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Cite This: J. Phys. Chem. C 2018, 122, 11862−11869

Investigation on Enhanced Moisture Resistance of Two-Dimensional Layered Hybrid Organic−Inorganic Perovskites (C4H9NH3)2PbI4 Ying-Bo Lu,*,† ChengBo Guan,† Hui sun,† Wei-Yan Cong,† Haozhi Yang,‡ and Peng Zhang† †

School of Space Science and Physics and ‡Supercomputing Center, Shandong University, Weihai 264209, China S Supporting Information *

ABSTRACT: Here, we choose (C4H9NH3)2PbI4 to represent the two-dimensional layered hybrid organic−inorganic perovskites and investigate its excellent moisture resistance by theoretical analyses. (C4H9NH3)2PbI4 adsorbs H2O molecules easily on all surfaces. However, the most energetic surface, i.e., C4H9NH3-terminated surface, provides an energy barrier of 3.46 eV to prevent the penetration of H2O molecules, which is 1 order higher than the barrier in MAPbI3 surface. This obstacle for the invasive H2O molecule comes from the methyl/ methylene radicals of C4H9NH3 groups. These methyl/methylene radicals are chemically inactive and only generate repulsive interactions with the H2O molecule, leading to the outstanding moisture resistance of (C4H9NH3)2PbI4. Herein, we propose a practical strategy to obtain good moisture-resistant perovskite solar cell materials, i.e., modulate the shape and symmetry of organic molecules in perovskites to let as many methyl radicals as possible prevent the penetration of adsorbed H2O molecules from the surface.

active materials in optoelectronic devices.27−30 Among these layered perovskites, (C4H9NH3) 2PbI4 has already been synthesized and showed outstanding optoelectronic properties.19,29,31,32 Although these great improvements have been made, a clear understanding on the mechanism of the moisture resistance of 2D layered perovskites is still lacking, which is crucial to increase and control the stability of PSCs in future. Theoretical simulations are proved to be useful to understand either the degradation rate or decomposition mechanism of MAPbI3 perovskites because it can separate contributions to the degradation from individual layers in the lattice, which is difficult in real experimental measurements.31,33−36 Therefore, in this work, we take (C4H9NH3)2PbI4 as a representation of the 2D layered perovskites and perform a series of ab initio studies to provide an atomistic-level understanding of the moisture-induced degradation mechanism of 2D layered perovskites, particularly regarding the role of water molecules in the layered perovskite lattice.

1. INTRODUCTION Due to the excellent electronic and optical properties, perovskite solar cells (PSCs) reached their highest power conversion efficiency (PCE) of 22.1% in 5 years,1−3 making PSCs the most promising solar cells.4 However, for the most investigated perovskites, i.e., CH3NH3PbI3 (MAPbI3) solar cells, one of the main disadvantages is the degradation induced by moisture.5,6 For instance, PCE of MAPbI3 solar cells under 90% humidity decreases from 12 to 1% in only 3 days.7,8 Numerous attempts have thus been made to develop perovskite materials with superior moisture resistance,9 including using hydrophobic heterojunction contacts, a fully covered waterresisting layer, etc.10−12 Recent investigations on elemental substitution at cation, anion, and halogen sites in the MAPbI3 lattice are also reported.5,13−18 Another strategy is to develop new perovskite materials, such as the two-dimensional (2D) layered perovskite materials (RNH3)2(CH3NH3)n−1MnX3n+1, in which R is a long-chain hydrocarbon (CnH2n+1) or a phenethyl group C6H5C2H4, M is a divalent metal, and X is a halogen element. This layered structure is packed by one or more sheets of corner-shared MX6 octahedra and bilayers of organic cations alternately along the c axis. Such layered perovskites yield high in-plane carrier mobilities and good optical absoptions,19−22 making the PCE of these 2D PSCs achieve a high value of 15.3%.23−25 Most importantly, 2D layered PSCs exhibit significantly enhanced moisture resistance. For example, the properties of PSCs based on (BA)2(MA)2Pb3I10 film do not change for 2 months under 40% humidity.26 (C8H9NH3)2(CH3NH3)n−1PbnI3n+1 devices preserve the high PCE for over 2 weeks under high humidity.23 The structure of 2D-(C6H5(CH2)2NH3)2(CH3NH3)2Pb3I10 shows no obvious change for over 46 days of humidity exposure.24 Hence, these 2D PSCs are promising candidates as © 2018 American Chemical Society

2. METHODS AND MODELING DETAILS The quasi-two-dimensional hybrid organic−inorganic perovskites, formulated by (RNH3)2(CH3NH3)n−1MnX3n+1, are stacked by one or more sheets of corner-shared MX6 octahedra and bilayers of long-chain (RNH3)2(CH3NH3) alternately, where R is either a hydrocarbon chain CH2+q or a phenethyl group C6H5C2H4. By varying the organic cations and the value of n, the interlayer separation and thickness of the inorganic layers can be tuned. 2 5 In this study, we choose (C4H9NH3)2PbI4 as a representation of the 2D layered Received: March 8, 2018 Revised: May 6, 2018 Published: May 14, 2018 11862

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Figure 1. (a) Top view and (b) side view of 3D MAPbI3 lattice. (c) Top view and (d) side view of 2D layered (C4H9NH3)2PbI4 lattice. H atoms in (c) are removed to make it concise.

3. RESULTS AND DISCUSSION The degradation induced by moisture can be simplified into considerations of interactions between the water molecule and the solid lattice. The interaction of the water molecule on the solid surface is illustrated by two separate but continuous processes: adsorption on the surface and penetration through the surface. Hence, to find out the origin of the excellent moisture resistance of 2D perovskite materials, we select (C4H9NH3)2PbI4 as the representation of 2D layered perovskites and study the adsorption and penetration of H2O molecule near the surface. Performances of 3D MAPbI3 perovskites are also presented to make an elaborate comparison between 2D and 3D perovskite materials. The structures of both 2D and 3D perovskites are shown in Figure 1. For MAPbI3, Figure 2a,b shows (001) slabs29 covered by organic MAI group and inorganic PbI group, respectively, signaled as MAI-terminated (MAI-T) and PbI-terminated (PbIT) surfaces, respectively. The calculated surface energies indicate that the MAI-T surface is energetic and more stable than the PbI-T one. However, the outmost CH3NH3 (MA) molecule for the MAI-T surface can be rotated or moved easily by the adsorbed H2O molecule, resulting in the decomposition of the surface. However, on the PbI-T surface, the outmost inorganic frameworks act as a protective layer against the

organic−inorganic perovskites. The geometric structure of (C4H9NH3)2PbI4 is illustrated in Figure 1, in which each atomic layer of PbI4 is sandwiched by (C4H9NH3)2 organic chains. The adjacent layers are connected by hydrogen-bonding interactions. For comparison, the structure of three-dimensional (3D) MAPbI3 perovskites is also shown in Figure 1. All first-principle calculations in this work are performed via Vienna ab initio simulation package (VASP). Generalized gradient approximation functional parameterized with the Perdew−Burke−Ernzerhof method is used to describe the electron−electron exchange and correlation effects. The electronic wave functions are expanded using the PAW method. The cutoff energy, SCF energy difference, and residual force criterion are set as 400 eV, 10−4 eV, and 10−2 eV/Å, respectively. For the emergence of organic components, the van der Waals interaction between the organic cation molecule and the PbI inorganic frameworks should be taken into account, so all structures are optimized with functional optb88vdW. To determine the minimum energy migration pathway of water molecules into the subsurface of MAPbI 3 and (C4H9NH3)2PbI4 lattices, the climbing image nudged elastic band (CI-NEB) method implemented in the VASP package is used. Γ-point sampling and K-point meshes are adopted in calculations on the surface and bulk crystal, respectively. 11863

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Figure 2. (a) PbI-terminated (PbI-T) and (b) MAI-terminated (MAI-T) surfaces of MAPbI3 materials. (c) PbI-terminated (PbI-T) and (d) C4H9NH3-terminated (nBU-T) surfaces of (C4H9NH3)2PbI4 materials.

Figure 3. Relaxed structures of PbI-terminated surfaces with adsorbed/penetrated H2O molecules. (a) H2O in the green dashed circle denotes the locations just above the hollow site of the surface before geometric relaxation, whereas the H2O outside the circle denotes the location after relaxation. (b) H2O locates above the Pb atom. (c) H2O penetrates into the cage below the outmost PbI layer; the red arrow denotes the initial orientation of the MA molecule before relaxation.

layer is weak. However, when the adsorbed H2O molecule migrates further to the interior of the PbI-T surface of MAPbI3, the upside CH3 moiety shows a large energy barrier owing to its inactive bonding with the H2O molecule, while the upside NH3 moiety shows a nearly zero energy barrier due to the active bonding with H2O. That is, the orientation of the MA molecule plays a critical role in the surface performance of MAPbI3 materials. Hence, the PbI-T surface with CH3 moiety upside is the most moisture-resistant surface for MAPbI3, which is already proved by previous reports.35 Hence, we adopt this slab to represent the strongest moisture resistance of MAPbI3 systems, which is shown in Figure 2a. All adsorption positions for H2O molecules on MAPbI3 surface (illustrated in Figure 3) contribute negative adsorption energy. The most favorable adsorption site locates above the hollow site of the surface, but closes to one of the corner Pb atoms, as plotted in Figure 3a. To see if the H2O molecule can

invasive H2O molecule, which is attributed to the moderate bonding interactions between Pb and I atoms. Because CH3 moiety in the MA molecule has a closed-shell electronic configuration, it is chemically inactive with H2O, which is just the reverse for the case of NH3 moiety. As we have reported previously,4 the dipole direction of the CH3NH3 molecule comes from the imbalance of charge distribution between the methyl radical and ammonia radical. For the surface slab, this dipole direction predominantly affects the adsorption of H2O molecule. As revealed in ref 35, in the PbI-T surface with CH3 moiety upside, the chemically inactive CH3 moiety bonds with PbI frames weakly, whereas the adsorbed H2O molecule shows slightly stronger bonds with the outmost PbI layers, making the adsorption of the H2O molecule easy. However, for the PbI-T surface with NH3 moiety upside, the more chemically active NH3 bonds with PbI frames are more strong, so the bonding interaction between the adsorbed H2O and the outmost PbI 11864

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In general, by adjusting the size and symmetry of the organic molecule A, materials with ABX3 composition will change to different crystal structures. As illustrated in Figure 1, if we substitute the MA molecule in the MAPbI3 lattice by two headto-head long-chain n-butylamine C4H9NH3 molecules, the length of the (C4H9NH3)2 bilayer is so long that it inflates the 3D corner-shared PbI6 octahedron frameworks to isolated PbI4 layers, so (C4H9NH3)2PbI4 is packed by one layer of PbI6 inorganic octahedra and an organic (C4H9NH3)2 bilayer alternatively. With the Goldschmidt tolerance factor t ∼ 2.8, it is the 2D layered (C4H9NH3)2PbI4 perovskite. As in the case of MAPbI3 materials, there are also two kinds of surfaces for (C4H9NH3)2PbI4: PbI-terminated (PbI-T) and C4H9NH3Iterminated (nBU-T) slabs, which are illustrated in Figure 2c,d, respectively. The calculated surface energies show that the nBU-T surface is favorable during the equilibrium growth.31 Nevertheless, the key difference between the (C4H9NH3)2PbI4 and MAPbI3 systems is the C4H9NH3 molecule. So, the enhanced moisture resistance of 2D (C4H9NH3)2PbI4 must originate from the C4H9NH3 molecule or the nBU-T surfaces. Therefore, we study interactions between H2O materials and nBU-T surface elaborately, including the adsorption and penetration of H2O molecules. We test four adsorption positions of H2O on the nBU-T surface and plot them in Figure 7a−d. All adsorption energies are negative, indicating that the adsorption of H2O molecule on nBU-T surface is also a spontaneous process. Among these configurations, the one with a H2O molecule located at the top center of the hollow surrounded by four C4H9NH3 molecules has the lowest adsorption energy, which is shown in Figure 7d. Compared to other three configurations, this one exhibits a more tight bonding interaction between H2O and C4H9NH3 molecules, i.e., the O atom of H2O bonds with a H atom of C4H9NH3 molecule with a length of 1.76 Å, and one H atom of H2O bonds with a H atom of the C4H9NH3 molecule with a length of 2.37 Å. The charge density map in Figure 4c also supports this conclusion. There is also a question that whether the adsorbed H2O molecule on the nBU-T surface can penetrate through the channel surrounded by C4H9NH3 molecules to reach the interior space of (C4H9NH3)2PbI4. As shown in Figure 7e,f, we consider two penetrated H2O positions in the vacant space between the outmost (C4H9NH3)2 organic bilayer and the inner PbI4 layer and pick the configuration with H2O along the a-axis (Figure 7e), which is energetically favored. The possibility of the adsorbed H2O on the nBU-T surface penetrating into the subsurface is indicated by the energy barrier of the H2O molecule. This is a rather long migration path, and thus, ions below the outmost organic layer play minor roles during the penetration process. As illustrated in Figure 7, to simplify the calculation, we employ the surface configuration containing only the outmost organic layer, which is a rational way to describe the penetration process with the least computational demands. The migration traces of H2O are shown in Figure 5c,d (more elaborate atomic structures during the penetrating path are illustrated in Figure S3 in the Supporting Information). Figure 6b shows the penetration barrier with a substantial high value of 3.46 eV. This means that the direct transfer of the adsorbed H2O molecule from the nBU-T surface to the sublayer or deeper into (C4H9NH3)2PbI4 is unlikely. This penetration barrier is 1 order higher than the barrier (0.39 eV) of the PbI-T surface in the MAPbI3 system. We should note that the PbI-T surface is the most stable

transfer into the interior of the MAPbI3 lattice, we put a H2O molecule into the cage below the outmost PbI layer and show the relaxed structure in Figure 3c, where the CH3 moiety of the MA molecule in the cage is repelled downward. To keep the linear polarity of the MA molecule, NH3 radical shifts upward simultaneously. The penetration of the H2O molecule makes the orientation of the MA molecule deviate obviously from the original direction before relaxation (marked by the red arrow in Figure 3c). In the penetrated configurations, the oxygen atom of H2O bonds strongly with the NH3 moiety of the MA molecule, whereas the hydrogen atoms of H2O bond with idiom atoms. These bonding interactions are demonstrated by charge density map in Figure 4a and differential charge density map in Figure S1 in the Supporting Information.

Figure 4. Charge density maps for configurations of MAPbI3 with (a) penetrated H2O below the first PbI layer and (b) H2O located at the position where it encounters the highest energy barrier. Configurations of nBU-T surfaces in (C4H9NH3)2PbI4 with H2O molecule (c) located at the top center of the hollow surrounded by four C4H9NH3 molecules and (d) H2O located at the position with maximum energy barrier.

The rotation of the MA molecule gives two results. First, the downward shift of the CH3 radical provides extra vacant space to accommodate the penetrated H2O molecule. Second, the upward shift of the NH3 radical leads to the stronger bonding interaction between NH3+ and the H2O molecule. When the adsorbed H2O molecule migrates from the position above the surface into the cage below, the energy barrier calculated using CI-NEB methods is 0.39 eV, consistent with previous reports.36 The penetration pathway and the energy barrier are exhibited in Figures 5 and 6, respectively (more elaborate atomic structures during the penetrating path are illustrated in Figure S2 in the Supporting Information). It is agreed that the hydrogen bonds between the inorganic PbI frameworks and the organic MA molecules keep MAPbI3 stable.37 In other words, if the H2O molecule overcomes this energy barrier to penetrate through the PbI layers, then it will destroy the original bonding interaction between the PbI framework and MA molecules, giving rise to the weakened stability of the MAPbI3 lattice. 11865

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Figure 5. Penetration paths for adsorbed H2O molecules. (a) Side view and (b) top view of penetration paths for PbI-T surface in MAPbI3 materials. (c) Side view and (d) top view of penetration paths for nBU-T surface in (C4H9NH3)2PbI4 materials. The red dashed lines are guidelines for the penetration paths. The green dashed circles in (a) and (c) denote the position where penetrated H2O encounters the highest energy barriers.

Figure 6. Penetration energy barriers of adsorbed H2O. (a) H2O migrates from the PbI-T surface into the interior of the MAPbI3 system. (b) H2O migrates from the nBU-T surface into the interior of (C4H9NH3)2PbI4 materials.

layer to the space below the surface, it encounters the CH3 radical of the MA molecule. Because of the chemical inactivity of the CH3 radical, the H2O molecule repels this CH3 radical downward and thus rotates the MA molecule to accommodate itself in the cage below the PbI layer. As is well known, the hydrogen bonds between I atoms and H atoms in NH3 radicals of MA molecules contribute importantly to the structural stability of the hybrid inorganic−organic perovskite. The displacement of the NH3 radical during the invasion of H2O

surface in MAPbI3 materials. This indicates that the moisture resistance of 2D (C4H9NH3)2PbI4 perovskites is much stronger than that of 3D MAPbI3 perovskites. As mentioned above, the resistance against the invasion of the H2O molecule is significantly different in MAPbI3 and (C4H9NH3)2PbI4 lattices, which can be understood via analyses of the structural variation during the H2O penetration process. For the PbI-T surface of MAPbI3, when the adsorbed H2O molecule migrates from the position above the outmost PbI 11866

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Figure 7. Positions of the adsorbed H2O molecules on the nBU-T surfaces of (C4H9NH3)2PbI4 materials. (a, b) Positions of H2O sites just above two different NH3 radicals of C4H9NH3 molecule. (c) H2O sites just above the methylene radical of C4H9NH3 molecule. (d) H2O locates at the top center of the hollow surrounded by four C4H9NH3 molecules. (e, f) Positions of penetrated H2O molecules under outmost (C4H9NH3)2 bilayers, which align the a-axis and b-axis, respectively. Bond lengths are presented in each figure for the critical bonding interactions between H2O and host lattice, in units of angstrom.

needs energy to break the hydrogen bonds between the inorganic and organic groups. Therefore, during the NEBsimulated penetration process of H2O on PbI-T surface, the configuration with the maximum energy barrier is actually the case where the hydrogen bonds between PbI and MA groups begin to break and thus the MA molecule starts to rotate. The geometric structure and the charge density map of this configuration are illustrated in Figure 4b. However, these hydrogen bonds are relatively weak and the MA molecules can be rotated easily, so the penetration barrier for H2O molecules on the PbI-T surface of MAPbI3 materials has a moderate value. The outmost layer of the nBU-T surface of (C4H9NH3)2PbI4 is composed of the organic bilayer (C4H9NH3)2. As depicted by the simplified illustration in Figure 7, the NH3 radical of each C4H9NH3 molecule is located at the top and bottom surfaces of this bilayer. Due to the chemical activity of the NH3 radical, the hydrogen bonds between these NH3 radicals and PbI4 layers hold this hybrid lattice stable. The NH3 radical on the top of the nBU-T surface also plays a crucial role in the adsorption of H2O because it can form hydrogen bonds with H2O, leading to the negative adsorption energy. However, when the adsorbed H2O molecule keeps going through the bilayer (C4H9NH3)2 downward, it will pass through the channel constructed by the surrounding methyl radicals of C4H9NH3. Unfortunately, because of the chemically inactive nature of methyl radicals, the H2O molecule is hindered by the repulsive interaction originated from the electron cloud of the methyl radicals. As aforementioned in MAPbI3 systems, the repulsive forces between the invasive H2O and the MA molecule can rotate the MA molecule easily, which weakens the resistance against the penetration of H2O. However, in 2D (C4H9NH3)2PbI4

systems, the (C4H9NH3)2 group is so long that the repulsive interactions cannot bring about manifest rotation or translation effect on it. So, there is no obvious structural change to weaken the resistance against the penetration of H2O molecules. The only effect of the repulsive interactions is to prevent the downward migration of the H2O molecule. The highest energy barrier is found in the configuration with the H2O molecule located almost at the middle of the (C4H9NH3)2 bilayer, where the H2O molecule is surrounded by four nearest methyl ends from C4H9NH3 groups and is furthest from the NH3 ends. Orbital electrons from methyl radicals form a dense and localized electron cloud to impose the strongest repulsive force to the invasive H2O molecule, which is shown in Figure 4d. We should note an interesting phenomenon that there is a nearly flat energy surface in Figure 6b, which means that when the adsorbed H2O molecule starts to migrate from the outmost surface to the sublayer, it will encounter the almost maximum energy barrier very soon, and this high barrier will last for almost the whole migration path. This performance makes the nBU-T surface in the 2D (C 4 H 9 NH 3 ) 2 PbI 4 materials impermeable to H2O molecules.

4. CONCLUSIONS By theoretical calculation and analyses, we get the origin of the excellent moisture resistance of 2D layered (C4H9NH3)2PbI4 perovskite solar cell materials. The case in 3D MAPbI3 materials is also investigated for comparison. In 3D MAPbI3 materials, the most stable PbI-T surface provides an energy barrier of 0.39 eV to hinder the penetration of H2O, whereas in 2D (C4H9NH3)2PbI4 systems, the penetration barrier of H2O through the nBU-T surface equals 3.46 eV, which is a really 11867

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(3) Sessolo, M.; Bolink, H. J. Perovskite solar cells join the major league. Science 2015, 350, 917. (4) Lu, Y.-B.; Kong, X. H.; Chen, X. B.; Cooke, D. G.; Guo, H. Piezoelectric scattering limited mobility of hybrid organic-inorganic perovskites CH3NH3PbI3. Sci. Rep. 2017, 7, No. 41860. (5) Murali, B.; Saidaminov, M. I.; Abdelhady, A. L.; Peng, W.; Liu, J.; Pan, J.; Bakr, O. M.; Mohammed, O. F. Robust and air-stable sandwiched organo-lead halide perovskites for photodetector applications. J. Mater. Chem. C 2016, 4, 2545. (6) Bryant, D.; Aristidou, N.; Pont, S.; Sanchez-Molina, I.; Chotchunangatchaval, T.; Wheeler, S.; Durrant, J. R.; Haque, S. A. Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells. Energy Environ. Sci. 2016, 9, 1655. (7) Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V. Transformation of the excited state and photovoltaic efficiency of CH3NH3PbI3 perovskite upon controlled exposure to humidified air. J. Am. Chem. Soc. 2015, 137, 1530. (8) Yang, J.; Kelly, T. L. Decomposition and Cell Failure Mechanisms in Lead Halide Perovskite Solar Cells. Inorg. Chem. 2017, 56, 92. (9) Leguy, A. M. A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; Docampo, P.; Barnes, P. R. F. Reversible Hydration of CH3NH3PbI3in Films, Single Crystals, and Solar Cells. Chem. Mater. 2015, 27, 3397. (10) Pan, J.; Mu, C.; Li, Q.; Li, W.; Ma, D.; Xu, D. RoomTemperature, Hydrochloride-Assisted, One-Step Deposition for Highly Efficient and Air-Stable Perovskite Solar Cells. Adv. Mater. 2016, 28, 8309. (11) Yang, S.; Wang, Y.; Liu, P.; Cheng, Y.-B.; Zhao, H. J.; Yang, H. G. Functionalization of perovskite thin films with moisture-tolerant molecules. Nat. Energy 2016, 1, No. 15016. (12) Kim, I. S.; Martinson, A. B. F. Stabilizing hybrid perovskites against moisture and temperature via non-hydrolytic atomic layer deposited overlayers. J. Mater. Chem. A 2015, 3, 20092. (13) Berhe, T. A.; Su, W.-N.; Chen, C.-H.; Pan, C.-J.; Cheng, J.-H.; Chen, H.-M.; Tsai, M.-C.; Chen, L.-Y.; Dubale, A. A.; Hwang, B.-J. Organometal halide perovskite solar cells: degradation and stability. Energy Environ. Sci. 2016, 9, 323. (14) Zhao, X.; Park, N.-G. Stability Issues on Perovskite Solar Cells. Photonics 2015, 2, 1139. (15) Pearson, A. J.; Eperon, G. E.; Hopkinson, P. E.; Habisreutinger, S. N.; Wang, J. T.-W.; Snaith, H. J.; Greenham, N. C. Oxygen Degradation in Mesoporous Al2O3/CH3NH3PbI3-xClxPerovskite Solar Cells: Kinetics and Mechanisms. Adv. Energy Mater. 2016, 6, No. 1600014. (16) Li, Z.; Yang, M.; Park, J.-S.; Wei, S.-H.; Berry, J. J.; Zhu, K. Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys. Chem. Mater. 2016, 28, 284. (17) Yi, C.; Luo, J.; Meloni, S.; Boziki, A.; Ashari-Astani, N.; Gratzel, C.; Zakeeruddin, S. M.; Rothlisberger, U.; Gratzel, M. Entropic stabilization of mixed A-cation ABX3 metal halide perovskites for high performance perovskite solar cells. Energy Environ. Sci. 2016, 9, 656. (18) Nagabhushana, G. P.; Shivaramaiah, R.; Navrotsky, A. Direct calorimetric verification of thermodynamic instability of lead halide hybrid perovskites. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 7717. (19) Yaffe, O.; Chernikov, A.; Norman, Z. M.; Zhong, Y.; Velauthapillai, A.; van der Zande, A.; Owen, J. S.; Heinz, T. F. Excitons in ultrathin organic-inorganic perovskite crystals. Phys. Rev. B 2015, 92, No. 045414. (20) Dwivedi, V. K.; Vijaya Prakash, G. Fabrication and roomtemperature exciton photoluminescence stability studies of inorganic− organic hybrid (C12H25NH3)2SnI4 thin films. Solid State Sci. 2014, 27, 60. (21) Huang, T. J.; Thiang, Z. X.; Yin, X.; Tang, C.; Qi, G.; Gong, H. (CH3NH3)2PdCl4: A Compound with Two-Dimensional Organic− Inorganic Layered Perovskite Structure. Chem. - Eur. J. 2016, 22, 2146.

high value and accounts for the experimentally observed enhanced moisture resistance of 2D layered perovskites. In both 2D and 3D perovskite surfaces, the predominant resistances against the invasive H2O are generated by the methyl radical of organic groups. For the chemically inactive nature of the methyl radical, it affects repulsive interactions on the penetrated H2O. In slabs of 3D perovskites, the MA molecule is rotated by this repel interaction and thus reduces the resistance against the penetration of H2O. However, in 2D perovskite (C4H9NH3)2PbI4, the (C4H9NH3)2 bilayer surface is too long to be rotated or translated, and the only effect of the repulsive interactions is to prevent the downward migration of the H2O molecule. This is the key issue contributing to the distinguished moisture resistance of the 2D layered (C4H9NH3)2PbI4. Although investigations in this work are focused on the (C4H9NH3)2PbI4 system, we believe that in other 2D layered perovskites (RNH3)2(CH3NH3)n−1MnX3n+1, the enhanced moisture resistance comes from the same origin, i.e., the repulsive interactions generated by the methyl racial of organic groups. So, this provides a practical approach to produce hybrid inorganic−organic perovskite solar cell materials with excellent moisture resistance, i.e., modulate the shape and the symmetry of organic molecules, as well as the external factors, to let as many methyl radicals as possible resist the penetration of the adsorbed H2O molecule from the surface.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02299. Differential charge density maps (Figure S1); key configurations during the penetration paths of adsorbed H2O for PbI-T surface in MAPbI3 materials (Figure S2); key configurations during the penetration paths of adsorbed H2O for nBU-T surface in (C4H9NH3)2PbI4 materials (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ying-Bo Lu: 0000-0001-7799-8751 Peng Zhang: 0000-0002-1099-6310 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science foundation of China under Grant 11504202. The calculations in this work are carried out at the High Performance Computer Center of McGill University, CalculQuebec, Compute Canada, and the Supercomputing Center of Shandong University (Weihai).

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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. (2) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar cell efficiency tables (version 48). Prog. Photovoltaics 2016, 24, 905. 11868

DOI: 10.1021/acs.jpcc.8b02299 J. Phys. Chem. C 2018, 122, 11862−11869

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The Journal of Physical Chemistry C (22) Herz, L. M. Charge-Carrier Mobilities in Metal Halide Perovskites: Fundamental Mechanisms and Limits. ACS Energy Lett. 2017, 2, 1539. (23) Quan, L. N.; Yuan, M.; Comin, R.; Voznyy, O.; Beauregard, E. M.; Hoogland, S.; Buin, A.; Kirmani, A. R.; Zhao, K.; Amassian, A.; Kim, D. H.; Sargent, E. H. Ligand-Stabilized Reduced-Dimensionality Perovskites. J. Am. Chem. Soc. 2016, 138, 2649. (24) Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. A layered hybrid perovskite solar-cell absorber with enhanced moisture stability. Angew. Chem., Int. Ed. 2014, 53, 11232. (25) Ahmad, S.; Kanaujia, P. K.; Beeson, H. J.; Abate, A.; Deschler, F.; Credgington, D.; Steiner, U.; Prakash, G. V.; Baumberg, J. J. Strong Photocurrent from Two-Dimensional Excitons in Solution-Processed Stacked Perovskite Semiconductor Sheets. ACS Appl. Mater. Interfaces 2015, 7, 25227. (26) Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G. 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications. J. Am. Chem. Soc. 2015, 137, 7843. (27) Sichert, J. A.; Tong, Y.; Mutz, N.; Vollmer, M.; Fischer, S.; Milowska, K. Z.; García Cortadella, R.; Nickel, B.; Cardenas-Daw, C.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets. Nano Lett. 2015, 15, 6521. (28) Mitzi, D. B.; Wang, S.; Feild, C. A.; Chess, C. A.; Guloy, A. M. Conducting Layered Organic-inorganic Halides Containing ⟨110⟩Oriented Perovskite Sheets. Science 1995, 267, 1473. (29) Dou, L.; Wong, A. B.; Yu, Y.; Lai, M.; Kornienko, N.; Eaton, S. W.; Fu, A.; Bischak, C. G.; Ma, J.; Ding, T.; Ginsberg, N. S.; Wang, L.W.; Alivisatos, A. P.; Yang, P. Atomically thin two-dimensional organic-inorganic hybrid perovskites. Science 2015, 349, 1518. (30) Niu, W.; Eiden, A.; Vijaya Prakash, G.; Baumberg, J. J. Exfoliation of self-assembled 2D organic-inorganic perovskite semiconductors. Appl. Phys. Lett. 2014, 104, No. 171111. (31) Wang, D.; Wen, B.; Zhu, Y. N.; Tong, C. J.; Tang, Z. K.; Liu, L. M. First-Principles Study of Novel Two-Dimensional (C4H9NH3)2PbX4 Perovskites for Solar Cell Absorbers. J. Phys. Chem. Lett. 2017, 8, 876. (32) Mitzi, D. B. Synthesis, Crystal Structure, and Optical and Thermal Properties of (C4H9NH3)2MI4 (M = Ge, Sn, Pb). Chem. Mater. 1996, 8, 791. (33) Ohmann, R.; Ono, L. K.; Kim, H.-S.; Lin, H.; Lee, M. V.; Li, Y.; Park, N.-G.; Qi, Y. Real-Space Imaging of the Atomic Structure of Organic−Inorganic Perovskite. J. Am. Chem. Soc. 2015, 137, 16049. (34) Zhang, L.; Sit, P. H. L. Ab Initio Study of Interaction of Water, Hydroxyl Radicals, and Hydroxide Ions with CH3NH3PbI3 and CH3NH3PbBr3 Surfaces. J. Phys. Chem. C 2015, 119, 22370. (35) Koocher, N. Z.; Saldana-Greco, D.; Wang, F.; Liu, S.; Rappe, A. M. Polarization Dependence of Water Adsorption to CH3NH3PbI3 (001) Surfaces. J. Phys. Chem. Lett. 2015, 6, 4371. (36) Tong, C.-J.; Geng, W.; Tang, Z.-K.; Yam, C.-Y.; Fan, X.-L.; Liu, J.; Lau, W.-M.; Liu, L.-M. Uncovering the Veil of the Degradation in Perovskite CH3NH3PbI3 upon Humidity Exposure: A First-Principles Study. J. Phys. Chem. Lett. 2015, 6, 3289. (37) Grancini, G.; D’Innocenzo, V.; Dohner, E. R.; Martino, N.; Srimath Kandada, A. R.; Mosconi, E.; De Angelis, F.; Karunadasa, H. I.; Hoke, E. T.; Petrozza, A. CH3NH3PbI3 perovskite single crystals: surface photophysics and their interaction with the environment. Chem. Sci. 2015, 6, 7305.

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DOI: 10.1021/acs.jpcc.8b02299 J. Phys. Chem. C 2018, 122, 11862−11869