Investigations on Enhanced Moisture Resistance of Two-Dimensional

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Investigations on Enhanced Moisture Resistance of Two-Dimensional Layered Hybrid Organic-Inorganic Perovskites (CHNH)PbI 4

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Ying-Bo Lu, Chengbo Guan, Hui Sun, Wei-Yan Cong, Haozhi Yang, and Peng Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02299 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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The Journal of Physical Chemistry

Investigations on Enhanced Moisture Resistance of Two-dimensional Layered Hybrid Organic-inorganic Perovskites (C4H9NH3)2PbI4 Ying-Bo Lu1,*, ChengBo Guan1, Hui sun1, Wei-Yan Cong1, Haozhi Yang2 and Peng Zhang1 1 2

School of Space Science and Physics, Shandong University, Weihai 264209, China Supercomputing Center, Shandong University, Weihai 264209, China

ABSTRACT: We choose (C4H9NH3)2PbI4 to represent the two-dimensional (2D) 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 one 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 chemical inactive and only generate repulsive interactions to the H2O molecule, leading to the outstanding moisture resistance of (C4H9NH3)2PbI4. Herein, we propose a practical strategy to obtain good moisture resistant perovskites solar cell materials, i.e., modulate the shape and the symmetry of organic molecules in perovskites to let as more methyl-radicals as possible prevent the penetration of adsorbed H2O molecule from the surface.

1 Environment ACS Paragon Plus

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1. Introduction Due to the excellent electronic and optical properties, perovskites solar cells (PSCs) reached its highest power conversion efficiency (PCE) of 22.1% in five years,1-3 leading PSCs to be the most promising solar cell.4 However, for the most investigated perovskites, i.e., CH3NH3PbI3 (MAPbI3) solar cell, one of the main disadvantages is the degradation induced by the moisture.5,6 For instance, PCE of MAPbI3 solar cell under 90% humidity decreases from 12% to 1% in only three days.7,8 Numerous attempts have thus been carried out to develop perovskites materials with superior moisture resistance,9 including using hydrophobic heterojunction contacts and a fully covered water-resisting layer, etc.10-12 Recent investigations on elemental substitution at cation, anion and halogen sites in MAPbI3 lattice are also reported.5,13-18 Another strategy is to develop new perovskites materials, such as the two-dimensional (2D) layered perovskites 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 c axis. Such layered perovskites yield high inplane carrier mobilities and good optical absoptions,19-22 making PCE of these 2D PSCs achieves a high value of 15.3%.23-25 Most importantly, 2D layered PSCs exhibit significant enhanced moisture resistance. For examples, 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

over

2

weeks

under

high

humidity.23

Structure

of

2D-

(C6H5(CH2)2NH3)2(CH3NH3)2Pb3I10 shows no obvious change over 46 days of humidity exposure.24 Hence these 2D PSCs are promising candidates as the active materials in optoelectronic devices.27-30 Among these layered perovskites, (C4H9NH3)2PbI4 has been


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synthesized already 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 lack, 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 lattice, which is difficult in the real experimental measurements.31,33-36 Therefore, in this work, we take (C4H9NH3)2PbI4 as a representation of 2D layered perovskites and perform series of ab initio studies to provide an atomistic-level understanding of the moisture degradation mechanism of 2D layered perovskites, particularly regarding the role of water molecules in the layered perovskites lattice.

2. Methods and Modeling details The

quasi

two-dimensional

hybrid

organic-inorganic

perovskites,

formulated

by

(RNH3)2(CH3NH3)n-1MnX3n+1, is 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.25 In this study, we choose (C4H9NH3)2PbI4 as a representation of the 2D layered 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. Adjacent layers are connected by hydrogen bonding interactions. For comparison, the structure of 3D perovskites MAPbI3 is also plotted in Figure 1.



All first principle calculations in this work are performed via Vienna Ab initio Simulation Package (VASP). GGA functional parameterized with Perdew-Burke-Ernzerhof (PBE) method is used to describe the electron-electron exchange and correlation effects. The electronic wave functions are expanded using 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 emergency of organic components, the Van de Waals interaction between the organic cation molecule and the PbI inorganic frameworks should be accounted, so all structures are optimized with functional optb88-vdW. To determine the minimum energy migration pathway of water molecule into the subsurface of MAPbI3 and (C4H9NH3)2PbI4 lattices, climbing image nudged elastic band (CI-NEB) method implemented in VASP package is used. Gamma-point sampling and K-Point meshes are adopted in calculations on surface and bulk crystal, respectively. 3. Results and Discussions The degradation induced by moisture can be simplified into considerations of interactions between the water molecule and the solid lattice. The interaction played by water molecule on the solid surface is illustrated by two separate but continuous processes: the adsorption on the surface and the penetration through the surface. Hence, to find out the origin of the excellent moisture resistance of 2D perovskites 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 perovskites MAPbI3 are also presented to make an elaborate comparison between 2D- and 3D- perovskites materials. Structures of both 2D- and 3Dperovskites are shown in Figure 1.


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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. For MAPbI3, Figure 2a and 2b show (001) slabs29 covered by organic MAI group and inorganic PbI group, respectively, signaled as MAI-terminated (MAI-T) surface and PbIterminated (PbI-T) surfaces, respectively. The calculated surface energies indicate that MAI-T surface is energetic more stable than the PbI-T one. However, the outmost CH3NH3 (MA) molecule for MAI-T surface can be rotated or moved easily by the adsorbed H2O molecule, resulting to the decomposition of surface. While on PbI-T surface, the outmost inorganic frameworks act as a protective layer against the invasive H2O molecule, which is accounted to the moderate bonding interactions between Pb and I atoms. Because CH3-moiety in MA molecule has the closed-shelled electronic configuration, it is chemical inactive with H2O, which is just reverse for the case of NH3-moiety. As we report previously,4 the dipole direction of



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 PbI-T surface with CH3-moiety upside, the chemical inactive CH3 moiety bonds with PbI frames weakly, while the adsorbed H2O molecule show a little more strong bonds with outmost PbI layers, making the adsorption of H2O molecule easily. However, for the PbI-T surface with NH3-moity upside, the more chemical active NH3 bonds with PbI frames more strong, so the bonding interaction between adsorbed H2O and the outmost PbI layer is weak. However, when the adsorbed H2O molecule migrates further to the interior of PbI-T surface of MAPbI3, the upside CH3-moiety shows large energy barrier owing to its inactive bonding with H2O molecule, while the upside NH3-moiety gives a nearly zero energy barrier due to the active bonding with H2O.That is, the orientation of MA molecule plays a critical role on 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 So we adopt this slab to represent the most strong moisture resistance of MAPbI3 systems, which is shown in Figure 2a.


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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. 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, plotted in Figure 3a. To see if H2O molecule can transfer into the interior of 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 CH3moiety of MA molecule in the cage is repelled downward. To keep the linear polarity of MA molecule, NH3-radical shifts upward simultaneously. The penetration of H2O molecule makes the orientation of 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 MA molecule, while 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 supplementary materials..

Figure 3. Relaxed structures of PbI-terminated surfaces with adsorbed/penetrated H2O molecules. (a) H2O in green dashed circles denotes the locations just above the hollow site of surface before geometric relaxation, while 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 in (c) denotes the initial orientation of MA molecule before relaxation.

Figure 4. Charge density maps. For configurations of MAPbI3 with (a) penetrated H2O below the first PbI layer, (b) H2O locating at the position where it encounters the highest energy barrier. Configurations of nBU-T surfaces in (C4H9NH3)2PbI4 with H2O molecule (c) locating at the top center of the hollow surrounded by four C4H9NH3 molecules, (d) H2O locating at the position with maximum energy barrier. The rotation of MA molecule gives two results. Firstly, the downward shift of CH3-radical provides extra vacant to accommodate the penetrated H2O molecule. Secondly, the upward shift of NH3-radical leads to the stronger bonding interaction between NH3+ and 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, consisting with previous reports.36 The penetration pathway and the energy barrier are exhibited in Figure 5 and Figure 6,


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respectively (More elaborate atomic structures during the penetrating path are illustrated in Figure S2 in Supplementary materials). 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, it will destroy the original bonding interaction between PbI framework and MA molecules, giving rise to the weakened stability of MAPbI3 lattice.

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 to present the penetration paths. The green 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 MAPbI3 system. (b) H2O migrates from the nBU-T surface into the interior of (C4H9NH3)2PbI4 materials. In general, by adjustment of the size and the 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 MAPbI3 lattice by two head-to-head long-chain n-butylamine C4H9NH3 molecules, the length of (C4H9NH3)2 bilayer is so long that it inflates the 3D cornershared PbI6 octahedron frameworks to isolated PbI4 layers, so (C4H9NH3)2PbI4 is packed by one layer of PbI6 inorganic octahedra and organic (C4H9NH3)2 bilayer alternatively. With the Goldschmidt tolerance factor t~2.8, it is the 2D layered (C4H9NH3)2PbI4 perovskites. As the case in MAPbI3 materials, there are also two kinds of surfaces for (C4H9NH3)2PbI4: PbI-terminated (PbI-T) and C4H9NH3I-terminated (nBU-T) slab, which are illustrated in Figure 2c and 2d, respectively. The calculated surface energies show that nBU-T surface is favorable during the equilibrium growth.31 Nevertheless, the key difference between (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 nBU-T surface and


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plot them in Figure 7a-d. All adsorption energies are negative, indicating the adsorption of H2O molecule on nBU-T surface is a spontaneous process too. Among these configurations, the one with a H2O molecule locating at the top center of the hollow surrounded by four C4H9NH3 molecules has the lowest adsorption energy, which is shown in Figure 7d. Compared with other three configurations, this one exhibits more tight bonding interaction between H2O and C4H9NH3 molecules, i.e., the O atom of H2O bonds with a H atom of C4H9NH3 molecule in length of 1.76Å and one H atom of H2O bonds with a H atom of C4H9NH3 molecule in length of 2.37Å. The charge density map in Figure 4c also supports this conclusion.

Figure 7. Positions of adsorbed H2O molecules on the nBU-T surfaces of (C4H9NH3)2PbI4 materials. (a) and (b) describe 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) and (f) present positions of penetrated H2O molecules under outmost (C4H9NH3)2 bilayers, which align



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 Å. There is also a question that if 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 Figure7e and 7f, we consider two penetrated H2O positions in vacant space between the outmost (C4H9NH3)2 organic bilayer and the inner PbI4 layer, and pick the configuration with H2O along a-axis (Figure7e), which is energetically favored. The possibility of the adsorbed H2O on nBU-T surface penetrates into the subsurface is indicated by the energy barrier of 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 and 5d simultaneously (More elaborate atomic structures during the penetrating path are illustrated in Figure S3 in Supplementary materials). Figure 6b gives 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 the (C4H9NH3)2PbI4 is unlikely. This penetration barrier is one order higher than the barrier (0.39 eV) of PbI-T surface in MAPbI3 system. We should note that PbI-T surface is the most stable surface in MAPbI3 materials. This indicates that the moisture resistance of 2D perovskites (C4H9NH3)2PbI4 is much stronger than that of 3D MAPbI3 perovskites. As mentioned above, the resistance against the invasion of H2O molecule is significantly different in MAPbI3 and (C4H9NH3)2PbI4 lattices, which can be understood via analyses of the


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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 layer to space below surface, it encounters the CH3-radical of MA molecule. Because of the chemical inactivity of CH3-radical, the H2O molecule repels this CH3-radical downward and thus rotates MA molecule to accommodate itself in the cage below 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 hybrid inorganic-organic perovskite. The displacement of NH3-radical during the invasion of H2O needs energy to break the hydrogen bonds between inorganic and organic groups. So during the NEB simulated penetration process of H2O on PbI-T surface, the configuration with the maximum energy barrier is actually the case that the hydrogen bonds between PbI- and MA- groups begin to be broken and thus the MA molecule starts to rotate. The geometric structure and the charge density map of this configuration are illustrated in Figure 4b. But these hydrogen bonds are relative weak and MA molecules can be rotated easily, so the penetration barrier for H2O molecules on the PbI-T surface of MAPbI3 materials is a moderate value. The outmost layer of nBU-T surface of (C4H9NH3)2PbI4 is composed by the organic bilayer (C4H9NH3)2. As depicted by the simplified illustration in Figure 7, the NH3-radical of each C4H9NH3 molecule locates at the top and the bottom surfaces of this bilayer. Ascribing to the chemical activity of NH3-radical, the hydrogen bonds between these NH3-radicals and PbI4layers hold this hybrid lattice stable. The NH3-radical on the top of nBU-T surface also plays 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 surrounding



methyl radicals of C4H9NH3. Unfortunately, because of the chemical inactive nature of methyl radicals, H2O molecule is hindered by the repulsive interaction originating 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 groups 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 H2O molecule. The highest energy barrier is found in the configuration with H2O molecule locating almost at the middle of (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 surfaces in Figure 6b, which means that when the adsorbed H2O molecule start 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 2D (C4H9NH3)2PbI4 materials impermeable to H2O molecules. 4. Conclusion By theoretical calculation and analyses, we get the origin of the excellent moisture resistance of 2D layered (C4H9NH3)2PbI4 perovskites 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. While in 2D


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(C4H9NH3)2PbI4 systems, the penetration barrier of H2O through the nBU-T surface equals to 3.46 eV, which is a really high value and accounts for the experimentally observed enhanced moisture resistance of 2D layered perovskites. In both 2D- and 3D- perovskites surfaces, the predominant resistances against the invasive H2O are generated by the methyl-radical of organic groups. For the chemical inactive nature of methyl-radical, it impacts repulsive interactions on the penetrated H2O. In slabs of 3D perovskites, the MA molecule is rotated by this repel interactions and thus lessens the resistance against the penetration of H2O. But in 2D perovskite (C4H9NH3)2PbI4, the (C4H9NH3)2 bilayer surface is too long to be rotated or translated, the only effect of the repulsive interactions is to prevent the downward migration of H2O molecule. This is the key issue contributing to the distinguished moisture resistance of 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)n1MnX3n+1,

the enhanced moisture resistance comes from the same origin, i.e., the repulsive

interactions generated by 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 more methyl-radical as possible resist the penetration of adsorbed H2O molecule from the surface.

AUTHOR INFORMATION CORRESPONDING AUTHOUR: Email: [email protected] (To Ying-Bo Lu) Notes



The authors declare no competing financial interests. ACKNOWLEDGMENT This work is supported by the National Natural Science foundation of China under Grant 11504202. The calculations in this work are carried out on the High Performance Computer Center of McGill University, CalculQuebec, Compute Canada and the Supercomputing Center of Shandong University (Weihai). 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 Res. Appl. 2016, 24, 905. (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, 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. Room-Temperature, Hydrochloride-Assisted, One-Step Deposition for Highly Efficient and Air-Stable Perovskite Solar Cells Adv. Mater. 2016, 28, 8309.


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TOC GRAPHICS



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 389x414mm (300 x 300 DPI)

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Figure 2. (a) PbI-terminated (PbI-T) and (b) MAI-terminated (MAI-T) surfaces of MAPbI3 materials. (c) PbIterminated (PbI-T) and (d) C4H9NH3-terminated (nBU-T) surfaces of (C4H9NH3)2PbI4 materials. 508x304mm (300 x 300 DPI)



Figure 3. Relaxed structures of PbI-terminated surfaces with adsorbed/penetrated H2O molecules. (a) H2O in green dashed circles denotes the locations just above the hollow site of surface before geometric relaxation, while 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 in (c) denotes the initial orientation of MA molecule before relaxation. 660x177mm (300 x 300 DPI)


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Figure 4. Charge density maps. For configurations of MAPbI3 with (a) penetrated H2O below the first PbI layer, (b) H2O locating at the position where it encounters the highest energy barrier. Configurations of nBU-T surfaces in (C4H9NH3)2PbI4 with H2O molecule (c) locating at the top center of the hollow surrounded by four C4H9NH3 molecules, (d) H2O locating at the position with maximum energy barrier. 194x190mm (300 x 300 DPI)



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 to present the penetration paths. The green circles in (a) and (c) denote the position where penetrated H2O encounters the highest energy barriers. 1411x1305mm (72 x 72 DPI)


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Figure 6. Penetration energy barriers of adsorbed H2O. (a) H2O migrates from the PbI-T surface into the interior of MAPbI3 system. (b) H2O migrates from the nBU-T surface into the interior of (C4H9NH3)2PbI4 materials. 1446x599mm (72 x 72 DPI)



Figure 7. Positions of adsorbed H2O molecules on the nBU-T surfaces of (C4H9NH3)2PbI4 materials. (a) and (b) describe 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) and (f) present positions of penetrated H2O molecules under outmost (C4H9NH3)2 bilayers, which align 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 Å. 524x397mm (300 x 300 DPI)


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Investigations on Enhanced Moisture Resistance of Two-Dimensional

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