Stability in Perovskite Photovoltaics: A Paradigm for Newfangled

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Perspective

Stability in Perovskite Photovoltaics: A Paradigm for Newfangled Technologies Jeffrey A Christians, Severin N. Habisreutinger, Joseph J. Berry, and Joseph M. Luther ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Energy Letters

Stability in Perovskite Photovoltaics: A Paradigm for Newfangled Technologies Jeffrey A. Christians, Severin N. Habisreutinger, Joseph J. Berry, Joseph M. Luther

National Renewable Energy Laboratory, Golden, CO 80401, USA

Abstract: While halide perovskites embody some of the most ideal photovoltaic properties, the breakthrough into the commercial marketplace is still uncertain and high risk. The major technical hurdle that still must be overcome is durability. This perspective lays out our view of the ultimate needs for perovskite solar cell stability research to flourish. We outline a paradigm for conceptualizing stability as a tiered system of material, cell, and module. In cell-level studies in particular, we propose the adoption of transferrable and flexible testing protocols. We believe that the adoption of standard testing protocols could dramatically improve the translatability of insights between laboratories working in this area worldwide; however, improved protocols alone will by no means replace the need for mechanistic insight into degradation mechanisms at all levels of perovskite solar cell stability. We believe that knowledge of degradation science is what will ultimately push perovskite solar cell modules from promise to production.

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When Chapin, Fuller, and Pearson reported on the first practical Si solar cell in 1954,1 they characterized their solar cell with limited knowledge of the solar spectral distribution and an incomplete description of the solar radiation incident on the cell. As the field developed over the coming decade, it became clear that characterization of photovoltaic devices had to be performed in such a way that it was broadly transferrable across locations and laboratories and that characterization of devices under natural sunlight presented a vast array of complicating variables.2 However, it took over two decades to finally implement common standards for solar cell characterization which are now universally agreed upon, carefully followed, and largely taken for granted by the solar cell research community.3 Like the move toward standard solar cell efficiency measurement protocols, standard stability measurement protocols are required, particularly for new or emerging photovoltaic technologies. This is especially true considering the recent developments in the field of perovskite solar cells (PSCs), the focus of this Perspective. Transitioning a technology from a research project to a commercialized product is often fraught with difficulties. This challenge is described as crossing the technology “Valley of Death” because of its high degree of difficulty. In the laboratory, PSCs have demonstrated performance equivalent to other technologies which have already been commercialized,4 far exceeding the previous performance expectations for a solution-processed photovoltaic (PV). Thus, PSC systems have all the potential in the world to become a major player on the field of power generation. They tick some of the most important boxes for a new PV technology: high performance,5 low cost,6 and terawatt scalability.7 They can be made in many form factors including on low-cost, lightweight flexible substrates or integrated into building façades. They can be processed in myriad ways which enable scalability and integration into unique form factors.7

In the light of all this potential, the question for

researchers, journal editors, reviewers, public funding agencies, venture capitalists, and other interested parties then becomes: what is ultimately required for the promise of this technology to be realized? There are many important issues and associated research questions which must be addressed in PSCs, yet the critical hurdle this emerging PV technology needs to overcome is stability: the undiminished long-term ability to produce power from solar radiation. Device stability has loomed over the entire PSC field dating back to its relatively modest inception,8,9 and has emerged as the primary question given the demonstrations of efficiency and scalability. 2


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ACS Energy Letters

Astounding gains in device stability have been achieved in relatively little time. Just five years ago state-of-the-art operational lifetimes of even encapsulated PSCs could be measured in tens of hours,10 whereas unencapsulated operational lifetimes of thousands of hours are now being reported.11–13 Nevertheless, reconciling stability studies to truly understand advances, assess remaining challenges, and determine the readiness of PSC to become a deployed technology is currently problematic.14 Current studies in stability have inconsistencies in reported experimental details and non-standardized test conditions which, although rational in the context of any one study, make comparison difficult if not impossible. Addressing these issues as a field will be critical as the community moves to tackle the challenges of accelerated testing, activated behavior, and real-world stability. Advances are being made, yet the current state of PSC stability in many ways mirrors the initial development of solar cells during the 1950s, 1960s, and 1970s. The wide array of measurement protocols confounds the advances being made and limits the transferability of research developments.14 Our aim in this Perspective is to discuss (a) challenges currently facing the PSC community, (b) the value choices and goals related to stability testing, (c) the implementation of stability test protocols, and (d) the development of accelerated testing protocols directly linked to real-world stability. We present an outline for a tiered definition of stability we hope will aid researchers thinking about stability research and in designing stability studies. We discuss the context which requires rigorous stability testing regimes and stability analysis with a focus on how researchers can proceed forward to provide valuable insight that results in de-risking of the technology and ultimately in commercialization. In fact, in the past, the PSC field experienced similar conceptual problems when initially observing the hysteretic behavior in the current-voltage sweeps of perovskite solar cells which introduced ambiguity in the determination of the actual power conversion efficiency of the devices.15 The research community was able to largely address the issue by arriving at consensus standard testing and reporting protocols.16 Our hope is for a similar actionable outcome in the case of PSC stability so that in the end the stability research being conducted in laboratories around the world will propel PSCs across the “Valley of Death” and toward commercial viability.

Defining stability. In some ways, the biggest challenge is that thus far there is not a universal definition of the concept of stability in the perovskite field. This definition can range from the 3



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structural integrity of the absorber alone, merely the device performance in shelf-life studies, or the ability of a packaged device to produce power over time. Ultimately, stability ought to be a metric which quantifies the operational lifetime of a photovoltaic device under typical real-world conditions. In practice, this would be determined by the decrease of the ability of the device to perform its primary function (i.e., produce power) over time. With this definition, a commercial product could be characterized by essentially two metrics: nominal power output (W) and stability (∆PCE/time, PCE = power conversion efficiency). During the current transition phase from a laboratory technology toward commercial products, this end-goal metric should be considered but is perhaps too ambitious and overly restrictive at the expense of other insightful quantities. Nevertheless, ∆PCE/time can serve as reference point for a more practical definition suited for the current stage of PSC development. The instability of a perovskite PV module can arise from a hierarchy of major sources. 1. Material: Component materials and interfaces (perovskite absorbers, charge selective contacts, electrodes) 2. Cell: Operating conditions and stresses in devices (working PV cells under stress) 3. Module: Module and packaging (including encapsulation, interconnects, etc.)

This list reflects both the order of importance and of vulnerability. That is, without an understanding of the stability of the constituent materials and operating cells under stress, efforts to understand the stability at the module level will be futile as these define the operational requirements for the module package. Consequently, the need for scientific insight and understanding scales accordingly. The implication of this hierarchy, shown schematically in Figure 1, is that degradation processes and the detrimental interaction of extrinsic factors with the perovskite absorber need to be well characterized to design a device architecture initially, and a module structure eventually, such that real-world operational lifetime can be understood, predicted, and maximized at the module level. This suggests that we need to design stability protocols with different specific foci depending on which aspect of stability is under investigation. Conversely, if the component and operational device level stability is truly understood, the requirements for the packaging can be specified. The dearth of data on perovskite solar cell stability relative to the abundance of data on module packaging also supports this prioritization for the field. 4


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ACS Energy Letters

Figure 1. Scheme representing the tiered nature of stability studies with studies focused on the materials at the base, operating cells built upon these foundational studies, and module-level studies at the pinnacle. The terms at the right represent key aspects of studies at each level. The figures contained in this scheme are emblematic of the type of work at each level. At the material level are depicted studies by Hoke et al. (bottom left), adapted from reference 17, published by The Royal Society of Chemistry, and by Yang et al. (bottom right), adapted with permission from reference 18. Copyright 2015 American Chemical Society. At the cell level is depicted a study by Christians et al., adapted from reference 12, with permission by Springer Nature. The photograph under the module level is of solar cell modules under outdoor test at the NREL outdoor test facility. Level 1 stability: Material The materials at the core of devices can be investigated for their resilience when exposed to relevant stressors such as heat, (UV-) light, moisture, oxygen, mechanical stress and combinations thereof. There is a wide parameter space of testing at this level within which valuable insights can be obtained through various tests with different levels of sophistication. The main objective at this level is to obtain important basic insight on the operational 5



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ranges and limitations of the materials themselves, as well as the underlying mechanism of degradation. This type of stability testing

does not necessitate strict standardized testing

conditions because the focus of these tests is to elucidate and probe the material specific stability regimes as well as fundamental degradation mechanisms. The tests can therefore be tailored to answering material specific questions rather than complying with the operating regime of solar cell. Of course, how these studies correlate to the full device stack can be tenuous, placing a premium on insight rather than simple demonstration. Furthermore, given the relative simplicity of the samples, a wide range of metrics can be used to assess the impact of the stressing process on the investigated material (i.e., absorption spectroscopy, X-ray diffraction, photoluminescence, microscopy, etc.), as depicted in Figure 1. The results can be generalized across systems, thereby increasing the impact of such studies, if undertaken in a rigorous manner. For example, studies providing insight into the process of photoinduced halide segregation in mixed halide perovskites have contributed significantly to the field’s understanding of these materials17,19,20 which has altered and informed the approach to developing stable wide band gap absorber materials for tandem solar cells.21,22

Level 2 stability: Cell Akin to the requirement of standardized testing conditions for determining the powerconversion efficiency of devices mentioned at the outset of this manuscript, cell level stability needs to be tested under a set of standardized conditions. We can probe material photophysics under a wide array of conditions (ultrafast laser spectroscopy, cryogenic temperatures, etc.) which provides invaluable understanding of device operation, but we can only accept reports of device efficiency which are obtained using standard testing conditions (calibrated white light illumination, apertured device area, etc.). As a field, we should progress toward viewing operational device stability through a similar lens. For cell level stability tests, standardized test conditions are absolutely necessary for direct interlaboratory comparisons and to establish benchmarks. When standardized tests are followed, improvements relevant to the entire field can be achieved and tracked. This will allow the field to establish indicators of the maturation of the technology (e.g., t80 > 1 year1), which subsequently allow a direct, comparison and assesment of different cell architectures. Naturally, 1

t80 is the time at which a solar cell operates at 80% of its initial performance 6


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there will be an unavoidable variability in the results from different institutions and setups, however, standardization can minimize the number of unconsidered factors, and thus establish fundamental trends. This will enable, researchers to be able to more clearly define the appropriat e design parameter space for future improvements regardless of the increase in complexity at the device level due to the emergence of multiple interdependent, coupled degradation pathways.

The ability to bring relevant analytical techniques to bear at this level of sophistication becomes significantly more difficult due to technical aspects associated with device integration. For example, the interfaces can become more important but are less accessible. Here insight from Level 1 studies can and should inform these Level 2 studies where scientific understanding can enable the identification of specific degradation mechanisms within the device stack. However, it is worth emphasizing that Level 2 stability is qualitatively very different from Level 1 stability, because it places a premium on strict rigor but in return can produce tangible metrics.

Level 3 stability: Module Module stability tests aim for real-world deployment, where the focus lies on eliminating primary mechanisms as determined from Level 1 and Level 2 studies. Here, the primary failures will be external components such as interconnects and encapsulation materials ability to isolate the device from key extrinsic factors. At this stage, degradation processes should be largely known and understood at the lower levels to devise strategies for packaging design and such that the modules can withstand the conditions of field operation for the targeted application. For example, the choice of glass versus polyethylene terephthalate (PET) substrates must be made with knowledge of application conditions, water vapor transmission rate, and water-mediated degradation mechanisms. Breaches in packaging can also result in activated degradation modes. Moreover, it is at this stage of development that durability tests must be designed which stress the weakest links of the module stability and ideally to translate test performance to the realworld stability definition required by a commercial product (∆PCE/time). Testing at the module level is required to manage the capital risks of PV deployment and their associated performance 7



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warranties for the successful transition of technology to market. Deployment of PV technologies at scale necessitates large capital investment which is only feasible with rigorous understanding of the underlying science of PV module stability and failure: failure modes, failure consequence, failure time, failure cause, and failure scale.23

Goals of Stability Testing. Perhaps the largest hurdle to date for PSC stability testing has been the myriad and diverse goals of testing, closely followed by the functional constraints of stability tests. While an ideal situation would be to perform a simple quick test which would provide certainty of how the device will work after >30 years in the field, this already presumes that we normalize over the various real-world test data at various locations and climates and have data collected on timescales of many years. While the former is laudable, clearly the second is untenable, at least in the near-term. At this stage in the technology development, value choices must be made by researchers as to the most applicable stability tests for a given study. When designing experiments, researchers must carefully evaluate a variety of competing goals, such as: (i) the desire for an experiment to accurately approximate real-world conditions against the need for reduced experimental complexity and a limited number of uncontrolled variables; (ii) the need to test a specific hypothesis against the desire to utilize existing infrastructure and equipment; (iii) timely data acquisition against the desire to demonstrate long-term stability. Invariably, individual researchers will arrive at different decisions when confronted with these and other considerations during the experimental design. The emphasis placed on each aspect of experimental design can diverge widely based on the specific circumstances of the researcher or group (existing equipment/facilities, funding, expertise, established collaborations, etc.), and the hypotheses or goals implicit in the study at the outset (thermal stability, moisture stability, photostability, temperature cycling, etc.). These types of value choices are embedded in all aspects of scientific research, they are both inescapable and implicit in every experiment. This reality is not in and of itself a failure or shortcoming, in fact, in many cases the perspectives or perceived limitations of research groups uncover new experimental techniques, analyses, and insights. Nevertheless, understanding and acknowledging the choices that are made is critical to the experimental design process. In the context of PSC stability, tailoring experiments such that they explore the unique and relevant characteristics of materials, and elucidate the processes underlying material degradation can be an enormous asset if researchers report their value 8


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ACS Energy Letters

judgements carefully and in great detail to ensure that their results can be understood in the proper context and allow straightforward reproducibility such that they provide general insights which move the entire field towards the common goal of inexpensive clean energy.

Transferrable and Flexible Stability Testing Protocols. While there are no perfect stability tests which can be suggested for all studies, it is critical that stability measurements and studies, especially at the cell and module level (Level 2 and Level 3 in Figure 1, are both applicable and accessible to others working toward the same goal. Ideally, a set of stability protocols would be designed which are as broadly transferrable between project and/or group as possible. In the PSC field today, stability testing protocols span nearly the entire gamut of possible testing protocols. This situation makes interlaboratory comparisons essentially impossible. Even compiling research in a coherent framework is fraught with difficulties, and in many cases renders even intra-laboratory comparisons doubtful due to variations in test protocols even within individual research groups.14 Attempting a direct comparison of some of the most notable literature results, shown in Figure 2, quickly reveals the differences and caveats in each measurement which render this direct comparison confounding.

Figure 2. Direct comparison of key perovskite solar cell stability results adapted from data presented in references 11–13,24–26. The data were adapted by digitizing the image files from the publication or by using the original vector image file when possible. While this data is overlaid onto a single plot, we caution that direct comparison of these stability traces is not possible, the different conditions of each experiment must be noted. We have summarized the illumination, 9



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load, atmosphere, and temperature in the table below the plot. MPP indicates maximum power point tracking. While differing test conditions are less of an issue at the level of material stability (Level 1 stability, above), we contend that minimizing the variation of stability tests from the current set to a widely applicable subset of tests is not only necessary for the advancement of the field from material stability (Level 1) to cell stability (Level 2) but eminently possible. Yet, even at the level of material stability, if it is not possible to extrapolate others’ results to one’s own work, the speed of scientific development is dramatically slowed with each group left working essentially independently on complex and multifaceted issues. This major problem is only exacerbated as stability studies increase to higher orders of sophistication. In the organic photovoltaic (OPV) field, the international summit on OPV stability (ISOS) encountered a similar challenge and issued a report outlining a consensus strategy for stability analysis.27 The protocols developed by ISOS outline standard test conditions for common experiments which can significantly improve the current state of the PSC stability literature. While we believe following the ISOS protocols for OPV stability measurements would be a marked improvement over current practices, we consider them mainly a potential starting points for a broader discussion in the PSC community, which may very well be closely modeled on the ISOS conferences with the aim to develop stability protocols and recommendations specific for stability studies on PSCs. Below, we summarize a subset of the ISOS guidelines to specifically highlight standard testing and reporting protocols. In Table 1 we outline the ISOS protocols for testing stability in the dark while Table 2 shows the ISOS protocols for testing stability under illumination. These protocols cover cases ranging from simple shelf-life experiments to significantly more complex experiments which stress devices under constant illumination at their maximum power point (MPP) while exposed to temperature and high relative humidity (RH). We have chosen to reproduce this subset of the ISOS protocols in brief here, but the full protocols and a much more detailed discussion can be found in the original work.27 The ISOS protocols, are organized within three levels of sophistication with Level 1 requiring very little specialized equipment, Level 2 utilizing standard equipment found in many laboratories, and Level 3 demanding highly sophisticated test equipment necessary for complex analysis under rigorous standards. This flexibility allows researchers around the world to conform their experiments to the protocols regardless of their situation or the specific goals of their experiment. 10


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Table 1. Summary ISOS protocols for dark stability testing. Test Conditions

ISOS-D-1

ISOS-D-2

ISOS-D-3

Light source

None (Dark)

None (Dark)

None (Dark)

Load

Open circuit

Open circuit

Open circuit

Temperature

Ambient

65/85 °C

65/85 °C

Atmosphere

Ambient or Inert

Ambient or Inert

85% RH

Table 2. Summary ISOS protocols for laboratory weathering testing. Test Conditions

ISOS-L-1

ISOS-L-2

ISOS-L-3

Light source

Simulator

Simulator

Simulator

Load

Open circuit or MPP

Open circuit or MPP

MPP

Temperature

Ambient

65/85 °C

65/85 °C

Atmosphere

Ambient or Inert

Ambient or Inert

50% RH

This framework can go a long way toward improving reproducibility in the field and coordinating research progress if researchers investigating PSC durability would conform their experiments to these protocols, like the way researchers around the globe have conformed to best practices regarding the reporting of current density-voltage measurements and stabilized power output. Researchers explicitly using the ISOS protocols for stability tests would have the added benefit of allowing other researchers to fully understand their experimental conditions. Eventually, at the commercial module level, PSCs will be required to pass the various International Electrotechnical Commission (IEC) standards for PV module stability. The fact that PSCs have already been demonstrated which pass these strenuous tests is promising for the technology as a whole;28 however, these standards are only of limited value at the current state of PSC development. That is, passing the IEC tests is necessary, yet passing these tests does not directly equate to confidence that the product will survive 30+ years of real-world outdoor operation nor provide the insight into degradation mechanisms necessary to understand outdoor stability and adequately de-risk the technology for commercial investors.

The Need for Mechanistic Insight. While standardized testing conditions could homogenize the stability literature and allow for interlaboratory comparisons, making a plot akin to Figure 2 both feasible and meaningful, it is certainly not all that is required of the field. 11



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Fundamentally, we view the role of those working in this space at academic and national laboratories to be the building of scientific understanding. The role of publicly funded research groups is generally not to develop a product, but to de-risk the development of a product. Ultimately, a scientific understanding of PSC stability will allow, as discussed subsequently, for the design of the appropriate test conditions and reliability tests which will allow companies to say with certainty that their products will perform as expected in the field. While empiricism can be an effective tool for achieving gains, our role as publicly funded researchers is not best suited to empiricism; any moderately well-funded corporation is much better suited to empiricism than an academic research group. For this reason, we urge the researchers, authors, reviewers, journal editors, funding agencies, and others in this burgeoning field to strive for scientific understanding in their PSC stability efforts. A single demonstration of a “stable” device under a single set of test conditions is not sufficient to push the field forward in any meaningful manner. We must begin to uncover where things fail, why things fail, and why things succeed. What we believe the field really needs is hypothesis driven science which is supported by thorough investigation. If a study shows that Device A is more stable than Device B, this is not enough. Researchers must go the extra mile and explore the degradation mechanisms present in Device A which are reduced or mitigated in Device B and then investigate the degradation mechanisms which are still present in Device B to gain a firm knowledge of the system. The exploration of failure conditions, even in the more stable configuration of Device B, can provide the some of the most important and invaluable insights. We believe that standardized testing and reporting conditions are critical to the goal of assessing progress of the field, but they need not be followed in every case in which the focus lies on mechanistic insights. If test conditions are tailored in a way which artificially enhances the “stability” of a device but does not allow for easy comparison to the work of other research groups, this work becomes much less valuable to the broader community. On the other hand, there are times when specific mechanistic insight can be achieved using non-standardized test conditions. As an analogy, when measuring solar cell efficiency, it is the universally accepted standard to use a well-calibrated solar simulator and reference cell, yet when designing a solar cell for use in indoor light one may with good reason deviate from this convention. Deviations which are made from standard solar cell test conditions are only accepted in cases where there is 12


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a sound technical reasoning. A similar approach is required for stability testing. As a community, we should rely mainly on standardized test protocols whenever possible and depart from these standards with sound scientific justification such as exploring unique material properties with relevance for their use in PSCs. If other researchers are unable to compare the results in their own laboratories to published work the impact becomes diminished. Related to this, the scientific community does not accept a single device as evidence for improved performance, statistical significance is demanded and we must do so for stability as well.

Accelerated Testing, Activated Behavior, and Real-World Stability. In addition to understanding degradation and improving stability, a key area where research can make important contributions is in the realm of accelerated testing and activated behavior. Practical commercial modules must have an accurately predicted real-world lifetime to spur capital investment which is, at least for grid-scale PV applications, of a timescale dramatically longer than reliability tests can reasonably be expected to last at the outset of a technology. Additionally, the typically iterative process of improving on a certain design based on previous failures and improved mechanistic understanding becomes a much more feasible endeavor if the iteration frequency is of the order of days or weeks rather than months or years. The question is: can we devise a testing protocol with “enhanced” testing parameters which compresses thousands of hours of testing, into a much shorter time period? It is critical to find accelerated test regimes that produce results which do in fact correspond to “real-world” testing and do not introduce additional degradation pathways which are solely a function of the artificial test conditions. Prior to establishing accelerated testing regimes, it is necessary to understand the degradation mechanisms which couple in devices in ways which may be non-linear and interdependent. If these challenges are met, the demonstration and assessment of 30-year lifetime can be made with a higher degree of certainty in a reduced, perhaps dramatically so, timeframe, but tackling this challenge requires a solid understanding of device-relevant degradation mechanisms so that accelerated degradation tests can stress these most critical factors in ways which accurately predict real-world performance. Moreover, the coupling of multiple intrinsic and extrinsic degradation factors (light and water, oxygen and heat, light and heat, etc.) must be understood in much greater detail than is currently the case. For example, the activated behavior of water vapor requires significant further 13



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investigation and understanding. As discussed previously, while water can be sufficiently encapsulated out by glass-glass encapsulation methods, the water vapor transmission of other substrates, such as PET, or encapsulation strategies may mean that the modules will be exposed to water vapor and/or oxygen during their operational lifetime. While tests in inert atmosphere can be instructive, tests should also expose devices to these extrinsic factors so that their effects can be more fully understood.23

Summary and Future Outlook. PSCs already achieve high power conversion efficiency and recent efforts toward scalability are proceeding rapidly, making stability the biggest remaining hurdle for the technology. The stability gains witnessed in the past few years have been very encouraging for researchers in the PSC community. Devices now last thousands of hours under constant illumination,12,13,25 can withstand UV-illumination,29 are much more resistant to extrinsic degradation factors such as water and oxygen,11,12 and have even been shown to pass many of the common IEC PV module tests.28,30 However, the work is not yet complete. Progress in perovskite solar cell stability necessitates coordinated effort across the many different research groups around the world. The universality of basic scientific understanding is at a premium and is required to lower the risk factors currently associated with this PV technology. The most impactful and useful publications will be those containing clear understanding and transferable knowledge to help others improve the stability of their materials/devices/modules. The adoption of degradation protocols throughout the stability community is an important first step on this path. The broad nature of the ISOS protocols allows researchers to easily fit their current protocols into this framework and would thus greatly improve the translatability of work done in labs around the world. Researchers must constantly assess and reassess their understanding at the various levels of complexity: materials, cells, and modules. Figure 3 depicts a flow chart outlining a paradigm for undertaking stability studies, an approach which we believe is applicable to PSCs as well as all other PV technologies. Under this framework, moving to systems of greater complexity (viz., materials to cells, or cells to modules) is only undertaken once a sound hypothesis can be built upon or as validation of the understanding derived at the lower level(s) of investigation. Moreover, we view this entire process as cyclical and iterative. That is, even once products are 14


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on the market there will be need for continued studies at the material, cell, and module level to further improve product durability.

Figure 3. Flow chart showing best practices for the design of perovskite solar cell stability experiments adhering to the three levels of stability experiments outlined in this Perspective. For PSCs specifically, the major material- and device-level degradation pathways and mechanisms must be understood in more detail and informed by module-level considerations that are still largely unexplored for these devices. The nascent state of the technology makes the interplay between materials processing, degradation mechanisms and device/module design still largely unknown at this point. Demonstrating clear causality, not just correlations, has thus far 15



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been insufficiently shown in the majority of the stability literature, making predictions difficult if not impossible. The coupling of degradation factors is uncharted territory. The impact of device packaging on thermally activated processes, such as organic cation volatility, has not yet been described in detail. While challenging, it is only by confronting these issues that we can move PSCs from an interesting research project to an energy technology that matters.

Acknowledgements: This work was authored by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding was provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office under the Hybrid Perovskite Solar Cell Program. J.A.C. was supported by the DOE Office of Energy Efficiency and Renewable Energy Postdoctoral Research Award through the Solar Energy Technologies Office under DOE contract number DE-SC00014664. S.N.H. acknowledges support from the Director’s Fellowship program of the National Renewable Energy Laboratory. We thank A. Hicks for assistance with the graphics. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. References: (1) (2) (3) (4) (5)

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Author Biographies: Jeffrey A. Christians is an EERE postdoctoral fellow at the National Renewable Energy Laboratory where he has participated in an array of halide perovskite research, much of which has been focused on questions of stability. Jeff received his Ph.D. in Chemical and Biomolecular Engineering from the University of Notre Dame in 2015. Severin N. Habisreutinger is a Director’s postdoctoral fellow at the National Renewable Energy Laboratory where his research is focused on spectroscopically characterizing the interfaces of metal halide perovskites with respect to charge transfer and material stability. Severin received his Ph.D. in Condensed Matter Physics from the University of Oxford in 2016. Joseph J. Berry is a senior staff scientist in the Materials Science Center and has worked at contact and interfaces for photovoltaics since joining NREL’s National Center for Photovoltaics in 2006. He current leads efforts on metal halide perovskite technologies. He previously was a NRC postdoctoral fellow at NIST Boulder, and received his Ph.D. in Physics from the Penn State University. Joseph M. Luther is a senior staff scientist in the Chemical Materials and Nanoscience team at NREL since 2009 and leads efforts on solution processed PV technologies including metal halide perovskites. He previously was a postdoctoral researcher at Lawrence Berkeley National Laboratory and the University of California at Berkeley and received his Ph.D. in Physics from Colorado School of Mines. TOC Graphic

Quotes to Highlight - Ultimately, stability ought to be a metric which quantifies the operational lifetime of a photovoltaic device under typical real-world conditions - At this stage in the technology development, value choices must be made by researchers as to the most applicable stability tests for a given study. - We should rely mainly on standardized test protocols whenever possible and depart from these standards with sound scientific justification such as exploring unique material properties with relevance for their use in PSCs - The universality of basic scientific understanding is at a premium and is required to lower the risk factors currently associated with this PV technology.

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Stability in Perovskite Photovoltaics: A Paradigm for Newfangled

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