Synergistic Effects of Water and Oxygen Molecule Co-adsorption on

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Synergistic Effects of Water and Oxygen molecules Co-Adsorption on (001) Surfaces of Tetragonal CHNHPbI: A First-Principle Study 3

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Wei Hao, Xiaodong Chen, and Shuzhou Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09231 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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The Journal of Physical Chemistry

Synergistic Effects of Water and Oxygen molecules Co-Adsorption on (001) Surfaces of Tetragonal CH3NH3PbI3: a First-Principle Study Wei Hao, Xiaodong Chen, and Shuzhou Li* School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore *E-mail: [email protected]

1 Environment ACS Paragon Plus


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Abstract

The poor environmental stability of organometallic halide perovskite solar cells presents a big challenge for its commercialization, which is mainly due to the degradation of perovskite materials in humid air. The role played by water molecules has been extensively studied in the degradation processes, where strong interactions between water molecules and perovskite surfaces are found. Using first-principles simulations, we find that oxygen molecules also have strong interactions with (001) surfaces of tetragonal CH3NH3PbI3 through the formations of chemical Pb-O bond on PbI2-terminated surface and hydrogen bond on CH3NH3I-terminated surface. The adsorbed oxygen molecules introduce empty states near the Fermi level of the surfaces, which can facilitate charge transfer between the surface and oxygen molecules. Furthermore, when an oxygen molecule located atop of Pb atom on PbI2-terminated surface, the calculated adsorption energies indicate that the surface is more attractive to water molecules, making the surface even more sensitivity to humidity. These findings reveal that oxygen molecules also play an important role in the initial stage of the degradation of perovskite materials.


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INTRODUCTION As the most abundant clean and renewable energy, the solar energy has drawn intensive attentions since the last century. In order to convert the solar energy directly into electricity, many kinds of solar cell devices have been developed, such as the silicon-based solar cell, dyesensitized solar cell, and the perovskite solar cell based on organometallic perovskite materials.1 Because of its fast improvement in power conversion efficiency and the low fabrication cost, the perovskite solar cell becomes a superstar in the field of photovoltaic recently.2-8 The perovskite solar cell usually employs the organometallic halide perovskite materials as the light absorber, such as CH3NH3PbI3 and its analogies. Numerous experimental and theoretical studies have proved that the outstanding optical and electronic properties of these perovskite materials, such as long diffusion length of electrons and holes, high optical absorption, and benign defect properties, are crucial to the high photo-conversion efficiency and high open circuit voltage of perovskite solar cell.9-15 Despite these great advantages, one of the most significant concerns for perovskite solar cell is their long-term stability, which is a key hinder of commercialization. Experimental results have shown that the efficiency dropped dramatically when the cells exposed to humid environment and this is mainly due to the degradation of perovskite materials.16-19 By using time-domain ab initio simulations, Long and co-workers have proven that the electron-hole recombination rate can be influenced by humidity.20 The sensitivity of perovskite materials to the humid environment makes the application of perovskite solar cells a big challenge. Besides, the degradation of perovskite materials can also lead to the toxic Pb leaking into the environment. In order to radically prevent the degradation process, the interaction details between perovskite materials and humid environment would be very important for understanding the degradation



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mechanisms and instructing to fabricate perovskite solar cells with a greater resistance to humidity. Recently, Frost and co-workers have proposed that the water molecule may catalyze the degradation of CH3NH3PbI3 by assisting the formation of gaseous CH3NH2, PbI2 and HI, which means that even a small amount of water is fatal to perovskite materials.21 These decomposition products were soon observed by Yu’s experiments, in which they also found that the subsequent reaction of hydroiodic acid with the metal electrode may further accelerate the perovskite degradation process.22 Furthermore, by detailed XRD examinations, Niu and co-workers have pointed out that the generated HI may dissociate into I2 and H2 gas under sunlight irradiation, also facilitating the degradation process.17 While the degradation products of CH3NH3PbI3 can be easily detected experimentally, the process details that CH3NH3PbI3 is destroyed by water molecule are much more difficult to be revealed directly by experiment methods, especially at the initial stage of degradation. Complementarily, valuable information of the degradation process on atomic level can be provided by simulations based density functional theory, through which people have already gained important understanding of properties of the perovskite surfaces.23-28 For instance, Jun and co-workers have investigated the termination dependence of tetragonal CH3NH3PbI3 surfaces and found that both the PbI2-terminated (110) and (001) surfaces would facilitate the hole transfer, thus improving the performance of perovskite solar cells.29 Mosconi and co-workers have studied the growth orientation of CH3NH3PbI3 and CH3NH3PbClxI3-x on TiO2 and demonstrated that the growth in (110)-oriented films is preferred on TiO2, due to the better structural matching between interfacial halide atoms of perovskite materials and interfacial titanium atoms of TiO2.30 The adsorption of water molecules to CH3NH3PbI3 surface would be the first step of the CH3NH3PbI3 material degradation. Recently,


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Tong and co-workers’ first-principles calculations have shown that water molecules can be attracted by CH3NH3I-terminated (001) surface of tetragonal CH3NH3PbI3 and then easily penetrate into the surface, leading to corrode down the whole structure gradually.31 These results well explained the sensitivity of CH3NH3PbI3 to water. Koocher and co-workers’ calculation results indicate that the water adsorption on surfaces of CH3NH3PbI3 can be influenced by the orientation of the CH3NH3+ cations close to the surface.32 However, the surfaces with PbI2 termination have not been explored systematically, which is also the possible constituent structure of perovskite surfaces. On the other hand, the role of molecular oxygen, which is abundant in air, has not been included in these theoretical studies. Recently, Aristidou and coworkers’ experimental results have shown that the degradation can be initiated by the reaction of superoxide with the CH3NH3+ cations.33 Yang and co-workers’ experiment results have indicated that the combination of oxygen and light can trigger the decomposition of CH3NH3PbI3.19 Similarly phenomenon has been observed by Daniel Bryant and co-workers, who claimed that the light and oxygen-induced degradation is the main reason for the low operational stability of CH3NH3PbI3 perovskite solar cells in ambient conditions.34 To this regard, the interactions between oxygen molecules and perovskite surfaces should also be importance to understand the degradation mechanisms of perovskite materials. More simulation investigations are needed for degradation processes of CH3NH3PbI3 upon humidity exposure so that the degradation products in experiments could be reproduced in simulations. In this study, we investigated the adsorption of water and oxygen, as well as their coadsorption on tetragonal CH3NH3PbI3 (001) surfaces on atomic scale by employing the state-ofthe-art first-principles calculations. The stable tetragonal phase of CH3NH3PbI3 (Figure 1) at room temperature is chosen. We focus on (001) surface of tetragonal CH3NH3PbI3 because



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previous studies have shown that the (001) surface is more stable due to the alternative stacking of neutral CH3NH3I and PbI2 planes.29 Our calculations results suggest that both water and oxygen molecules can be strongly attracted by both CH3NH3I-terminated and PbI2-terminated tetragonal CH3NH3PbI3 (001) surfaces. We also found that the interplay of water and oxygen molecules on the surfaces plays an important role in the degradation process of perovskite materials. COMPUTIONAL DETAILS Our density functional theory calculations were carried out within the Vienna ab-initio simulation package (VASP).35-36 The projector augmented wave (PAW) method was used to describe the electron–ion interaction and the exchange-correlation between electrons was described by the generalized gradient approximation (GGA) in the Perdew–Burke–Ernzerhof (PBE) form.37-38 A cutoff energy of 400 eV was used for the plane-wave basis set in all calculations. As the spin-orbit coupling (SOC) effect is reported to have little influence on the geometric structures,39 the spin-polarization was not included in the calculations except for those involving oxygen molecule, which has an intrinsic spin in its ground state. The lattice parameters of bulk tetragonal CH3NH3PbI3 were optimized through a k-mesh of 3×3×2 in Monkhorst-Pack scheme40 and a tetragonal unit cell containing 48 atoms, whose initial positions were adopted from Baikie and co-workers’ experimental results.41 The atomic positions and the cell parameters were then relaxed until the force on each atom is below 0.01 eV/Å. The relaxed lattice constants are summarized in Table 1. The PBE functional slightly overestimates the lattice parameters relative to experimental results,41 which has been observed by previous calculations.29, 42 The crystal structure relaxations were also performed with the van der Waals


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corrections in optB86b-vdW form,43 and the lattice parameters (see Table 1) are slightly smaller than, but closer to experiment results. Although the lattice constant results with van der Waals corrections show no obvious improvement for tetragonal CH3NH3PbI3, the PBE functional with optB86b-vdW correction was adopted in the following calculations to yield safer results in the view of water and oxygen molecules that involved in the surface calculations.

Figure 1. The supercells of tetragonal CH3NH3PbI3. (a) Side and (b) top views of relaxed bulk tetragonal CH3NH3PbI3 structures. The rectangles in (b) highlight the supercells used. The solid blue lines indicate the (1×1) supercell while the dotted blue lines give the (√2×√2 R 45°) supercell. Color coding of the atoms are Grey: Pb, Purple: I, Brown: C, Light blue: N, White: H. Table 1. The calculated lattice constants of tetragonal CH3NH3PbI3 by PBE functional without and with optB86b-vdW correction.

a(Å) This work

PBE 8.925 PBE+vdW 8.646 41 Exp. values 8.853

b(Å)

c(Å)

Cell Volume(Å3)

8.798 8.654 8.853

12.866 12.691 12.443

1009.765 949.609 975.228

After obtained the geometric structure of bulk tetragonal CH3NH3PbI3, the slab models were established based on the optimized lattice parameters to represent the CH3NH3PbI3 surfaces. The



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PbI2-terminated (001) surface of tetragonal CH3NH3PbI3 (Figure 2(a)) was modeled by a slab supercell with 5 atomic layers, including 3 PbI2 layers and 2 CH3NH3I layers. And the CH3NH3Iterminated (001) surface (Figure 2(b)) was modeled by a slab supercell with 6 atomic layer, including 3 PbI2 layers and 3 CH3NH3I layers. A thickness of 15 Å vacuum was inserted between adjacent slabs to safely overlook their interactions. The bottom two atomic layers of both slabs (inside the dashed rectangle region in Figure 2(a) and (b)) were fixed to their bulk positions during relaxation, while other layers were fully relaxed. The (1×1) slab supercells with (3×3×1) Monkhorst-Pack k-point grids and the (√2 ×√2 R 45°) slab supercells with (2×2×1) Monkhorst-Pack k-point grids, as displayed in Figure 1(b), were used in this study. Since these two types of supercells generate same optimized structures, all the calculations below were carried out using the (1×1) slab supercells with (3×3×1) Monkhorst-Pack k-point grids. For the details of the validity check of the models, please refer the supporting information. Note that for the CH3NH3I-terminated surface, the orientation of the surface CH3NH3+ cations turned out to be aligned with the CH3-end pointed toward the surface (see Figure 2(b)). Details discussions about the orientation effects of organic cations on the adsorption can be found in the supporting information. The adsorption energies of water and oxygen molecules at different adsorption sites were investigated by initially placing the molecules at the sites displayed in Figure 2(c) and (d), and then relaxing the system to find out their stable adsorption sites. On the PbI2-terminated surface, these initial sites include: atop of Pb atom (Site A), atop of I atom (Site B), and the hollow site of the surface (Site C). On CH3NH3I-terminated surface, they are: atop of I atom (Site D), atop of CH3NH3+ cation (Site E), and the hollow site of the surface (Site F). The adsorption energies of molecule on slab surface ∆Eads(molecule) were calculated as


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∆Eads(molecule) = Emolecule/slab - Eslab - Emolecule

(1)

where Emolecule/slab, Eslab, and Emolecule are the relaxed total energy of the adsorption system, the clean slab model, and the free molecule in vacuum, respectively. Under this definition, the negative value of ∆Eads means the attraction behavior of molecule to the slab surface, while the positive value means the repulsion.

Figure 2. Slab models of tetragonal CH3NH3PbI3 (001) surfaces. (a) Side and (c) top views of the (1×1) 5-layer slab model of PbI2-terminated CH3NH3PbI3 (001) surface. (b) Side and (d) top views of the (1×1) 6-layer slab model of CH3NH3I-terminated CH3NH3PbI3 (001) surface. The dashed black rectangle in (a) and (b) specifies the layers which are fixed to bulk positions. In (c) and (d), the green balls indicate studied molecule adsorption sites. On PbI2-terminated surface (panel c), they are namely atop of Pb (Site A), atop of I (Site B) and hollow (Site C). On CH3NH3I-terminated surface (panel d), they are namely atop of I (Site D), atop of CH3NH3 (Site E) and hollow (Site F). RESUTLS AND DISCUSSIONS



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The adsorption of a single water molecule on (001) surfaces of tetragonal CH3NH3PbI3 was firstly investigated. The adsorption energies of water molecule on the above mentioned six absorption sites were listed in Table 2, which clearly show that water molecule is attractive to both kinds of (001) surfaces of tetragonal CH3NH3PbI3, where results of CH3NH3I-terminated surfaces are consistent with previous studies.31-32 On a CH3NH3I-terminated surface, all three adsorption sites were capable of holding a water molecule and the hollow site (Site F) is most preferred. Here, our calculated adsorption energy of water at Site F on CH3NH3I-terminated surface (−0.333 eV) is rather close to Tong and co-worker’s result (−0.30 eV).31 Our adsorption energies of water at Site D and at Site E are −0.131 eV and −0.137 eV, respectively. On a PbI2terminated surface, the most stable configuration is a water molecule at Site A, with the adsorption energy of −0.627 eV. When the water molecule was initially placed at Site B on PbI2terminated surface, it moves automatically to Site A during relaxation. This phenomenon was consistent with Koocher’s results that there is only one local minimum for water molecule around the surface Pb atom on (001) surface of CH3NH3PbI3.32 We also found a metastable configuration where the water molecule is adsorbed at Site C with the adsorption energy of −0.224 eV where a similar observation has been reported near the surface CH3NH3+ cations in Figure 2 of Reference 32. The most stable adsorption site is Site A, the atop of Pb atom on PbI2terminated surface among all the studied adsorption sites for a water molecule on (001) surface of CH3NH3PbI3. Table 2. The calculated adsorption energies, ∆Eads in eV, of molecular water and oxygen on (001) surfaces of tetragonal CH3NH3PbI3. The italic value in parentheses is from Ref. 31.

PbI2-terminated surface

CH3NH3I-terminated surface


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Atop of Pb (Site A)

Atop of I (Site B)

Hollow (Site C)

Atop of I (Site D)

Atop of CH3NH3+

H2O

-0.627

-(to Site A)

-0.224

-0.137

-0.131

-0.333 (0.30)

O2

-0.398

-0.096

-0.120

-0.144

-0.150

-0.287

(Site E)

Hollow (Site F)

To explore the origins of different adsorption abilities for water adsorbed at Site A, Site C, and Site F, the electronic analysis were performed for the relaxed geometric structures of water molecule, which are in Figure 3. For Site A on PbI2-terminated surface (inset panel of Figure 3(a)), the adsorbed water molecule stays almost flat atop of Pb atom with the Pb-O distance of 2.60 Å, which is similar to the case of close-packed transition and noble metal surfaces.44 The projected densities of states (PDOS) in Figure 3(a) show a large degree of overlap between Pb atom and O atom, which indicates strong bonding character and contributes to the strong attraction ability of Pb atom to water molecule. When water molecule locates at Site C on PbI2terminated surface (inset panel of Figure 3(b)), the plane of water molecule is found to be perpendicular to the surface, with one of its H atom towards surface I atom. The small degree of overlap in PDOS between H atom and I atom in Figure 3(b) indicates the weak interactions between them, thus leading to the weak attraction of I atom to water. For Site F on CH3NH3Iterminated surface (inset panel of Figure 3(c)), the plane of water molecule is also perpendicular to the surface but with the O atom downward to form hydrogen bond with the hydrogen atom from NH3-group. As a hydrogen bond is much weaker than the Pb-O chemical bond, the water molecule is more preferable to PbI2-terminated surface than CH3NH3I-terminated surface.



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Figure 3. Geometric structure and PDOS of water on surfaces. (a) The relaxed geometric structure of water at Site A (inserted panel) and the calculated projected density of state (PDOS) of oxygen atom and its nearest Pb atom. (b) The relaxed geometric structure of water at Site C (inserted panel) and the calculated PDOS of labeled H atom from water and its nearest I atom. (c) The relaxed geometric structure of water at Site F (inserted panel) and the calculated PDOS of O atom and labeled H atom from CH3-group. The Fermi levels are set to the energy origin. As a strong oxidant and one of the main constituents in air, the oxygen molecule is inevitable to interact with the surface of CH3NH3PbI3 when exposing to air. Similar to the case of water molecule, we also investigated the adsorption of oxygen molecule on (001) surfaces of tetragonal CH3NH3PbI3 and the calculated adsorption energies are tabulated in Table 2. The adsorption energies are negative for all the six adsorption sites. The preferred adsorption sites for oxygen


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molecule are Site A on PbI2-terminated surface and Site F on CH3NH3I-terminated surface, which are same as water molecule. On a CH3NH3I-terminated surface, the adsorption energy of oxygen (−0.287 eV) is comparable to that of water (−0.333 eV). This means that the oxygen molecule is a competitor of water molecule in adsorbing on (001) surfaces of tetragonal CH3NH3PbI3 and the oxygen molecule can also play a role in CH3NH3PbI3 degradation. Figure 4(a) displays the geometric structures of oxygen at Site A on PbI2-terminated surface and corresponding PDOS of labeled O atom and Pb atom. And again, the strong interactions between the labeled O atom and Pb atom lead to the strong attraction of PbI2-terminated surface to oxygen molecule. When an oxygen molecule locates at Site C on PbI2-terminated surface, the nearest distance between oxygen atom and surface atom is as far as 3.67 Å, thus there is no strong interaction between them (see Figure 4(b)). However, empty states near the Fermi level in both cases may cause charge transfer from the surface to oxygen molecule, which has been observed in experiments.33 The geometric structure of oxygen at Site F on CH3NH3I-terminated surface and its corresponding PDOS of O and H atoms are plotted in Figure 4(c). In PDOS plots, it’s obvious that the hydrogen bond formed between the lower oxygen atom and H atom from the NH3-group and the existence of empty states near the Fermi level as well. In summary, the adsorption site and geometry of oxygen molecule on (001) surfaces of tetragonal CH3NH3PbI3 are similar to those of water molecule. The adsorbed oxygen molecule introduces empty states near the Fermi level, which would facilitate the charge transfer between oxygen molecule and surface.



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Figure 4. Geometric structures and PDOS of an oxygen molecule on surfaces. (a) The relaxed geometric structure of an oxygen molecule at Site A on PbI2-terminated surface (inserted panel) and the calculated projected density of state (PDOS) of labeled O and Pb atom. (b) The relaxed geometric structure of an oxygen molecule at Site C on PbI2-terminated surface (inserted panel) and the calculated PDOS of labeled O and I atom. (c) The relaxed geometric structure of an oxygen molecule at Site F on CH3NH3I-terminated surface (inserted panel) and the calculated PDOS of labeled O and H atom. The Fermi levels are set to the energy origin.

The adsorption of oxygen in the presence of water is then investigated. A water molecule is pre-adsorbed at Site A on PbI2-terminated surface and at Site F on CH3NH3I-terminated surface,


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respectively. And an oxygen molecule is placed at different possible adsorption sites that illustrated in Figure 5(a) and (c). The adsorption energy of oxygen in the presence of a water molecule, ∆Eads(O2: H2O/slab) is calculated as ∆Eads(O2: H2O/slab) = E(H2O+O2)/slab – EH2O/slab – EO2

(2)

where E(H2O+O2)/slab is the total energy of the slab with both water and oxygen molecules on slab surface. The calculated results are showed in Table 3. On the PbI2-terminated surface, the site 6 (atop of Pb atom) is most preferred by the oxygen molecule when a water molecule locates at Site A because of the strong Pb-O bonding. The adsorption energies of oxygen, −0.421 eV is very close to that on the clean PbI2-terminated surface, −0.398 eV. For the sites near the water molecule (Site 1~5), all of them are more attractive to oxygen molecule with the presence of water molecule at site A than the cases without water molecule. On the CH3NH3I-terminated surface with a water molecule at Site F, the most stable site for oxygen molecule is the Site 15, with weak preference over the Site 13 and Site 14. The adsorption energies of oxygen on these three sites are all comparable to that of oxygen at Site F on clean CH3NH3I-terminated surface, −0.287 eV. Besides, the Site 17 is also more attractive to oxygen molecule with the help of water molecule as the adsorption energy of oxygen decreases from −0.144 eV to −0.236 eV. Our results suggest that the presence of water makes both the PbI2- and CH3NH3I-terminated surfaces more attractive to oxygen molecules.



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Figure 5. Possible adsorption sites for water molecule or oxygen molecule co-adsorption. (a) The studied adsorption sites for an oxygen when a water molecule is located at Site A on a PbI2terminated surface. (b) The studied adsorption sites for a water molecule when an oxygen molecule is located at Site A on a PbI2-terminated surface. (c) The studied adsorption sites for an oxygen molecule when a water molecule is located at Site F on a CH3NH3I-terminated surface. (d) The studied adsorption sites for a water molecule when an oxygen molecule is located at Site F on a CH3NH3I-terminated surface.


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Table 3. The calculated adsorption energies, ∆Eads in eV, of water or oxygen molecule on (001) surfaces of tetragonal CH3NH3PbI3 with oxygen or water molecule adsorbed beforehand. PbI2-terminated surface with H2O @ Site A Site

∆Eads(O2: H2O/slab)

with O2 @ Site A Site

∆Eads(H2O: O2/slab)

CH3NH3I-terminated surface with H2O @ Site F Site

∆Eads(O2: H2O/slab)

with O2 @ Site F Site

∆Eads(H2O: O2/slab)

1

-0.096

7

-0.953

13

-0.281

18

-0.147

2

-0.242

8

-0.676

14

-0.268

19

-0.147

3

-0.297

9

-0.221

15

-0.333

20

-0.350

4

-0.229

10

-0.257

16

-0.242

21

-0.103

5

-0.134

11

-0.194

17

-0.236

22

-0.296

6

-0.421

12

-0.637

The adsorption behaviors of water molecule were also investigated in the presence of an oxygen molecule on (001) surfaces of tetragonal CH3NH3PbI3. The oxygen molecules were placed at Site A and at Site F for PbI2-terminated and CH3NH3I-terminated surfaces, respectively. Figure 5(b) and (d) show the possible adsorption sites for water molecules. Similarly, the adsorption energy of water in the presence of an oxygen molecule, ∆Eads(H2O: O2/slab) is calculated as ∆Eads(H2O: O2/slab) = E(H2O+O2)/slab – EO2/slab – EH2O

(3)

The calculated adsorption energies are also listed in Table 3. On the PbI2-terminated surface, the difference of adsorption energy of water at Site A with and without the presence of oxygen molecule is very small (0.01 eV). However, the presence of oxygen molecule at Site A greatly changes the site preference of water molecule. The Site 7 and Site 8 present much stronger attraction than the hollow sites (Site 9~11), and the adsorption energies of water molecule on these two sites were even lower than that of water at Site A on clean surface. Unlike the PbI2-



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terminated surface, the presence of oxygen at Site F on CH3NH3I-terminated surface has a smaller influence on the adsorption energies of water at different sites. The adsorption energies of water decreases compared to the case of clean surface for all sites except for Site 21. As Site 18 and Site 19 are equilibrium positions, we got the same adsorption energy for them. From these adsorption energies, we conclude that the presence of oxygen increase the adsorption ability of water for both PbI2- and CH3NH3I-terminated surfaces. Several theoretical investigations have proved that the water molecule can easily penetrate into the perovskite surface and cause lattice distortion, leading the decomposing of the materials.31-32 And with the increase of water concentration, the distortion of the lattice is increased, making the decomposing even easier.32 As our calculation results showed that the presence of oxygen molecule can enhance the adsorption of water molecule, the above decomposing process could be accelerated by oxygen molecule.

CONCLUSIONS In conclusion, we intensively investigated the adsorption of molecular water and oxygen, as well as their co-adsorptions on (001) surfaces of tetragonal CH3NH3PbI3 using first-principle simulations. Our calculation results showed that not only the water can be attracted, but also the oxygen molecule can be easily adsorbed by the surfaces. On PbI2-terminated surface, the attraction is mainly from strong interactions between O and surface Pb atom, while on CH3NH3Iterminated surface it is from the weak hydrogen bonds between molecules and H atom from NH3-group. With the presence of a water molecule, the CH3NH3I-terminated surface presents more attractive to oxygen molecule, while the attraction of PbI2-terminated surface to oxygen molecule stays almost unchanged. On the contrary, with the presence of an oxygen molecule, the


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PbI2-terminated surface is more favorable of water molecule adsorption, while small influence can be found for adsorption of water molecule on CH3NH3I-terminated surface. These findings revealed the possible effect of oxygen on the degradation process of CH3NH3PbI3 when exposing to air.



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SUPPORTING INFORMATION Supporting Information Available: the validity check of the slab models and the orientation effects of organic cations on the adsorption. ACKNOWLEDGMENTS S.L. acknowledges support from MOE Tier 2 (ACR12/12) and MOE Tier 1. 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, 6050-6051. (2) Liu, M.; Johnston, M. B.; Snaith, H. J., Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. (3) Hodes, G., Perovskite-Based Solar Cells. Science 2013, 342, 317-318. (4) Green, M. A.; Ho-Baillie, A.; Snaith, H. J., The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506-514. (5) He, M.; Zheng, D.; Wang, M.; Lin, C.; Lin, Z., High Efficiency Perovskite Solar Cells: From Complex Nanostructure to Planar Heterojunction. J. Mater. Chem. A 2014, 2, 5994-6003. (6) You, J.; Hong, Z.; Yang, Y. M.; Chen, Q.; Cai, M.; Song, T.-B.; Chen, C.-C.; Lu, S.; Liu, Y.; Zhou, H., Low-Temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility. ACS nano 2014, 1674-1680. (7) Jung, H. S.; Park, N. G., Perovskite Solar Cells: From Materials to Devices. Small 2015, 11, 10-25. (8) Niu, G.; Guo, X.; Wang, L., Review of Recent Progress in Chemical Stability of Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 8970-8980. (9) 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.


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Synergistic Effects of Water and Oxygen Molecule Co-adsorption on

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