessing the lifetime of perovskite solar cells and where comparability comes to its limits.

The present situation with respect to current–voltage measurement standards for metastable photovoltaics, including perovskites, is discussed. New draft updates to the IEC 60904-1 standard do not fully capture the needs of metastable devices. A new document within the 60904 series capturing the academically favoured SPO and MPPT methods would go a long way toward solving the present ambiguity, however the lack of an effective stabilisation procedure remains the greatest hurdle for perovskite PV.

Organometallic lead-halide solar cells exhibited immense potential over the past years and reached the transition point from lab to industry-scale fabrication. However, bridging this gap and establishing perovskites as a viable competitor to conventional Si-based photovoltaics, hinges on the success of cost-effective upscaling process. The key factor impeding this transition is operational stability of solar cells under realistic photoconversion conditions. To this extent, reducing the dimensionality of cell constituents appears as a promising and very attractive approach to tackle this issue. The beneficial influence of such materials on device stability, which is explicitly tied to the engineered interface quality with underlying layers, comes as a result of complex interplay between energy alignment, strain-induced interactions and barrier-like properties of 2D components. The aim of this perspective is to briefly outline key challenges regarding the exploitation of 2D materials within the framework of perovskite photovoltaics, as well as to suggest further development directions.