Structural and Electronic Properties of Two-Dimensional Organic

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Structural and Electronic Properties of Two-Dimensional Organicinorganic Halide Perovskites and their Stability against Moisture Zi-Qian Ma, Yangfan Shao, Pak Kin Wong, Xingqiang Shi, and Hui Pan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06673 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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Structural and Electronic Properties of Two-Dimensional Organic-

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inorganic Halide Perovskites and their Stability against Moisture

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Zi-Qian Ma1, Yangfan Shao1, 2, Pak Kin Wong3, Xingqiang Shi2, and Hui Pan1* 1

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Institute of Applied Physics and Materials Engineering, University of Macau, Macao SAR, P. R. China

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Department of Physics, Southern University of Science and Technology, Shenzhen, China

Department of Electromechanical Engineering, Faculty of Science and Technology, University of Macau, Macao

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SAR, P. R. China

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Abstract: Organic-inorganic halide perovskites have attracted increasing interest

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for solar-energy harvesting because of the simple fabrication process, high

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efficiency, and low cost. In this work, we systematically investigate the structural

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and electronic properties, and stability of two-dimensional (2D) hybrid organic-

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inorganic perovskites (HOIPs) based on density-functional-theory calculations. We

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explore a general rule to predict the bandgap of the 2D HOIP: its bandgap

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decreases as the thickness increases, the size of metal atom decreases as well as

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that of halide atom increases. We find that effective mass of hole increases as the

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thickness of 2D HOIP increases. Importantly, the 2D HOIPs exhibit high stability

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on the resistance of water and oxygen than bulk HOIPs due to high positive

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adsorption energy. Our results confirm that the 2D HOIPs may be excellent

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alternatives to the unstable bulk HOIPs in solar energy harvesting with improved

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performance due to suitable bandgap, small carrier effective mass, and high

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resistance to water and oxygen molecule.

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Keywords: 2D HOIPs; solar-energy harvesting; electronic properties; effective

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mass; stability; first-principles calculation

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* H. Pan ([email protected]); Tel: (853)88224427; Fax: (853)88222426

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ACS Paragon Plus Environment

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1. Introduction

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Finding materials to efficiently harvest solar power has been a challenge to satisfy

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the increasing demand on energy. Basically, the materials for high efficiency

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should satisfy: (a) optimal bandgap for maximal sun-light absorption, (b) high

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carrier mobility, and (c) efficient electron-hole separation.1-14 Hybrid organic-

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inorganic perovskites (HOIPs) have triggered considerable interests due to

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remarkable photovoltaic efficiency and low cost.15-16 The bulk HOIP (3D HOIP)

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has a chemical formula of ABX3 (A: CH3NH3+ (MA+) or CH(NH2)+ (FA+); B:

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Pb2+or Sn2+; X: Cl–, Br– or I–). The A cation sits at the eight corners of the cubic

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unit, while the B cation is at the center of an octahedral [BX6]4− cluster. These 3D

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HOIPs can be efficient absorber in photonic devices because of their optimal

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bandgaps (~1.5 eV), high carrier mobility, long carrier lifetime, and long diffusion

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length (~1 µm), which satisfy the basic requirements well. A power conversion

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efficiency (PCEs) of 23.6% was recently reported on an HOIP-silicon

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heterojunction.17 However, the poor stability of the bulk HOIPs with respect to

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moisture, oxygen, heat, and light is a major challenge for their commercial

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applications.18-22 Although the stability can be improved by new fabrication

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method,23 surface treatment,24 and coating,25 it is still far away from commercially

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viable devices in comparison to the conventional silicon solar cell.

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Alternatively, 2D and quasi-2D layered HOIPs show tunable bandgap, diverse

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physical properties and high stability when subjected to moisture, oxygen, heat,

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and radiation.26-32 For example, the excitons in 2D HOIPs are more strongly

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confined to inorganic layer, leading to a binding energy up to 200 meV.33-34 The

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PCE of a 2D HOIP solar cell could reach to 12.52% with no hysteresis, and the

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devices exhibited greatly improved stability in comparison to their three2


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dimensional counterparts.27 The 2D HOIPs can be directly derived from the related

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bulk HOIPs by conceptually slicing along different crystallographic planes.30, 35

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The 2D layered HOIPs have the general formula of (RNH3)2(MA)n − 1BnX3n+1,

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where R is a long-chain alkyl or aromatic group at surfaces, B cations and X

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anions are the same as those in 3D system (B: Pb2+or Sn2+; X: Cl–, Br– or I–), and n

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is the number of metal-halide sheets (inorganic sheets). The 2D HOIP looks like an

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ideal quantum well with [(MA)n−1BnX3n+1]2- stacking separated by [RNH3]+, and

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the adjacent layers are held together by weak van der Waals force. When n = ∞, the

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2D HOIP transfers to the 3D HOIP. As a result, the optoelectronic property of 2D

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layered perovskites can be tuned by changing n.29,

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(CH3(CH2)3NH3)2(CH3NH3)n − 1PbnI3n+1 with an ultra-smooth and high-surface

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coverage exhibited tunable bandgaps with n.29 Theoretically, Wang et al. reported

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that 2D (C4H9NH3)2PbI4 had relatively lower surface energy than bulk counterpart,

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confirming the convenient preparation of atomically thin 2D hybrid perovskites.36

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Ma et al. found that the bandgaps of 2D (C4H9NH3)2MI4 (M = Ge2+, Sn2+, and Pb2+)

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were between 1.5 and 2.0 eV.37 Yang et al. found that the bandgaps of 2D HOIPs

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were increased by replacing Cs+ with CH3NH3+, or Sb with Pb, and reduced as

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changing halide atom from Cl to Br to I.32 Liu et.al reported that the line defects

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with fixed orientation could be tuned in the 2D HOIPs from electron acceptors to

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inactive sites by varying synthesis conditions.38 Although there are a few

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theoretical studies on these 2D HOIPs, a systematic and comprehensive study with

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various structures are necessary to fully explore their physical properties and

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practical applications.

33-34

For example, 2D

3



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In this work, we present a first-principles study on the structural and electronic

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properties of the 2D HOIPs, (RNH3)2(MA)n−1BnX3n+1 (R = CH3CH2, CH3(CH2)3,

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and C6H5CH2; B = Pb2+and Sn2+; X = Br– and I–), with varied n values and phases,

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and their stability against water and oxygen molecules. We find that the bandgap

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of the 2D HOIP strongly depends on the thickness, structure, and the size of halide

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atom. We show that the 2D HOIPs exhibit the higher resistance on moisture and

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oxygen than the 3D HOIPs. We further show that SnI-based 2D HOIPs are better

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than PbI-based, SnBr-based and PbBr-based 2D HOIPs in solar energy harvesting

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due to narrow bandgap, high carrier mobility, and high stability.

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2. Methods

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First-principles calculations based on density-functional theory (DFT) and Perdew-

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Burke-Ernzerhof generalized gradient approximation (PBE-GGA) were carried out

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to study the structural and electronic properties of (RNH3)2(MA)n−1BnX3n+1 with

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various phases, thicknesses and surface molecules. The projector augmented wave

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(PAW) scheme as incorporated in the Vienna ab initio simulation package (VASP)

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was used. The first Brillouin zone was sampled by Monkhorst-Pack method to

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generate k-point meshes, in which 3 × 3 × 1 was used for all considered systems.

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Considering the weak interaction between the organic molecule and inorganic

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matrix, a nonlocal density functional, vdW-DF (proposed by Dion et.al39), was

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employed in this study.40-41 A cut-off energy of 500 eV was consistently used for

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the expansion of plane-wave basis. Good convergence was obtained by using these

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parameters. Meanwhile, the spin–orbit coupling (SOC42) and Hybrid Functionals

4


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(HSE0643) were also used to investigate the electronic structures and effective

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masses of 2D HOIPs.

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3. Results and discussion

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3.1 Optimized structures and layer distance. It was reported that the 2D

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HOIPs can be derived from the 3D parent compound by taking single 〈100〉-

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oriented layers from the ABX3 structure.44 However, different from the structures

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of the 3D HOIPs that have three phases (cubic, tetragonal and orthorhombic phase)

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at different temperatures,45 the 2D HOIPs have only tetragonal or orthorhombic

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phases due to the accommodation of much larger and more complex organic

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cations.30 Furthermore, the distorted and rotated MA cations also give rise to the

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different crystal structures on the 2D HOIPs with respect to the orientations of C-N

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bonds. Therefore, the tetragonal phases with the C-N bonds parallel and vertical to

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each other (Figures 1a & 1b) are considered.

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In this study, CH3CH2NH3+ (EA), CH3(CH2)3NH3+ (BA) and C6H5CH2NH3+

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(BEN) were used as three organic terminal cations to cover the surfaces of 2D

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HOIPs. We first focus on EA in four systems (SnI-based, PbI-based, SnBr-based

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and PbBr-based 2D HOIPs), (EA)2(MA)n−1B nX3n+1 (B = Pb2+or Sn2+; X = Br– or I–)

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(Figures 1a, b, c, d & f). The optimized geometries show that the lattice constants

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of the tetragonal parallel (TETP) structures in x and y directions (a and b, a = b)

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are slightly larger than those of the tetragonal vertical (TETV) structures at same

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thickness, while that in z direction (c) is relatively small (Supporting data, Table S1,

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S2, S3 & S4). Similar to the 3D HOIP 46, the lattice constant c strongly depends on

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the orientations of C–N bonds, and is larger in TETV structure than in TETP 5



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structure. When the temperature decreases, the tetragonal phases will transfer to

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the orthorhombic phase. The SnX6 octahedron has a rotation direction in z axis that

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is obviously different from the TETP & TETV (Figure 1c), consistent with

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literatures.30, 37-38 Furthermore, the thinnest 2D HOIPs (n = 1) can only have the

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orthorhombic phase because the organic terminal cations cover the surfaces of 2D

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HOIPs.30, 47 In the three structures (TETP, TETV and orthorhombic phases), our

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calculated results show that the TETP structure is most stable (Table S1, S2, S3 &

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S4), which is the same as the 3D HOIPs.45

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We also find that the thickness of 2D HOIPs plays an important role on the layer

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distance. The layer distances (d) (Figure 1f) of TETP and orthorhombic 2D HOIPs

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decrease as the thickness increases (Figure 2). While for TETV phases, the layer

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distance decreases for n =1~3, and increases slightly for n = 3~5. The reduction of

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layer distance is contributed to the enhanced van der Waals force within 2D HOIPs

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as the thickness increases. Our calculations show that the layer distance decreases

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significantly as the size of halide atom decreases (from I to Br) and the size of

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metal atom decreases (from Pb to Sn). As a result, the TETP (EA)2(MA)n −

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1SnnBr3n+1

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1PbnI3n+1

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HOIP materials may be able to be tuned by changing the thickness, halide atom,

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metal atom and the orientation of MA cations. In the following parts, the electronic

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properties of the 2D HOIPs with different phases and thicknesses are investigated

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and analyzed.

exhibits the smallest layer distance, while the TETV (EA)2(MA)n −

has the largest layer distance. Therefore, the layer distances of the 2D

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6


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3.2 Electronic Properties of the 2D HOIPs. The calculated band structures

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(Supporting data, Figures S1-S12) show that the 2D HOIPs are direct

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semiconductors with both the conduction band bottom (CBB) and valence band top

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(VBT) located at Γ points in the Brillouin zone. As the thickness increases, the

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bandgap of the 2D HOIP gradually decreases and converges to the value of the

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corresponding 3D counterpart (Figure 3), consistent with the experimental

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results.29 For example, the bandgaps of 2D (EA)2(MA)n − 1SnnI3n+1 (SnI-based

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system) with TETP phase (Figure 4) are 1.57, 1.30 eV, 1.17eV, 1.09 eV, and 1.07

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eV as n = 1 to 5, respectively, which are larger than that of bulk MASnI3 (0.73 eV)

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(Table 1). To check if the results are reliable, we also calculated the band

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structures by considering SOC and using HSE method. Similar to the PBE-GGA

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method result, the bandgaps of 2D TETP (EA)2(MA)n − 1SnnI3n+1 with SOC

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decreases as n increases (1.43 eV, 1.11 eV, 0.95 eV, 0.93 eV and 0.84 eV for n = 1

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to 5, respectively), which are still larger than that of bulk MASnI3 (0.50 eV)

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(Figure 5). At the same thickness, SnI-based system exhibits the smallest bandgap,

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while the PbBr-based system is the largest, which are the same as the 3D HOIPs

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(MASnI3 & MAPbBr3 45). In addition, we find that the Rashba-Dresselhaus effect

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is quite obvious in the 2D HOIP system.44 For example, the Rashba parameter at

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the valence-band maximum along the Γ→X direction (Figure 5) is 0.93 eV·Å for

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2D SnI-based HOIPs with n = 3 (Table S5).48 The Rashba parameter of bulk-TETV

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HOIPs is 0.23 eV·Å,49 but that of bulk-TETP is 0 eV·Å. Our calculations with

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HSE functional show that the HSE bandgap is larger than PBE one by 0.55 eV

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(Figure S13). For example, the HSE bandgap of (EA)2(MA)n−1SnnI3n+1 (n = 1) is

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2.14 eV, while the PBE result is 1.57 eV. 7



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Similar to the 3D HOIPs, the metal and halide atoms play key roles on the

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bandgaps of the 2D HOIPs. As the size of metal atom increases (from Sn to Pb) or

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the size of halide atom decreases (from I to Br), the bandgaps of these 2D

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compounds increase (Figure 3). For example, the bandgaps of SnBr-based systems

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(Figure 3c) are smaller than those of PbI-based systems at the same thickness

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(Figure 3b). At the same time, the band structures of the 2D HOIPs are also

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affected by the crystal structure (Figure 3). The bandgap of TETP structure (named

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as p-n) is smaller than those of the TETV (named as v-n) and orthorhombic

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(named as o-n) structures, which is the same as the 3D HOIPs.45 For example, the

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bandgap of the TETP (EA)2(MA)4Sn5I16 is 1.07 eV, which is smaller than those of

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TETV and orthorhombic structures by 0.08 eV and 0.15 eV, respectively (Figure

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4e, Figure S1e & S3e). Our calculations clearly show that the bandgap of the TETP

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(EA)2(MA)n − 1SnnI3n+1 is the smallest, while the orthorhombic (EA)2(MA)n −

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1PbnBr3n+1 is

the largest in all the 2D compounds.

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To investigate the origin of the shift of band edge sates, we analyze partial

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densities of states (PDOSs) (Figures S4-S6 & S10-S12). For the PbI-based and

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PbBr-based 2D HOIPs with EA as the terminal molecule, their CBBs are mainly

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contributed to the p states of Pb atoms (Figures S4-S6 & S10-S12). Their VBTs are

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dominated by the s electrons of Pb atoms and the p electron of I/Br atoms at n = 1.

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As n increases, the p electrons of halide atoms dominate the VBTs. For the SnI-

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based and SnBr-based 2D HOIPs (Figures S1-S3 & S7-S9), their CBBs are

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contributed to the p states of metal atoms (Sn), which is the same as the PbI-based

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and PbBr-based systems. Differently, their VBTs are dominated by the p electrons

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of halide atoms and contributed partially to the s electrons of Sn atoms. At n = 5,

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their VBTs are totally contributed to the p states of I/Br atoms. We also find that 8


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the organic molecules have negligible contributions to the band edges states in

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these four systems.

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In addition, to investigate the effect of terminal organic molecule on the

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electronic properties, we replace the EA cations by the BA and BEN cations in the

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TETP (EA)2(MA)2Sn3I10 (Figure 1e). Our calculations show that the size of

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terminal molecule has negligible effect on the bandgap of the 2D HOIP (Figures 6a,

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b & c). However, when EA in the TETP (EA)2(MA)2Sn3I10 is replaced by BA or

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BEN, the PDOSs (Figures 6f & g) show that the VBTs are contributed to the s

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electrons of Sn atoms and partially to the p electrons of I atoms, which is different

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from the EA system (mainly contributed the p electrons of halide atoms and

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partially to the s electrons of Sn atoms (Figure 6e)). The CBBs is still totally

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contributed by the p states of Sn atoms. The BA and BEN molecules have

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negligible contribution to the band edges states.

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To find out the origin of improved charge transfer, we investigate the position of

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fermi levels of these 2D TETP HOIPs compounds with different thicknesses (n =

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1-6) (Figure 7). We find that the valence band edges in TETP-SnI system increase

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dramatically as n increases (from 1 to 3), then keep slightly increasing at n

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increases from 4 to 6. The conduction band edges also increase slightly as n

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increases (Figure 7a). When a solar cell is composed of TETP-SnI 2D HOIPs with

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different thicknesses, the hole carrier can easily transfer from the thin layer to thick

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layer, while the electron carrier from thick layer to thin layer, leading to efficient

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electron-hole separation and high mobility as well as high PCE. For other systems,

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the valence band edges increase as n increases, but the conduction band edges have

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a minor fluctuation. The hole can transfer freely from the thin to thick layer.

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However, the transportation of electron in 2D HOIP multilayers strongly depends

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on the thickness chosen. For example, the conduction band edge of TETP-PbBr 9



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system with n=2 is the highest (Figure 7d). If the solar cell is composed of layers

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with n = 2 and n > 2, the separation of electron-hole pair is difficult because both

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electron and hole should transfer from thin to thick layer, leading to low PCE. To

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demonstrate our analysis, we built a heterojunction with a two-layer (n = 2) and

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four-layer (n = 4) structures of the TETP (EA)2(MA)n-1SnnI3n+1 (named as p-2-4,

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Figure 1f). The PDOSs show (Figure 8a) that the VBT of the 2D HOIP

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heterojunction is mainly contributed to the p states of I atoms (4-I_p) and partially

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to the p states of Sn atoms (4-Sn_p) from the four-layer structure, but the CBB is

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mostly dominated by the p states of Sn atoms from the two-layer structure (2-

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Sn_p). We see that the PDOS for two-layer 2D HOIP is below that of four-layer

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one in valence band, while above that of four-layer 2D HOIP in conduction band

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(Figure 8b), which is consistent with our analysis in Figure 7. Our calculations

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show that the thickness of 2D HOIP is critical to improve the harvesting efficiency.

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Only by choosing suitable layer with optimal thickness, the PCE of the solar cell

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can be maximized.

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3.3 Carrier effective mass. Carrier effective mass is an important factor in

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photovoltaic cell. To give a quantitative estimation, the carrier (electron and hole)

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effective mass (m*) of the 2D HOIPs are calculated from their band structures

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based on the following equation

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 2  * 2 ∂ ε (k ) m =h   2 k ∂  

−1

(1)

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where ε ( k ) are the eigenvalues at band edges around the CBB or VBT and k is the

241

wave vector. The calculated hole effective mass (mh*) of the 2D HOIPs slightly 10


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increases and slowly converges to the value of bulk HOIPs with the increase of the

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thickness, while the electron effective mass (me*) shows no trend (Figure 9)

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because DFT cannot exactly calculate the excited states. We also find that as the

245

size of metal atom increases (from Sn to Pb), the absolute value of mh* (|mh*|)

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increases sharply. However, the absolute value of mh* is unaffected significantly

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by halide atoms, which is different from the 3D HOIPs.45 In addition, the structure

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also plays a complicated role on the effective mass of the 2D HOIP. We find that

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|mh*| in the orthorhombic phase is heavier than that in the tetragonal phase. For the

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tetragonal phase, the TETP structure exhibits a relatively small |mh*| compared to

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the TETV structure. Finally, the impact of terminal organic molecule on |mh*| of

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the 2D HOIPs is negligible (Table S6). After considering all the factors, we see

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clearly that (EA)2(MA)n-1SnnI3n+1 with TETP phase has the lightest effective mass

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among all the studied 2D systems, and (EA)2(MA)n-1PbnBr3n+1 with TETV and

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orthorhombic phases have the heaviest hole effective mass. Meanwhile, we also

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considered the effects of SOC and hybrid functions on the effective mass. We find

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that the absolute values of both mh* and me* of 2D TETP (EA)2(MA)n−1SnnI3n+1

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with SOC slightly decrease and slowly converges to the values of bulk HOIPs with

259

the increase of the thickness, which is different with the PBE calculated results

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(only mh* slowly converges to the value of bulk HOIPs) (Table S7). For the

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calculated effective mass by HSE (Table S8), |mh*| of bulk is the same to the PBE

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and SOC results, while |me*| is larger than those with PBE and SOC. Differently,

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for 2D TETP (EA)2(MA)n-1SnnI3n+1 (n = 1), the HSE effective masses are almost

264

same to PBE data, but larger than SOC data by 0.1.

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3.4 Stability of the 2D HOIPs. The stability is one of the advantages for the 2D

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HOIPs compared to the 3D HOIPs. In this study, the adsorption energies

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(including dissociation and physisorption) of water and oxygen molecules at

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different adsorption sites (RNH3-H2O/O2: close to the terminal molecule RNH3;

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Insert- H2O/O2: between the two terminal molecules RNH3; I- H2O/O2: close to the

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halide atoms; and Sn- H2O/O2: close to the metal atoms) were investigated to find

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out their stable adsorption sites and adsorption energies by initially placing the

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molecules at the sites (Figure 10). Here, the TETP (RNH3)2(MA)2Sn3I10 systems

274

with three different terminal molecules, EA, BA and BEN, are studied. The

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adsorption energies of molecule on slab surface (ΔEads(molecule)) are calculated

276

as21

277

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

(2)

278

where Emolecule/slab, Eslab, and Emolecule are the total energies of the slab adsorbed with

279

molecules, the clean slab model (Figure 10a), and the free molecule in vacuum,

280

respectively. Under this definition, a negative Δ Eads indicates the attraction

281

behavior of molecule to the slab surface, while a positive value means repulsion.

282

Our calculations (Figure 10d & e) show that all of ΔEads are positive in our studied

283

systems, which are absolutely different from the 3D HOIPs (all of ΔEads are

284

negative),21 indicating that the 2D HOIPs can resist effectively the erosion of the

285

H2O and O2 molecules than the 3D HOIPs. Our calculations also demonstrate that

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the H2O molecule is easier to erode the 2D HOIP compounds than the O2 molecule

287

because of the low adsorption energy, in good agreement with the reports.50-51

288

Most importantly, the terminal molecule plays an important role on the stability of

289

the 2D HOIPs on water and oxygen molecules. We find (Figure 10d & e) that the

290

water molecule is much easier to adsorb at insert-H2O, I-H2O and Sn-H2O sites for 12


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291

EA2(MA)2Sn3I10, BA2(MA)2Sn3I10 and BEN2(MA)2Sn3I10 compounds due to the

292

low adsorption energies (1.59 eV, 1.59 eV and 1.49 eV). Different from the water

293

molecule, the oxygen molecule is easier adsorbed at Sn-O2 site in these three

294

compounds. We also find that the ΔEads at Sn-O2 site on 2D HOIPs with EA (1.89

295

eV) and BEN (1.93 eV) terminal molecule system are significantly lower than that

296

on the BA system (3.00 eV). The relaxed crystal structures (Figure 10c) show that

297

the oxygen bonds are broken (dissociation) at Sn-O2 in the systems with EA and

298

BEN terminal molecules, while the oxygen molecule keeps unchanged

299

(physisorption) in the system with BA terminal molecule. We see that the stability

300

of 2D HOIPs against water and oxygen strongly depends on the terminal molecule.

301

Although it is possible for Sn+2 oxidizes towards Sn4+ in 2D Sn-based HOIPs, the

302

adsorption energies (ΔEads) of oxygen molecules at Sn-O2 site on 2D SnI-based

303

HOIPs with EA, BA and BEN terminal molecule system is 1.89 eV, 3.00 eV and

304

1.93 eV, indicating Sn atom is difficult to be oxidized due to the high adsorption

305

energies. From the above analysis, we can conclude that the 2D HOIPs exhibit

306

better performance on the stability against water and oxygen than the 3D HOIPs

307

due to the high (positive) adsorption energy. Especially, BA2(MA)2Sn3I10 can resist

308

more effectively the erosion of the H2O and O2 molecules than EA and BEN

309

terminal molecule systems.

310 311

4. Conclusion

312

In summary, we present a first-principles study on the crystal structures, electronic

313

properties of 2D (RNH3)2(MA)n−1BnX3n+1 (R = CH3CH2, CH3(CH2)3, C6H5CH2; B

314

= Pb2+, Sn2+; X = Br – or I – ) with different phases and thicknesses and their 13



Page 14 of 27

315

stabilities in moisture. We find that their electronic properties are greatly affected

316

by the thickness, metal atoms, halide atoms and structures. We show that the

317

bandgap decreases as the thickness increases, the sizes of metal atoms decrease,

318

and the sizes of halide atoms increase. 2D HOIP with TETP phase has the smallest

319

bandgap and lightest carrier effective mass. Especially, the TETP (EA)2(MA)n −

320

1SnnI3n+1

321

smallest bandgap, lightest effective mass, and efficient charge separation. By

322

comparing with the 3D HOIPs, we predict that the 2D HOIPs may be more

323

efficient in solar-energy harvesting because of high stability on water and oxygen.

324

It is expected that (RNH3)2(MA)n−1BnX3n+1, especially (EA)2(MA)n−1SnnI3n+1 with

325

TETP phase, may be excellent alternatives to the unstable 3D HOIPs and have

326

many potential applications in solar energy technology.

327

328

Hui Pan thanks the supports of the Science and Technology Development Fund

329

from Macao SAR (FDCT-068/2014/A2, FDCT-132/2014/A3, and FDCT-

330

110/2014/SB) and Multi-Year Research Grants (MYRG2015-00017-FST and

331

MYRG2017-00027-FST) from Research & Development Office of the University

332

of Macau. The DFT calculations were performed at High Performance Computing

333

Cluster (HPCC) of Information and Communication Technology Office (ICTO) of

334

the University of Macau.

may show the best performance in solar energy harvesting because of the

Acknowledgments

335 336

References

337

(1)

338

Correa-Baena, J.P., Tress, W.R., Abate, A., Hagfeldt, A. and Grätzel, M., Incorporation of Rubidium

339

Cations into Perovskite Solar Cells Improves Photovoltaic Performance. Science 2016, 354, 206-209.

Saliba, M., Saliba, M., Matsui, T., Domanski, K., Seo, J.Y., Ummadisingu, A., Zakeeruddin, S.M.,

14


Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


340

(2)

Pan, H., Bandgap Engineering of Oxygen-Rich TiO2+X for Photocatalyst with Enhanced Visible-

341

Light Photocatalytic Ability. J. Mater. Sci. 2015, 50, 4324-4329.

342

(3)

343

Energy 2016, 1, 16048.

344

(4)

345

Splitting. Renew. Sust. Energ. Rev. 2016, 57, 584-601.

346

(5)

347

Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476-480.

348

(6)

349

Study. J. Chem. Theory Comput. 2009, 5, 3074-3078.

350

(7)

351

A.; Snaith, H. J., Efficient, Semitransparent Neutral-Colored Solar Cells Based on Microstructured

352

Formamidinium Lead Trihalide Perovskite. J. Phys. Chem. Lett. 2015, 6, 129-138.

353

(8)

354

Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643.

355

(9)

356

Z.; Eres, G.; Gu, B., Fabrication and Characterization of Brookite-Rich, Visible Light-Active TiO2 Films

357

for Water Splitting. Appl. Catal. B: Environ. 2009, 93, 90-95.

358

(10)

359

Properties of Perovskite-Type CH3NH3PbI3 Compounds. Appl. Phys. Express 2014, 7, 121601.

360

(11)

361

Generation: A First-Principles Study on Electronic and Magnetic Properties. Nano Energy 2012, 1, 488-

362

493.

363

(12)

364

Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar

365

Cells. Energ. Environ. Sci. 2014, 7, 982-988.

366

(13)

367

Featuring Ambipolar Transport in Methylammonium Lead Iodide Perovskite: A Density Functional

368

Analysis. J. Phys. Chem. Lett. 2013, 4, 4213-4216.

369

(14)

370

Functionalized Graphitic Carbon Nitride Nanotube. ACS Catal. 2011, 1, 99-104.

371

(15)

372

A., The Rapid Evolution of Highly Efficient Perovskite Solar Cells. Energ. Environ. Sci. 2017.

Zhang, W.; Eperon, G. E.; Snaith, H. J., Metal Halide Perovskites for Energy Applications. Nat.

Pan, H., Principles on Design and Fabrication of Nanomaterials as Photocatalysts for Water-

Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., Compositional

Pan, H.; Gu, B.; Zhang, Z., Phase-Dependent Photocatalytic Ability of TiO2: A First-Principles Eperon, G. E.; Bryant, D.; Troughton, J.; Stranks, S. D.; Johnston, M. B.; Watson, T.; Worsley, D.

Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid Solar

Pan, H.; Qiu, X.; Ivanov, I. N.; Meyer, H. M.; Wang, W.; Zhu, W.; Paranthaman, M. P.; Zhang,

Takeo, O.; Masahito, Z.; Yuma, I.; Atsushi, S.; Kohei, S., Microstructures and Photovoltaic

Pan, H.; Zhang, Y.-W., Gan/Zno Superlattice Nanowires as Photocatalyst for Hydrogen

Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J.,

Giorgi, G.; Fujisawa, J.-I.; Segawa, H.; Yamashita, K., Small Photocarrier Effective Masses

Pan, H.; Zhang, Y.-W.; Shenoy, V. B.; Gao, H., Ab Initio Study on a Novel Photocatalyst:

Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jesper Jacobsson, T.; Gratzel, M.; Hagfeldt,

15



Page 16 of 27

373

(16)

Polman, A.; Knight, M.; Garnett, E. C.; Ehrler, B.; Sinke, W. C., Photovoltaic Materials: Present

374

Efficiencies and Future Challenges. Science 2016, 352, aad4424.

375

(17)

376

McMeekin, D.P., Hoye, R.L., Bailie, C.D., Leijtens, T. and Peters, I.M., 23.6%-Efficient Monolithic

377

Perovskite/Silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2017, 2, 17009.

378

(18)

379

CH3NH3PbI3 Hybrid Perovskite Thin Film. J. Phys. Chem. C 2017, 121, 3904-3910.

380

(19)

381

S.; Anokhin, D. V.; Kurmaev, E. Z.; Stevenson, K. J.; Troshin, P. A., Probing the Intrinsic Thermal and

382

Photochemical Stability of Hybrid and Inorganic Lead Halide Perovskites. J. Phys. Chem. Lett. 2017,

383

1211-1218.

384

(20)

385

Methylammonium Lead Iodide Perovskite Degradation by Water. Chem.Mater. 2015, 27, 4885-4892.

386

(21)

387

(001) Surfaces of Tetragonal CH3NH3PbI3: A First-Principles Study. J. Phys. Chem. C 2016, 120, 28448-

388

28455.

389

(22)

390

and Sn): A First-Principles, Computational Study. Chem.Mater. 2017, 29, 682-689.

391

(23)

392

G., Amassian, A., Stingelin, N. and Colella, S., Organic Gelators as Growth Control Agents for Stable

393

and Reproducible Hybrid Perovskite-Based Solar Cells. Adv. Energy Mater. 2017, 1602600.

394

(24)

395

Enhance Perovskite Solar Cell Efficiency. ACS Appl. Mater. Inter. 2017, 9, 13181-13187.

396

(25)

397

Fully Infiltration of MAPbI3-xClx into Mesoporous TiO2 for Stable Hybrid Perovskite Solar Cells.

398

Electrochimica Acta.

399

(26)

400

Perovskite Solar-Cell Absorber with Enhanced Moisture Stability. Angew. Chem. Int. Ed. Engl. 2014, 53,

401

11232-5

402

(27)

403

Verduzco, R., Crochet, J.J., Tretiak, S. and Pedesseau, L., High-Efficiency Two-Dimensional

404

Ruddlesden–Popper Perovskite Solar Cells. Nature 2016, 536, 312-316.

405

(28)

406

Organic-Inorganic Perovskite Semiconductors. Appl. Phys. Lett. 2014, 104, 171111.

Bush, K.A., Palmstrom, A.F., Zhengshan, J.Y., Boccard, M., Cheacharoen, R., Mailoa, J.P.,

Li, Y.; Xu, X.; Wang, C.; Ecker, B.; Yang, J.; Huang, J.; Gao, Y., Light-Induced Degradation of

Akbulatov, A. F.; Luchkin, S. Y.; Frolova, L. A.; Dremova, N. N.; Gerasimov, K. L.; Zhidkov, I.

Mosconi, E.; Azpiroz, J. M.; De Angelis, F., Ab Initio Molecular Dynamics Simulations of

Hao, W.; Chen, X.; Li, S., Synergistic Effects of Water and Oxygen Molecule Co-Adsorption on

He, Y.; Galli, G., Instability and Efficiency of Mixed Halide Perovskites Ch3nh3ai3–Xclx (a = Pb

Masi, S., Rizzo, A., Munir, R., Listorti, A., Giuri, A., Esposito Corcione, C., Treat, N.D., Gigli,

Aeineh, N.; Barea, E. M.; Behjat, A.; Sharifi, N.; Mora-Seró, I., Inorganic Surface Engineering to

Kim, W.; Park, J.; Kim, H.; Pak, Y.; Lee, H.; Jung, G. Y., Sequential Dip-Spin Coating Method:

Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I., A Layered Hybrid

Tsai, H., Nie, W., Blancon, J.C., Stoumpos, C.C., Asadpour, R., Harutyunyan, B., Neukirch, A.J.,

Niu, W.; Eiden, A.; Vijaya Prakash, G.; Baumberg, J. J., Exfoliation of Self-Assembled 2d 16


Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


407

(29)

Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G., 2d Homologous

408

Perovskites as Light-Absorbing Materials for Solar Cell Applications. J. Am. Chem. Soc. 2015, 137,

409

7843-7850.

410

(30)

411

Ding, T. and Ginsberg, N.S., Atomically Thin Two-Dimensional Organic-Inorganic Hybrid Perovskites.

412

Science 2015, 349, 1518.

413

(31)

414

Perovskites. Nat. Photonics 2015, 10, 115-121.

415

(32)

416

Dimensional Halide Perovskites and Their Potential Applications for Electronics and Optoelectronics. J.

417

Phys. Chem. C 2016, 120, 24682-24687.

418

(33)

419

Quasi 2d Perovskite Based Solar Cells. Adv. Funct. Mater. 2017, 27, 1604733.

420

(34)

421

Efficient Visible Quasi-2d Perovskite Light-Emitting Diodes. Adv. Mater. 2016, 28, 7515-20.

422

(35)

423

Postsynthetic Transformations in Hybrid Perovskites. Chem. Mater. 2017.

424

(36)

425

Novel Two-Dimensional (C4H9NH3)2PbX4 Perovskites for Solar Cell Absorbers. J. Phys. Chem. Lett.

426

2017, 8, 876-883.

427

(37)

428

Perovskite Semiconductors. Adv. Energy Mater. 2016, 1601731.

429

(38)

430

Activities of Defects. Nano Lett. 2016, 16, 3335-3340.

431

(39)

432

Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401.

433

(40)

434

Application to Double-Wall Carbon Nanotubes. Phys. Rev. Lett. 2009, 103, 096102.

435

(41)

436

Phys. Rev. B, 2011, 83, 195131.

437

(42)

438

with Projected-Augmented-Wave Methodology and the Case Study of Disordered Fe1-xCox Alloys. Phys.

439

Rev. B, 2016, 93, 224425.

Dou, L., Wong, A.B., Yu, Y., Lai, M., Kornienko, N., Eaton, S.W., Fu, A., Bischak, C.G., Ma, J.,

Ha, S.-T.; Shen, C.; Zhang, J.; Xiong, Q., Laser Cooling of Organic–Inorganic Lead Halide

Yang, J.-H.; Yuan, Q.; Yakobson, B. I., Chemical Trends of Electronic Properties of Two-

Cohen, B.-E.; Wierzbowska, M.; Etgar, L., High Efficiency and High Open Circuit Voltage in

Byun, J.; Cho, H.; Wolf, C.; Jang, M.; Sadhanala, A.; Friend, R. H.; Yang, H.; Lee, T. W.,

Smith, I. C.; Smith, M. D.; Jaffe, A.; Lin, Y.; Karunadasa, H. I., Between the Sheets:

Wang, D.; Wen, B.; Zhu, Y.-N.; Tong, C.-J.; Tang, Z.-K.; Liu, L.-M., First-Principles Study of

Ma, L.; Dai, J.; Zeng, X. C., Two-Dimensional Single-Layer Organic-Inorganic Hybrid

Liu, Y.; Xiao, H.; Goddard, W. A., Two-Dimensional Halide Perovskites: Tuning Electronic

Dion, M.; Rydberg, H.; Schröder, E.; Langreth, D. C.; Lundqvist, B. I., Van Der Waals Density

Román-Pérez, G.; Soler, J. M., Efficient Implementation of a Van Der Waals Density Functional:

Klimeš, J.; Bowler, D. R.; Michaelides, A., Van Der Waals Density Functionals Applied to Solids.

Steiner, S.; Khmelevskyi, S.; Marsmann, M.; Kresse, G., Calculation of the Magnetic Anisotropy

17



Page 18 of 27

440

(43)

Krukau, A. V.; Vydrov, O. A.; Izmaylov, A. F.; Scuseria, G. E., Influence of the Exchange

441

Screening Parameter on the Performance of Screened Hybrid Functionals. J. Chem. Phys. 2006, 125,

442

224106.

443

(44)

444

Chem. Soc. Dalton Trans. 2001, 1-12.

445

(45)

446

Properties of Sn-Based Organic–Inorganic Halide Perovskites. J. Electron. Mater. 2016, 45, 5956-5966.

447

(46)

448

Iodide Perovskite Tetragonal and Orthorhombic Phases for Photovoltaics. J. Phys. Chem. C, 2014, 118,

449

19565-19571.

450

(47)

451

Layered Organohalide Lead Perovskites. J. Chem. Phys. 2016, 144, 164701.

452

(48)

453

Ferroelectric Oxide BiAlO3. Phys. Rev. B, 2016, 93.

454

(49)

455

Ferroelectric Halide Perovskites. Proc. Natl. Acad. Sci. USA 2014, 111, 6900-4.

456

(50)

457

Uncovering the Veil of the Degradation in Perovskite CH3NH3PbI3 Upon Humidity Exposure: A First-

458

Principles Study. J. Phys. Chem. Lett. 2015, 6, 3289-3295.

459

(51)

460

Water Adsorption to CH3NH3PbI3 (001) Surfaces. J. Phys. Chem. Lett. 2015, 6, 4371-4378.

Mitzi, D. B., Templating and Structural Engineering in Organic-Inorganic Perovskites. J. Am.

Ma, Z.Q.; Pan, H.; Wong, P. K., A First-Principles Study on the Structural and Electronic

Geng, W.; Zhang, L.; Zhang, Y.-N.; Lau, W.-M.; Liu, L.-M., First-Principles Study of Lead

Fraccarollo, A.; Cantatore, V.; Boschetto, G.; Marchese, L.; Cossi, M., Ab Initio Modeling of 2D

da Silveira, L. G. D.; Barone, P.; Picozzi, S., Rashba-Dresselhaus Spin-Splitting in the Bulk

Kim, M.; Im, J.; Freeman, A. J.; Ihm, J.; Jin, H., Switchable S = 1/2 and J = 1/2 Rashba Bands in

Tong, C.-J.; Geng, W.; Tang, Z.-K.; Yam, C.-Y.; Fan, X.-L.; Liu, J.; Lau, W.-M.; Liu, L.-M.,

Koocher, N. Z.; Saldana-Greco, D.; Wang, F.; Liu, S.; Rappe, A. M., Polarization Dependence of

461

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462

Table 1. Calculated bandgaps of 2D HOIPs

SnI

PbI

SnBr

PbBr

n

Bandgap (eV)

n

Bandgap (eV)

n

Bandgap (eV)

n

Bandgap (eV)

TETP

Bulk 1 2 3 4 5

0.73 ----1.30 1.17 1.09 1.07

Bulk 1 2 3 4 5

1.60 ----2.09 1.94 1.90 1.78

Bulk 1 2 3 4 5

1.06 ----1.63 1.49 1.46 1.42

Bulk 1 2 3 4 5

1.92 ----2.42 2.31 2.21 2.12

TETV

Bulk 1 2 3 4 5

0.93 ----1.35 1.25 1.17 1.15

Bulk 1 2 3 4 5

1.82 ----2.08 1.96 1.93 1.89

Bulk 1 2 3 4 5

1.36 ----1.71 1.65 1.54 1.43

Bulk 1 2 3 4 5

2.13 ----2.44 2.26 2.23 2.15

Orthorhombic

Bulk 1 2 3 4 5

1.08 1.57 1.44 1.42 1.23 1.22

Bulk 1 2 3 4 5

1.90 2.28 2.19 2.17 2.00 1.98

Bulk 1 2 3 4 5

1.50 1.84 1.83 1.69 1.51 1.57

Bulk 1 2 3 4 5

2.27 2.68 2.56 2.51 2.29 2.29

463 464 465 466

19



Page 20 of 27

467

Figure caption:

468

Figure 1. Relaxed structures of (a) TETP, (b) TETV and (c) orthorhombic structures of

469

(EA)2(MA)n−1BnX3n+1 (B = Pb2+, Sn2+; X = Br– or I–). (d) Relaxed structures of HOIPs with

470

different thicknesses (n = 1, 2, 3, 4 and 5). (e) Relaxed structures of HOIPs by replacing the

471

terminal molecule EA with BA and BEN at n = 3. (f) Relaxed structures of heterojunction with a

472

two-layers and four-layers 2D HOIPs. Sn/Pb = silver grey; I/Br = purple; N = blue; C = grey; H

473

= white.

474

Figure 2. Layer distance of (a) SnI-based 2D HOIP, (b) PbI-based 2D HOIP, (c) SnBr-based 2D

475

HOIP and (d) PbBr-based 2D HOIP with different thicknesses (n = 1, 2, 3, 4 and 5).

476

Figure 3. Bandgap of (a) SnI-based 2D HOIP, (b) PbI-based 2D HOIP, (c) SnBr-based 2D HOIP

477

and (d) PbBr-based 2D HOIP with different thicknesses (n = 1, 2, 3, 4 and 5).

478

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

479

n = 2, (c) n = 3, (d) n = 4, (e) n = 5.

480

Figure 5. Calculated band structures of TETP SnI-based HOIP with SOC: (a) Bulk, (b) n = 1, (c)

481

n = 2, (d) n = 3, (e) n = 4, (f) n = 5.

482

Figure 6. Calculated band structures of SnI-based 2D TETP HOIP with different terminal

483

molecules at n = 3: (a) EA, (b) BA, and (c) BEN. Calculated partial densities of states (PDOSs)

484

of SnI-based 2D TETP HOIP with different terminal molecules at n = 3: (e) EA, (f) BA, and (g)

485

BEN.

486

Figure 7. Band alignment of (a) SnI-based 2D TETP HOIP, (b) PbI-based 2D TETP HOIP, (c)

487

SnBr-based 2D TETP HOIP and (d) PbBr-based 2D TETP HOIP with different thicknesses (n =

488

1, 2, 3, 4, 5 and 6).

489

Figure 8. Calculated partial densities of states (PDOSs) for heterojunction of SnI-based 2D TETP

490

HOIP with a two-layer and four-layer structures (named as p-2-4). (a) PDOSs of atoms and (b)

491

PDOSs of layers.

20


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492

Figure 9. Carrier effective mass for different crystal structures: (a) SnI-based 2D HOIP, (b) PbI-

493

based 2D HOIP, (c) SnBr-based 2D HOIP and (d) PbBr-based 2D HOIP with different

494

thicknesses (n = 1, 2, 3, 4 and 5).

495

Figure 10. (a) Relaxed structures of the clean slab models with different terminal molecules. (b)

496

Adsorption sites of water and oxygen molecules. (c) Relaxed crystal structures of the systems

497

with O2 at Sn-O2 sites. (d) Adsorption energies for water molecule on SnI-based 2D HOIP with

498

n=3 and different terminal molecules (EA, BA and BEN). (e) Adsorption energies for oxygen

499

molecule on SnI-based 2D HOIP with n=3 and different terminal molecules (EA, BA and BEN).

500 501

502 503

Figure 1.

504

21



Page 22 of 27

505 506

Figure 2.

507

508 509

Figure 3. 22


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510 511

Figure 4.

512

513 514

Figure 5.

515

23



Page 24 of 27

516 517

Figure 6.

518

24


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519


Figure 7.

520 521

Figure 8.

522

523 524

Figure 9.

525 25



Page 26 of 27

526 527

Figure 10.

26


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The TETP (EA)2(MA)n−1SnnI3n+1 may show the best performance in solar energy harvesting because of the lowest band gap, the 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 low bandgap, small carrier effective masses and 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. 299x190mm (96 x 96 DPI)


Structural and Electronic Properties of Two-Dimensional Organic

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