Consensus Statement https://doi.org/10.1038/s41560-019-0529-5 Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures Mark V. Khenkin 1,2, Eugene A. Katz 1,3*, Antonio Abate4, Giorgio Bardizza5, Joseph J. Berry 6, Christoph Brabec7,8, Francesca Brunetti9, Vladimir Bulović10, Quinn Burlingame11, Aldo Di Carlo 9, Rongrong Cheacharoen12, Yi-Bing Cheng13, Alexander Colsmann 14, Stephane Cros15, Konrad Domanski 16, Michał Dusza17, Christopher J. Fell 18, Stephen R. Forrest19,20,21, Yulia Galagan 22, Diego Di Girolamo9,23, Michael Grätzel24, Anders Hagfeldt 25, Elizabeth von Hauff26, Harald Hoppe27, Jeff Kettle28, Hans Köbler 4, Marina S. Leite 29,30, Shengzhong (Frank) Liu31,32, Yueh-Lin Loo 11,33, Joseph M. Luther 6, Chang-Qi Ma 34, Morten Madsen 35, Matthieu Manceau15, Muriel Matheron 15, Michael McGehee 6,36, Rico Meitzner27, Mohammad Khaja Nazeeruddin 37, Ana Flavia Nogueira 38, Çağla Odabaşı 39, Anna Osherov10, Nam-Gyu Park 40, Matthew O. Reese6, Francesca De Rossi9,41, Michael Saliba 42,43, Ulrich S. Schubert 27,44, Henry J. Snaith 45, Samuel D. Stranks 46, Wolfgang Tress 25, Pavel A. Troshin47,48, Vida Turkovic35, Sjoerd Veenstra 22, Iris Visoly-Fisher 1,3, Aron Walsh 49,50, Trystan Watson 41, Haibing Xie51, Ramazan Yıldırım 39, Shaik Mohammed Zakeeruddin24, Kai Zhu 6 and Monica Lira-Cantu51* Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great empha- sis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mecha- nisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing quali- fication standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis. T o ensure economic feasibility and competitive levelized stability of these devices cannot be fully assessed by the procedures cost of electricity, new photovoltaic (PV) technologies must developed for conventional PV products4–10, which led to the publica- offer long-term stability alongside high power conversion tion of various studies that attempted to understand the degradation efficiency (PCE). For instance, the lifetime expectation for a PV mechanisms in emerging PV systems. Unfortunately, these stud- module in a power plant is 20–25 years, to match the reliability of ies lacked consistency in the assessment and reporting procedures, silicon-wafer-based modules. At present, the long-term stability of which prevented data comparison and, consequently, the identifica- emerging technologies such as organic photovoltaic (OPV) cells, tion of various degradation factors and failure mechanisms. dye-sensitized solar cells (DSSCs) and halide perovskite solar cells In light of such shortcomings, in 2011, a broad consortium of (PSCs) is not meeting this target and improvements are hampered researchers developed recommendations for evaluating the stabil- by a lack of understanding of the module failure modes. ity of OPV cells11. These standardized ageing experiments (the so- The existing qualification tests described in the International called ISOS protocols) were established at the International Summit Electrotechnical Commission (IEC) standards on terrestrial PV on Organic PV Stability (ISOS) held in Roskilde, Denmark, in 2010. modules (such as IEC 61215)1–3 are designed for the field perfor- They outline a consensus between researchers in the OPV field on mance of silicon panels to screen for well-understood degradation performing and reporting degradation studies in a controlled and modes generally associated with issues at the module level. These reproducible way. These protocols are not intended to be a standard tests, however, are unlikely to be well-suited to OPV cells, DSSCs and qualification test, nor are they suited for application by industry or PSCs because of their fundamentally different material properties insurance agencies; however, it is worth mentioning that tests based and device architectures. In fact, various reports have shown that the on the ISOS protocols were recently considered at the IEC level A full list of affiliations appears at the end of the paper. Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy 35 Consensus Statement NaTure Energy (see IEC TS 62876-2-13). Unlike IEC qualification standards, a solar Existing ISOS stability protocols cell cannot pass or fail ISOS stability tests. Instead, ISOS protocols ISOS protocols designed for OPV cells, which are reported in detail are intended primarily for lab-scale devices and aim to ensure the elsewhere11, are grouped in terms of the applied stresses into five comparability of PV testing performed at different laboratories, categories, shelf-life or dark storage testing, outdoor testing, light enabling successful round-robin experiments12–14, and to assist soaking testing, thermal cycling testing and light–humidity–ther- in improving the quality and relevance of published data. This mal cycling testing, all of which are also highly relevant to PSCs as approach has subsequently allowed a more comprehensive under- discussed below. Each category has three levels of sophistication, standing of device degradation. While these protocols are most fre- basic, intermediate and advanced, the goal of which is to cover differ- quently applied at the cell level, their application to neat materials, ent levels of laboratory infrastructure. The first, basic level (level 1) ‘half ’ cells (incomplete PV stacks) and mini modules can also pro- requires only commonly available equipment and provides rela- vide valuable information on degradation processes. tively low control over the stress factors. While limited insights into PSCs are among the most promising emerging PV technologies, the PSC degradation can be gathered from these tests, we suggest especially when used in high-efficiency multi-junction architec- such first-level procedures as the minimum requirement for sta- tures. The PCE of these potentially inexpensive, solution-process- bility testing. The second and third levels of sophistication (inter- able devices has grown tremendously over the past decade, reaching mediate and advanced, level 2 and 3, respectively) require more over 25% in a single junction PSC and 28% in a perovskite-on-sili- specialized tools, such as environmental chambers and maximum con tandem15,16. The next major challenge for PSC technology, along power point (MPP) trackers, but provide higher levels of confidence with large-area processing and manufacturing upscaling, is improv- in the results and, in most cases, more stringent test conditions. The ing their reliability. protocols can be applied to both encapsulated and unencapsulated PSC degradation is affected by multiple parameters whose devices provided it is clearly reported (see the detailed discussion impact on device stability is still not fully understood or appreciated below in ‘Intrinsic stability testing (ISOS-I)’). Stress tests specific to by IEC standards. These parameters include exposure to visible17 encapsulated modules (including hail tests, potential-induced deg- and ultraviolet (UV) light18, high temperature19–21, contamination radation, bypass diode stability and so on1,2), as well as mechanical from the ambient environment (oxygen, humidity)22–24 and elec- stability and special consideration for space applications, are outside trical bias4,25,26. Stability studies for PSCs are drawing increasing the scope of this report, and indeed will likely be leveraged from the attention but, despite the already large number of publications, it is existing IEC 61215 standard. difficult to compare the available results, mostly because of differ- Dark storage studies (ISOS-D) provide information on the toler- ences in the control and reporting of parameters and the inconsis- ance of the solar cells to oxygen, moisture, other aggressive atmo- tent application of statistics to stability data27. The development of spheric components naturally present in air (for example, CO2, unified PSC stability evaluation procedures by the research commu- NOx, H2S), and elevated temperatures. In other words, ISOS-D nity, similar to the procedures recently developed for PSC efficiency tests estimate a cell’s shelf life under ambient conditions when it is measurements28–31, is therefore a priority4–10,27,32,33. Given the anal- not exposed to light. The ambient atmosphere plays a crucial role ogy to the OPV case, ISOS protocols are an excellent starting point in determining the lifetime of perovskite absorbers and of some for unifying PSC stability testing, provided that they are adapted of the transport layers used in PSC architectures23,37,38. In particu- to address the following particularities of PSCs: recovery processes lar, interaction with ambient species can promote the formation after stress removal; the presence of mobile charged species (ions); of traps39 or charge carrier barriers40 (as a result of increased den- and differentiation between the processes related to exposure to sity of mobile defects or ions and electronic traps within the active the ambient atmosphere from intrinsic device-related factors. PSCs layer) as well as perovskite decomposition, which quickly deterio- are also known to exhibit hysteresis in their current density–volt- rate device performance22. Atmospheric species were also shown to age (J–V) characteristics (that is, the J–V measurements depend on charge the perovskite surface, affecting the ion distribution across the direction and rate of the voltage sweep34,35). This imposes con- the device41. The impact of these factors is taken into consideration straints on cell performance and stability assessment, but does not in the ISOS-D-1 tests, where the cell environment is monitored but necessarily signify less stable devices36. not explicitly controlled (room temperature in the laboratory is During the 11th International Summit on Organic and Hybrid assumed to be 23 ± 4 °C). Photovoltaics Stability held in October 2018 in Suzhou, China, a dis- Another important stress factor is temperature. Elevated tem- cussion panel involving many researchers working on PSCs laid the peratures are applied to study the thermal stability of solar cells and groundwork for the development of procedures for testing and report- to accelerate the degradation induced by other stressors42. Thermal ing PSC stability, which are presented in this Consensus Statement. degradation in the dark occurs in PSCs at elevated temperatures First, we discuss the suitability of existing ISOS stress protocols due to chemical and structural instabilities of the absorber materi- designed for OPV cells to understand degradation pathways in PSCs. als10,43 or transport layers44,45. Notably, some metal halide perovskites Second, we propose additional stress tests to account for factors that undergo phase transitions in the temperature range relevant to PV detrimentally affect the stability of PSCs, such as electrical bias and applications46,47. At the moment, the impact of phase transitions light–dark cycling. Additionally, to discriminate the impact on device on the device lifetime is unclear and therefore so is the impact of stability of intrinsic factors from extrinsic ones, we suggest a sub- different temperatures during (accelerated) ageing. As protocols set of tests to be conducted under inert atmosphere. Next, we iden- should be applicable to any type of perovskites regardless of their tify key technical and procedural information about stability testing phase transitions, ISOS protocols cannot accommodate all the pos- that should be reported in publications to improve transparency and sible temperature options. We are therefore suggesting maintaining reproducibility in the field. We also discuss the use of figures-of-merit the temperature settings described in the original ISOS protocols for device stability in light of the various PCE evolutions reported for that would be above the tetragonal to cubic phase transition for PSCs. Finally, we envision that improvements in consistency of stabil- MAPbI3. The effects of elevated temperatures on the device stability ity testing and reporting by adopting the unified set of protocols and are assessed with the ISOS-D-2 test that is carried out at controlled reporting recommendations proposed will enable the consolidation of elevated temperatures of 65 or 85 °C. While not explicitly controlled a large volume of data published in the future into a single database in ISOS-D-1 and D-2, we stress that monitoring and reporting the that could, in turn, be used to reliably compare stability studies, to ambient relative humidity (RH) is of critical importance because dry analyze the relative significance of various degradation factors and to (RH < 20%) and humid air represent dramatically different stress ultimately identify key failure mechanisms in devices. conditions for PSCs38,48. The ISOS-D-3 damp heat test specifically 36 Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy NaTure Energy Consensus Statement a b 16 28 Light intensity J–V measurements 1.0 26 PCE (%) 14 T (°C) 0.5 12 24 MPP tracking 0 0 50 100 150 200 Time (h) 1.0 c Normalized PCE (a.u.) 18 Cycled 6/6 h 0.6 PCE (%) 16 0.2 Continuous light exposure 14 0 100 200 300 0 50 100 150 200 250 300 Time (h) Time (h) d e 10 1.0 0.6 V Fresh 0.8 V 5 After reverse bias Normalized efficiency 0.8 Recovered 0 J (mA cm–2) 0.6 −5 1.0 V 0.4 −10 1.2 V Forward bias 0.2 in the dark −15 Reverse bias in the dark 0.0 −20 0 0 40 160 180 200 −0.5 0 0.5 1 Time (h) V (Volts) Fig. 1 | Specific features of PSC stability studies. a, PCE extracted from continuous MPP tracking (red curve) versus periodic J–V scans collected from forward to reverse bias (black circles) for the same PSC. b, Normalized PCE of PSC subjected to repeated 12 h light on–off cycles at 25 °C and 10% RH. c, PCE evolution of PSCs exposed to continuous (blue curve) or cycled (6/6 h, red curves) illumination by white-light-emitting diodes. d, Normalized PCE changes of PSCs exposed to different forward bias in the dark. e, Light J–V curves for a fresh PSC, after 1 min at −20 mA cm–2 and after recovering for over 3 h of MPP tracking. Panel a was adapted with permission from ref. 8, Elsevier; panel b was adapted with permission from ref. 82, Elsevier; panel c was adapted with permission from ref. 4, Springer Nature Ltd; panel d was adapted with permission from ref. 26, the American Chemical Society; panel e was adapted with permission ref. 88, John Wiley and Sons. takes into account the impact of humidity (set at 85% RH) when conditions to the lifetime obtained under the accelerated stress devices are kept at high elevated temperatures (65 or 85 °C). conditions. Previously, this approach was pursued to help establish Light soaking tests (ISOS-L, ‘Laboratory weathering’ in the origi- IEC 61215 by correlating outdoor tests results for Si modules with nal ISOS protocols11) promote ion and defect migration in PSCs49–54 results obtained from various acceleration tests69–71. To date, stud- as well as phase segregation55,56 in the perovskite photoactive layer, ies of PSC outdoor stability are scarce72–75, but the community has which reduces efficiency. Additionally, light can catalyse or acceler- gained some critical insights with ISOS-O experiments, such as the ate harmful chemical reactions, which lead to perovskite decom- importance of light–dark cycling74 and the unexpectedly high open- position57 or defect formation58. Detrimental changes in organic circuit voltage at low illumination intensities73. charge extraction layers, material intermixing at the interfaces59 and Under the ISOS-O-1 protocol, periodic measurements of ion exchange between adjacent solar cell layers can also be caused J–V curves are done under illumination by a solar simulator. In by cell illumination52,60,61. As with OPV62 and DSSC63, the spectral ISOS-O-2, the J–V measurements are periodically acquired under composition of the light source also merits special attention, espe- illumination by natural sunlight. ISOS-O-3 requires both in situ cially in the UV range. UV light assists perovskite decomposition64 MPP tracking under natural sunlight and periodic performance and increases the non-radiative recombination rate in PSCs based measurements under a solar simulator. The results obtained by on mesoporous TiO2 (ref. 65,) which may thus require UV-blocking J–V measurements and MPP tracking do not necessarily coincide layers to become more stable. Recently, PSCs with novel transport in PSCs (Fig. 1a)8, although they generally have similar trends4,76. layers have been shown to be tolerant to UV irradiation66,67. PSC Therefore, it is crucial when characterizing PSCs to rigorously stability may be affected differently by light in the UVA and UVB describe the load and recovery time before J–V measurements. We spectral ranges68. encourage the use of MPP tracking, whenever possible, both as the In outdoor stability studies (ISOS-O), ageing occurs by illumina- most practical electrical-bias condition for ageing and as a reliable tion with natural sunlight in the ambient environment. Although tool for PSC performance assessment (see further discussion below these conditions are not necessarily reproducible (they depend on in ‘Checklist for PSC stability studies’). However, we indicate MPP weather, location, season, and so on), the results of outdoor tests are tracking as mandatory only at the third, most advanced level of the most relevant to device operation. Unlike other protocols, they ISOS protocols. For lower levels of sophistication, we give options of can be directly applied to obtain realistic assessments of device life- exposure under open-circuit condition or using a fixed voltage bias time, albeit specific to a given climate. Field tests can also determine near the MPP (instead of active MPP tracking) in line with what was whether the list of failure modes identified in the lab is complete and suggested in the original ISOS protocols11. adequate for understanding reliability of the solar cell under real Thermal cycling in the dark (ISOS-T) and light–humidity– operation and, furthermore, can provide reference points for calcu- thermal cycling (‘solar thermal humidity cycling’ in the original lating acceleration factors that correlate lifetime under real-weather ISOS protocols11) (ISOS-LT) allow evaluation of the damage to PV Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy 37 Consensus Statement NaTure Energy Table 1 | Overview of existing ISOS protocols and suggested additional protocols that account for the properties of perovskite materials and devices Test ID Light source Temperature Rel. humidity Environment/Set-up Characterization Load light source Dark storage (ISOS-D) ISOS-D-1 None Ambient Ambient Ambient air Solar simulator or OC (23 ± 4 °C) sunlight ISOS-D-2 None 65, 85 °C Ambient Oven, ambient air Solar simulator OC ISOS-D-3 None 65, 85 °C 85% Env. chamber Solar simulator OC Bias stability (ISOS-V) ISOS-V-1 None Ambient Ambient Ambient air Solar simulator Positive: (23 ± 4 °C) VMPP; Voc; ISOS-V-2 None 65, 85 °C Ambient Oven, ambient air Solar simulator Eg/q; JSC Negative: ISOS-V-3 None 65, 85 °C 85% Env. chamber Solar simulator –Voc, JMPPa Light-soaking (ISOS-L) ISOS-L-1 Solar simulator Ambient Ambient Light only Solar simulator MPP or OC (23 ± 4 °C) ISOS-L-2 Solar simulator 65, 85 °C Ambient Light & temperature Solar simulator MPP or OC ISOS-L-3 Solar simulator 65, 85 °C ~ 50% Light, temperature & RH Solar simulator MPP Outdoor stability (ISOS-O) ISOS-O-1 Sunlight Ambient Ambient Outdoor Solar simulator MPP or OC ISOS-O-2 Sunlight Ambient Ambient Outdoor Sunlight MPP or OC ISOS-O-3 Sunlight Ambient Ambient Outdoor Sunlight and Solar MPP simulator Thermal cycling (ISOS-T) ISOS-T-1 None RT to 65, 85 °C Ambient Hot plate/ oven Solar simulator OC ISOS-T-2 None RT to 65, 85 °C Ambient Oven/env. chamber Solar simulator OC ISOS-T-3 None −40 to + 85 °C < 55%b) Env. chamber Solar simulator OC Light cycling (ISOS-LC) ISOS-LC-1 Solar simulator/ Ambient Ambient Light only Solar simulator MPP or OC Dark Cycle (23 ± 4 °C) ISOS-LC-2 period: 2, 8, or 65, 85 °C Ambient Light & temperature Solar simulator MPP or OC 24 h Duty cycle: ISOS-LC-3 1:1 or 1:2 65, 85 °C < 50% Light, temperature & RH Solar simulator MPP Solar-thermal cycling (ISOS-LT) ISOS-LT-1 Solar simulator Linear or step Monitored, uncontrolled Weathering chamber Solar simulator MPP or OC ramping between room temp. and 65 °C ISOS-LT-2 Solar simulator Linear ramping Monitored, controlled at 50% Env. chamber with sun Solar simulator MPP or OC between 5 °C and beyond 40 °C simulator 65 °C ISOS-LT-3 Solar simulator Linear ramping Monitored, controlled at 50% Env. chamber with sun Solar simulator MPP or OC between −25 °C beyond 40 °C simulator and freezing and 65 °C Each test group is divided into three levels of sophistication that reflect the complexity of required equipment and the harshness of the applied stress. Reported ISOS protocols are taken from ref. 11. Proposed additional ISOS protocols are printed in bold. aVOC, VMPP, and JMPP are determined from light J–V curves measured under standard solar cell testing conditions on a fresh device. Eg and q are the band gap of the active layer and elementary charge, respectively. bRelative humidity is controlled at temperatures above 40 °C and is not controlled for the remainder of the cycle. Env., environmental; OC, open-circuit condition; MPP, maximum power point; RT, room temperature; RH, relative humidity. devices caused by diurnal and seasonal variations in the weather that under constant extreme temperatures, which is attributed to in terms of solar radiation, temperature and humidity. These tests the effect of ion accumulation at the contacts20. Delamination from are relevant for any outdoor-dedicated PV technology (including thermal cycling was mitigated by adding a flexible polymer buffer PSCs) because they simulate realistic conditions, stimulate failure layer around the mechanically fragile perovskite, which resulted mechanisms related to delamination of layers or contacts77 and are in PSCs retaining more than 90% of their performance after 200 included in the qualification standards78. Particularly for PSCs, cycles between –40 and 85 °C77. Temperature and illumination degradation under varying temperature could be more severe than cycling, which resembles weather conditions in central Europe 38 Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy NaTure Energy Consensus Statement for several representative days, was applied to PSCs under a nitro- Table 2 | Overview of suggested ISOS-I protocols for ageing gen atmosphere79. This study enabled from a temperature point experiments in an inert atmosphere of view insight into the real-world operation of PSCs and empha- sized the complex interplay of temperature-dependent transient Test ID Elevated temperature Electrical bias Light effects during the day with reversible and irreversible degradation ISOS-D-1I – – – processes. Depending on the available equipment, temperature cycling varies from simple turning on and off a hotplate installed ISOS-D-2I ✓ – – in an ambient environment to complex temperature and humidity ISOS-V-1I – ✓ – cycles in an environmental chamber. Examples of such cycles are ISOS-V-2I ✓ ✓ – available elsewhere11,78. ISOS-L-1I – – ✓ Suggested specific ISOS protocols relevant for PSCs ISOS-L-2I ✓ – ✓ Recently some ageing protocols for PSC were suggested ; they 10,28 ISOS-T-1I Cycled – – mostly feature subsets of the original ISOS protocols described ISOS-T-2I Cycled – – above, although some additional ideas, particularly concerning the ISOS-T-3I Cycled – – application of electrical-bias10, were also introduced. Below we sug- gest extensions of the ageing procedures, summarized in Table 1, ISOS-LC-1I – – Cycled to account for the specific properties of perovskite materials and ISOS-LC-2I ✓ – Cycled solar cells (proposed additional ISOS protocols in Table 1 are in ISOS-LC-3I ✓ – Cycled bold). Although a major part of the original motivation for the Ticks and dashes denote the presence and absence of the stress, respectively. In case of ISOS protocols was to limit conditions for each level to facilitate temperature, ‘–‘ refers to room temperature. comparison, data are limited on the protocols discussed below. We therefore propose a reporting framework, give some example condi- tions, and discuss why these might be relevant. As in the case of the existing ISOS protocols, we recommend testing the device following maintained at ambient conditions while the temperature and RH the basic procedure (level 1) as a minimum requirement. A similar are monitored, but not controlled. At the ISOS-LC-2 level, the cell framework is likely to be useful for OPV and other emerging PV is maintained at a fixed set point temperature of 65 or 85 °C in the technologies. If these protocols become widespread over the next ambient atmosphere. At the ISOS-LC-3 level, RH is held at 50% few years, the community can adopt a more informed decision on a and high temperatures. limited number of consensus conditions. Electrical bias in the dark (ISOS-V) Light–dark cycling (ISOS-LC) Electrical bias causes PSC degradation25,26,88,89 by triggering ion Various PSC degradation modes have repeatedly been shown to be migration26 or charge carrier accumulation that result in thermally entirely or partly reversible in the dark (often referred to as metasta- activated traps formation90 or detrimental electrochemical reac- bility)20,36,51,58,74,80–83. Therefore, cycling through light–dark periods to tions23. Electric fields also promote moisture-initiated perovskite simulate day–night cycles constitutes a significantly different stress degradation89,91 because moisture ingress can lead to the formation test than applying constant illumination (ISOS-L)4,74. Two opposite of hydrated perovskite phases containing mobile ions, whose drift types of dynamics are reported in the literature: reversible photo- accelerates the degradation92. Both positive (Fig. 1d) and negative induced PCE increase with subsequent decrease in the dark74,82 (Fig. 1e) biases are potentially harmful26,88 and might occur during (Fig. 1b) and photo-induced degradation with recovery in the the operation of solar panels. In our view, ISOS protocols revisited dark51,58 (Fig. 1c). Reversible performance loss is attributed to cat- for PSCs should include the ISOS-V group of testing in which the ion redistribution51, metastable defect formation58 and reversible behaviour of the cell is analysed when exposed to a certain electrical chemical reactions57. The PCE improvement under illumination bias in the dark (Table 1). after storage in the dark is commonly attributed to the neutraliza- Usually, a solar cell is kept near its MPP (that is, positively tion of interfacial defects by photogenerated charge carriers or to biased with voltage V < VOC); however, disconnected cells under changes in the built-in electric field due to ion migration84–86. The illumination are biased at open-circuit voltages (typically, ~ 1 V PCE dynamics during a cycle depends on the status of cell degrada- for iodine single-junction PSCs). Therefore, we suggest applying a tion81. For example, PSCs have shown a ‘fatigue’ effect: while the voltage equal to VMPP or VOC (as measured under AM1.5 G one sun PCE decreased in the dark and recovered under illumination, the illumination on a fresh device) as a positive bias condition. Since rate of PCE restoration reduced with each consecutive light–dark bias-induced degradation effects may have a threshold behavior26, cycle (Fig. 1b)82. Such metastability is attributed to the migration of we recommend voltages below the bandgap energy divided by the ions, which is known to be pronounced in metal halide perovskites83. charge of the electron to avoid unnatural overstressing. In Si PVs, Reversible and irreversible degradation mechanisms may co-exist constant-current stress was shown to mimic MPP operation under in a given PSC79,81,87. full sunlight exposure, while testing current-induced degradation is The ISOS protocols revised for PSCs should, therefore, include technically easier93. To date, however, no similar data exist for PSCs, a group of light–dark cycling protocols to account for the recovery so such stability tests might also be useful. phenomena (ISOS-LC in Table 1). For the ISOS-LC experiments, For a partly shaded solar module (shaded by clouds, dirt, nearby we suggest exposing the cells to simulated sunlight turned on and trees, and so on), shaded solar cells can be forced to operate under off with cycle periods of 2, 8, or 24 h and duty cycles (light:dark) reverse bias to match the current flow through the rest of the mod- of 1:1 or 1:2. Of the suggested conditions, 24-h-long cycles (12 h ule88,94,95. The choice of negative-bias stressing conditions depends light and 12 h dark or 8 h light and 16 h dark) mimic the diurnal on the anticipated module connection scheme, particularly on sun cycle. However, because the interplay between degradation the use and choice of bypass diodes. At present, experience with and recovery in realistic conditions can be complex79 and depend perovskite modules is insufficient, so it is reasonable to learn from on cell history, varying cycle duration and duty cycle should pro- both experiments: with a constant negative bias applied (for exam- vide additional information on the extent of reversibility and suf- ple, −VOC), which is relevant for modules with bypass diodes, and ficient recovery times. At the ISOS-LC-1 level, the solar cell is with the current enforced up to −JMPP (which in the dark would Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy 39 Consensus Statement NaTure Energy Table 3 | Suggested checklist for reporting data from stability tests Key aspects Characteristics Details to be reported Initial solar cell Current–voltage Light source (light source type, intensity, spectrum, filters applied, calibration), scan speed, direction, characterization (J–V) curves dwelling time, the number of power line cycles (NPLC), preconditioning, and so on MPP tracking or Hardware, tracking algorithm, delay time, tracking duration photocurrent at MPP EQE/IPCE spectra Indicating the lock-in frequency and light bias if used, and if monochromatic light is smaller than active area, or larger and an optical mask applied. Calculated JSC and its comparison to JSC obtained from the J–V data Encapsulation Wiring Materials, processing conditions, addition of a protective sealant Front- and back- Materials (reference or composition, thickness), processing conditions (environment, temperature, side encapsulation duration) layer(s) Edge sealant Materials (reference, thickness, width), processing conditions (environment, curing and so on) Geometry Rim (minimum distance between encapsulation edge and active area edge), device active area; picture or a scheme of the device Ageing conditions Light Light source type, intensity, spectrum, filters applied, calibration Temperature In shadow and/or under illumination, temperature sensor type Atmosphere Air/glovebox/sealed pouch/environmental chamber, and so on; RH (controlled or monitored) Electrical bias Open circuit/maximum power point/short circuit/constant load condition Cycling procedure Dwell and period times (if applicable) Comply with known protocols (IEC, ISOS and so on)? Ageing time Stress duration and corresponding performance loss, resting times (for example, without stress). Measurements Periodically recorded Recording frequency, scan parameters (similar to ‘Initial solar cell characterization’) during ageing J–V curves Recovery before Recovery time and conditions prior to the measurements. measurements MPP tracking Tracking parameters (similar to ‘Initial solar cell characterization’) Other periodic measurements (if applied) Number of samples Number of solar cells of each type tested under each ageing condition, statistical analysis (if applicable). Number of samples still operating above a specified efficiency level at the end of the ageing test. Outdoor stability Location and time of Place or coordinates; dates and total amount of hours of exposure. exposure Weather conditions Temperature, humidity, sunlight irradiance (preferably in tabulated format), wind speed, and so on. throughout the exposure period mean a relatively high negative bias applied to the cell). The latter It may also be informative to report the evolution of the in situ dark condition simulates the situation of a partially shaded module in current (or voltage in constant current density mode) during stress- the absence of a bypass diode. Practical negative-bias conditions ing in addition to periodic J–V measurements in ISOS-V protocols. depend on the details of the module layout. The three sophistication levels of the ISOS-V protocols differ by the level of control over the Intrinsic stability testing (ISOS-I) sample temperature and atmosphere and the required equipment, The stress factors can be divided into two groups: intrinsic and which is similar to the corresponding ISOS-D protocols (Table 1)11. extrinsic. The intrinsic factors include light, temperature, and elec- Electrical bias can redistribute charged species across the PSC, trical bias (relevant regardless of the cell encapsulation or protec- which might be reversible after stress removal26. Thus, we recom- tive environment), and the extrinsic factors are governed by the cell mended tracing the solar cell recovery after ageing by storing it in interactions with ambient species such as oxygen and/or moisture the dark under open-circuit (disconnected) conditions and peri- (which are relevant assuming imperfect device encapsulation). odically checking its performance until it reaches saturation. For a Generally, ‘encapsulation’ of devices refers to the protection of similar reason, measuring the steady state of the J–V curve may be the solar cells by gas-barrier materials, which delays the contact difficult after electrical biasing. We recommend using MPP track- between the cell and ambient air (especially moisture). Encapsulation ing (or a stabilized current at a constant voltage close to MPP for can be done by glass–glass sealing, lamination of rigid or flex- ISOS-V-1 and ISOS-V-2) to account for possible transient effects. ible gas-barrier materials, direct deposition of protective layers 40 Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy NaTure Energy Consensus Statement Table 4 | Conditions of stability analysis of selected PSCs analyzed under continuous illumination for over 1000 h. Only cells losing less than 15% of initial PCE are included Light UV filter Reported Intensity T (°C) Atmosphere Encap- Cell structure Initial Time Ref. source (Suns) sulation PCE (%) (h) Sulfur – ‘Triple A class’ sulfur 0.77 30 °C Ambient Air No ITO/SnO2 (FA0.76MA0.15 16.6 1500 49 plasma plasma lamp, Plasma-I 10%–20% RH Cs0.09)0.97Pb(I0.89Br0.11)2.97 AS 1300 Light Engine EH44/MoOx/Al (http://www.plasma-i. com/plasma-i- products.htm). – Sulfur plasma lamp 1 35 °C Ambient Air No ITO/NiO/LiF Cs0.17FA0.83 14.5 1000 148 from LG (6000 K 40% RH Pb(I0.83Br0.17)3 LiF/PC60 blackbody). BM/SnO2/ZnSnO2/ITO/LiF/Ag White LED – White LED illumination 1 RT N2 filled No ITO/SnO2/PCBM:PMMA/ 20.4 1000 149 chamber Rb0.05Cs0.1FAPbI3 PMMA/ Spiro-OMeTAD/Au – White light LED array 1 55– Ar filled No FTO/c-TiO2/mp-TiO2/(FAI)0.9 20.9 1000 150 60 °C chamber Cs0.1(PbI2)1.05: 3-(5-mercapto- 1H-tetrazol-1-yl)benzenaminium iodide/Spiro-OMeTAD/Au – White LED (XLamp 1 RT N2 filled No ITO/C60-SAM FA0.83MA0.17 21.2 1050 151 CXA2011 1300 K CCT) chamber Pb1.1Br0.5I2.8 PDCBT/Ta-WOx/Au – Array of white LEDs 1 60 °C N2 filled No FTO/c-TiO2/mp-TiO2 20.4 1000 131 was powered by a chamber CsFAMAPbI3-x constant current Brx CuSCN/r-GO/Au – White LED lamp 1 55 °C N2 filled No FTO/c-TiO2/mp-TiO2/ 21.5 1500 152 chamber (FAI)0.81(PbI2)0.85(MABr)0.15 (PbBr2)0.15/ doped with N-(4-bromophenyl)-thiourea Spiro-OMeTAD/Au – White LED lamp 1 RT N2 filled No FTO/c-TiO2/mp-TiO2/ 16.4 1000 21 chamber MAPbI3/PTAA/Au Metal No Light-soaking chamber, 1 RT N2 filled Yes FTO/La-BaSnO3/MAPbI3/ 21.2 1000 66 halide or K3600-MH300, chamber NiO/FTO/glass xenon- McScience Inc. plasma Yes Class AAA solar sim. 1 55 °C Ar filled Yes FTO/c-TiO2/mp-TiO2/ 11.9 10,000 133 lamp from Newport equip. chamber or ZrO2(5-AVA)x(MA)1-x with a 1000 W Xenon encapsulated PbI3/Carbon lamp. AAA class simulator using a plasma lamp with a spectrum that exactly superimposes to the standard. No Newport solar 1 Ambient air, No FTO/c-TiO2/mp-TiO2/ZrO2 10 1008 153 simulator (model unspecified (5-AVA)x(MA)1–xPbI3/Carbon 91192) giving light with AM 1.5 G spectral distribution No Atlas SUNTEST 0.76 70– Ambient air Yes FTO/NiO/(FA0.83MA0.17)0.95Cs0.05 ~ 19 1885 154 XLS + (1,700 W air- 75 °C 40%–50% RH Pb(I0.9Br0.1)3 + BMIMBF4/ cooled xenon lamp) PCBM/Cr(Cr2O3)/Au light-soaking chamber under simulated full-spectrum AM1.5 sunlight. Solar Yes ‘Solar cell light 1 45– Yes FTO/NiMg(Li)O/MAPbI3/ 18.3 1000 155 simulator resistance test system 50 °C PCBM/Ti(Nb)Ox/Ag with (Model BIR– 50, unspecified Bunkoh–Keiki Co., LTD) light source equipped with a Class AAA solar simulator’ Yes ‘Sun Simulator 1 35 °C N2 Yes ITO/ BDPSO/MAPbI3/ 17.2 1300 156 Sumitomo Heavy C60/BCP/Ag Industries Advanced Machinery’ Not ‘AM.1.5 G solar 1 Yes FTO/PEDOT:PSS/(BA)2(MA)3 12.5 2250 157 specified simulator’ Pb4I13/ PCBM/Al No ‘AM 1.5 G illumination’ Air 38% RH PET/Gr/TiO2/PCBM MAPbI3/ 11.9 1014 158 Spiro-OMeTAD:cross-stacking carbon nanotube/All Carbon Electrode Outdoor No Variable Variable Yes FTO/c-TiO2/mp-TiO2/ZrO2/ 12.9 1056 72 up to (5-AVA)x(MA)1-xPbI3/mcarbon 45 °C RT, room temperature; RH, relative humidity Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy 41 Consensus Statement NaTure Energy a e 1.0 Normalized PCE PCE (a.u.) ∼100% 0.8 100% 0.6 0 250 500 750 1,000 1,250 1,500 End of 'burn-in' b 16 100% 100% PCE (%) 14 12 ∼100% T0 Tburn-in Time T80 T80 TS80 c 1.2 0 200 400 600 800 1,000 f PCE (a.u.) Normalized PCE 1.0 95.5% Max PCE 0.8 d 0 192 384 576 786 960 100% 100% PCE (%) 10 100% ∼50% 5 0 200 400 600 800 1,000 T0 Tmax T80 T80 Time TS80 Fig. 2 | Different degradation dynamics in perovskite solar cells and common practices for estimating T80. a–d, Examples of some of the most stable PSCs reported so far, demonstrating a variety of possible PCE time profiles upon ageing under 1 sun illumination. e,f, Common practices in the T80 estimations. The black curves show schematically how PCE evolves with ageing time in the case of ‘burn-in’ effect (e) and in the case of nonmonotonic PCE (f). The three strategies depicted to estimate T80 differ in how the initial PCEs (‘100%’) are estimated and the choice of starting point of the ageing timer (shown in the figure as circles of corresponding colours). In the first strategy, 80% PCE is counted from the initial PCE at t = T0 (shown in orange). In the second strategy, 80% PCE is counted from the back-extrapolated ‘after burn-in’ PCE in panel e (red dashed line indicates the extrapolation) or from the PCE maximum in panel f (red dashed line used to guide the eyes); in both cases the timer starts at t = T0 (shown in green). In the final strategy, 80% PCE is counted from the PCE at t = Tmax in panel f or at t = Tburn-in in panel e, and the ageing timer starts at corresponding timestamps (shown in blue). Panel a is adapted with permission from ref. 49, Elsevier; panel b is adapted with permission from ref. 148, Elsevier; panel c is adapted with permission from ref. 72, Springer Nature Ltd; panel d is adapted with permission from ref. 65, IEEE. or a combination of these processes. Extrinsic stability depends into PSC degradation mechanisms and is helpful for differentiat- on the device sensitivity to air and on the properties of the barrier ing between how thermal stress, light, electrical bias and the cycles material (including their chemical compatibility with the device). A thereof affect device degradation4,106,107. full understanding of degradation of encapsulated devices requires We suggest including protocols to address the intrinsic stability knowledge of the stability and properties of the gas barrier. Specific of solar cells in inert atmospheres (nitrogen, argon, and so on). The tools exist to characterize the gas-barrier properties and to deter- protocol is labelled by the index ‘I’ at the end of the protocol name mine the amount of moisture that has permeated orthogonally96–99 (Table 2) to indicate an inert atmosphere in the corresponding test, and laterally100,101 within the encapsulation. Any measurement of with the other parameters kept the same. For example, ISOS-L-1I the lateral permeation from the encapsulation edge should mimic, stands for the intrinsic photo-stability at room temperature (similar to the extent possible, the operational encapsulation and take into to ISOS-L-1 except that the atmosphere is inert), ISOS-L-2I stands account the interfacial permeation that is not considered in gas-bar- for the intrinsic photo-stability at elevated temperature, and so on. rier measurements of bulk materials. Control of the self-resistance The latter protocol is essential because it is often used to determine of encapsulation is particularly important because ageing tests, the lifetime of a given PSC in research papers. Notably, the new which involve high temperature, high humidity and UV irradiation, family of protocols includes ageing experiments with a single stress could degrade the gas-barrier protection, leading to a dramatic drop factor (only heat in ISOS-D-2I; only electrical bias in ISOS-V-1I; in PV performance. Therefore, we recommend ageing encapsulat- only light in ISOS-L-1I, and so on), which simplifies the analysis of ing materials under the same ageing conditions as done with encap- degradation modes. We also included ISOS-LC-3I and ISOS-T-3I sulated devices and to determine the gas-barrier properties after protocols in Table 2 despite the original conditions requiring a cer- ageing. Developing dedicated encapsulation procedures constitutes tain relative humidity value. The advanced, level 3 protocols differ a separate technological challenge, especially for PSCs77,102–105. from LC-1,2 and T-1,2 in the temperature cycle and some techni- In the vast majority of studies, the barrier properties of the calities that are reported elsewhere11. For instance, ISOS-T-3 goes to encapsulants are unknown, which inhibits the ability to differenti- –40 °C, while ISOS-T-2 only down room temperature. We therefore ate between intrinsic and extrinsic cell stability. Even if the device suggest performing ISOS-LC-3I and ISOS-T-3I tests in the absence is nominally ‘unencapsulated’, the top evaporated electrode can play of humidity, given that I-tests are conducted in an inert atmosphere, the role of barrier (with unknown properties). This has motivated to stress the device with a harsher temperature cycle. numerous studies to focus on intrinsic PSC stability by stressing Note that encapsulation not only inhibits reactions with ambi- the cells in an inert atmosphere, for example, in sealed pouches, or ent species but may also prevent out-diffusion of volatile perovskite with equipment installed in inert atmosphere glove boxes or envi- decomposition products from the device. Recently the light-induced ronmental chambers. This approach provided important insights degradation of unencapsulated PSCs was also shown to accelerate 42 Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy NaTure Energy Consensus Statement Table 5 | Possible figures of merit for evaluating PSC stability FOM options Stress Conditions Description and applicability 1 T80 Continuous stress 20% of PCE decay from initial PCE (t = 0). 2 TS80 20% of PCE decay from a certain PCE value during the ageing experiment, corresponding to t = Tmax or t = Tburn-in: from the extrapolated t = 0 value from the post- burn-in decay fitting (see Fig. 2e,f). For cells with an increase in PCE, T80 should be estimated for time at which the efficiency has dropped to 80% of the maximum PCE, with the complete time from t = 0 to this point quoted as the T80 value. 3 η1000 (PCE after 1000 h) In case T80 is not reached within the timeframe of the ageing experiment, so the decrease observed over first 1000 h should be reported in addition to (optionally) an extrapolation applied to determine T80 and/or TS80. 4 T80 analogue, corrected for If the restoration process has been tested after the stress removal. the recovery processes 5 T95 and TS95 Analogous to T80 and TS80, apart from to 95% of the t = 0, maximum, or post-burn-in back-extrapolated t = 0 PCE. 6 T80 analogue for energy output Cycled stress For cycled illumination conditions, especially in the case of non-monotonic PCE versus per cycle or average PCE value time curves. during the cycle in an ultrahigh vacuum108. Therefore, encapsulated PSCs may have for consistency. If suitable equipment is available, a third approach a longer lifetime than unencapsulated PSCs, even for degradation to logging the steady-state performance of the device over time is experiments conducted in an inert atmosphere. In addition, the MPP tracking (MPPT)6,30,35. MPPT simultaneously holds the device environment in which the PSC is encapsulated may also play a role. at its normal operating voltage and measures the output. However, Thus, reporting the presence or absence and the details of encapsu- due to hysteresis, standard perturb and observe MPPT algorithms lation is also mandatory in I protocols. might be suboptimal for PSCs112. Modifications of a perturb and observe algorithm were suggested for effective MPPT in solar cells Checklist for PSC stability studies exhibiting hysteresis, including extended thresholds for switching To compare results and ensure reproducibility, sufficient informa- the voltage sweep direction112 and predictive MPPT algorithms with tion must be reported about ageing experiments, in addition to lowered settling times113,114.If any form of MPPT is used during an giving a detailed description of device preparation27. Table 3 pro- experiment, the hardware and MPPT algorithm should be clearly poses a checklist for reporting stability data in accordance with referenced. Similar to organic solar cells115, preconditioning the PSC that introduced by Nature journals for reporting PV performance (with light and/or electrical bias) prior to each J–V scan may affect data109. We stress that, even if a parameter is not controlled during the outcome and should be reported. the ageing experiment (for example, temperature or RH in ‘ISOS-1’ MPPT is also recommended as a bias condition for ageing protocols), it is still important to monitor and report the param- experiments done under illumination (mandatory only at the third eters listed in Table 3. sophistication level). When MPPT is performed throughout the In particular, we recommend specifying the number of samples entire ageing test, it is recommended to periodically measure J–V studied in each ageing test. According to a critical analysis of the curves, because such measurements provide information on the quality of PSC stability studies27, nearly half of the studies consider degradation mechanism that is more detailed. Additional non- only a single sample of each kind, which is particularly worrisome destructive characterizations at the intermediate stages of PSC age- for PSCs that are typically characterized by relatively low reproduc- ing are also encouraged, although care must be taken to account for ibility. Ideally, statistics should be provided to account for sample- possible cell recovery during the measurement. to-sample and batch-to-batch variations. The same work provides Ideally, light sources with an irradiance of 800–1000 W m–² estimates of the desired sample size27. (1 sun = 1000 W m–²) should be applied, and the exact irradiance, Stability data are often reported in the form of normalized the type of light source and its spectrum should be reported. Table 4 parameters as a function of ageing time, while only specifying the shows a collection of the most stable reported PSCs to date that performance of a fresh representative device (the ‘champion’ or can withstand over 1000 h of light-soaking while losing <15% average). Thus, the reported stabilities and efficiencies may be mea- of their initial PCE. Five main types of light sources were used: sured on different devices and thus cannot be directly related. Any sulfur plasma lamp, white light-emitting diodes (LEDs), metal plot with normalized parameter variation should include the values halide xenon lamp, solar simulator and outdoor (that is, real sun- to which each parameter was normalized9. light). The solar simulator section encompasses devices analysed Due to the ongoing development of best practices for measuring under unspecified conditions of irradiation. Sulfur plasma and J–V curves and efficiency of PSCs28–31,110, the procedure for mak- white LED illumination typically do not include UV light, so it ing periodic measurements during ageing tests should be clearly is redundant to report the use (or not) of a UV filter (except for described. Typically, measurements of the J–V curve (or parts some modern LED sources with extended range, which may have thereof) are made with a periodicity that depends on the charac- components in the range 300–400 nm); in fact, reporting that “no teristic degradation timescale of each given device. Because J–V UV filter was applied” would be misleading in this case. Metal hysteresis is common in PSCs, steps should be taken to ensure that halide and xenon arc lamps have UV light in their spectra, so the measurements are taken under (quasi) steady-state conditions. any filtering used must be reported. Reporting the company and This can usually be done by using a dynamic J–V approach29,111, model of the solar simulator is good practice. Note that ASTM’s which allows time at each voltage step for the current to settle (sta- class ‘A’ for simulated solar spectrum is only relevant in the range bilize), or, alternatively, by using a very slow J–V scan in the vicin- 400–1100 nm116. Therefore, the type of light source used and ity of the MPP, usually repeated in the reverse direction to check its spectrum should always be clearly specified. For example, Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy 43 Consensus Statement NaTure Energy Effect of light and/or bias ISOS parameters Light: OFF Light: OFF Light: ON Light: cycled Bias: OFF (OC) Bias: ON Bias: OC or MPP Bias: OC or MPP Temperature Ambient 65 or 85 Cycled Ambient 65 or 85 Cycled Ambient 65 or 85 Cycled Ambient 65 or 85 Cycled Atmosphere (RT) °C (RT) °C (RT) °C (RT) °C Effect of atmosphere Inert atmosphere D-1I D-2I T-1I V-1I V-2I L-1I L-2I LC-1I LC-2I, 3I T-2I T-3I Ambient humidity D-1 D-2 T-1 V-1 V-2 L-1 L-2 LC-1 LC-2 LT-1 T-2 O-1 O-2 O-3 Controlled D-3 T-3 V-3 L-3 LC-3 LT-2 RH = 85 or 50% LT-3 Effect of T Effect of T Effect of T Effect of T Fig. 3 | Stress factors affecting solar cells in different ISOS protocols. Rows correspond to different atmospheres, columns correspond to different combinations of light and electrical bias, and sub-columns depict different temperature regimes. Arrows alongside the table guide the eye for comparing protocols to identify the effects of atmosphere, temperature, light or electrical bias. reporting ‘AM 1.5 G illumination’ without specifying the light For the most stable PSCs, T80 exceeds 1000 h (42 days) or even source, the solar simulator details and the calibration procedure, 10000 h ( > 1 year) for two dimensional (2D)–3D perovskites133 is not sufficient. under certain stress conditions, including illumination (see Furthermore, some of the light sources described above (espe- Table 4). Considering recent advancements in PSC stability, we cially xenon lamps) may degrade significantly on the timescale of strongly recommend that reviewers and journal editors discour- stability experiments, so it is recommended to periodically check age the use of the word ‘stable’ in the title of scientific papers in an the light intensity with a reference cell. unspecific manner (thermal, photo-, operational, and so on) and Several reports claim that PSCs, as well as DSSCs117,118 and without matching the state of the art for a specific device type. organic solar cells119–122, might be suitable for indoor and outdoor Notably, such long exposure times are challenging to realize. If T80 low-light-intensity applications123,124. Indoor-light illuminance is is not reached, it is difficult to predict the lifetime based on the significantly lower (100 to 200 lux in a typical home, and 250 to observed ‘PCE versus time’ trend because of the variety of pos- 1000 lux in an office125, which corresponds to ~ 0.01 sun irradiance). sible curve shapes (see Fig. 2). In this case, we suggest ageing the The spectra of indoor-light sources also differ significantly from sample for at least 1000 h and using the PCE after 1000 h of stress natural sunlight, and there is still no standard spectrum for indoor (η1000, as a percentage of the initial PCE) as the stability figure of PV testing126,127. The share of LEDs in the indoor lighting market merit. If researchers choose to apply any type of extrapolation to is expected to increase due to their high lighting efficiency128. For determine T80 or TS80, it must be clearly differentiated from the PSCs intended for low-intensity illumination, we encourage device measured data. We recommend limiting the extrapolation times characterization at several intensity levels (for example, 200, 500, to less than one order of magnitude beyond the actual ageing 1000 lux) and reporting the light source (preferably LED) spectrum time. As PSC stability improves, it may become common to quote in accordance with the original ISOS procedures11. T95, which is the time required to degrade to 95% of the initial efficiency. This will also be in accordance with the IEC proce- Stability figures of merit and acceleration factors dures, where the pass criterion is for the modules, after exposure The time required to drop to 80% of the initial efficiency is com- to stress, to operate at > 95% of their starting performance. monly denoted T80 and often serves as a figure of merit for solar The presence of reversible degradation in PSCs makes it harder cell stability. It would, therefore, be an optimal minimum ageing to assess their stability. Although no broadly accepted figures of test time. Extrapolation of degradation data (or readily achieved T80 merit currently exist to account for partial reversibility, some proce- lifetimes) can be used to evaluate the cell lifetime energy yield129, dures are currently under debate8,74. Recovery effects can be studied which is an important parameter for calculating return on invest- in two types of experiments: continuous ageing followed by perfor- ment and life cycle analysis. Despite the apparent simplicity, there mance tracking after stress removal, or cycled stress experiments. are several approaches129,130 to determine T80 for devices (see Fig. 2 In the former case, it was suggested to correct T80 to account for and Table 5 for a summary). Figure 2 explains three approaches for the restoration done during the rest period8. In cycling experiments calculating T80, which are applicable to different ageing scenarios. (such as ISOS-LC), an analogue of the T80 metric might be intro- It is, therefore, vital to detail the metrics used when reporting sta- duced for the energy output per cycle74. bility studies. In particular, the original ISOS protocols11 suggested The ISOS testing protocols do not provide direct information on the use of a ‘stabilized T80 time’ (denoted TS80), which is the time the expected lifetime of solar cells under operational conditions. For during which the PCE decreases by 20% of its magnitude after an such evaluations, the concept of acceleration factor (AF) has been arbitrarily defined stabilization time (indicated in blue in Fig. 2f). used134. The AF is a constant that relates the times to failure in an This suggestion is based on the widely known shape of the ‘PCE accelerated stress test with that in a reference stress test. Once deter- versus time’ curve in OPVs, which reflects a rapid initial degrada- mined for an ageing protocol (and validated through real-world tion (‘burn-in’)129 followed by a stabilized region. Although similar operation), the AF provides an estimate of the solar cell lifetime dynamics occur in some PSCs4,131,132, it is not a universal trend for in a fast and reproducible manner in the laboratory. AF for each these devices. Figure 2 shows various examples, including the non- stress should, in principle, be derived from a physical model, such monotonic evolution of PCE with time. as the activation energy (that is, the Arrhenius factor) in thermal 44 Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy NaTure Energy Consensus Statement accelerated lifetime tests. To date, no reports have been made of Unified procedures for stability studies and consistency in data AFs of PSCs subjected to accelerated ageing, but they should appear reporting could lead to the creation of a large machine-readable as the technology matures. For organic solar cells, more attempts database on PSC stability. Machine learning (ML) methods145–147 have been made to deduce AFs for accelerated ageing42,134,135. AFs could potentially identify patterns in such data, detect statistically for most ISOS tests were determined relative to outdoor conditions significant stress factors, correlate repeated phenomena in different in northern Europe using OPV mini modules134. Overall, dark-stor- studies to detect universal degradation mechanisms and stabilizing age ISOS-D tests can yield AFs between 0.45 (ISOS-D-1) and 12 approaches, and predict lifetimes and failure modes. Information (ISOS-D-3). Light soaking ISOS-L tests gave an AF of 15 and 24 in from ageing measurements under the relevant stressors can optimize ISOS-L-1 and ISOS-L-2, respectively. the steps required for supervised learning algorithms. Ultimately, To further ramp up the development of stable PSCs, some sug- ML relies heavily on having sufficient quantity and diversity of gest using high-intensity light5,136–139, which has been used for deg- information from ageing tests to provide an accurate prediction of radation studies of perovskite absorbers136 and solar cells138 with device performance and degradation. Thus, ML algorithms must intensities up to 100 and 10 suns, respectively. Rough estimates be trained with extensive laboratory data so that predictions can be show that even moderate light intensities of several suns can tre- compared with actual performance measurements. Once working, mendously accelerate the degradation. For example, a PV operat- ML tools should provide knowledge extraction without the need to ing for 1000 h under continuous illumination with an intensity of do all the tests in Fig. 3. only 5 suns under 85 °C is estimated to provide an equivalent of tens To accommodate the large number of perovskites possibly of years of outdoor testing if degradation scales linearly with light suitable for PV, a shared-knowledge repository database has been intensity5. Such intensities are easily achievable with commercial proposed145, where positive and negative results from stability light sources and solar concentration. tests are considered equally important for ML (that is, not only Although high light intensities (up to hundreds of suns) can the champion cell but also the suboptimal or underperforming provide substantially higher AFs, such tests require careful, inde- cells aged under similar conditions), because they all represent pendent control of the solar cell temperature and illumination valuable training data. It is therefore critical that a trend emerges intensity140,141. Moreover, care should be taken to check whether the that all cells that undergo ageing tests are measured and reported same degradation mechanisms occur under high light intensity and for the duration, even total failures. At the laboratory scale, this under 1 sun. Such experiments were done for OPVs135,142,143 and pre- information could be shared if researchers report the conditions dicted lifetimes in the range of tens of years143,144. To the best of our used for device fabrication and testing through a common web- knowledge, no similar studies have yet been reported for PSCs. site. Progress in this direction might be significantly accelerated if researchers, who foresee their data being useful for ML, provide Protocols applications and outlook complete information (on device fabrication, performance and We suggest that future stability studies include at least one or, ide- stability) in standardized tabulated format (a possible template is ally, multiple ISOS tests with level 1 procedures being the minimum available in the Supplementary Information). A similar strategy requirement for testing. Using the labelling we proposed would can be extended from PSCs to other emerging PV systems. allow easy identification of the testing conditions used. We recom- mend reporting all the data indicated in Table 3 to foster transpar- Conclusions ency and reproducibility. We have presented our consensus on procedures for studying the Getting a better understanding of solar cell failure modes is criti- stability of perovskite solar cells. The protocols we suggest pri- cal for PSCs as they are still at the early stage of technology develop- marily rely on the original ISOS standards developed for OPV ment. The ISOS protocols we have discussed in the present article cells11, which have proven to be highly relevant for uncovering can be used to screen a variety of stressor combinations. Figure 3 various degradation pathways in PSCs. We have further extended summarizes the relationship between the protocols with the major the protocols with a set of testing procedures in accordance with currently known degradation factors for PSCs: atmosphere, tem- specific stability features of PSCs, including light–dark cycling perature, electrical bias and light. For each combination of light and (ISOS-LC) mimicking the diurnal cycle, study of solar cell behav- bias, nine types of ageing protocols might be suggested with respect iour under continuously applied bias in the dark (ISOS-V) and to temperature (ambient, elevated or cycled) and atmosphere (inert, protocols for studying intrinsic solar cell stability (indexed with ambient or controlled humidity). Apart from determining device ‘I’). We have indicated which figures-of-merit for device stability lifetime under specific conditions, the impact of the stress factors should be used to take into account the evolution of the PSC per- can be understood by comparing the results of different ageing pro- formance over time. To improve reproducibility, we also propose cedures with each other. For example, comparing protocols D-1 and a checklist for reporting results from PSC stability studies more D-2 provides insight on the effect of elevated temperature, compar- consistently. ing protocols D-2I, D-2, and D-3 provides insights on the effect of We hope that the guidelines for conducting and reporting sta- the atmosphere, comparing protocols D-2 and L-2 provides insights bility studies described in this paper will improve comparisons on the combined effect of light and heat, and so on. The arrows in between data from different laboratories and from different device Fig. 3 schematically depict these relationships. architectures. The set of procedures and practices suggested here The blank spaces in Fig. 2 can be filled with the corresponding serves as an intermediate stage in perovskite solar cell technology ageing procedures (by analogy with the additional ISOS tests we maturation, aimed at the identification of degradation pathways have suggested in this Consensus Statement). Other protocols can and the prospects for their mitigation. be constructed by varying the ‘fixed’ parameters (for example, tem- perature, light intensity, RH). We do not aim to cover all possibili- Methods ties with the ISOS protocols nor discredit studies with a systematic The work extends the outcomes of the round table discussion on PSC stability variation of a particular stressor. Nevertheless, these investigations assessment that took place during the 11th International Summit on Organic and would benefit from having common ‘reference points’ with other Hybrid Photovoltaics Stability (ISOS 11) in Suzhou, China, in October 2018 (http:// studies conducted at different laboratories, with different device isos11.csp.escience.cn/dct/page/1) The round table was followed by drafting of the discussed procedures and manuscript text, which were circulated between architectures, different perovskite materials and even different the contributing authors multiple times until a consensus was reached among the research questions. Consensual conditions, like ISOS protocols, authors. In cases when no single opinion was possible between all the co-authors may serve as such references. we extended the number of options presented. 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Energy Manufacturing Innovation under award no. 1824674. S.D.S. acknowledges the Mater. 3, 424–427 (2013). European Research Council (ERC) under the European Union’s Horizon 2020 research 141. Visoly-Fisher, I. et al. Concentrated sunlight for accelerated stability testing and innovation programme (HYPERION, grant agreement no. 756962), and the Royal of organic photovoltaic materials: towards decoupling light intensity and Society and Tata Group (UF150033). The work at the National Renewable Energy temperature. Sol. Energy Mater. Sol. Cells 134, 99–107 (2015). Laboratory was supported by the US Department of Energy (DOE) under contract 48 Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy NaTure Energy Consensus Statement DE-AC36-08GO28308 with Alliance for Sustainable Energy LLC, the manager and Competing interests operator of the National Renewable Energy Laboratory. The authors (J.J.B, J.M.L., S.D.S. is a co-founder of Swift Solar Inc; H.J.S. is a co-founder and CSO of Oxford PV M.O.R, K.Z.) acknowledge support from the ‘De-risking halide perovskite solar cells’ Ltd.; A.H. is a co-founder of Dyenamo; M.D. is an employee of Saule Tech; K.D. is an program of the National Center for Photovoltaics, funded by the US Department of employee at Fluxim AG. Energy, Office of Energy Efficiency and Renewable Energy, Solar Energy Technology Office. The views expressed in the article do not necessarily represent the views of the DOE or the US Government. H.J.S. acknowledges the support of EPSRC UK, Additional information Engineering and Physical Sciences Research Council. V.T. and M.Madsen acknowledge Supplementary information is available for this paper at https://doi.org/10.1038/ ‘Villum Foundation’ for funding of the project CompliantPV, under project no. 13365. s41560-019-0529-5. M.Madsen acknowledges Danmarks Frie Forskningsfond, DFF FTP for funding of the Correspondence and requests for materials should be addressed to E.A.K. or M.L.-C. project React-PV, no. 8022-00389B. M.G. and S.M.Z. thank the King Abdulaziz City Reprints and permissions information is available at www.nature.com/reprints. for Science and technology (KACST) for financial support. S.V. acknowledges TKI-UE/ Ministry of Economic Affairs for financial support of the TKI-UE toeslag project Open Access This article is licensed under a Creative Commons POP-ART (no. 1621103). RC thanks the grants for Development of New Faculty Staff, Attribution 4.0 International License, which permits use, sharing, adap- Ratchadaphiseksomphot Endowment Fund. A.D.C. gratefully acknowledges funding tation, distribution and reproduction in any medium or format, as long from the European Union’s Horizon 2020 Research and Innovation Program (grant as you give appropriate credit to the original author(s) and the source, provide a link to agreement no. 785219-GrapheneCore2 and no. 764047-ESPResSo). M.L.C. and H.X. the Creative Commons license, and indicate if changes were made. The images or other acknowledges the support from Spanish MINECO for the grant GraPErOs (ENE2016- third party material in this article are included in the article’s Creative Commons license, 79282-C5-2-R), the OrgEnergy Excellence Network CTQ2016-81911- REDT, the unless indicated otherwise in a credit line to the material. If material is not included in Agència de Gestiód’Ajuts Universitaris i de Recerca (AGAUR) for the support to the the article’s Creative Commons license and your intended use is not permitted by statu- consolidated Catalonia research group 2017 SGR 329 and the Xarxa de Referència en tory regulation or exceeds the permitted use, you will need to obtain permission directly Materials Avançats per a l’Energia (Xarmae). ICN2 is supported by the Severo Ochoa from the copyright holder. To view a copy of this license, visit http://creativecommons. program from Spanish MINECO (Grant no. SEV-2017-0706) and is funded by the org/licenses/by/4.0/. CERCA Programme/Generalitat de Catalunya. © The Author(s) 2020 1 Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel. 2Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, PVcomB, Berlin, Germany. 3Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Be’er- Sheva, Israel. 4Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany. 5European Commission, Joint Research Centre (JRC), Ispra, Italy. 6National Renewable Energy Laboratory, Golden, CO, USA. 7Department of Materials Science and Engineering, Friedrich Alexander University Erlangen Nürnberg, Erlangen, Germany. 8 Helmholtz Institute Erlangen-Nürnberg (HI-ErN), Forschungszentrum Jülich (FZJ), Erlangen, Germany. 9CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome Tor Vergata, Rome, Italy. 10Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA. 11Andlinger Center for Energy & The Environment, Princeton University, Princeton, NJ, USA. 12 Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok, Thailand. 13State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China. 14Light Technology Institute, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany. 15University of Grenoble Alpes, CEA, LITEN, INES, Le Bourget du Lac, France. 16Fluxim AG, Winterthur, Switzerland. 17Saule Technologies, Wroclaw Technology Park, Wroclaw, Poland. 18CSIRO Energy, Mayfield West, New South Wales, Australia. 19Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, USA. 20Department of Physics, University of Michigan, Ann Arbor, MI, USA. 21 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, USA. 22TNO - Solliance, High Tech Campus, Eindhoven, The Netherlands. 23Department of Chemistry, University of Rome La Sapienza, Rome, Italy. 24Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. 25Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. 26Department of Physics and Astronomy, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands. 27Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Jena, Germany. 28School of Electronic Engineering, Bangor University, Bangor, Gwynedd, Wales, UK. 29Department of Materials Science and Engineering, Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD, USA. 30Department of Materials Science and Engineering, University of California, Davis, CA, USA. 31Dalian National Laboratory for Clean Energy/Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning, Shaanxi, China. 32Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology; Institute for Advanced Energy Materials; School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, Shaanxi, China. 33Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ, USA. 34Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), Suzhou, China. 35SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark, Sønderborg, Denmark. 36University of Colorado Boulder, Boulder, Colorado, USA. 37Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Sion, Switzerland. 38Laboratório de Nanotecnologia e Energia Solar, Chemistry Institute, University of Campinas – UNICAMP, Campinas, São Paulo, Brazil. 39Department of Chemical Engineering, Boğaziçi University, Bebek, Istanbul, Turkey. 40School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, Korea. 41SPECIFIC, College of Engineering, Swansea University, Bay Campus, Swansea, UK. 42Institute of Materials Science, Technical University of Darmstadt, Darmstadt, Germany. 43IEK-5 Photovoltaik, Forschungszentrum Jülich GmbH, Jülich, Germany. 44Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Jena, Germany. 45Clarendon Laboratory, University of Oxford, Oxford, UK. 46Cavendish Laboratory, University of Cambridge, Cambridge, UK. 47Skolkovo Institute of Science and Technology, Moscow, Russia. 48IPCP RAS, Chernogolovka, Russia. 49Department of Materials, Imperial College London, London, UK. 50Department of Materials Science and Engineering, Yonsei University, Seoul, Korea. 51Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Bellaterra, Barcelona, Spain. *e-mail: keugene@bgu.ac.il; monica.lira@icn2.cat Nature Energy | VOL 5 | January 2020 | 35–49 | www.nature.com/natureenergy 49