First-Principles Insight into the Degradation Mechanism of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

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First-Principles Insight Into The Degradation Mechanism of CHNHPbI Perovskite: Light-Induced Defect Formation and Water Dissociation Chao Peng, Jian-Fu Chen, Haifeng Wang, and Peijun Hu

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07294 • Publication Date (Web): 12 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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The Journal of Physical Chemistry

First-Principles Insight into the Degradation Mechanism of CH3NH3PbI3 Perovskite: Lightinduced Defect Formation and Water Dissociation Chao Peng,†,‡ Jianfu Chen,*,† Haifeng Wang*,† and P. Hu†,‡ †

Key Laboratory for Advanced Materials, Center for Computational Chemistry and Research

Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China ‡

School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast

BT9 5AG, UK

ABSTRACT: The organic/inorganic hybrid CH3NH3PbI3 perovskite shows promising features in light harvesters, but its instability under humid environment seriously restricts its application. To improve the long-term stability, it is imperative to understand the degradation mechanism at the atomic level. Here, we apply the DFT+U method to systematically investigate the effect of light and the chemical interaction between the orthorhombic CH3NH3PbI3 (ort-CH3NH3PbI3) and water, and some key roles of water/light causing instability of ort- CH3NH3PbI3 are identified. Firstly, we demonstrate that the identified U values (I: 8 eV, Pb: 9 eV) in the DFT+U approach together with spin-orbit coupling can well describe the electronic and basic chemical properties of ort-

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CH3NH3PbI3, which is a good balance between the accuracy and computational cost. Secondly, the photogenerated hole is revealed to thermodynamically promote the formation of surface and bulk iodine vacancies in ort-CH3NH3PbI3(100) system that may induce the hysteresis behavior in current-voltage (J-V) curves and aging of perovskite solar cells. More importantly, the formed defect of iodine vacancy would induce H2O to undergo irreversible dissociation. Thirdly, on the perfect ort-CH3NH3PbI3(100) surface in the dark, H2O is found to be difficult to dissociate and incline to molecularly adsorb; in contrast, with a photogenerated electron involved, the dissociation of H2O molecule becomes favorable with a decreased barrier of 0.57 eV. H2O dissociation results in the formation of hydroxyl anion (OH-), which can strongly interact with CH3NH3+ and lead to the formation of CH3NH2, thereby accelerating the ort-CH3NH3PbI3 degradation.

1. Introduction Methyl ammonium lead iodide organic/inorganic hybrid perovskite (CH3NH3PbI3), acting as one of the most promising photovoltaic materials recently, has attracted considerable attention due to its unique photovoltaic properties such as proper band gap, long charge carrier diffusion length, low effective masses and high light absorption coefficient. Serving as an outstanding light harvester, the power conversion efficiency (PCE) of CH3NH3PbI3 perovskite has exceeded 22% in recent years.1-12 Despite its high performance and great progress in efficiency, the instability of CH3NH3PbI3 perovskite remains a critical obstacle.13-17 The ionic defect migration was identified to affect PCE and stability.18-21 The calculations of the anionic and cationic vacancy migrations showed that I‒ anions can easily diffuse in iodic vacancies with a low barrier of 0.45 eV in both CH3NH3PbI3 and (NH2)2CHPbI3.18 The diffusivity of iodine was even reported as high as 4.3 × 10-6 cm2 s-1 at 300 K in iodic defective system.20 The high mobility of iodine in vacancies was


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found to result in the occurrence of the well-known hysteresis phenomenon in current-voltage (JV) curves.21 In addition, easy diffusion of ions in perovskites was also identified to induce aging of perovskite solar cells.22 Thus, understanding the formation process of defects is important to the stability and development of perovskite solar cells. Another critical obstacle is the environmental humidity and is considered as one of the murderers for perovskite degradation.15,23-30 When perovskite solar cells were stored under ambient air, more than an 80% drop of the initial performance of solar cells was found over 24 hours, and a 95% drop of the performance was identified after six days. However, when it stored in either dry air or nitrogen, the solar cells can retain 80% of the initial performance after 24 hours and 20% after 6 days.11 Under sun illumination (100 mWꞏcm-2), Han et al. verified the good performance of the encapsulated CH3NH3PbI3 perovskite solar cell at room temperature and zero humidity, but the degradation of CH3NH3PbI3 phase became significant with the increase of humidity.29 Several possible degradation pathways of CH3NH3PbI3 under humid condition have been proposed recently. By monitoring phase change in perovskite degradation, Yang et al. reported a hydrated intermediate containing an isolated PbI64- octahedra formation during the degradation.23 Based on the interaction between CH3NH3PbI3 and H2O vapor, it was found that H2O vapor does not induce CH3NH3PbI3 to PbI2 in the dark, but promotes the formation of a hydrate product which is similar to (CH3NH3)4PbI6ꞏH2O.27 Although CH3NH3PbI3 perovskite shows sensitivity to H2O and this might be due to its solubility, its stability was also found to be highly affected by light. Under illumination, the CH3NH3PbI3 perovskite was reported to dissociate into CH3NH3I and PbI2 in the presence of H2O, and the decomposition of formed CH3NH3I can continue, leading to CH3NH2 and HI.30 Although understanding on the degradation mechanism of CH3NH3PbI3 perovskite has been improved to some extent,21,23,24,28-36 many fundamental issues of the stability



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of CH3NH3PbI3 perovskite in the presence of moisture on the atomic-scale remain elusive. In particular, the following questions remain to be answered: How does H2O interact with the CH3NH3PbI3 surface? Why does CH3NH3PbI3 perovskite show difference in degradation with and without light? Thus, it is very desirable to reveal the role of H2O molecules and uncover the degraded mechanism of CH3NH3PbI3. In this work, we present a first-principles study on the stability of orthorhombic CH3NH3PbI3 (ort-CH3NH3PbI3) based on the state-of-the-art density functional theory (DFT) calculations. Particularly, the DFT+U approach, with an appropriate U (U-J) value successfully screened, was employed to describe well CH3NH3PbI3 instead of time-consuming hybrid functional calculations. Using this efficient approach, we characterized the surface and defect properties of ortCH3NH3PbI3 with and without light. By investigating the interaction between H2O and ortCH3NH3PbI3 surface, some insights into the degradation mechanism of CH3NH3PbI3 were obtained. 2. Model and computational details To model ort-CH3NH3PbI3, the stable orthorhombic bulk, containing 24 H, 4 C, 4 N, 12 I and 4 Pb atoms, was used.37 A p (1×2) stoichiometric ort-CH3NH3PbI3(100) surface (a= 12.60 Å, b = 17.09 Å, c = 35.85 Å; α= β= γ= 90°) with 5 atomic layers was constructed to investigate the degradation of perovskite.38 The vacuum layer is 15 Å as shown in Figure 1. In structural optimization, the bottom two layers were fixed, while the top three layers were fully relaxed. All the spin-polarized calculations were performed with Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation using the Vienna ab-initio simulation package (VASP) code.39-41 The project-augmented wave method was used to represent the corevalence electron interaction.42,43 The valence electronic states were expanded in plane-wave basis


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sets with energy cutoff at 450 eV, and the Pb 6s, 6p and 5d orbitals, I 5s and 5p orbitals, C 2s and 2p orbitals, N 2s and 2p orbitals, and the H 1s orbital were treated as valence states. The transition states were searched using a constrained optimization scheme, and were verified when (i) all forces on atoms vanish and (ii) the total energy is a maximum along the reaction coordination but a minimum with respect to the rest of the degrees of freedom.44-47 All the transition states were verified by vibration analysis. The force convergence criterion in structural optimization was set to be 0.05 eV/Å. The (2×2×2) k-point was used for U screening, and 61 k points was computed along the A-G symmetry line for the calculations of band structures. A (2×1×1) k-point was used for surface optimization. To correctly describe the electronic structure of ort-CH3NH3PbI3, an DFT+U method including the on-site Coulomb interaction, together with spin-orbit coupling (SOC), was employed,37,48-51 where the Hubbard-type correction was applied on Pb’s 6p orbitals and I’s 5p orbitals. The absolute energy levels of ort-CH3NH3PbI3 were extensively studied to achieve an accurate group of U, and PBE0 (with an optimized fraction of exact exchange by α= 0.188) and HSE06 functionals were adopted as benchmarks to screen U values.52 DFT-D3 method was employed to describe the weak interaction in that inorganic-organic system,53,54 which shows great consistence with experiments for the optimized lattice constant of orthorhombic CH3NH3PbI3 and more approachable than that from DFT-D2, as shown in Table S2.53,54 To simulate a photogenerated hole or electron in the consideration of simplifying the complicated photoexcited system, we removed or added an electron from the periodic ort-CH3NH3PbI3 system, and similar approaches were used and verified in previous work.18,55,56 The adsorption energy is defined as Eads= E(adsorbate/surf)- E(adsorbate)- E(surf), where E(adsorbate/surf), E(adsorbate) and E(surf) represent the energies of CH3NH3PbI3 surface with adsorbate, adsorbate in the gas and clean CH3NH3PbI3 surface, respectively. The formation energy



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of I vacancy (IV) is defined as EV(I) = Esys_vac+ 1/2EI2 –Esys, where Esys_vac and Esys are the energies of systems with and without IV, respectively, while EI2 is the energy of I2 molecule in gas phase. For the calculations of IV formation, we also used larger supercells to calculate the formation energies and found consistent results (see Supporting Information).

Figure 1. Side view and front view of p (1×2) orthorhombic CH3NH3PbI3(100) surface. The light black, purple, brown, silver and gray balls represent Pb, I, C, N and H atoms, respectively. This notation is used throughout the paper. 3. Results and discussion 3.1 U screening in DFT+U approach To calculate CH3NH3PbI3 perovskite, the standard DFT coupled with SOC would underestimate the band gap, and the self-interaction error in standard DFT prefers the delocalization of photogenerated electrons and holes. To better describe the basic properties of ort-CH3NH3PbI3 with a balance of computational cost, we made the effort to employ DFT+U approach in ortCH3NH3PbI3 perovskite, where the electronic structures and chemical properties were taken as the guide to screen the U values.


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In ort-CH3NH3PbI3 perovskite, the valence band maximum (VBM) is mainly composed of I-5p orbitals while the conduction band minimum (CBM) is dominated by Pb-6p orbitals. To obtain an accurate band gap, the Hubbard U terms were added on 6p orbitals of Pb and 5p orbitals of I atoms to adjust the energy levels of CBM and VBM. Figure 2a shows a contour map describing the change of band gap as a function of U values of I and Pb atoms. The black curve represents the reported band gap of 1.63 eV, 37,50,57,58 and the intersection on this curve extending to the X and Y axes are the appropriate U values of Pb and I atoms. To keep the band gap at 1.63 eV, the U value of Pb-6p varies in a narrow range from 7.4 to 9.7 eV, while the U value of I-5p changes considerably from 0 to 10 eV. The U value of Pb atoms plays a more decisive role in adjusting the band gap than that of I atoms in ort-CH3NH3PbI3. This can be seen from the shift of energy levels of CBM and VBM (see Table S3). As the U value of I-5p increases, the energy level of VBM increases slightly and that of CBM decreases mildly, resulting only in a small reduction of band gap. By contrast, the energy level of VBM decreases greatly and that of CBM rises sharply with the increasing of Pb-6p U value, leading to the variation of band gap from 0.75 to 1.88 eV and thereby determining the band gap.



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Figure 2. (a) the contour map of the band gap with respect to the U values of I-5p and Pb-6p in the ort-CH3NH3PbI3. The color bar represents the value of band gap. The black line shows the band gap at 1.63 eV. (b) the energy levels of VBM and CBM by different theoretical methods. (c~f) four band structures calculated by different U groups; the high symmetry points are A (0.5, 0, 0), G (0, 0, 0), Z (0, 0.5, 0), Q (0, 0.5, 0.5), X (0.5, 0.5, 0.5) and U (0, 0, 0.5), and the Hubbard U pairs are U1 (I: 1 eV, Pb: 7.5 eV), U2 (I: 3 eV, Pb: 8 eV), U3 (I: 6 eV, Pb: 8.5 eV) and U4 (I: 8 eV, Pb: 9 eV), respectively. It is clear from above discussions that more than one pair of U can adjust band gap to 1.63 eV (the band structures adjusted by these U values are shown in Figure S1). Moreover, the edges of band gap including VBM and CBM on different computational levels were also compared. Four pairs of U values were achieved that agree well with the results from PBE0, as shown in Figure 2b. The SOC term has a dominant effect on band gap and reduces the band gap to 0.76 eV by standard PBE functional, which is highly consistent with other reports.50,51 More importantly, we find that the absolute energy levels of VBM by these four selected U pairs are all around 1.6 eV


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and those of CBM are about 3.2 eV, which are also in good agreement to EVBM and ECBM from the reported high-precision PBE0 functional.50 The corresponding band structures acquired by these four U pairs are shown in Figure 2(c~f). It is found that these band structures have similar parabolas along the high symmetry points and suitable values of band gap at the Gamma point. Therefore, all these four U pairs can accurately describe electronic structures of CH3NH3PbI3. We also calculated the effective masses of electron and hole (me and mh, respectively) in ortCH3NH3PbI3 using these four pairs of U values. The effective masses are derived by parabolic band fitting at Gamma point in Brillouin zone. As shown in Table 1, both me and mh tend to decrease mildly as U values increase (from U1 to U4). Despite the small difference of effective masses from U1 to U4 identified by the homogeneous band dispersion of CBM and VBM at Gamma points (see Figure 2c~f), the effective masses given by U4 (0.21 and 0.32) are quite close to previous results of the tetragonal phase using high-precision GW+SOC method.51 The standard PBE calculations with and without SOC are also presented in Table 1 for comparison. The values of me and mh with and without SOC are larger than that from U4, indicating the overestimated effective masses from the standard PBE functional. Thus, judging from the term of me and mh in ort-CH3NH3PbI3, U4 setting (9 eV and 8 eV for Pb-6p and I-5p, respectively) seems to be more applicable. Table 1. Formation energies (EV(I)) of I vacancy (IV) and effective masses (relative to the electron mass m0) of electrons and holes (me and mh) in ort-CH3NH3PbI3 bulk. Four pairs of U values are U1 (I: 1 eV, Pb: 7.5 eV), U2 (I: 3 eV, Pb: 8 eV), U3 (I: 6 eV, Pb: 8.5 eV) and U4 (I: 8 eV, Pb: 9 eV).

PBE

EV(I)/ eV

me

mh

none SOC

2.58

0.23

0.37

SOC

1.84

0.26

0.33



U1

1.93

0.21

0.34

U2

2.10

0.21

0.33

U3

2.32

0.21

0.33

U4

2.56

0.21

0.32

HSE06 (SOC)

2.67

/

/

GW(SOC)

/

0.19a

0.25a

PBE+U (SOC)

a

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Those effective masses obtained from tetragonal phase of CH3NH3PbI3 in Ref. 51

In addition to the electronic structure, chemical properties are also important for U screening. Thus, we examined the dependence of an important parameter relating to the stability of ortCH3NH3PbI3, i.e. the formation energy of bulk IV (EV(I)), on U values. As can be seen from Table 1, EV(I) increases from 1.93 to 2.56 eV as U value changes from U1 to U4; compared with the highprecision HSE06 hybrid functional method, U4 group (I: 8 eV, Pb: 9 eV) is found to be the most accurate one in describing the formation energy of IV (2.56 eV) and thus was chosen to investigate the degradation mechanism of ort-CH3NH3PbI3 perovskite. For comparison, we also examined IV formation energies with and without SOC using standard PBE functional. EV(I) were calculated to be 1.84 and 2.58 eV, respectively. The formation energy of with SOC is smaller than that without SOC by 0.74 eV. Although SOC has been previously reported to have an insignificant effect on structure optimization in CH3NH3PbI3,59,60 it has a substantial impact on IV formation. The SOC effect should not be neglected in exploration of chemical properties. 3.2 Properties of charge carriers on the CH3NH3PbI3 surface It was reported that the (100) surface is one of the most stable surfaces of ort-CH3NH3PbI3.38 By performing PBE+U calculations, we efficiently explored basic properties of ort-CH3NH3PbI3(100)


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surface with and without light, aiming to well understand the role of water in the degradation of CH3NH3PbI3 under sunlight. With a photogenerated electron (e-) introduced into the system, we find that it is delocalized in the system and no spin density was identified (see Figure 3b). In the case of a photogenerated hole (h+), the hole is localized on two surficial I anions (I¯) with an I2¯ dimer configuration, inducing a local structural distortion (see Figure 3c), which is similar to that in the bulk of CH3NH3PbI3 we previously reported.61 The spin densities on these two I anions are 0.45 and 0.53 B, respectively. The I-I distance is shortened to 3.3 Å from original 4.4 Å. The electrostatic potentials of slabs are shown in the right panels of Figure 3 (a~c). In the presence of a hole (see Figure 3b), a small perturbation of the average potential curve at the position of an I dimer can be seen (the red dotted circle), whereas introducing the photogenerated electron shows no shift and a similar curve to that of the neutral system, implying a local change of the electrostatic potential resulting from the hole trapping.

Figure 3. Structures (the left panel) and averaged electrostatic potentials (the right panel) of the ort-CH3NH3PbI3(100) systems. (a) the neutral system. (b) the system involved a photogenerated electron, and (c) contains a photogenerated hole. For the electrostatic potential calculations, the x axis is the averaged electrostatic potential (Ep/eV) of the plane determined by a and b directions



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and the y axis (Z(Å)) represents the c direction of supercell. The value of iso-surfaces in (b) and (c) is 0.002 e/Å3. In CH3NH3PbI3 perovskite, the defect formation has a considerable effect on stability.62-66 Firstly, it can easily become an electron-hole recombination center because of forming a deep trapped state,67,68 which is undesirable. Furthermore, the easy diffusion of ions in defective system, such as the iodine defective perovskite, would result in the instability of perovskite solar cells.18,21,22 Therefore, IV which is one of the main defects in CH3NH3PbI3 was investigated in this work. In the bulk phase of ort-CH3NH3PbI3, each I anion binds with two nearest neighbor Pb2+ at a Pb-I distance about 3.3 Å. As an IV generates, two Pb-I bonds need be broken to release one I atom, inducing local structural relaxation by Pb-Pb distance shortened from 6.22 to 5.32 Å (see Figure 4d). In the dark, IV formation is difficult in the bulk because of a large formation energy (2.56 eV). In contrast, the surface I¯ binds with one five-coordinated Pb2+ and protrudes from the ortCH3NH3PbI3(100) surface (see Figure 1). If a surface IV forms, one Pb-I bond requires to be broken with a four-coordinated surface Pb cation left, as shown in Figure 4a, which reduces the formation energy of IV to 2.03 eV (versus 2.56 eV for bulk IV, see Table 2). Therefore, one can see that the exposed surface promotes the formation of iodine vacancy defects in CH3NH3PbI3.


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Figure 4. Structures and spin densities of IV formation on the (100) surface and in the bulk of CH3NH3PbI3. (a~c) correspond to the surface IV formation in the absence and presence of extra electron and hole, respectively, and (d~f) illustrate the corresponding ones of bulk IV. The level of iso-surfaces is 0.0017 e/Å3. In IV system, distribution of the left electron from the released I atom is also an important issue which may induce structural distortion (I¯ → 1/2I2 + e-). Based on charge analysis for the surface IV system (see Figure 4a), the spin charge accumulates on the p orbital of the surface Pb cation near IV with spin density of 0.84 B, indicating the left electron trapping on an adjacent Pb cation. For bulk IV formation, the left electron distributes on two nearest neighboring Pb cations with spin densities of 0.42 B on each. Moreover, as illustrated in Figure 4d, there are some interactions between these two Pb cations because the spin densities on the two Pb cations are considerably overlapped and the Pb-Pb distance is shortened from the original 6.18 Å to 5.32 Å.



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Table 2. Formation energies of IV and thermodynamics of the H2O interactions on the CH3NH3PbI3(100) surface are summarized in the absence and presence of an extra electron and hole, respectively. IV_surf and IV_bulk represent the surface and bulk I vacancies, respectively. ΔHads and ΔHdis are the enthalpy changes of H2O adsorption and dissociation. Ea-dis is the dissociation barrier. The unit is eV.

IV

H2O

Dark

e-

h+

IV_surf

2.03

1.96

0.57

IV_bulk

2.56

2.31

0.88

ΔHads

-0.56

-0.51

-0.62

Ea-dis

0.75

0.57

1.86

ΔHdis

0.38

0.38

1.57

We also examined the effect of sunlight on IV formation by involving one photogenerated electron and one hole separately. In the presence of one photogenerated electron, the formation energy of surface IV becomes 1.96 eV (see Table 2), which is similar to that in the dark because the photogenerated electron is fully delocalized. For the bulk IV, its formation energy is 2.31 eV, which decreases slightly in contrast to that in the dark (2.56 eV). This facilitation can be rationalized as follows. With a photogenerated electron introduced, the charges of two adjacent Pb cations around bulk IV are +0.67 and +0.68 |e|, respectively, which are both lower than the original +1.20 |e|. It indicates that the photogenerated electron and the left electron from one released iodine atom are separately trapped on these two Pb cations; the two reduced Pb sites also undergo considerably relaxation towards the IV center by large local structural distortion. In comparison with Pb-Pb distance at 5.32 Å in the dark, the distance here is shortened to 4.16 Å, implying formation of a local Pb-Pb dimer configuration in the presence of a photogenerated electron.


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Moreover, the Pb-Pb dimer possesses an actual character of (Pb-Pb)2+ state because no significant spin charge densities appear around these two Pb cations (see Figure 4e). From the projected DOS of IV-containing CH3NH3PbI3 bulk (see Figure 5), one can see that there is a gap state near the CBM in contrast with the perfect bulk (see Figure S2), resulting from the (Pb-Pb)2+ dimer. This chemical interaction within the (Pb-Pb)2+ dimer accounts for that reduced formation energy of IV in the CH3NH3PbI3 bulk.

Figure 5. The total density of state (TDOS) of (Pb-Pb)2+ dimer in the presence of a photogenerated electron in ort-CH3NH3PbI3 bulk calculated by DFT+U with SOC. The TDOS has been aligned by the carbon 2s orbital of bulk CH3NH3+. Interestingly, IV formation in the presence of a photogenerated hole (I¯ + h+ → 1/2I2) becomes much easier both in the bulk and on the surface of ort-CH3NH3PbI3. As shown in Table 2, the formation energies of surface and bulk IV were calculated to be 0.57 and 0.88 eV, respectively. Both are evidently lower than those in the dark (corresponding to 2.03 and 2.56 eV, respectively),



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showing an assistant role of the photogenerated hole for IV formation. On the (100) surface, the hole is well trapped on one surface I anion and one sub-surface I anion with an I dimer configuration. The trapping of hole induces elongation of Pb-I bonds to 3.4 Å (see Figure 3c), and thus the weakened Pb-I bonds promote surface IV formation. After the surface I is removed, the adjacent Pb near IV shows a charge of +1.15 |e| which is higher than that in the dark (+0.69 |e|). Furthermore, there is no spin density accumulation around the Pb site (see Figure 4c), indicating that the hole may recombine with the electron left from IV. That gives rise to a favorable thermodynamics of surface IV formation. Similarly, for IV formation in the bulk, we found two near-neighbor Pb cations at the IV site have more positive charges (+1.17 and +1.16 |e|, respectively). It demonstrates that the photogenerated hole also combines with one left electron. Therefore, the illumination of sunlight could induce iodic defect formation thermodynamically. However, due to two Pb-I bonds breaking, the formation energy of bulk IV is a little higher than that of surface IV. Our calculations also show that, once an IV forms, the left electron is well trapped at an adjacent Pb site, which forms a new electron donor site (recombination center) to annihilate hole or capture another photo-excited electron to form the local (Pb-Pb)2+. 3.3 Interaction between H2O and the ort-CH3NH3PbI3(100) surface As mentioned in the introduction, the quick degradation of CH3NH3PbI3 perovskite in humid condition is one key issue, in which moisture is expected to be one of the chief factors. To improve the stability of CH3NH3PbI3, it is very desirable to reveal the role of H2O and to understand its interaction with CH3NH3PbI3 perovskite. 3.3.1 Adsorption/dissociation of H2O on the perfect surface in the dark The adsorption and dissociation of H2O in the dark was investigated first. When H2O approaches to the surface, it adsorbs on a surface five-coordinated Pb site with the Pb-O bond of 2.74 Å (see


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Figure 6). This configuration gives rise to an adsorption energy of -0.57 eV (see Table 2), in which some evident charge transfer between Pb and H2O (see Figure S3) occurs, indicative of the formation of chemical bond. H2O dissociation starts with cleavage of one O-H bond and coupling of one leaving H with an adjacent I¯ anion. Figure 6b shows the optimized transition state (TS) structure. It possesses a four-membered ring [–Pb–O…H…I–] configuration perpendicular to the surface, in which the forming H…I bond is 1.67 Å, while the Pb-O bond length decreases from original 2.74 Å to 2.23 Å. Accompanied by cleavage of one O-H bond and one Pb-I bond, the final state (FS, see Figure 6c) shows that one HI molecule and an adsorbed OH species forms in the system. This dissociation type of H2O is energetically unfavorable by 0.38 eV, and the reaction barrier was calculated to be as high as 0.88 eV (see Table 2). It demonstrates that H2O is difficult to dissociate and inclines to molecularly adsorb on the surface without light. Recently, the hydrated intermediates like (CH3NH3)4PbI6ꞏ2H2O were found to form on the surface when CH3NH3PbI3 is exposed to humid condition in the dark, and the material can partly recover its performance when the environment becomes dry.23 This result is consistent with our finding that H2O can adsorb on the CH3NH3PbI3(100) surface in the dark, but cannot erode the surface by dissociation.



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Figure 6. (a)~(c) show optimized structures of adsorption, transition state and final state of H2O dissociation on CH3NH3PbI3(100) surface in the dark. The red numbers are charges carried by the adjacent atoms, and the black numbers are bond lengths. (d) the energy profile for the adsorption and dissociation of H2O molecule in the dark and with a photogenerated electron (e-) and hole (h+) involved, respectively. 3.3.2 Adsorption/dissociation of H2O on the perfect surface under sunlight To explore possible synergistic effects of water/light on the degradation of CH3NH3PbI3, the adsorption and dissociation of H2O under sunlight was investigated on ort-CH3NH3PbI3(100) surface by involving photogenerated e- and h+. In the presence of a photogenerated e-, H2O possesses a similar adsorption configuration to that in the dark (see Figure S4a). The adsorption


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energy was calculated to be -0.51 eV, being comparable to that (-0.56 eV) in the dark. With one photogenerated h+ involved, the adsorption energy of H2O (-0.62 eV) also shows small difference. Both e- and h+ have minimal influences on H2O adsorption, and this is mainly due to the fact that H2O molecule cannot well trap a hole or an electron.69 For example, spin density analysis shows that the hole traps on two surface I anions instead of H2O with spin densities of 0.37 and 0.25 µB, respectively, thereby showing little effect of the hole on the H2O adsorption configuration. Particularly, the charge density differences (CDD) for the H2O adsorption in these three cases appear to be comparable (see Figure S3), which are consistent with the similar adsorption energies. As H2O dissociates in the presence of an e-, the involved Pb-I bond in the optimized TS is elongated from 3.28 Å to 3.73 Å and a new H-I bond is forming at 1.68 Å in a four-membered ring configuration, as shown in Figure S4b. Compared with that in the dark, the dissociation barrier of H2O was calculated to be 0.57 eV, which becomes evidently lower, implying that H2O can more easily dissociate on the surface when a photogenerated e- participates. After overcoming the barrier, an adsorbed OH¯ (charge: -0.72 |e|) and HI generate with reaction energy change of 0.38 eV. Differently, the presence of h+ would seriously hinder the dissociation of H2O with a barrier as high as 1.86 eV. The optimized TS structure with h+ involved is shown in Figure S4e, which presents a similar configuration to that in the dark, except that the forming OH carries more positive charge (-0.42 |e|) in relative to that in the dark or in the presence of an e- (-0.72 |e|). It is worth pointing out that the effects of photogenerated h+ and e- on the H2O dissociation barrier can be rationalized from both electronic and geometrical structures. As shown in Figure 6b and Figure 4 (b, e), one can see the following common features from the optimized TS configurations; as H is leaving from H2O towards a surface I anion, the O-H bond is highly



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elongated, and a HI molecule-like species is forming with a short H-I bond length around 1.68 Å, while the Pb-I bond also breaks basically, showing a very late TS. On the one hand, H and I in the forming H-I species carry charges of -0.86 |e| and +0.87 |e|, respectively. The whole unit is nearly electroneutral (+0.01 |e|), which is very different from that in the initial state. For example, in the case without light, the H and surface I in the initial state carry charges of +1.00 |e| and -0.67 |e|, respectively, giving a total charge of +0.33 |e|. The difference (+0.01 |e| versus +0.33 |e|) implies that it requires to capture electrons from the slab to form H-I bond, which inherently contributes to the high dissociation barrier of H2O in the dark. When a photogenerated e- is present, the required electrons are easier to be donated from the slab, resulting in a decreased energy barrier. On the contrary, in the presence of h+, the whole system is electron-deficient, and it is difficult to extract electron from the slab to the forming HI species; moreover, owing to the trapping of h+ on the surface I in the initial state (I: -0.30 |e|), it requires more electron transfer (+0.70 |e|) to form a HI unit in the TS, thus giving rise to a barrier as high as 1.86 eV. On the other hand, based on the above charge analysis, the lower barrier when e- exists can be reflected by the different distance of the breaking O…H bond of H2O in the TSs. In the dark, the O…H bond distance is 2.58 Å in the TS, while it reaches a length of 2.72 Å in the presence of an e-, which shows reduced Coulomb repulsion between them (H: -0.86 |e|, O: -1.72 |e|; see Figure S4b) with an e- involved and thus possesses a lower barrier. In the presence of one h+, as H of H2O approaches to the hole-trapped I anion on the surface, the hole transfers to the forming OH species in the TS, leading to a less negative charge (-0.42 |e|) on OH relative to the cases in the dark and in the presence of an e- (-0.72 |e|) (see Figure S4c). It indicates the formation of an actual unstable OH· radical, which evidently weakens the Pb-OH bond and thus increases the energy barrier.


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Recently, Yang and coworker suggested that surface hydroxyl groups can accelerate the decomposition of perovskite solar cells.25 Theoretically, Zhang et al. proposed that OH· radical and OH¯ groups could spontaneously abstract hydrogens/protons from top-layer CH3NH3+ upon adsorption.35 Our work shows that the dissociation of H2O in the presence of a photogenerated eleads to a detached HI molecule and an upright OH¯ anion adsorbing on the surface Pb2+ site. With escape of the HI molecule, a surface IV site is exposed near the Pb2+ cation. Our calculations show that OH¯ is thermodynamically favorable to fill into the surface IV site with an energy change of 0.50 eV, resulting from the attraction between OH¯ and CH3NH3+ group, as shown in Figure 7a. This strong interaction induces dissociation of a N-H bond in CH3NH3+ and leads to the formation of CH3NH2 and H2O (OH¯ + CH3NH3+ → CH3NH2 + H2O, see Figure 7b). The reaction energy of this step was calculated to -0.04 eV. The formed CH3NH2 molecule can escape from the system, accelerating the degradation of CH3NH3PbI3 perovskite. It is worth noting that OH¯, as an effective hole scavenger in photocatalytic reactions, could form an OH· radical by trapping a hole.69, 74 Since the OH· radical has strong oxidizing ability, it may oxidize HI to I2 as shown in the following equation: OH· + HI → 1/2I2 + H2O. The reaction energy is -1.53 eV, indicating a thermodynamic character. Therefore, the CH3NH3PbI3 perovskite surface may irreversibly degrade by both the interaction between OH¯ and CH3NH3+ and the strong oxidizing ability of hole-trapped OH· radical.



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Figure 7. (a) the adsorbed OH¯ on the IV site. (b) the structure of the formed CH3NH2 and H2O in the ort-CH3NH3PbI3 perovskite. 3.3.3 Adsorption/dissociation of H2O on the defective surface We also investigated the adsorption and dissociation of H2O on IV site which can generate with assistance of h+, as discussed in Section 3.2. On the IV site, one H2O molecule binds with a fourcoordinated Pb cation (see Figure S5), and the adsorption energy is -0.73 eV in the dark. In contrast to the perfect ort-CH3NH3PbI3(100) surface, there are some improvements for H2O adsorption here; more importantly, it is found that the dissociation of H2O on the IV site is easier by a reaction barrier of 0.60 eV, through interacting with a sub-surface I¯ (see the TS structure in Figure S5). Using the transition state theory (k=(kBT/h)ꞏexp(∆Ea/RT)), we compared the effective barrier of H2O dissociation on the defective and perfect surface, and estimated their rate constants at room temperature, in which kdefective/kperfect gives an order of ~3×102. Therefore, the surface IV defect would largely promote the dissociation of H2O molecule. Similar with the case on the perfect surface, the formed OH and leaving HI molecule would induce a series of adverse reactions, such as formation of CH3NH2 (OH¯ + CH3NH3+ → CH3NH2 + H2O) and yield of another sub-surface IV defect, severely destroying the CH3NH3PbI3 structure.


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3.4 Roles of water/light on the stability of CH3NH3PbI3 perovskite Having presented detailed results of H2O interaction with CH3NH3PbI3 with and without light involved, we are at a position to summarize possible roles of water and light responsible for the degradation of CH3NH3PbI3 perovskite. As shown in Figure 8, two roles of H2O, together with photogenerated h+ or e-, were identified to damage the surface structure. Role I: the H2O molecule can dissociate on the perfect ort-CH3NH3PbI3(100) surface slightly facilitated by the photogenerated e-. After its dissociation, the HI molecule escapes from the system (resulting in a surface IV defect), and the formed OH¯ can easily interact with a CH3NH3+ to regenerate one H2O and a gas-phase CH3NH2 molecule, thereby destroying the structure. Role II: it is a stepwise mechanism for the degradation of CH3NH3PbI3. In this process, the photogenerated h+ can induce the formation of a surface IV first. On the IV site, H2O can efficiently adsorb and dissociate, forming the harmful OH¯ species and HI molecule that can easily desorb from the surface and leave another IV in the system. Similar to role I, the formed OH¯ in role II will also induce CH3NH2 formation and damage the structure. Based on these understandings, to improve the long-term stability of CH3NH3PbI3 perovskite, one may realize the following possible strategies. On the one hand, a chemically bonded interface should be well constructed between CH3NH3PbI3 and the hole transport material in perovskite solar cells, which guarantees the holes to diffuse outward effectively and avoids their accumulation in CH3NH3PbI3 to assist the IV defect formation; on the other hand, one should reduce the exposed surface of CH3NH3PbI3, especially the direct contact with H2O, which may be overcome to some extent by depositing a hydrophobic layer on CH3NH3PbI3.



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Figure 8. Scheme of initial degradation mechanism of CH3NH3PbI3 in the presence of H2O and light. Two crucial roles are illustrated. Role I: the photogenerated electron facilitates the adsorption and dissociation of H2O on the perfect ort-CH3NH3PbI3(100) surface. Role II: the photogenerated hole facilitates the formation of I vacancy (IV) on the ort-CH3NH3PbI3(100) surface, which serves as an active site to highly promote the adsorption and dissociation of H2O. The I¯(surf) and I¯(sub) represent the surface and sub-surface I¯ anions, respectively. 4. Conclusions In summary, we have performed the DFT+U approach together with the spin-orbit coupling for the first time to explore the basic properties of ort-CH3NH3PbI3 perovskite, aiming to uncover the origin of light- and water-mediated degradation of CH3NH3PbI3. A suitable pair of the U terms (I: 8, Pb: 9) has been identified, which can well describe the electronic and chemical properties of CH3NH3PbI3 perovskite, and a comprehensive understanding on how the photogenerated holes/electrons affect the formation of I vacancy defect and their interplay with water dissociation has been achieved as follows. Firstly, the photogenerated hole can evidently promote IV formation both in the bulk and on the surface, while the photogenerated electron can only enhance IV


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formation in the bulk slightly owing to the existence of [Pb-Pb]2+. Secondly, sunlight has been found to play an insignificant role for H2O adsorption on the CH3NH3PbI3(100) surface. However, H2O dissociation has been found to be promoted by light, in which two roles have been identified. On the perfect surface, H2O can dissociate directly, which is driven by the photogenerated electron with a barrier as low as 0.57 eV, being lower than the barrier (0.75 eV) in the dark. On the defective surface, the IV defect formed by assistance of h+ can serve as a highly reactive site for H2O dissociation. In both cases, as one H2O dissociates, the formed OH¯ species has been found to strongly interact with an adjacent CH3NH3+ to form a CH3NH2 molecule; meanwhile, the HI desorption would induce the formation of an IV defect. In addition, the OHꞏ radical formed by trapping a hole at OH¯ was found to have a strong ability to oxidize HI into I2 molecule. In general, the current work demonstrates an efficient approach to simulate CH3NH3PbI3, and the new understanding on the degradation of CH3NH3PbI3 from this work may yield some guidance to the modification of perovskite solar cells.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Formation energies of I vacancies in the surface and bulk system, lattice constants optimized from DFT-D2 and DFT-D3 methods, energy levels of valence band minimum and conduction band minimum and band gaps from different U values, band structures from 8 groups of U values, total density of states of ort-CH3NH3PbI3 bulk in the dark, charge differences of H2O adsorption, structures of H2O dissociation in the presence of photogenerated electron and hole, structures for



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the H2O adsorption and dissociation in surface iodine vacancy site, and H2O dissociation and I vacancy formation in tetragonal CH3NH3PbI3 phase with and without light AUTHOR INFORMATION Corresponding Authors E-mail for *H. W.: [email protected] E-mail for *J. C.: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This project was supported by National Natural Science Foundation of China (21333003, 21622305, 21873028), National Ten Thousand Talent Program for Young Top-notch Talents in China, The Shanghai “Shu Guang” project (17SG30), and the Fundamental Research Funds for the Central Universities (WJ1616007). REFERENCES (1)

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