Stability and Degradation in Hybrid Perovskites: Is the Glass Half

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Stability and Degradation in Hybrid Perovskites: Is the Glass Half-Empty or Half-Full? Ioannis Deretzis, Emanuele Smecca, Giovanni Mannino, Antonino La Magna, Tsutomu Miyasaka, and Alessandra Alberti J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00120 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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The Journal of Physical Chemistry Letters

Stability and Degradation in Hybrid Perovskites: Is the Glass Half-Empty or Half-Full? Ioannis Deretzis†, Emanuele Smecca†, Giovanni Mannino†, Antonino La Magna†, Tsutomu Miyasaka‡, and Alessandra Alberti†,*. †

Institute for Microelectronics and Microsystems (CNR-IMM), Zona Industriale - VIII Strada 5,

Catania 95121, Italy. ‡

Graduate School of Engineering, Toin University of Yokohama, 1614, Kuroganecho, Aoba,

Yokohama 225-8503, Japan.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ABSTRACT. Methylammonium lead iodide (CH3NH3PbI3) is an extensively used perovskite material with a remarkable potential for solar energy conversion. Despite its high photovoltaic efficiency, the material suffers from fast degradation when aging in atmospheric conditions and/or under sunlight. Here we review the principal degradation mechanisms of CH3NH3PbI3, focusing on the thermodynamic, environmental and polymorphic parameters that impact on the

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stability of the material. A critical analysis of the available data indicates that degradation under ambient conditions is a defect-generation process that is highly localized on surfaces and interfaces, while it is further enhanced above the tetragonal-cubic transition at ∼54 °C. Within this context, we discuss the conservative role of N2 and propose strategies for the emergence of industrially viable hybrid photovoltaics.

TOC GRAPHICS

KEYWORDS. CH3NH3PbI3, defects, surfaces, light, humidity, nitrogen.


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Hybrid perovskite photovoltaics have originally emerged as an innovation in the dye solar cell technology through the substitution of the sensitizing dye with methylammonium lead iodide and bromide nanocrystals1. Since then, academic and industrial research has oscillated between overwhelming enthusiasm for the outstanding solar cell efficiencies2,3 (nowadays above 22%) and increased skepticism for the poor stability of the related structures. Organic-inorganic perovskites exhibit remarkable physical properties for energy harvesting applications, like direct band-gaps4, large absorption coefficients and a wide absorption range (from infrared to ultraviolet)5, photon recycling mechanisms6 and micrometer diffusion lengths7 for the photogenerated electron-hole pairs. Moreover, hybrid perovskites are solution processable at low temperatures starting from inexpensive precursors, enabling a truly low-cost technology. There have been concerns, however, on the long-term stability of these materials under device operation that undermine their commercialization potential. Methylammonium lead iodide (CH3NH3PbI3), a most popular material in perovskite photovoltaics, is the target of our investigation. When exposed to atmospheric conditions or operating in solar cell architectures, CH3NH3PbI3 can rapidly degrade into solid and gas byproducts, although the exact degradation pathway is still a matter of scientific debate8,9; it includes decomposition mechanisms triggered by chemical interactions with molecular species (e.g. H2O, O2)10,11,12,13, often mediated by electromagnetic radiation14,15 or simply by the temperature16,17. In addition, a thermodynamic deterioration has been observed even in the absence of reactive agents or under vacuum conditions18,19. Ion migration20 and the diffusivity of external atoms within the perovskite lattice21,22,23 can further complicate the stability issue. Under typical solar cell operating conditions (20-80° C and full illumination), the decomposition process leaves an important structural fingerprint over the perovskite lattice: the ABX3 octahedral network (where A=


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CH3NH3+, B=Pb2+ and X=I-) turns into a photon-inactive hexagonal BX2-type layered structure, with completely different properties when compared to the initial perovskite (see Fig. 1a). Such transformation is irreversible, due to the high volatility of the gaseous organic/halide byproducts of the degradation process. The stabilization of CH3NH3PbI3 is therefore a fundamental question for its use in photovoltaics.

Figure 1: (a) Scheme of the CH3NH3PbI3 crystal structure and its transformation to the layered PbI2 crystal after degradation. (b) Overall XRD pattern of a CH3NH3PbI3 sample stored in N2 ambient for 15 months. The inset shows the (211) peak that is present only for the tetragonal phase of the material. (c) XRD pattern for the previous sample under thermal annealing in air at T=120° C, showing fast degradation towards the yellow PbI2 phase. All measurements were performed under dark conditions.


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Within this framework, a very first question that may arise regards the temporal limits of intrinsic stability for a non-encapsulated CH3NH3PbI3 sample. Fig. 1(b) shows the crystallographic structure for a CH3NH3PbI3 layer deposited on a TiO2 substrate and aged in a nitrogen environment for 15 months at room temperature (RT). The sample was initially prepared in a dry room with monitored humidity (< 20%) by a Cl-assisted one step deposition from a 40 wt % solution of PbCl2 and CH3NH3I (ratio 1:3)16. The X-ray diffraction (XRD) pattern of Fig. 1(b) shows the characteristic perovskite peaks located at 2θ=14.11° and 2θ=28.41°, along with a secondary peak at 2θ=23.5° that unequivocally identifies the tetragonal polymorph of CH3NH3PbI3. The intensity and texturing of the main XRD peaks are markers of a growth process that occurred through the formation of large-domains (microns in diameter, visible in the SEM image of Fig. 2(a)) along preferential directions17. Both the crystallographic and the morphological data show no evident sign of degradation, notwithstanding the long maturation period of the sample. Degradation can instead be progressively induced by annealing the sample at temperatures above RT or leaving the sample under vacuum conditions. The eventual degradation process depends on many parameters, including the morphology and quality of the starting perovskite, the boundary materials, the environment and its temperature. Fig. 1(c) shows selected diffraction patterns collected in air after annealing at T=120°C (isothermal annealing) during a three days experiment on the sample shown in figure 1(b). Under these conditions, the perovskite peaks gradually retreat and a new peak emerges at 2θ=12.6°, which is characteristic of PbI2. The degradation procedure continues until the whole sample collapses to PbI2 in ∼4100 min. The final morphology of the sample (see Fig. 2(c)) shows a mostly fractured layer with a granular habitus and additional elongated nanocrystals. Its formation is accompanied by a huge contraction of the initial mass (∼54%), which is responsible


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for the numerous fractures throughout the layer. The remarkable stability of CH3NH3PbI3 at RT under a controlled N2 ambient as well as its fast deterioration in atmospheric conditions under thermal annealing raise reasonable questions on the mechanisms that lead to degradation. This perspective focuses on the prototype organic-inorganic perovskite CH3NH3PbI3 due to its simple stoichiometry, facile growth and competitively high yield, attempting to examine its chemical, structural and thermodynamic strengths and weaknesses. In addition, strategies for the enhancement of its reliability in solar cell applications are proposed, based on the current literature status.

Figure 2: (a) SEM morphology of the initial CH3NH3PbI3 sample. (b) SEM image after a 4week storage period in air (the inset shows the effect of ageing at surfaces and grain boundaries). (c) Morphology of the PbI2-degraded sample after annealing in air at 120 °C.


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Figure 3: (a) Optical absorption coefficient for a CH3NH3PbI3 sample before and after a thermal cycle in humid air between 50-70 °C. (b) Normalized absorption coefficient integral at the 2.13.1 eV range for CH3NH3PbI3 samples annealed at 60°C under different gas environments (N2, Ar, O2, humid air). (c) The (002)/(001) XRD peak area of CH3NH3PbI3 in air and N2 under gradual annealing from 40 to 80 °C. (d) Percentage of remaining CH3NH3PbI3 as a function of time for a sample annealed at 120 °C in humid air, N2 and vacuum. The humidity in the air was 55 ± 5 %. Optical data were collected using a Xe lamp with irradiance of 10-2 mW/cm2; XRD data were collected under dark conditions. We initially address the thermodynamic behavior of CH3NH3PbI3. To this purpose, in situ experiments under inert gas environments and continuous heating allow to selectively study the intrinsic lattice modifications of the material (both reversible and irreversible) due to the


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temperature. A relevant approach was followed in Ref. 13. In this work, a CH3NH3PbI3 sample was subjected to isothermal annealing from 50°C to 70°C under a moderate overpressure of two different inert gases, N2 and Ar. A spectroscopic ellipsometer equipped with a heating stage system measured the optical constants of the material, allowing the calculation of the absorption coefficient during the annealing period. Results (Fig. 3(a-b)) interestingly showed that the behavior of the absorption coefficient under N2 and Ar was diverging. These data were compared with analogous experiments performed in the presence of interacting environments (humid air, O2), allowing to disentangle the thermodynamic deterioration of CH3NH3PbI3 occurring under Ar from that mediated by catalyst species like water and oxygen. The presence of nitrogen instead increased the optical response of the material under heating. In a second experiment, a CH3NH3PbI3 sample was loaded inside a close dome equipped with a heating stage integrated within an XRD chamber. The area under the diagnostic perovskite peak at 2θ=14.11° was used as a parameter to measure the perovskite mass-loss as a function of time during isothermal annealing under N2 or vacuum conditions16. In agreement with the optical experiment, nitrogen extended the stability of the material throughout the 30-80° C range of temperatures (Figure 3c). Only exceeding this range16 activated degradation (Fig. 3d), but, with a significantly slower rate than in vacuum, indicating that the external pressure can significantly affect the degradation process. Both experiments demonstrate that the degradation of CH3NH3PbI3 (within the temperature range of solar cell operation) is a surface and/or interface-initiated process that progressively proceeds towards the core of the material, rather than an intrinsic instability of the bulk. Otherwise, the degradation behavior upon equal thermodynamic conditions would have similar characteristics regardless of the type of testing environment. We additionally notice that the


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vulnerability of the material can potentially be related to any type of non-bulk region, namely free surfaces, grain boundaries, extended defects and interfaces with confining materials. In particular, the role of grain boundaries is still highly debated in the literature24,25. Our findings indicate that both grain boundaries and free surfaces can be centers of degradation. Hence, highquality growth and surface/interface engineering26 are primary aspects to take into account for building stable perovskite solar cells. In addition, the measurements show that the non-reversible degradation of CH3NH3PbI3 is a desorption process, as differences in the pressure of the inert ambient result in different degradation rates and kinetics. A related issue regards the gas byproducts arising from the decomposition events. Dualeh et al.27 initially proposed HI and CH3NH2 as the possible byproducts, based on stoichiometric considerations as well as on the experimental evidence of a two-step mass loss, attributed to the molecular masses of HI and CH3NH2, respectively. Such hypothesis was further corroborated by density functional theory calculations18, where the thermodynamic degradation was described as a statistical volatilization phenomenon at surfaces that breaks the defect equilibria of the CH3NH3PbI3 system. An experimental validation of this assumption took place by Knudsen effusion mass spectrometry within a temperature range of 53-134° C28. More recently, Juarez-Perez et al.8 performed thermogravimetric measurements coupled to mass spectrometry in the range of T=25-700 °C and heating rates from 5-20 °C per minute. They found that the degradation byproducts were CH3I and NH3. Alternatively, Nickel et al.29 using ionic current mass spectrometry, proposed the dissociation of the MA+ ions in CH3NH2 and molecular hydrogen. A possible explanation of these apparently diverging results may lie in the different experimental conditions (temperature, heating rate, illumination, pressure, ambient) that should give rise to different degradation pathways9. Moreover, the byproducts (as well as the degradation paths) are expected to further


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change under extreme conditions, e.g. under ultrahigh vacuum or e-beam solicitation occurring in a TEM chamber19,30. In this last case, the intensification of the degradation process causes the generation of metallic aggregates that should otherwise be absent. As an example, Fig. 4 shows a portion of a CH3NH3PbI3 grain aged for three days in ultrahigh vacuum (∼10-11 Torr). At the end of this period, a chemical analysis using energy-dispersive X-ray spectroscopy (see the inset of Fig. 4) showed the presence of Pb clusters throughout the sample.

Figure 4: STEM image taken from a portion of a large CH3NH3PbI3 grain after 3 days of storage in ultra-high vacuum (10-11 Torr) at room temperature, showing the formation of small Pb clusters. The thermodynamic behavior of hybrid perovskites within an N2 ambient deserves a separate comment. Comparative experiments in various gas environments have demonstrated that N2 has a stabilizing effect on CH3NH3PbI313. We argue that this is related to the high diffusivity of N2 molecules at the perovskite grain boundaries in conjunction with a weak interaction between the


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N2 gas and the material’s cage that obstacles the desorption of volatile species. Accordingly, optical measurements have shown that the N2 interaction with the perovskite material influences the absorption coefficient at energies that are sensitive to surface-related phenomena (3.14.5eV)13. N2 encapsulation is therefore envisaged as a viable method for material preservation during long periods. A different scenario is depicted when the material interacts with H2O and O2, which are both likely to be present during preparation or solar cell operation. Water has been identified as the main source of material degradation since the first studies on this topic10 and, in contrast to other molecular species (e.g. O2), its action appears to be independent from the presence of light17. Müller et al. have used infrared spectroscopy22 to demonstrate that H2O molecules can infiltrate into the perovskite lattice and occupy the space between the MA+ and the I- ions, even at humidity concentrations as low as 10%. Similar results were obtained through ab initio molecular dynamics calculations31. Two main mechanisms have been proposed to explain the damage induced by H2O in CH3NH3PbI3. The first mechanism considers that H2O has a catalytic action that simply accelerates the thermodynamic degradation of the material towards PbI2, through the deprotonation of the MA+ ions and the consequent release of molecular gases16,19. The main argument that supports this hypothesis evidences the structural dynamics of the degradation process at room temperature, which is very similar in both air and vacuum conditions19. Indeed, XRD measurements19 have shown that degradation in both cases proceeds through the gradual transformation of the tetragonal crystal towards a “more cubic” arrangement, prior to conversion in PbI2 (see Fig. 5). This loss of the tetragonal symmetry at room temperature accounts for an initial deterioration step against the structural integrity of the lattice through the generation of defects. Moreover, zero-order degradation kinetics have been estimated in humid


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air (in contrast to first-order kinetics in vacuum)16, indicating that the presence of H2O molecules induces a catalytic action in the degradation process.

Figure 5: (a) XRD spectrum for a fresh CH3NH3PbI3 sample at RT in dark (tetragonal phase), showing details of the peak located at 2θ=14.11° with its two constituent components at 2θ002=13.97° and 2θ110=14.14°. (b) The same measurement after 24 h maturation in air, showing a displacement of the (002) component at 2θ002=14.03°. This displacement indicates a contraction of the z-axis of the system and a rearrangement of the tetragonal lattice towards a “more cubic” configuration. A second degradation mechanism proposed in the literature21 considers the formation of mono- (CH3NH3PbI3·H2O) and dihydrate [(CH3NH3)4PbI6·2H2O] metastable phases, which, under certain humidity and temperature conditions can be dissolved into byproducts. The formation of these phases takes place at rather high humidity conditions (usually >70-80%) and can be partially reversible through exposure to dry air; but uncertainty exists on the degradation pathway. Leguy et al.21 and Yang et al.32 have argued that the dihydrate phase can give rise to a non-reversible degradation into PbI2 and other byproducts. On the contrary, Zhao et al.33 have argued that the monohydrate phase is primarily responsible for the material’s decomposition,


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through further separation in PbI2 and aqueous CH3NH3I. In both cases, the presence of a hydrated phase should in principle increase the volume of the material, which is something that has not been verified experimentally34. Nonetheless, the hydration process can take place at RT even in dark conditions17, making the protection of the material from water infiltration one of the main issues to deal with when building perovskite solar cells.

Figure 6: Scheme of the rotational state for the MA+ ions at different temperatures corresponding to the tetragonal (165 K, 295 K) and cubic (395 K) phases, based on ab initio molecular dynamics simulations (Refs. 17,35,36). An atomistic perspective for the infiltration of H2O in CH3NH3PbI3 crystals stresses the role of the MA+ ions and in particular, their rotational state as a function of the external temperature. This aspect is closely related to the order-disorder character of the tetragonal-cubic transition17 that takes place around 54°C. Fig. 3(c) shows the structural behavior of a CH3NH3PbI3 sample subjected to thermal annealing by gradually increasing the target temperature from 40 to 80 °C in air. The trend of the area under the diagnostic XRD peak at 2θ=14.11° shows that the perovskite mass starts to significantly respond to the heat stress only above the temperature of the tetragonal-cubic transition. A possible explanation of this degradation enhancement above the transition temperature considers the rotational state of the MA+ ions at surfaces/interfaces or


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grain/domain boundaries. Recent ab initio molecular dynamics simulations35,36 have evidenced that ordered MA+ can exist in the orthorhombic and tetragonal phases of CH3NH3PbI3 as a result of medium-range coordination between nearby MA+ ions (considering mean MA+-MA+ distances of ∼6.3 Å). This order can lead to an increased stability for the material (also through the formation of ferroelectric domains37,38), a reduced volatilization rate for the gaseous byproducts, a reduced H2O incorporation rate within the perovskite lattice and, in general, a reduced chemical reactivity with external species.17 Temperature annealing towards and above the tetragonal-cubic transition leads to a gradual loss of this spontaneous MA+ ordering17,36 (see Fig. 6) and consequently to a sharp increase of the degradation dynamics. The presence of this weakness point in the degradation behavior of the material implies that durability could be prolonged if solar devices were operating only within the limits of the tetragonal CH3NH3PbI3 phase (i.e. slightly below ∼50 oC), or if the phase transition could be upwards shifted through viable lattice engineering. A link between the intrinsic modification of the perovskite lattice under heating and the phenomenon of atomic inter-diffusion from boundary materials above this thermal threshold is also plausible. Apart from the adverse impact of humidity, numerous studies have evidenced the negative role of O2 in the lifetime of the perovskite material. O2- species are expected to be a principal reason of chemical degradation when the material is fully illuminated11,14,29. Aristidou et al.11 have shown the generation of O2- in a dry air ambient under illumination, through fluorescence emission measurements of CH3NH3PbI3 samples immersed in solutions containing hydroethidine. They indicated that the efficient extraction of electrons from hybrid perovskites during photovoltaic operation is fundamental for both their operability and stability. A similar mechanism lies beneath the undesirable effect of degradation during ultraviolet (UV)


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irradiation15, when the electron transporting material is an oxide semiconductor with a wide bandgap (e.g. TiO2). In this case, the photogenerated carries can enhance oxygen reactivity at the boundaries between the perovskite and the confining material that can lead to interface modification and a consequent strong deterioration of the solar cell properties. The substitution of wide-bandgap oxides with electron transporting layers that are not influenced by the presence of UV light (such as fullerenes and other metal oxides2) could drastically reduce this problem. We note here that illumination can be a significant drawback for the preservation of the material even in the absence of ambient O2.29 On the other hand, Brenes et al.39 proposed the constructive use of photo-activated oxygen and H2O in passivating the surface defects of CH3NH3PbI3 crystals. Alike O2, an adverse impact of I2 molecules has been recently reported, mostly under illumination conditions12. This aspect could further complicate the stability problem, as I2 can itself be a degradation byproduct, giving rise to an internal catalytic mechanism that promotes the corrosion of the material. The minimization of primary degradation mechanisms is therefore crucial in order to significantly moderate this issue. Finally, a subject closely related to material degradation is the migration of ions often observed in perovskite devices. Theoretical calculations have shown that migration barriers, which are mediated by the presence of vacancies, are low for both I- and MA+ ions40 and can be easily surpassed at typical solar-cell temperatures. Ionic conductivity has been linked to hysteresis phenomena as well as to device degradation due to ion accumulation at the contacts41,42,43. Similarly, the source of deterioration for perovskite solar cells is not always the photoactive layer, but the confining materials and/or metallic electrodes. Within this context, recent studies have pointed out the instability of the spiro-OMeTAD hole-transporting layer above 70-80 °C.


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At these temperatures, Domanski et al.23 have demonstrated by secondary ion mass spectroscopy that metallic Au nanoparticles diffuse from the contact into the hole transporting material and the perovskite layer. Moreover, Kumar Jena et al44 have shown the creation of big voids within the spiro-OMeTAD layer, at the areas below the metallic contacts. Both studies suggest that the commercialization of perovskite solar cells requires the consolidation of stable solar-cell architectures beyond the photosensitive layer. Considering the complexity and multiplicity of the previously discussed degradation mechanisms, the main question that arises is if the stabilization of CH3NH3PbI3 is feasible. The demonstration of stable hybrid perovskite systems for more than a year either through storage/encapsulation in N2 or by interface engineering26 effectively shows that stability is a realistic objective. To our opinion, the key strategy for the engineering of stable perovskite materials is undoubtedly their proper surface passivation or functionalization, in order to block molecular desorption, hinder the infiltration of external species within the perovskite lattice and reduce chemical interactions with the surrounding environment. However, as understood from the N2 paradigm, passivation is expected to be more effective if the passivating agent is in the gas phase, in order to diffuse through grain boundaries, interfaces and areas that are out of reach for solid materials. Moreover, the use of gas species should avoid the technological drawbacks related with the complex and costly processing steps needed for solid/liquid passivation. Alternatively, we envisage the encapsulation of solar cell devices in a pressurized N2 ambient as a viable preservation method, even though the industrial realization of similar systems is still missing. Avoiding the negative impact of the tetragonal-cubic transition may also be important for the long-term stability of CH3NH3PbI3, indicating that the material is more suitable for indoor applications. Nonetheless, the persistence of the tetragonal phase at higher temperatures could be


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engineered either structurally (e.g. through the growth of alloyed perovskite systems) or by the installation of slightly refrigerating units in solar modules. Finally, we mention that alternative approaches are now running, concerning the use of mixed organic and inorganic ions (CH3NH3+, CH(NH2)2+, Cs+, I-, Br-) during the growth process45, which appear to increase the stability of the perovskite structure. Among others, the substitution of CH3NH3+ by CH(NH2)2+ drastically improves the thermal stability of the material16. In conclusion, in this paper we have reviewed the main degradation mechanisms for CH3NH3PbI3 and discussed on the positive impact of a proper passivating gas-based ambient as well as of the tetragonal phase on the lifetime of the material. The former should interfere with atomic processes that lead to degradation at exposed areas (i.e. at surfaces, grain boundaries, interfaces or defects), while the later should delay the diffusion of external species within the perovskite lattice. We expect that surface passivation along with an adequate encapsulation technology against H2O will greatly improve the stability issue and lead to the realization of affordable photovoltaic devices. Moreover, the ferroelectric character of the tetragonal phase37,38 and the possibility to tune/control the polarization field could open up new perspectives for applications in sensors, actuators and light emitting diodes. Nonetheless, the challenges for the production of stable perovskite solar cells are still numerous and have to face a series of fundamental and technological questions (e.g. atomistic understanding of the degradation pathways, local effects of passivating layers or gas species, reduction of defects by lattice engineering, integration of stable electron/hole transporting materials, interface optimization, etc.). We prospect further theoretical and experimental studies on these topics in the forthcoming years.


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AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank Dr. Corrado Bongiorno for expert technical assistance. This activity was partially supported by the national project BEYOND NANO Upgrade (CUP G66J17000350007). I.D and A. L. acknowledge computational support from the CINECA consortium under project MD-HYPER.

REFERENCES

1

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells J. Am. Chem. Soc. 2009, 131 (17), 6050–6051 2 Shin, S.S.; Yeom, E.J.; Yang, W.S.; Hur, S.; Kim, M.G.; Im, J.; Seo, J.; Noh, J.H.; Seok, S.I. Colloidally prepared La-doped BaSnO3 electrodes for efficient, photostable perovskite solar cells Science 2017, 356 (6334), 167-171 3 Arora, N.; Dar, M.I.; Hinderhofer, A.; Pellet, N.; Schreiber, F.; Zakeeruddin, S.M.; Grätzel, M. Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20% Science 2017, 358 (6364), 768-771 4 Umari, P.; Mosconi, E.; De Angelis, F. Relativistic GW calculations on CH3NH3PbI3 and CH3NH3SnI3 perovskites for solar cell applications Sci. Rep. 2014, 4, 4467 5 Kim, H.S.; Lee, C.R.; Im, J.H.; Lee, K.B.; Moehl, T.; Marchioro, A.; Moon, S.J.; Humphry-Baker, R.; Yum, J.H.; Moser, J.E.; Grätzel, M.. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2012, 2, 591 6 Pazos-Outón, L.M.; Szumilo, M.; Lamboll, R.; Richter, J.M.; Crespo-Quesada, M.; Abdi-Jalebi, M.; Beeson, H.J.; Vrućinić, M.; Alsari, M.; Snaith, H.J.; Ehrler, B. Photon recycling in lead iodide perovskite solar cells. Science 2016, 351, 1430-1433 7 Stranks, S.D.; Eperon, G.E.; Grancini, G.; Menelaou, C.; Alcocer, M.J.; Leijtens, T.; Herz, L.M.; Petrozza, A.; Snaith, H.J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342, 341-344 8 Juarez-Perez, E.J.; Hawash, Z.; Raga, S.R.; Ono, L.K.; Qi, Y. Thermal degradation of CH 3 NH 3 PbI 3 perovskite into NH 3 and CH 3 I gases observed by coupled thermogravimetry–mass spectrometry analysis Energy Environ. Sci. 2016, 9, 3406-3410


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Latini, A.; Gigli, G.; Ciccioli, A. A study on the nature of the thermal decomposition of methylammonium lead iodide perovskite, CH 3 NH 3 PbI 3: an attempt to rationalise contradictory experimental results Sustainable Energy Fuels 2017,1, 1351-1357 10 Niu, G.; Li, W.; Meng, F.; Wang, L.; Dong, H.; Qiu, Y. Study on the stability of CH 3 NH 3 PbI 3 films and the effect of post-modification by aluminum oxide in all-solid-state hybrid solar cells J. Mater. Chem. A 2014, 2, 705710 11 Aristidou, N.; Sanchez‐Molina, I.; Chotchuangchutchaval, T.; Brown, M.; Martinez, L.; Rath, T.; Haque, S.A. The role of oxygen in the degradation of methylammonium lead trihalide perovskite photoactive layers Angew. Chem. Int. Ed. 2015, 54, 8208-8212 12 Wang, S.; Jiang, Y.; Juarez-Perez, E.J.; Ono, L.K.; Qi, Y. Accelerated degradation of methylammonium lead iodide perovskites induced by exposure to iodine vapour Nat. Energy 2016 2, 16195 13 Mannino, G.; Alberti, A.; Deretzis, I.; Smecca, E.; Sanzaro, S.; Numata, Y.; Miyasaka, T.; La Magna, A. First Evidence of CH3NH3PbI3 Optical Constants Improvement in a N2 Environment in the Range 40–80° C J. Phys. Chem. C 2017 121, 7703-7710 14 Bryant, D.; Aristidou, N.; Pont, S.; Sanchez-Molina, I.; Chotchunangatchaval, T.; Wheeler, S.; Durrant, J.R.; Haque, S.A. Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells Energy Environ. Sci. 2016, 9, 1655-1660 15 Leijtens, T.; Eperon, G.E.; Pathak, S.; Abate, A.; Lee, M.M.; Snaith, H.J. Overcoming ultraviolet light instability of sensitized TiO2 with meso-superstructured organometal tri-halide perovskite solar cells Nat. Commun. 2013, 4, 2885. 16 Smecca, E.; Numata, Y.; Deretzis, I.; Pellegrino, G.; Boninelli, S.; Miyasaka, T.; La Magna, A.; Alberti, A. Stability of solution-processed MAPbI 3 and FAPbI 3 layers Phys. Chem. Chem. Phys. 2016, 18, 13413-13422 17 Alberti, A.; Deretzis, I.; Mannino, G.; Smecca, E.; Sanzaro, S.; Numata, Y.; Miyasaka, T.; La Magna, A. Revealing a Discontinuity in the Degradation Behaviour of CH3NH3PbI3 during Thermal Operation J. Phys. Chem. C 2017, 121, 13577–13585 18 Deretzis, I.; Alberti, A.; Pellegrino, G.; Smecca, E.; Giannazzo, F.; Sakai, N.; Miyasaka, T.; La Magna, A. Atomistic origins of CH3NH3PbI3 degradation to PbI2 in vacuum Appl. Phys. Lett. 2015, 106, 131904 19 Alberti, A.; Deretzis, I.; Pellegrino, G.; Bongiorno, C.; Smecca, E.; Mannino, G.; Giannazzo, F.; Condorelli, G.G.; Sakai, N.; Miyasaka, T.; Spinella, C. Similar structural dynamics for the degradation of CH3NH3PbI3 in air and in vacuum ChemPhysChem 2015, 16, 3064-3071 20 Yuan, H.; Debroye, E.; Janssen, K.; Naiki, H.; Steuwe, C.; Lu, G.; Moris, M.; Orgiu, E.; Uji-i, H.; De Schryver, F.; Samorì, P. Degradation of methylammonium lead iodide perovskite structures through light and electron beam driven ion migration J. Phys. Chem. Lett. 2016, 7, 561–566 21 Leguy, A.M.; Hu, Y.; Campoy-Quiles, M.; Alonso, M.I.; Weber, O.J.; Azarhoosh, P.; Van Schilfgaarde, M.; Weller, M.T.; Bein, T.; Nelson, J.; Docampo, P.; Barnes, P.R.F. Reversible hydration of CH3NH3PbI3 in films, single crystals, and solar cells Chem. Mater. 2015, 27, 3397-3407 22 Müller, C.; Glaser, T.; Plogmeyer, M.; Sendner, M.; Döring, S.; Bakulin, A.A.; Brzuska, C.; Scheer, R.; Pshenichnikov, M.S.; Kowalsky, W.; Pucci, A. Water infiltration in methylammonium lead iodide perovskite: Fast and inconspicuous Chem. Mater. 2015, 27, 7835–7841 23 Domanski, K.; Correa-Baena, J.P.; Mine, N.; Nazeeruddin, M.K.; Abate, A.; Saliba, M.; Tress, W.; Hagfeldt, A.; Grätzel, M. Not all that glitters is gold: Metal-migration-induced degradation in perovskite solar cells ACS Nano 2016, 10, 6306-6314 24 Wang, Q.; Chen, B.; Liu, Y.; Deng, Y.; Bai, Y.; Dong, Q.; Huang, J. Scaling behavior of moisture-induced grain degradation in polycrystalline hybrid perovskite thin films Energy Environ. Sci. 2017, 10, 516-522 25 Chu, Z.; Yang, M.; Schulz, P.; Wu, D.; Ma, X.; Seifert, E.; Sun, L.; Li, X.; Zhu, K.; Lai, K. Impact of grain boundaries on efficiency and stability of organic-inorganic trihalide perovskites Nat. Commun. 2017, 8, 2230 26 Grancini, G.; Roldán-Carmona, C.; Zimmermann, I.; Mosconi, E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.; Graetzel, M.; Nazeeruddin, M.K. One-Year stable perovskite solar cells by 2D/3D interface engineering Nat. Commun. 2017, 8, 15684 27 Dualeh, A.; Gao, P.; Seok, S.I.; Nazeeruddin, M.K.; Grätzel, M. Thermal behavior of methylammonium leadtrihalide perovskite photovoltaic light harvesters Chem. Mater. 2014, 26, 6160-6164 28 Brunetti, B.; Cavallo, C.; Ciccioli, A.; Gigli, G.; Latini, A. On the thermal and thermodynamic (in) stability of methylammonium lead halide perovskites Sci. Rep. 2016, 6, 31896


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Nickel, N.H.; Lang, F.; Brus, V.V.; Shargaieva, O.; Rappich, J. Unraveling the Light‐Induced Degradation Mechanisms of CH3NH3PbI3 Perovskite Films Adv. Electron. Mater. 2017, 3, 1700158 30 Rothmann, M.U.; Li, W.; Zhu, Y.; Bach, U.; Spiccia, L.; Etheridge, J.; Cheng, Y.B. Direct observation of intrinsic twin domains in tetragonal CH3NH3PbI3 Nat. Commun. 2017, 8, 14547 31 Mosconi, E.; Azpiroz, J.M.; De Angelis, F. Ab initio molecular dynamics simulations of methylammonium lead iodide perovskite degradation by water Chem. Mater. 2015, 27, 4885-4892 32 Yang, J.; Siempelkamp, B.D.; Liu, D.; Kelly, T.L. Investigation of CH3NH3PbI3 degradation rates and mechanisms in controlled humidity environments using in situ techniques ACS Nano 2015, 9, 1955-1963 33 Zhao, J.; Cai, B.; Luo, Z.; Dong, Y.; Zhang, Y.; Xu, H.; Hong, B.; Yang, Y.; Li, L.; Zhang, W.; Gao, C. Investigation of the hydrolysis of perovskite organometallic halide CH3NH3PbI3 in humidity environment Sci. Rep. 2016, 6, 21976 34 Shirayama, M.; Kato, M.; Miyadera, T.; Sugita, T.; Fujiseki, T.; Hara, S.; Kadowaki, H.; Murata, D.; Chikamatsu, M.; Fujiwara, H. Degradation mechanism of CH3NH3PbI3 perovskite materials upon exposure to humid air J. Appl. Phys. 2016, 119, 115501 35 Deretzis, I.; Di Mauro, B.N.; Alberti, A.; Pellegrino, G.; Smecca, E.; La Magna, A. Spontaneous bidirectional ordering of CH3NH3+ in lead iodide perovskites at room temperature: The origins of the tetragonal phase Sci. Rep. 2016, 6, 24443 36 Deretzis, I.; La Magna, A. Exploring the orthorhombic–tetragonal phase transition in CH 3 NH 3 PbI 3: the role of atom kinetics Nanoscale 2017, 9, 5896-5903 37 Rakita, Y.; Bar-Elli, O.; Meirzadeh, E.; Kaslasi, H.; Peleg, Y.; Hodes, G.; Lubomirsky, I.; Oron, D.; Ehre, D.; Cahen, D. Tetragonal CH3NH3PbI3 is ferroelectric Proc. Natl. Acad. Sci. 2017, 114, E5504–E5512 38 Röhm, H.; Leonhard, T.; Hoffmann, M.J.; Colsmann, A. Ferroelectric domains in methylammonium lead iodide perovskite thin-films Energy Environ. Sci. 2017, 10, 950-955 39 Brenes, R.; Guo, D.; Osherov, A.; Noel, N.K.; Eames, C.; Hutter, E.M.; Pathak, S.K.; Niroui, F.; Friend, R.H.; Islam, M.S.; Snaith, H.J.; Bulović, V.; Savenije, T.J.; Stranks, S.D. Metal Halide Perovskite Polycrystalline Films Exhibiting Properties of Single Crystals. Joule 2017, 1, 155-167. 40 Eames, C.; Frost, J.M.; Barnes, P.R.; O’regan, B.C.; Walsh, A.; Islam, M.S. Ionic transport in hybrid lead iodide perovskite solar cells Nat. Commun. 2015, 6, 7497. 41 Chen, B.; Yang, M.; Zheng, X.; Wu, C.; Li, W.; Yan, Y.; Bisquert, J.; Garcia-Belmonte, G.; Zhu, K.; Priya, S. Impact of capacitive effect and ion migration on the hysteretic behavior of perovskite solar cells J. Phys. Chem. Lett. 2015, 6, 4693-4700 42 Almora, O.; Zarazua, I.; Mas-Marza, E.; Mora-Sero, I.; Bisquert, J.; Garcia-Belmonte, G. Capacitive dark currents, hysteresis, and electrode polarization in lead halide perovskite solar cells. J. Phys. Chem. Lett. 2015, 6, 1645-1652 43 Guerrero, A.; You, J.; Aranda, C.; Kang, Y.S.; Garcia-Belmonte, G.; Zhou, H.; Bisquert, J.; Yang, Y. Interfacial degradation of planar lead halide perovskite solar cells ACS Nano 2015, 10, 218-224 44 Jena, A.K.; Ikegami, M.; Miyasaka, T. Severe Morphological Deformation of Spiro-OMeTAD in (CH3NH3) PbI3 Solar Cells at High Temperature ACS Energy Lett. 2017, 2, 1760-1761 45 Singh, T.; Miyasaka, T. Stabilizing the Efficiency Beyond 20% with a Mixed Cation Perovskite Solar Cell Fabricated in Ambient Air under Controlled Humidity Adv. Energy Mater. 2017, 8, 1700677


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Stability and Degradation in Hybrid Perovskites: Is the Glass Half

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