Prospects for Mitigating Intrinsic Organic Decomposition in

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Prospects for Mitigating Intrinsic Organic Decomposition in Methylammonium Lead Triiodide Perovskite John A McLeod, and Lijia Liu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00323 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry Letters

Prospects for Mitigating Intrinsic Organic Decomposition in Methylammonium Lead Triiodide Perovskite John A. McLeod∗ and Lijia Liu∗ Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, Jiangsu, 215123 China E-mail: [email protected]; [email protected]

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ACS Paragon Plus Environment


Abstract Organometallic lead halide perovskites seem to be on the threshold of becoming viable commercial photovoltaics, however further improvements to the stability of these materials must be made before they can compete with existing photovoltaic technologies. Of the organometallic lead halide perovskites used in photovoltaics, methylammonium lead triiodide perovskite (MAPI) is perhaps the most studied, and understanding how MAPI degrades is crucial for developing strategies to improve stability. We discuss the experimental evidence behind several possible routes for MAPI to degrade into PbI2 and various organics, and how the decomposition path of MAPI may strongly depend on substrate, precursors, intrinsic organic defects, and morphology. Exploring the conditions required for MAPI to degrade according to a particular pathway is important not only from a fundamental materials chemistry perspective, but also for understanding intrinsic instability in MAPI-based photovoltaics and to develop strategies to improve stability.

Graphical TOC Entry CH3NH3PbI3 Perovskite CH3NH2 HI om ec

D n

?

tio

CH3I

?

si po

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PbI2

NH3

2


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The impact of organometallic lead halide perovskites on solar cell research can hardly be understated, and is likely well-known by anyone choosing to read this Perspective. Similarly, the difficulties with stabilizing these perovskites for long-term commercial use is also wellknown, and has been discussed at length in several review articles. 1–5 Improving perovskite stability in a working solar cell requires careful control of a number of factors: film morphology, encapsulation, choice of materials for electrodes and charge transport layers, etc. 1–3,6 Herein we focus our discussion on the degradation of the most common organometallic lead halide perovskite photovoltaic material, methylammonium lead triiodide CH3 NH3 PbI3 (MAPI). MAPI decomposition can be driven by two basic mechanisms; either “extrinsic” decomposition driven by additional ions or molecules entering MAPI and causing chemical or structural changes, or “intrinsic” decomposition driven by rearrangement of the elements already present in MAPI. For the former category, ambient exposure to oxygen and water is obviously a readily available route to introduce new reactive species that may degrade MAPI. 3,4,7 However great progress has been made with encapsulating MAPI within photovoltaics to mitigate extrinsic decomposition. The focus of this Perspective is intrinsic decomposition, particularly of the organic components in MAPI. The intrinsic decomposition of MAPI is very problematic, and may even point to MAPI being fundamentally unsuitable for long-lifetime photovoltaics. 8

Pathways for Organic Decomposition in MAPI: MAPI is a relatively soft and flexible material, 13,14 with a relatively low formation enthalpy (predicted to be around 0.1 eV per formula unit). 15 This suggests there may be a relatively low barrier to the evolution of MAPI back to PbI2 and organic components. Several possible intrinsic decomposition pathways have been suggested in the literature, some of these are show schematically in Figure 1. Other than the reversible decomposition of MAPI back to CH3 NH3 I (MAI) and PbI2 (which is obviously endothermic), two additional simple direct decomposition pathways

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Pb I C N H

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Evaporate

n

PbI2

Surface

MPI* Trapped MAPI

Figure 1: Schematic of various decomposition pathways for MAPI to turn into PbI2 , adapted from the literature. MAPI may decay directly to PbI2 with the release of HI and CH3 NH2 (in which some CH3 NH2 may remain trapped in grain boundaries 9 ), 1 or with the release of NH3 and CH3 I. 10 This latter route may include an unstable CH3 PbI3 stage (MPI*), 11 and may further involve CH3 I decomposing to I2 and Cn H2n on the surface. 12 result in CH3 NH2 and HI, or CH3 I and NH3 as shown schematically in Figure 1; the corresponding reactions are given by Equations 1 and 2. 1,10,16–18 HI, CH3 I, and NH3 are expected to evaporate from the substrate, and CH3 NH2 may also evaporate or be trapped at grain boundaries. 9

CH3 NH3 PbI3 (s)

−−→

CH3 NH2 (g) + HI(g) + PbI2 (s)

(1)

CH3 NH3 PbI3 (s)

−−→

NH3 (g) + CH3 I(g) + PbI2 (s)

(2)

A modification to Equation 2 wherein the CH3 I remains connected to the Pb-I framework, leaving an unstable CH3 PbI3 (MPI*) perovskite structure was also suggested. 11 This pathway, shown in Equation 3 and schematically in Figure 1 implies the evaporation of NH3 and

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CH3 I at different stages.

CH3 NH3 PbI3 (s)

−−→

NH3 (g) + CH3 PbI3 (s)

CH3 PbI3 (s)

−−→

CH3 I(g) + PbI2 (s)

(3)

A second modification to Equation 2 involves further decomposition of multiple CH3 I molecules into gaseous HI and random alkanes at the surface of the crystal, 12 as shown in Equation 4 and schematically in Figure 1.

nCH3 I(s)

in PbI

2 −−−−→

Cn H2n (s) + nHI(g)

(4)

All of the decomposition pathways result in the reappearance of PbI2 . There are conditions (chiefly light exposure) which permit further decomposition of PbI2 into metallic Pb, 19,20 but as PbI2 is already unsuitable as a photovoltaic material we will not give much focus here to further decomposition. Early on, dissociation according to Equation 1 was considered the most likely pathway. 1 Preliminary investigations suggested that this reaction would only occur at relatively high temperatures (above 250◦ C), 21 well above photovoltaic operating temperatures. The enthalpy of intrinsic MAPI defects was also investigated by ab initio studies; most of these restricted themselves to “elemental” vacancies (i.e. Pb, I, or CH3 NH3 vacancies or interstitials), 15,22,23 finding that these kinds of defects lead to unintentional doping. Only a few studies have considered the possibility of CH3 NH3+ decomposition, finding that the reaction described by Equations 1 and 2 require rather high dissociation energies (> 1 eV). 17,24 Taken together this suggests that the decomposition reactions given above are unlikely to happen in MAPI within a photovoltaic cell. However subsequent experiments revealed MAPI decomposition at much lower temperatures (85◦ C), by a process that leaves only PbI2 remaining. 25 Other work demonstrated MAPI decomposition to PbI2 at even more modest temperatures (45◦ C to 55◦ C) in light and even when encapsulated. 26 These findings suggest that intrinsic 5



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MAPI degradation is an important concern during photovoltaic operation. 2 (a) C 1s XPS H

N 1s XPS M CH3I/CH3NH2

L

Expected

NH3/CH3NH2

Observed Initial (Org.) Initial (MOx) CH3NH3I 290 288

S 286

(b) C K-edge XAFS P

284 282 406 404 Binding Energy (eV)

400

(c) FTIR CH3NH3I N-? MAPI 1:1

A

Expected Obs. (ITO) Obs. (TiO2) Calc. (CH3I) CH3NH3I

402

2:1 3:1 CH3I/C=C/C-M

280 282 284 286 288 Excitation Energy (eV)

C-I N-C 1000 600 500 400 Wavenumber (cm-1)

Figure 2: Schematic of spectral evidence for organic decomposition in MAPI taken from the literature (actual measured spectra from the literature simulated herein using Gaussian profiles and arctangents, where appropriate): (a) C 1s XPS and N 1s spectra showing the low (L) and high (H) binding energy C 1s features, and main (M) and shoulder (S) N 1s features; 8,27,28 (b) C K-edge XAFS spectra showing the pre-edge (P) and edge absorption (A) features; 9,11 and (c) FTIR spectra showing a C-I vibrational mode in MAPI prepared under MAI-rich conditions (the MAI:PbI2 ratio is noted on the figure). 29

X-ray and Infrared Spectroscopic Studies Point to Organic Volatility in MAPI: In fact, the volatility of the organic constituents of MAPI has been hinted at by virtually every X-ray photoelectron spectroscopy (XPS) study reported on these materials: there is almost always two carbon 1s states present and typically only one nitrogen 1s state, as shown in Figure 2(a). The higher energy C 1s state (labelled H in Figure 2(a)) is attributed to CH3 NH3+ , while the lower energy C 1s state (labelled L in Figure 2(a)) was initially attributed to carbon contamination, which is almost always present. 30,31 This is certainly a reasonable explanation. However the H and L states are present even when MAI is deposited in situ on a carbon-free PbI2 substrate, 27,32 and furthermore the L state often manifests first during progressive MAI deposition, before MAPI is fully formed. 27 The studies mentioned

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herein performed vacuum deposition below 110◦ C, 27 but as MAI is stable below 234◦ C, 21 it is unlikely that MAI decomposes at vapour phase. In the absence of other information, the simplest explanation for the two C 1s states would be that volatile organic impurities from impure MAI are accumulating on the PbI2 /MAPI substrate; this interpretation is incorrect since when MAI is deposited first onto a PbI2 -free substrate, there is only one clear C 1s state (H, from CH3 NH3+ ), and subsequent PbI2 deposition onto the MAI substrate produces MAPI with only the H state (see Figure 2(a)). 27,32 This suggests that MAI decomposition can occur during intercalation into PbI2 . The resulting MAPI film will then host organic defects, which can lead to MAPI degradation during operating conditions. Interpreting the C 1s states in XPS measurements of MAPI is tricky because there almost certainly is carbon contamination on MAPI prepared ex situ, and because the L state overlaps with the binding energy of typical carbon-carbon bonding in amorphous carbon. However the evolution of C 1s states during MAPI deposition in vacuum suggest that reactions more complicated than those in Equations 1 and 2 can occur, and thus the L state is now associated with either CH3 I or CH3 NH2 . 27,28,32 In fact, evidence abounds that decomposition in MAPI involves chemical changes to the organic components. During extrinsic MAPI decomposition after exposure to water, nitrogen is released from MAPI while carbon is retained, but the C 1s XPS intensity shifts entirely to the L state. 18 Thus water helps catalyse Equation 4: gaseous NH3 evaporates from MAPI when the sample is placed in vacuum for XPS measurements, while the hydrocarbons remain stuck within the reformed PbI2 . 18 This reaction (albeit without water exposure) also occurs in vacuum deposited MAPI: the C 1s XPS L state presents before any N 1s states appear, 27 also suggesting that CH3 NH3+ from the precursor can decompose to NH3 (which evaporates) and some hydrocarbon radical. However since this was observed during MAPI formation rather than decomposition, it appears that CH3 I forms and is incorporated into the Pb-I lattice, according to Equation 3. 27 As a CH3+ radical is not large enough to support the perovskite structure, this type of defect is unstable and would eventually collapse, possibly

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accelerating further decomposition in neighbouring stoichiometric MAPI unit cells. A further complication is that chemical changes that happen to CH3 NH3+ during MAPI formation may be substrate-dependent: during the initial cycles of vacuum deposition on a metal oxide substrate (SiO2 or ITO) only the L state is visible in the C 1s XPS, and there are no N 1s states. 27,28 During deposition on an organic substrate, such as polyethylenimine ethoxylate (PEIE) or poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), however, both C 1s H and L states are visible, as well as the main N 1s state with a clear shoulder (labelled M and S, respectively, in Figure 2(a)); 28 the S state is interpreted as due to NH3 or CH3 NH2 . 28 Note that even during 2-step deposition of MAI onto an existing layer of PbI2 the underlying substrate still seems to influence CH3 NH3+ decomposition, 27 which is likely due to the fractal intercalation of MAI into PbI2 during MAPI formation. 33 There is also evidence of CH3 I-like components in MAPI used as a photovoltaic. 11 C K-edge X-ray absorption fine structure (XAFS) spectroscopy, performed on a MAPI photovoltaic active layer, exhibits features that cannot be from CH3 NH3+ , nor a hydrocarbon. However they are a good match to features expected from a MPI*-like complex, as shown in Figure 2(b). 11 Although another study has interpreted these XAFS features as due CH3 NH2 , 9 this was from starting with the assumption that CH3 NH2 is present, and simply attributing all features unrelated to CH3 NH3+ to CH3 NH2 . Evidence of NH3 evaporation, followed by CH3 I evaporation is present in in situ XAFS, XPS, and grazing-incidence, wide-angle X-ray diffraction (GIWAXD) measurements of MAPI during thermally-induced decomposition. 34 It is worth stressing that the organic decomposition observed in XAFS is not entirely due to X-ray beam damage during XAFS measurements, 11 unless MAPI is under prolonged exposure to the X-ray beam, so the conclusion that at least some of the decomposition products are present prior to the measurements is justified. It also bears mention that these data may also give evidence to substrate-dependent decomposition: the pre-edge feature in the C K-edge XAFS related to decomposition (labelled P in Figure 2(b)) is much more intense

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relative to the main absorption edge (labelled A in Figure 2(b)) in MAPI deposited on ITO than on TiO2 . 9,11 Finally, new vibrational modes in MAPI prepared under MAI-rich conditions are present in Fourier transform infrared (FTIR) spectroscopy, as shown in Figure 2(c). 29 These modes are not found in pure MAI, and are related to bonds terminating in nitrogen or carbon that are not present in MAI. 29 It is unlikely that these new modes are due to CH3 NH2 , as the intensity of the new N-related mode is at least two orders of magnitude weaker than that of the new C-related mode, suggesting C:N ≫ 1 (NH3 is expected to evaporate much more quickly than CH3 I); rather one of these new vibrational modes is from CH3 I, the other from an unknown nitrogen complex. 29 This parallels many in situ XPS measurements revealing an intense L state but only a single N 1s state, or even the complete absence of detectable N 1s states — if CH3 NH2 is produced the L and S states should be roughly equal in intensity, if there are no detectable N 1s states then the decomposition products must be N-free. A very recent report on light- and heat-induced MAPI decomposition also using FTIR (among other techniques) corroborated these findings. 19

Direct Investigation of MAPI Decomposition by Mass Spectroscopy: The aforementioned XPS, XAFS, and FTIR investigations all indirectly reveal the products of MAPI decomposition. An early direct measurement of the products from MAPI during thermal decomposition using temperature-dependent XRD and mass spectrometry (MS) revealed the presence of CH3 NH2 . 35 The presence of NH3 was also detected, however, which may be the result of decomposition according to Equation 2; unfortunately the measurements did not probe high enough to detect either HI or CH3 I. 35 Subsequently, direct evidence favouring MAPI decomposition following Equation 2 was found with thermogravitic analysis (TGA) and MS: substantial quantities of NH3 and CH3 I gas are released during thermal decomposition of MAPI, while the release of HI and CH3 NH2 is negligible. 10 Inducing degradation by bombarding MAPI with low-energy electrons follows Equation 4 (as a secondary reaction

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following initial decomposition according to Equation 2): both nitrogen and iodine are lost, and Cn H2n forms at the surface. 12 However MAPI decomposition following Equation 1 also can occur, as found by Knudsen effusion mass spectroscopy (KEMS): both CH3 NH2 and HI are detected evaporating from the sample. 36 Direct measurement of the products of MAPI decomposition, therefore, produced seemingly contradictory evidence favouring either Equations 1 9,36 or 2. 10,11,29,34 The data reported in these studies is typically of high quality, and appropriate experimental controls were present: these seemingly contradictory findings cannot be attributed to experimental error or mistakenly identifying decomposition products. A subsequent carefully planned follow-up study using KEMS over a larger temperature range found CH3 NH2 and HI as the dominant products that evaporate from MAPI during thermal decomposition, although CH3 I and NH3 are also present and increase in significance at low effusion rates. 16 Thermodynamic analysis of the Gibbs energy change occurring in Equations 1 and 2 in suggests that Equation 2 is favoured; the clear experimental observation that Equation 1 dominates MAPI decay under rapid effusion rates, and that Equation 2 only becomes significant (although still a minority) under low effusion rates (i.e. closer to thermodynamic equilibrium conditions), suggest that the decomposition of MAPI is kinetically controlled. 16

Products of MAPI Decomposition: In an attempt to combine all of the findings on MAPI decomposition discussed above, we present Figure 3, showing the logarithmic ratio of the concentrations of CH3 NH2 and CH3 I (as representative of Equations 1 and 2, respectively) as a function of temperature reported in various studies. The concentrations of CH3 NH2 and CH3 I obtained from XPS, XAFS, and FTIR studies 11,19,27–29 were estimated based on the assumption that CH3 NH2 or NH3 are the only decomposition products containing nitrogen, and these estimates are admittedly rather crude. The thermodynamic curve was calculated using tabulated formation energies, following the Arrhenius equation, 16 and should be accurate for the temperature range reported here. The kinetic curve is based on

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Figure 3: Concentration ratios of CH3 NH2 and CH3 I in MAPI estimated from data from the literature. Kinetic and thermodynamic curves obtained from Latini et al.’s results. 16 Experimental data sets from: (1) KEMS during thermal decomposition, 16 (2) MS during thermal decomposition, 35 (3) TGA during thermal decomposition, 21 (4) XAFS of MAPI film, 11 (5) XPS during MAPI formation, 28 (6) FTIR after thermal degrading, 19 (7) XPS during MAPI formation, 27 (8) FTIR after in situ annealing, 29 and (9) MS of MAPI thermal decomposition. 10 fitting KEMS measurements in a narrower temperature range to the Arrhenius equation, 16 the dotted red lines indicate possible margins of error. See the Supporting Information for more details on how the data in Figure 3 was estimated. The early TGA and MS measurements support CH3 NH2 dominance (decomposition following Equation 1), 21,35 however, as previously mentioned, there were some ambiguities in these findings. Latini et al.’s findings unambiguously detect decomposition following Equation 1, and these measurements also follow the Arrhenius equation as expected 16 — a very good sign of reliable data. However all of the XPS, XAFS, and FTIR data discussed herein reveal a paucity of nitrogen-related features that make it unlikely for CH3 NH2 (i.e. any molecule with equal parts carbon and nitrogen) to be present in significant quantities. It is admittedly possible that decomposition following Equation 1 is followed and that a subsequent reaction removes NH3 , according to Equation 5.

nCH3 NH2

in PbI

2 −−−−→

11

Cn H2n + nNH3


(5)


However we think that this reaction is unlikely; alkanes produced by Equation 4 seem more likely given the reactivity of CH3 I with the Pb-I lattice (i.e. forming unstable MPI*, or I – ion exchange). Furthermore, regardless of the viability of Equation 5, CH3 I was unambiguously detected by Juarez-Perez et al. when CH3 NH2 was not, 10 and, in our opinion, the signal-tonoise ratio in their reported data suggest that any CH3 NH2 that may be present is at least an order of magnitude lower in concentration than suggested by the kinetic curve obtained from Latini et al. To make sense of all this, one must recognize that it may not be necessary to regard these different results as contradictory. The XPS- and FTIR-based evidence for CH3 NH3+ decomposition during MAPI formation, and the sensitivity of this decomposition to substrates and precursors, is a clear indication that a significant concentration of organic defects can be contained with a MAPI film that is otherwise (i.e. by XRD patterns or photovoltaic performance) a “good” material. Subsequent decomposition of MAPI likely starts with these organic defects; the nature and distribution of these defects may be the most important factor in not just the stability of MAPI, but also the products produced by decomposition. To address the rather different results obtained by Juarez-Perez et al. and Latini et al., note that the former prepared millimetre-sized MAPI single crystals from equal parts of PbI2 and MAI in gamma-butyrolactone solution, 10 while the latter prepared MAPI powder from an aqueous solution of lead acetate trihydrate, HI, and CH3 NH2 . 16 The explanation for the differing results, then, may be as simple as a question of surface to volume ratios: perhaps for a smooth single crystal the decomposition is closer to thermodynamic equilibrium than in a rougher and smaller powder sample (with, presumably, more avenues for gaseous species to escape from the bulk). Furthermore, Juarez-Perez et al. buried their crystal in alumina powder during thermal decomposition, 10 while Latini et al. contained their MAPI powder in a graphite effusion cell. 16 It is known that metal oxides can react with MAPI; in particular MAPI solar cells using Al2 O3 scaffolds are known to have superior stability to other metal oxides (like TiO2 ). 2 Is it possible that decomposition at a MAPI:Al2 O3 interface under

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thermodynamic, rather than kinetic control? It is difficult to say, but it is within the realm of possibility. For a third speculation, recall that MAPI prepared in MAI-rich conditions exhibits substantial C-I bonding. 29 It is possible that decomposition following Equation 1 is favoured in I-deficient MAPI, one possible reason may be that I vacancies provide channels for the larger CH3 NH2 to diffuse through. Finally, as previously mentioned, water catalyses MAPI decomposition following Equations 2 and then 4. 37 Water is present in the MS spectra in Juarez-Perez et al.’s study, and this may play a role in the decomposition. Unfortunately, however, the presence of water was not investigated in the other studies. 16,35,36

Prospects for Controlling MAPI Decomposition: Ultimately, one of the goals of understanding MAPI decomposition is to develop strategies to mitigate this in photovoltaic devices. In this regard it is crucial to recognize that decomposition of MAPI single crystals or free powders may not be representative of decomposition of MAPI films, especially those supported by metal oxide scaffolds. It is worth keeping in mind, as was stressed in another recent study, that decomposition according to Equations 1 and 2 may be reversible if the products are unable to escape (i.e. are trapped by charge transport layers or other forms of encapsulation). 38 With regards to decomposition following Equation 2, the presence of CH3 I within MAPI observed by several spectroscopic studies 11,27,32 raises an important question: is the primary source CH3 NH3 I decomposition upon contact with PbI2 , resulting in CH3 I intercalating layered PbI2 prior to MAPI formation (as shown schematically in Figure 4(a)); or is it CH3 I and NH3 created after MAPI formation (possibly driven by the release of energy from MAPI formation, as shown schematically in Figure 4(b))? If it is the former case, then careful addition of sacrificial agents in the precursor mixture may prevent CH3 I intercalation, i.e. by reacting with the CH3 I to form other compounds that may be washed off after MAPI is formed. If it is the latter case, then careful control of growth conditions may mitigate CH3 I formation. CH3 I within the perovskite, producing the MPI* structure, is certainly

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structurally less stable than pristine MAPI, so understanding this issue is important for improving MAPI stability. (a)

(b)

(c)

(d)

Organics PbI2

PbI2 MAPI (e) Organics

PbI2

MAPI

Figure 4: Possible mechanisms for CH3 I to become incorporated in MAPI: (a) PbI2 catalysis decomposition of CH3 NH3 I, leading to CH3 I intercalating PbI2 layers prior to MAPI formation; (b) energy release from MAPI formation causes some CH3 NH3+ to dissociate into NH3 and a CH3 I complex; and (c) ion transport through MAPI lowers energy barrier for CH3 NH3+ dissociation. Possible means of MAPI thermal decomposition: (d) at the surface and (e) fractally throughout bulk, both leading to layered PbI2 and evaporated. Indeed, it has been emphasized that while ab initio calculations suggest CH3 NH3+ has a relatively large dissociation energy, making dissociation unlikely near thermodynamic equilibrium in perfect stoichiometric MAPI, one should not assume that these organic defects from CH3 NH3+ dissociation do not form during nonequilibrium processes such as MAPI formation. 24 The situation is further complicated because ab initio calculations and indirect experimental evidence suggest MAPI is rather efficient at ion transport (of I – through I vacancies). 39 Even after MAPI is formed, it is possible that other dynamical processes such as I – transport may also lower the energy barrier for CH3 NH3+ decomposition, as shown schematically in Figure 4(c). The non-equilibrium dynamical processes available in MAPI under photovoltaic operating conditions are thus much more complicated than in most inorganic semiconductor photovoltaics. The possible influence of dynamic processes such as (but 14


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not limited to) the build-up of long-diffusion length charge carriers 40 and ion transport 39 on MAPI degradation needs to be carefully considered. Further research on the influence of non-equilibrium dynamical processes is needed, as is research on how CH3 NH3 I may dissociate during MAPI formation. It is important to recognize that MS thermal decomposition experiments, while undoubtedly valuable, only measure the products that evaporate from the sample: intrinsic defects (like MPI* complexes) may undergo further reactions before evaporating. Inducing thermal decomposition in MAPI may therefore not be the best way to understand how MAPI decomposes in photovoltaic devices. That being said, the possibly contradictory results from MS studies do emphasize that further research investigating decomposition in MAPI with different morphologies (monocrystal, powder, film, etc.), prepared from different precursors, or formed from different phases (i.e. precipitated from solution, or from vapour deposition 32 ) is necessary. Further research also must be directed at examining the morphology of MAPI during decomposition. Does MAPI decomposition occur layer-by-layer from the surface inwards, as shown schematically in Figure 4(d)? On the other hand, as MAI fractally intercalates PbI2 during MAPI formation, 33 does the reverse prove true for MAPI decomposition, as shown schematically in Figure 4(e)? Knowing whether decomposition proceeds from the surface inwards, or spreads from defects within the bulk is crucial to understanding why different MAPI samples exhibit different decomposition products, and also essential to assessing to what degree the results from decomposition studies on pristine MAPI samples can be expected to apply to MAPI encapsulated within a photovoltaic device. In a similar vein, it is also especially important to understand how contact with electrode or carrier-transport layer materials influences MAPI decomposition.

Summary and Conclusions: Understanding MAPI decomposition is important to developing strategies for improving the performance and stability of MAPI-based photovoltaics. Experimental evidence makes it fairly clear that MAPI can decompose following Equations 1

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and 2. Which reaction dominates seems to be a question of whether the decomposition occurs under kinetic or thermodynamic control. Although the exact conditions that lead to one reaction over the other are not fully understood, the preparation method, morphology, and interfaces with other materials all have some influence. Experimental evidence also strongly suggests that newly-formed MAPI may already host organic defects. It seems likely that the defects accelerate decomposition in MAPI, so it is important to investigate how the preparation method, morphology, and interfaces with other materials influence the initial concentration of these defects. It is also important to consider that organic defects may not always have a negative influence on device properties. For example, deliberately adding CH3 I to the MAPI precursor solution results in MAPI with improved morphology, higher photoconversion efficiency, and dramatically higher photoluminescence lifetime. 41 As another example, annealing MAPI in CH3 NH2 fuses grain boundaries, improving stability as well as morphology and photoconversion efficiency. 42 On the other hand, exposing MAPI to I2 vapour leads to accelerated decomposition, 8 while undesirable this does suggest that MAPI may be more stable when prepared in iodine-poor conditions. In this context, investigating the effect of adding HI to MAI and PbI2 precursors in solution, or the effect of annealing MAPI in NH3 , are of immediate interest.

Acknowledgement This work is funded by National Natural Science Foundation of China (NSFC) project numbers U1432106 and 21550110188. We acknowledge the support from Soochow UniversityWestern University Center for Synchrotron Radiation Research, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), Soochow University.

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