Structural and Electronic Properties of Two-Dimensional Organic

Article Cite This: J. Phys. Chem. C 2018, 122, 5844−5853

pubs.acs.org/JPCC

Structural and Electronic Properties of Two-Dimensional Organic− inorganic Halide Perovskites and their Stability against Moisture Zi-Qian Ma,† Yangfan Shao,†,‡ Pak Kin Wong,§ Xingqiang Shi,‡ and Hui Pan*,† †

Institute of Applied Physics and Materials Engineering, University of Macau, Macao SAR, P. R. China Department of Physics, Southern University of Science and Technology, Shenzhen, China § Department of Electromechanical Engineering, Faculty of Science and Technology, University of Macau, Macao SAR, P. R. China ‡

S Supporting Information *

ABSTRACT: Organic−inorganic halide perovskites have attracted increasing interest for solar-energy harvesting because of the simple fabrication process, high efficiency, and low cost. In this work, we systematically investigate the structural and electronic properties, and stability of two-dimensional (2D) hybrid organic−inorganic perovskites (HOIPs) based on density-functional-theory calculations. We explore a general rule to predict the bandgap of the 2D HOIP: its bandgap decreases as the thickness increases and the size of metal atom decreases as well as that of the halide atom increases. We find that effective mass of hole increases as the thickness of 2D HOIP increases. Importantly, the 2D HOIPs exhibit high stability on the resistance of water and oxygen than bulk HOIPs due to high positive adsorption energy. Our results confirm that the 2D HOIPs may be excellent alternatives to the unstable bulk HOIPs in solar energy harvesting with improved performance due to suitable bandgap, small carrier effective mass, and high resistance to water and oxygen molecules. inorganic layer, leading to a binding energy up to 200 meV.33,34 The PCE of a 2D HOIP solar cell could reach to 12.52% with no hysteresis, and the devices exhibited greatly improved stability in comparison to their three-dimensional counterparts.27 The 2D HOIPs can be directly derived from the related bulk HOIPs by conceptually slicing along different crystallographic planes.30,35 The 2D layered HOIPs have the general formula of (RNH3)2(MA)n−1BnX3n+1, where R is a long-chain alkyl or aromatic group at surfaces, B cations and X anions are the same as those in 3D system (B = Pb2+or Sn2+; X = Cl−, Br− or I−), and n is the number of metal-halide sheets (inorganic sheets). The 2D HOIP looks like an ideal quantum well with [(MA)n−1BnX3n+1]2− stacking separated by [RNH3]+, and the adjacent layers are held together by weak van der Waals force. When n = ∞, the 2D HOIP transfers to the 3D HOIP. As a result, the optoelectronic property of 2D layered perovskites can be tuned by changing n.29,33,34 For example, 2D (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 with an ultrasmooth and high-surface coverage exhibited tunable bandgaps with n.29 Theoretically, Wang et al. reported that 2D (C4H9NH3)2PbI4 had relatively lower surface energy than bulk counterpart, confirming the convenient preparation of atomically thin 2D hybrid perovskites.36 Ma et al. found that the bandgaps of 2D (C4H9NH3)2MI4 (M = Ge2+, Sn2+, and Pb2+) were between 1.5 and 2.0 eV.37 Yang et al. found that the bandgaps of 2D

1. INTRODUCTION Finding materials to efficiently harvest solar power has been a challenge to satisfy the increasing demand on energy. Basically, the materials for high efficiency should satisfy the following: (a) optimal bandgap for maximal sun-light absorption, (b) high carrier mobility, and (c) efficient electron−hole separation.1−14 Hybrid organic−inorganic perovskites (HOIPs) have triggered considerable interests due to remarkable photovoltaic efficiency and low cost.15,16 The bulk HOIP (3D HOIP) has a chemical formula of ABX3 (A = CH3NH3+ (MA+) or CH(NH2)+ (FA+); B = Pb2+ or Sn2+; X = Cl−, Br−, or I−). The A cation sits at the eight corners of the cubic unit, while the B cation is at the center of an octahedral [BX6]4− cluster. These 3D HOIPs can be efficient absorber in photonic devices because of their optimal bandgaps (∼1.5 eV), high carrier mobility, long carrier lifetime, and long diffusion length (∼1 μm), which satisfy the basic requirements well. A power conversion efficiency (PCEs) of 23.6% was recently reported on an HOIP-silicon heterojunction.17 However, the poor stability of the bulk HOIPs with respect to moisture, oxygen, heat, and light is a major challenge for their commercial applications.18−22 Although the stability can be improved by new fabrication method,23 surface treatment,24 and coating,25 it is still far away from commercially viable devices in comparison to the conventional silicon solar cell. Alternatively, 2D and quasi-2D layered HOIPs show tunable bandgap, diverse physical properties and high stability when subjected to moisture, oxygen, heat, and radiation.26−32 For example, the excitons in 2D HOIPs are more strongly confined to © 2018 American Chemical Society

Received: July 7, 2017 Revised: December 8, 2017 Published: January 25, 2018 5844

DOI: 10.1021/acs.jpcc.7b06673 J. Phys. Chem. C 2018, 122, 5844−5853

Article

The Journal of Physical Chemistry C

Figure 1. Relaxed structures of (a) TETP, (b) TETV, and (c) orthorhombic structures of (EA)2(MA)n−1BnX3n+1 (B = Pb2+, Sn2+; X = Br− or I−). (d) Relaxed structures of HOIPs with different thicknesses (n = 1, 2, 3, 4, and 5). (e) Relaxed structures of HOIPs by replacing the terminal molecule EA with BA and BEN at n = 3. (f) Relaxed structures of heterojunction with a two-layers and four-layers 2D HOIPs. Sn/Pb = silver gray; I/Br = purple; N = blue; C = gray; H = white.

these parameters. Meanwhile, the spin−orbit coupling (SOC42) and Hybrid Functionals (HSE0643) were also used to investigate the electronic structures and effective masses of 2D HOIPs.

HOIPs were increased by replacing Cs+ with CH3NH3+, or Sb with Pb, and reduced as changing halide atom from Cl to Br to I.32 Liu et.al reported that the line defects with fixed orientation could be tuned in the 2D HOIPs from electron acceptors to inactive sites by varying synthesis conditions.38 Although there are a few theoretical studies on these 2D HOIPs, a systematic and comprehensive study with various structures are necessary to fully explore their physical properties and practical applications. In this work, we present a first-principles study on the structural and electronic properties of the 2D HOIPs, (RNH3)2(MA)n−1BnX3n+1 (R = CH3CH2, CH3(CH2)3, and C6H5CH2; B = Pb2+and Sn2+; X = Br− and I−), with varied n values and phases, and their stability against water and oxygen molecules. We find that the bandgap of the 2D HOIP strongly depends on the thickness, structure, and the size of halide atom. We show that the 2D HOIPs exhibit the higher resistance on moisture and oxygen than the 3D HOIPs. We further show that SnI-based 2D HOIPs are better than PbI-based, SnBr-based, and PbBr-based 2D HOIPs in solar energy harvesting due to narrow bandgap, high carrier mobility, and high stability.

3. RESULTS AND DISCUSSION 3.1. Optimized Structures and Layer Distance. It was reported that the 2D HOIPs can be derived from the 3D parent compounds by taking single ⟨100⟩-oriented layers from the ABX3 structures.44 However, different from the structures of the 3D HOIPs that have three phases (cubic, tetragonal and orthorhombic phase) at different temperatures,45 the 2D HOIPs have only tetragonal or orthorhombic phases due to the accommodation of much larger and more complex organic cations.30 Furthermore, the distorted and rotated MA cations also give rise to the different crystal structures on the 2D HOIPs with respect to the orientations of C−N bonds. Therefore, the tetragonal phases with the C−N bonds parallel and vertical to each other (Figure 1, parts a and b) are considered. In this study, CH3CH2NH3+ (EA), CH3(CH2)3NH3+ (BA), and C6H5CH2NH3+ (BEN) were used as three organic terminal cations to cover the surfaces of 2D HOIPs. We first focus on EA in four systems (SnI-based, PbI-based, SnBr-based and PbBrbased 2D HOIPs), (EA)2(MA)n−1B nX3n+1 (B = Pb2+or Sn2+; X = Br− or I−) (Figure 1, parts a, b, c, d, and f). The optimized geometries show that the lattice constants of the tetragonal parallel (TETP) structures in x and y directions (a and b, a = b) are slightly larger than those of the tetragonal vertical (TETV) structures at same thickness, while that in z direction (c) is relatively small (Supporting Information, Tables S1, S2, S3, and S4). Similar to the 3D HOIP,46 the lattice constant c strongly depends on the orientations of C−N bonds, and is larger in TETV structure than that in TETP structure. When the temperature decreases, the tetragonal phases will transfer to the orthorhombic phase. The SnX6 octahedron has a rotation direction in z axis that is obviously different from the TETP and TETV (Figure 1c), consistent with literatures.30,37,38 Furthermore, the thinnest 2D HOIPs (n = 1)

2. METHODS First-principles calculations based on density-functional theory (DFT) and Perdew−Burke−Ernzerhof generalized gradient approximation (PBE-GGA) were carried out to study the structural and electronic properties of (RNH3)2(MA)n−1BnX3n+1 with various phases, thicknesses and surface molecules. The projector augmented wave (PAW) scheme as incorporated in the Vienna ab initio simulation package (VASP) was used. The first Brillouin zone was sampled by Monkhorst−Pack method to generate k-point meshes, in which 3 × 3 × 1 was used for all considered systems. Considering the weak interaction between the organic molecule and inorganic matrix, a nonlocal density functional, vdW-DF (proposed by Dion et.al39), was employed in this study.40,41 A cutoff energy of 500 eV was consistently used for the expansion of plane-wave basis. Good convergence was obtained by using 5845


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calculations show that the layer distance decreases significantly as the size of halide atom decreases (from I to Br) and the size of metal atom decreases (from Pb to Sn). As a result, the TETP (EA)2(MA)n−1SnnBr3n+1 exhibits the smallest layer distance, while the TETV (EA)2(MA)n−1PbnI3n+1 has the largest layer distance. Therefore, the layer distances of the 2D HOIP materials may be able to be tuned by changing the thickness, halide atom, metal atom and the orientation of MA cations. In the following parts, the electronic properties of the 2D HOIPs with different phases and thicknesses are investigated and analyzed. 3.2. Electronic Properties of the 2D HOIPs. The calculated band structures (Supporting Information, Figures S1−S12)

can only have the orthorhombic phase because the organic terminal cations cover the surfaces of 2D HOIPs.30,47 In the three structures (TETP, TETV and orthorhombic phases), our calculated results show that the TETP structure is most stable (Tables S1, S2, S3 and S4), which is the same as the 3D HOIPs.45 We also find that the thickness of 2D HOIPs plays an important role on the layer distance. The layer distances (d) (Figure 1f) of TETP and orthorhombic 2D HOIPs decrease as the thickness increases (Figure 2). While for TETV phases, the layer distance decreases for n = 1−3, and increases slightly for n = 3−5. The reduction of layer distance is contributed to the enhanced van der Waals force within 2D HOIPs as the thickness increases. Our

Figure 2. Layer distance of (a) SnI-based 2D HOIP, (b) PbI-based 2D HOIP, (c) SnBr-based 2D HOIP, and (d) PbBr-based 2D HOIP with different thicknesses (n = 1, 2, 3, 4, and 5).

Figure 3. Bandgap of (a) SnI-based 2D HOIP, (b) PbI-based 2D HOIP, (c) SnBr-based 2D HOIP, and (d) PbBr-based 2D HOIP with different thicknesses (n = 1, 2, 3, 4 and 5). 5846


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with the experimental results.29 For example, the bandgaps of 2D (EA)2(MA)n−1SnnI3n+1 (SnI-based system) with TETP phase (Figure 4) are 1.57, 1.30, 1.17, 1.09, and 1.07 eV as n = 1 to 5, respectively, which are larger than that of bulk MASnI3 (0.73 eV) (Table 1). To check if the results are reliable, we also calculated the band

show that the 2D HOIPs are direct semiconductors with both the conduction band bottom (CBB) and valence band top (VBT) located at Γ points in the Brillouin zone. As the thickness increases, the bandgap of the 2D HOIP gradually decreases and converges to the value of the corresponding 3D counterpart (Figure 3), consistent

Figure 4. Calculated band structures of SnI-based 2D TETP HOIP (named as p-n): (a) n = 1, (b) n = 2, (c) n = 3, (d) n = 4, and (e) n = 5.

Table 1. Calculated bandgaps of 2D HOIPs SnI TETP

TETV

orthorhombic

PbI

SnBr

PbBr

N

bandgap (eV)

n

bandgap (eV)

n

bandgap (eV)

n

bandgap (eV)

bulk 1 2 3 4 5 bulk 1 2 3 4 5 bulk 1 2 3 4 5

0.73 − 1.30 1.17 1.09 1.07 0.93 − 1.35 1.25 1.17 1.15 1.08 1.57 1.44 1.42 1.23 1.22

bulk 1 2 3 4 5 bulk 1 2 3 4 5 bulk 1 2 3 4 5

1.60 − 2.09 1.94 1.90 1.78 1.82 − 2.08 1.96 1.93 1.89 1.90 2.28 2.19 2.17 2.00 1.98

bulk 1 2 3 4 5 bulk 1 2 3 4 5 bulk 1 2 3 4 5

1.06 − 1.63 1.49 1.46 1.42 1.36 − 1.71 1.65 1.54 1.43 1.50 1.84 1.83 1.69 1.51 1.57

bulk 1 2 3 4 5 bulk 1 2 3 4 5 bulk 1 2 3 4 5

1.92 − 2.42 2.31 2.21 2.12 2.13 − 2.44 2.26 2.23 2.15 2.27 2.68 2.56 2.51 2.29 2.29

Figure 5. Calculated band structures of TETP SnI-based HOIP with SOC: (a) bulk, (b) n = 1, (c) n = 2, (d) n = 3, (e) n = 4, and (f) n = 5. 5847


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Figure 6. Calculated band structures of SnI-based 2D TETP HOIP with different terminal molecules at n = 3: (a) EA, (b) BA, and (c) BEN. Calculated partial densities of states (PDOSs) of SnI-based 2D TETP HOIP with different terminal molecules at n = 3: (e) EA, (f) BA, and (g) BEN.

EA as the terminal molecule, their CBBs are mainly contributed to the p states of Pb atoms (Figures S4−S6 and S10−S12). Their VBTs are dominated by the s electrons of Pb atoms and the p electron of I/Br atoms at n = 1. As n increases, the p electrons of halide atoms dominate the VBTs. For the SnI-based and SnBrbased 2D HOIPs (Figures S1−S3 and S7−S9), their CBBs are contributed to the p states of metal atoms (Sn), which is the same as the PbI-based and PbBr-based systems. Differently, their VBTs are dominated by the p electrons of halide atoms and contributed partially to the s electrons of Sn atoms. At n = 5, their VBTs are totally contributed to the p states of I/Br atoms. We also find that the organic molecules have negligible contributions to the band edges states in these four systems. In addition, to investigate the effect of terminal organic molecule on the electronic properties, we replace the EA cations by the BA and BEN cations in the TETP (EA)2(MA)2Sn3I10 (Figure 1e). Our calculations show that the size of terminal molecule has negligible effect on the bandgap of the 2D HOIP (Figure 6, parts a−c). However, when EA in the TETP (EA)2(MA)2Sn3I10 is replaced by BA or BEN, the PDOSs (Figure 6, parts f and g) show that the VBTs are contributed to the s electrons of Sn atoms and partially to the p electrons of I atoms, which is different from the EA system (mainly contributed the p electrons of halide atoms and partially to the s electrons of Sn atoms (Figure 6e)). The CBBs is still totally contributed by the p states of Sn atoms. The BA and BEN molecules have negligible contribution to the band edges states. To find out the origin of improved charge transfer, we investigate the position of Fermi levels of these 2D TETP HOIPs compounds with different thicknesses (n = 1−6) (Figure 7). We find that the valence band edges in TETP-SnI system increase dramatically as n increases (from 1 to 3), then keep slightly increasing at n increases from 4 to 6. The conduction band edges also increase slightly as n increases (Figure 7a). When a solar cell is composed of TETP-SnI 2D HOIPs with different thicknesses, the hole carrier can easily transfer from the thin layer to thick layer, while the electron carrier from thick layer to thin layer, leading to efficient electron−hole separation and high mobility as well as

structures by considering SOC and using HSE method. Similar to the PBE-GGA method result, the bandgaps of 2D TETP (EA)2(MA)n−1SnnI3n+1 with SOC decreases as n increases (1.43, 1.11, 0.95, 0.93, and 0.84 eV for n = 1−5, respectively), which are still larger than that of bulk MASnI3 (0.50 eV) (Figure 5). At the same thickness, SnI-based system exhibits the smallest bandgap, while the PbBr-based system is the largest, which are the same as the 3D HOIPs (MASnI3 and MAPbBr345). In addition, we find that the Rashba-Dresselhaus effect is quite obvious in the 2D HOIP system.44 For example, the Rashba parameter at the valence-band maximum along the Γ → X direction (Figure 5) is 0.93 eV·Å for 2D SnI-based HOIPs with n = 3 (Table S5).48 The Rashba parameter of bulk-TETV HOIPs is 0.23 eV·Å,49 but that of bulkTETP is 0 eV·Å. Our calculations with HSE functional show that the HSE bandgap is larger than PBE one by 0.55 eV (Figure S13). For example, the HSE bandgap of (EA)2(MA)n−1SnnI3n+1 (n = 1) is 2.14 eV, while the PBE result is 1.57 eV. Similar to the 3D HOIPs, the metal and halide atoms play key roles on the bandgaps of the 2D HOIPs. As the size of metal atom increases (from Sn to Pb) or the size of halide atom decreases (from I to Br), the bandgaps of these 2D compounds increase (Figure 3). For example, the bandgaps of SnBr-based systems (Figure 3c) are smaller than those of PbI-based systems at the same thickness (Figure 3b). At the same time, the band structures of the 2D HOIPs are also affected by the crystal structure (Figure 3). The bandgap of TETP structure (named as p-n) is smaller than those of the TETV (named as v-n) and orthorhombic (named as o-n) structures, which is the same as the 3D HOIPs.45 For example, the bandgap of the TETP (EA)2(MA)4Sn5I16 is 1.07 eV, which is smaller than those of TETV and orthorhombic structures by 0.08 and 0.15 eV, respectively (Figure 4e and Figures S1e and S3e). Our calculations clearly show that the bandgap of the TETP (EA)2(MA)n−1SnnI3n+1 is the smallest, while the orthorhombic (EA)2(MA)n−1PbnBr3n+1 is the largest in all the 2D compounds. To investigate the origin of the shift of band edge sates, we analyze partial densities of states (PDOSs) (Figures S4−S6 and S10−S12). For the PbI-based and PbBr-based 2D HOIPs with 5848


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Figure 7. Band alignment of (a) SnI-based 2D TETP HOIP, (b) PbI-based 2D TETP HOIP, (c) SnBr-based 2D TETP HOIP, and (d) PbBr-based 2D TETP HOIP with different thicknesses (n = 1, 2, 3, 4, 5 and 6).

Figure 8. Calculated partial densities of states (PDOSs) for heterojunction of SnI-based 2D TETP HOIP with a two-layer and four-layer structures (named as p-2−4). (a) PDOSs of atoms and (b) PDOSs of layers.

high PCE. For other systems, the valence band edges increase as n increases, but the conduction band edges have a minor fluctuation. The hole can transfer freely from the thin to thick layer. However, the transportation of electron in 2D HOIP multilayers strongly depends on the thickness chosen. For example, the conduction band edge of TETP-PbBr system with n = 2 is the highest (Figure 7d). If the solar cell is composed of layers with n = 2 and n > 2, the separation of electron−hole pair is difficult because both electron and hole should transfer from thin to thick layer, leading to low PCE. To demonstrate our analysis, we built a heterojunction with a two-layer (n = 2) and four-layer (n = 4) structures of the TETP (EA)2(MA)n‑1SnnI3n+1 (named as p-2−4, Figure 1f). The PDOSs show (Figure 8a) that the VBT of the 2D HOIP heterojunction is mainly contributed to the p states of I atoms (4-I_p) and partially to the p states of Sn atoms (4-Sn_p) from the four-layer structure, but the CBB is mostly dominated by the p states of Sn atoms from the two-layer structure (2-Sn_p). We see that the PDOS for two-layer 2D HOIP is below that of fourlayer one in valence band, while above that of four-layer 2D HOIP in conduction band (Figure 8b), which is consistent with our analysis in Figure 7. Our calculations show that the thickness of 2D HOIP is critical to improve the harvesting efficiency. Only

by choosing suitable layer with optimal thickness, the PCE of the solar cell can be maximized. 3.3. Carrier Effective Mass. Carrier effective mass is an important factor in photovoltaic cell. To give a quantitative estimation, the carrier (electron and hole) effective mass (m*) of the 2D HOIPs are calculated from their band structures based on the following equation ⎤−1 ⎡ 2 2 ∂ ε(k ) * m =ℏ⎢ ⎥ ⎣ ∂k 2 ⎦

(1)

where ε(k) are the eigenvalues at band edges around the CBB or VBT and k is the wave vector. The calculated hole effective mass (mh*) of the 2D HOIPs slightly increases and slowly converges to the value of bulk HOIPs with the increase of the thickness, while the electron effective mass (me*) shows no trend (Figure 9) because DFT cannot exactly calculate the excited states. We also find that as the size of metal atom increases (from Sn to Pb), the absolute value of mh* (|mh*|) increases sharply. However, the absolute value of mh* is unaffected significantly by halide atoms, which is different from the 3D HOIPs.45 In addition, the structure also plays a complicated role on the effective mass of the 2D HOIP. We find that |mh*| in the orthorhombic phase is heavier 5849


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considered the effects of SOC and hybrid functions on the effective mass. We find that the absolute values of both mh* and me* of 2D TETP (EA)2(MA)n−1SnnI3n+1 with SOC slightly decrease and slowly converges to the values of bulk HOIPs with the increase of the thickness, which is different with the PBE calculated results (only mh* slowly converges to the value of bulk HOIPs) (Table S7). For the calculated effective mass by HSE (Table S8), |mh*| of bulk is the same to the PBE and SOC results,

than that in the tetragonal phase. For the tetragonal phase, the TETP structure exhibits a relatively small |mh*| compared to the TETV structure. Finally, the impact of terminal organic molecule on |mh*| of the 2D HOIPs is negligible (Table S6). After considering all the factors, we see clearly that (EA)2(MA)n‑1SnnI3n+1 with TETP phase has the lightest effective mass among all the studied 2D systems, and (EA)2(MA)n‑1PbnBr3n+1 with TETV and orthorhombic phases have the heaviest hole effective mass. Meanwhile, we also

Figure 9. Carrier effective mass for different crystal structures: (a) SnI-based 2D HOIP, (b) PbI-based 2D HOIP, (c) SnBr-based 2D HOIP, and (d) PbBr-based 2D HOIP with different thicknesses (n = 1, 2, 3, 4 and 5).

Figure 10. (a) Relaxed structures of the clean slab models with different terminal molecules. (b) Adsorption sites of water and oxygen molecules. (c) Relaxed crystal structures of the systems with O2 at Sn−O2 sites. (d) Adsorption energies for water molecule on SnI-based 2D HOIP with n = 3 and different terminal molecules (EA, BA, and BEN). (e) Adsorption energies for oxygen molecule on SnI-based 2D HOIP with n = 3 and different terminal molecules (EA, BA, and BEN). 5850


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(R = CH3CH2, CH3(CH2)3, C6H5CH2; B = Pb2+, Sn2+; X = Br− or I−) with different phases and thicknesses and their stabilities in moisture. We find that their electronic properties are greatly affected by the thickness, metal atoms, halide atoms and structures. We show that the bandgap decreases as the thickness increases, the sizes of metal atoms decrease, and the sizes of halide atoms increase. 2D HOIP with TETP phase has the smallest bandgap and lightest carrier effective mass. Especially, the TETP (EA)2(MA)n−1SnnI3n+1 may show the best performance in solar energy harvesting because of the smallest bandgap, lightest effective mass, and efficient charge separation. By comparing with the 3D HOIPs, we predict that the 2D HOIPs may be more efficient in solar-energy harvesting because of high stability on water and oxygen. It is expected that (RNH3)2(MA)n−1BnX3n+1, especially (EA)2(MA)n−1SnnI3n+1 with TETP phase, may be excellent alternatives to the unstable 3D HOIPs and have many potential applications in solar energy technology.

while |me*| is larger than those with PBE and SOC. Differently, for 2D TETP (EA)2(MA)n‑1SnnI3n+1 (n = 1), the HSE effective masses are almost same to PBE data, but larger than SOC data by 0.1. 3.4. Stability of the 2D HOIPs. The stability is one of the advantages for the 2D HOIPs compared to the 3D HOIPs. In this study, the adsorption energies (including dissociation and physisorption) of water and oxygen molecules at different adsorption sites (RNH3−H2O/O2, close to the terminal molecule RNH3; insertH2O/O2, between the two terminal molecules RNH3; I−H2O/O2, close to the halide atoms; and Sn−H2O/O2, close to the metal atoms) were investigated to find out their stable adsorption sites and adsorption energies by initially placing the molecules at the sites (Figure 10). Here, the TETP (RNH3)2(MA)2Sn3I10 systems with three different terminal molecules, EA, BA and BEN, are studied. The adsorption energies of molecule on slab surface (ΔEads(molecule)) are calculated as21 ΔEads(molecule) = Emolecule / slab − Eslab − Emolecule

■

(2)

ASSOCIATED CONTENT

S Supporting Information *

where Emolecule/slab, Eslab, and Emolecule are the total energies of the slab adsorbed with molecules, the clean slab model (Figure 10a), and the free molecule in vacuum, respectively. Under this definition, a negative ΔEads indicates the attraction behavior of molecule to the slab surface, while a positive value means repulsion. Our calculations (Figure 10d and e) show that all of ΔEads are positive in our studied systems, which are absolutely different from the 3D HOIPs (all of ΔEads are negative),21 indicating that the 2D HOIPs can resist effectively the erosion of the H2O and O2 molecules than the 3D HOIPs. Our calculations also demonstrate that the H2O molecule is easier to erode the 2D HOIP compounds than the O2 molecule because of the low adsorption energy, in good agreement with the reports.50,51 Most importantly, the terminal molecule plays an important role on the stability of the 2D HOIPs on water and oxygen molecules. We find (Figure 10, parts d and e) that the water molecule is much easier to adsorb at insert-H2O, I−H2O, and Sn−H2O sites for EA2(MA)2Sn3I10, BA2(MA)2Sn3I10 and BEN2(MA)2Sn3I10 compounds due to the low adsorption energies (1.59 eV, 1.59 and 1.49 eV). Different from the water molecule, the oxygen molecule is easier adsorbed at Sn−O2 site in these three compounds. We also find that the ΔEads at Sn−O2 site on 2D HOIPs with EA (1.89 eV) and BEN (1.93 eV) terminal molecule system are significantly lower than that on the BA system (3.00 eV). The relaxed crystal structures (Figure 10c) show that the oxygen bonds are broken (dissociation) at Sn−O2 in the systems with EA and BEN terminal molecules, while the oxygen molecule keeps unchanged (physisorption) in the system with BA terminal molecule. We see that the stability of 2D HOIPs against water and oxygen strongly depends on the terminal molecule. Although it is possible for Sn2+ oxidizes toward Sn4+ in 2D Sn-based HOIPs, the adsorption energies (ΔEads) of oxygen molecules at Sn−O2 site on 2D SnIbased HOIPs with EA, BA and BEN terminal molecule system is 1.89 eV, 3.00 and 1.93 eV, indicating Sn atom is difficult to be oxidized due to the high adsorption energies. From the above analysis, we can conclude that the 2D HOIPs exhibit better performance on the stability against water and oxygen than the 3D HOIPs due to the high (positive) adsorption energy. In particular, BA2(MA)2Sn3I10 can resist more effectively the erosion of the H2O and O2 molecules than EA and BEN terminal molecule systems.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06673. Calculated lattice constants and total energies, Rashba parameters, calculated bandgaps and effective masses, and calculated band structures(PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.P.) E-mail: [email protected]. Telephone: (853)88224427. Fax: (853)88222426. ORCID

Hui Pan: 0000-0002-6515-4970 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.P. expresses thanks for the support of the Science and Technology Development Fund from Macao SAR (FDCT-068/ 2014/A2, FDCT-132/2014/A3, and FDCT-110/2014/SB) and Multi-Year Research Grants (MYRG2015-00017-FST and MYRG2017-00027-FST) from the Research & Development Office of the University of Macau. The DFT calculations were performed at High Performance Computing Cluster (HPCC) of Information and Communication Technology Office (ICTO) of the University of Macau.

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4. CONCLUSION In summary, we present a first-principles study on the crystal structures, electronic properties of 2D (RNH3)2(MA)n−1BnX3n+1 5851


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Structural and Electronic Properties of Two-Dimensional Organic

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