Atomistic Origins of Surface Defects in CH3NH3PbBr3 Perovskite and

Jan 26, 2017 - Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various...
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Atomistic Origins of Surface Defects in CH3NH3PbBr3 Perovskite and Their Electronic Structures Yunxia Liu,† Krisztian Palotas,‡,§ Xiao Yuan,† Tingjun Hou,† Haiping Lin,*,† Youyong Li,*,† and Shuit-Tong Lee† †

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199 Ren’ai Road, Suzhou 215123, P. R. China ‡ Department of Theoretical Physics, Budapest University of Technology and Economics, Budafoki út 8, H-1111 Budapest, Hungary § Center for Computational Materials Science, Department of Complex Physical Systems, Institute of Physics, Slovak Academy of Sciences, SK-84511 Bratislava, Slovakia S Supporting Information *

ABSTRACT: The inherent instability of CH3NH3PbX3 remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CH3NH3PbX3 have been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface defects systematically with first-principles density functional theory calculations and STM simulations. The calculated STM images of the Br and Br-MA vacancies are both in good agreement with the experimental measurements. In vacuum conditions, the formation energy of Br-MA is 0.43 eV less than the Br vacancy. In the presence of solvation effects, however, the formation energy of a Br vacancy becomes 0.42 eV lower than the Br-MA vacancy. In addition, at the vacancy sites, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly higher than those on the pristine surfaces. This clearly demonstrated that the structural decomposition of perovskites are much easier to start from these vacancy sites than the pristine surfaces. Combining DFT calculations and STM simulations, this work reveals the chemical identities of the intrinsic defects in the CH3NH3PbX3 perovskite crystals and their effects on the stability of perovskite materials. KEYWORDS: CH3NH3PbBr3, STM simulations, surface defects, density functional theory perovskite remains an unavoidable technical barrier for industrial applications of CH3NH3PbX3-based solar cells.17−19 As the decomposition of crystal structures usually starts from the defects on surfaces, the characterization of surface defects and their chemical properties may provide useful information to understand the chemical insights of the poor stabilities of CH3NH3PbX3 perovskite.20 Recently, by using the highresolution, low-temperature scanning tunneling microscopy

In the past few years, the three-dimensional organic−inorganic hybrid lead halide perovskites, CH3NH3PbX3 (X = Br, I), have become a primary focus of research because these materials can be produced facilely and cheaply and, importantly, be used to fabricate high-efficiency solar cell devices.1−5 To date, the power conversion efficiency of CH3NH3PbX3-based solar cells has improved from 3.8% to 22.1%.6−11 Taking the advantages of high open-circuit voltages (1.13 V),12 efficient charge carrier transport,3,13,14 long carrier diffusion length,15,16 and low cost7,17,18 into consideration, these organic−inorganic hybrid perovskites have been regarded as an important basis for nextgeneration thin-film solar cells. Despite these attractive advantages, the notorious inherent instability of CH3NH3PbX3 © 2017 American Chemical Society

Received: December 9, 2016 Accepted: January 26, 2017 Published: January 26, 2017 2060

DOI: 10.1021/acsnano.6b08260 ACS Nano 2017, 11, 2060−2065

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ACS Nano (STM) technology, Ohmann et al.17 and She et al.21 have determined the stable surface structures of orthorhombic CH3NH3PbBr3 and CH3NH3PbI3 perovskites as the reconstructed Br-MA terminated (010) surfaces, in which the MA cations may align either perpendicular or parallel to their nearest neighbors. Interestingly, although the (010) surfaces of CH3NH3PbX3 perovskite were produced with different methods (thin film growth on Au (111) for CH3NH3PbI3 and crystal cleavage for CH3NH3PbBr3), the same surface defects were observed in the high-resolution STM images: a dark depression at the surface Br or I site (see Figure 4 and Figure 1d,e in refs 17 and 21, respectively). Such surface defects, correspondingly, can be the dominating intrinsic defects of the CH3NH3PbX3 perovskite. Thus, the determination of the chemical identity and electronic structures of these surface defects may not only indicate a possible interpretation of the decomposing process of CH3NH3PbX3 perovskites but also provide important guidelines to improve their stabilities by ion doping. For instance, Grätzel et al. reported that the embedding of small amount of Rb+ can significantly improve the stability and reduce the number of surface defects in photoactive perovskite.11 In this work, we focus on the Br-MA-terminated (010) surfaces of the CH3NH3PbBr3 perovskite. Four vacancy defects and 11 adsorption structures have been investigated with the DFT and STM simulations. By comparing the formation energies and simulated STM images, we demonstrate that (i) the pristine CH3NH3PbBr3 (010) surface is rather inert toward the adsorption of atoms or molecules and (ii) the experimentally observed surface defects are most likely to be the Br vacancies or the Br-MA double vacancies. Subsequent electronic structure analysis indicates that such vacancy defects introduce an electronic state in the bandgap of the pristine CH3NH3PbBr3 (010) surface, very close to the conduction band minimum. The band gap of the CH3NH3PbBr3, however, is not significantly affected by the existence of the vacancy defects. Significantly, at the surface vacancy defects, the adsorption energies of H2O, O2, and CH3CN molecules are much larger than those on the pristine surface. The increased attractive interactions are attributed to the formation of hydrogen bonds between the adsorbates and the MA cations around the vacancy defects. As a result, these MA cations are found to move toward the adsorbates during structural optimizations. Therefore, we believe that the attractive interaction of hydrogen bonds between the molecules containing O and N atoms may play a vital role in the structural decomposition of the orthogonal CH3NH3PbBr3 crystals.

Ef = Edefective − Epristine

(1)

Here, Epristine represents the energy of the pristine CH3NH3PbBr3 (010) surface, while the Edefective represents the energy of the decoupled system, in which the surface ions are moved 5 Å away from the surface. Such a method allows us to model the charged defective surface and charged ions with a neutral slab model.24 Correspondingly, the adsorption energies of charged ions and molecules on the pristine surface or defective surface are calculated by Eads = Eadsorbed − Edecoupled

(2)

Here, Eadsorbed is the energy of an adsorbed system, while Edecoupled is the energy when the adsorbate is placed 5 Å higher than the binding site of pristine surface or defective surface. For the adsorption of a single atoms (e.g., Br, Pb, or H), the adsorption energies are calculated with Eads = Eadsorbed − Epristine − Eadsorbate

(3)

where Eadsorbate refers to the atomic energy in their elemental state. As shown in Table 1, in a vacuum, the Br-MA double vacancy (VBr‑MA) has the lowest formation energy of 1.96 eV, which is Table 1. Calculated Formation Energies of the Defects on the CH3NH3PbBr3 (010) Surfaces surface vacancy defecta

Ef(vacuum)

Ef(water)

VBr VMA VBr‑Pb VBr‑MA

2.39 3.32 4.65 1.96

1.05 1.14 1.47

VX represents a defective surface with an “X” vacancy defect. VX‑Y represents the defective surface with a double vacancy of “X-Y”. All energies are in eV. a

0.43 eV lower than that of VBr. It suggests that without water the formation of a neutral defect is easier than the charged defects. However, when the solvent effect of water was taken into consideration, the formation energies of all vacancy structures were significantly decreased. Such a decrease of vacancy formation energies is consistent with the experimental observations that the hybrid perovskite materials decompose quickly in a humid environment. In the presence of water, the VBr (1.05 eV) and VMA (1.14 eV) vacancies exhibit the lowest formation energies, which indicate that the formation of charged defects become easier. The calculated STM images of all vacancy structures are shown in Figure 1. The dark STM feature observed in the VBr, VBr‑Pb, and VBr‑MA vacancies is consistent with the experimental observations, in which one bright peak at the surface site of Br is replaced by a depression (see Figure 4 in ref 17). The calculated STM image of the MA vacancy, however, does not agree with the experimental measurements because the number of bright features remains unchanged. The depressions observed at a Br vacancy (Figure 1b) and the Br−Pb double vacancy (Figure 1d) have similar sizes with the nearby protrusions resulting from surface Br atoms, while the depression of a Br-MA double vacancy appears more extended toward the missing MA cation. Interestingly, we found that the STM tip plays a vital role in calculating the correct STM image for the VBr structure. As seen in Figure S1, the vacancy appears as a bright peak and a dark depression when the images were

RESULTS AND DISCUSSION Because of the rotation of the surface MA molecules, two coexisting stable surface structures of CH3NH3PbBr3 (010) have been reported.17,21 Since there is no significant difference in the stabilities and the electronic structures of the two surface phases, all calculations in this work were performed based on the “antiferroelectric” surface structure, in which backbone of MA cations align parallel to each other. As the observed surface defect in STM experiments may either be caused by a surface vacancy defect or an adsorbate (known as the reversed contrast),22,23 we have calculated four vacancy structures and 11 adsorption structures to determine the chemical identities of the surface defects. The vacancy formation energies were calculated with the following equation: 2061

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Figure 1. The top layer atoms and simulated STM images of (a) the pristine CH3NH3PbBr3 (010) surface and a surface with (b) a Br vacancy VBr, (c) a MA vacancy VMA, (d) a Br−Pb double vacancy VBr−Pb, and (e) a Br-MA double vacancy VBr‑MA. These STM images were all calculated with a W (111) tip. The squares show the position of the missing Br, Pb atoms or MA cations. Vbias = −3.0 V. Color code: N (blue), C (gray), H (white), Br (brown).

calculated without and with taking into account the effect of the tip, i.e., employing the Tersoff−Hamann (TH) method and the revised Chen’s derivative rule, respectively.25−27 Such tipinduced reversed contrast has been widely observed when there is an energy mismatch between the local density of states (LDOS) of the tip and the surface28−30 or when m ≠ 0 states dominate the electronic structure of the tip.31 The adsorption energies of atoms and molecules above the CH3NH3PbBr3 (010) surface are listed in Table 2. Note the

Table 3. Adsorption Energies of Ions and Molecules on the Br and Br-MA Vacancies of the CH3NH3PbBr3 (010) Surface adsorptions on VBr or VBr‑MA vacancy sitesa

Table 2. Adsorption Energies of Atoms and Molecules on the Pristine CH3NH3PbBr3 (010) Surface adsorptions on surface Br or MA sitesa

Eads (eV)

Bra‑Br Bra‑Pb Bra‑H Bra‑H2O Bra‑O2 MAa‑Br MAa‑Pb MAa‑H MAa‑H2O MAa‑O2 MAa‑CH3CN

0.40 2.72 2.46 −0.21 −0.10 0.41 2.72 2.41 −0.36 −0.18 −0.30

Eads (eV)

VBra‑O2

−2.33

VBra‑H2O

−0.52

VBra‑CH3CN

−0.41

VBra‑OH

−3.72

VBrMAa‑2O2

−0.80

VBrMAa‑2H2O

−0.61

VBrMAa‑CH3CN

−0.90

VBrMAa‑OH‑H

0.27

VBra‑X and VBrMAa‑X represent the adsorption of ion or molecule “X” above the Br and Br-MA vacancy sites. The adsorption energy values are given per molecule adsorbed above vacancy sites following eq 2.

a

images, the observed surface defects in STM measurements are most likely to be the Br vacancy (when the sample is prepared in water environment) and the Br-MA double vacancy (when the sample is prepared in vacuum). The effect of vacancy defects on the band structure of CH3NH3PbBr3 (010) surfaces is shown in Figure 3, revealing that the vacancies have introduced an extra defective electronic state very close to the bottom of the conduction band (shown by the red line). As a result, the band gaps of the defective CH3NH3PbBr3 (010) surfaces are decreased by 0.19 and 0.01 eV (2.31 eV for pristine surface) for the Br vacancy and Br-MA vacancy, respectively. Such a small decrease of band gaps is expected to have small effects on the open-circuit voltages. Thus, the corresponding photon-electron conversion efficiency would not be strongly affected by those vacancy defects. The real-space expansion of the defective state shown in Figure 3 can be visualized with the decomposed band calculations by considering the partial charge at the given defect energy level. As seen in Figure 4, the electronic states produced by vacancies are localized near the Pb atoms, which are beneath the missing Br atoms. The electronic states of other atoms near the vacancy sites, however, are not significantly affected. This, again, confirms that the device properties of CH3NH3PbBr3 perovskite will not be significantly affected by these vacancy defects. As the decomposition of CH3NH3PbX3 perovskite can be significantly promoted in the presence of other molecules (e.g.,

a

The Bra‑X and MAa‑X represent the adsorption of an atom or a molecule “X” above the Br and MA site of the pristine CH3NH3PbBr3 (010) surface, respectively.

difference in the definition of Eads in eqs 2 and 3. From the results, it is clear that atomic adsorption is energetically unfavorable (positive Eads values). On the other hand, the adsorptions of H2O, O2, and CH3CN molecules are favored but quite weak. It indicates that the pristine CH3NH3PbBr3 (010) surface is chemically inert. This finding is a bit unexpected as the CH3NH3 PbBr3 perovskite materials are known to decompose in the presence of water and oxygen19 but possibly through different mechanisms than considered in Table 2; see our later analysis and Table 3. The simulated STM images of adsorbed atoms and molecules above the Br and MA sites of the pristine CH3NH3PbBr3 (010) surface are shown in Figure 2, where the adsorbates appear as bright protrusions, which clearly disagree with the experimentally measured depression images.17 Taking into account the formation energies and simulated STM 2062

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Figure 2. Simulated STM images of (a) the pristine CH3NH3PbBr3 (010) surface and a surface with (b) Bra‑Br, (c) Bra‑Pb, (d) Bra‑H, (e) Bra‑H2O, (f) Bra‑O2, (g) MAa-Br, (h) MAa‑Pb, (i) MAa‑H, (j) MAa‑H2O, (k) MAa‑O2, (l) MAa‑CH3CN. These STM images were all calculated by the Tersoff− Hamann method. Vbias = −3.0 V.

Figure 3. Calculated band structure of (a) the pristine CH3NH3PbBr3 (010) surface, (b) the VBr defective surface, and (c) the VBr‑MA defective surface.

the Br vacancy defects in CH3NH3PbBr3 may be the general starting sites of the degradation in the water environment.32

CONCLUSIONS With first-principles DFT calculations and STM simulations, we determine the experimentally observed surface defects of CH3NH3PbBr3 to be the Br vacancy or Br-MA double vacancy. These two vacancy defects show the lowest formation energy, and the calculated STM images are consistent with experimental observations. The presence of water will significantly reduce the formation energy of the vacancy defects. The neutral vacancy site (VBr‑MA) and charged vacancy sites (VBr, VMA) are preferred in the vacuum and in aqueous solutions, respectively. The pristine CH3NH3PbBr3 (010) surfaces are shown to be quite inert toward the adsorption of molecules or atoms. In the presence of vacancy sites, in contrast, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly increased due to the formation of hydrogen bonds between the adsorbates and the defective surface. It is expected that in comparison with the pristine surfaces, structural decomposition of perovskite materials is much easier to start from the surface vacancy sites. This finding may therefore provide important implications for the development of stable perovskite materials.

Figure 4. Decomposed band calculations of the defective state of the (a) VBr and (b) VBr‑MA defective surfaces. The isosurface values are all 0.01 e/Å3. Color code: N (blue), C (gray), H (white), Br (brown), Pb (black with white highlight).

water and oxygen),19 the calculations of the adsorption of molecules on the defective CH3NH3PbBr3 (010) surface may provide useful information to understand the origin of this process. Table 3 shows that O2, H2O, and CH3CN molecules and OH ions strongly bind to the vacancy sites. Structural analysis shows that the enhanced adsorption can be attributed to the formation of hydrogen bonds of the O and N atoms of the adsorbates and the hydrogen atoms of the defective CH 3 NH 3 PbBr 3 (010) surface. As a consequence, the surrounding MA cations are found to move toward the adsorbates (see Figure 5). Such structural changes are expected to reduce the formation energy of other vacancy defects and thus accelerate the decomposition of CH3NH3PbBr3 perovskite materials. Recently, Park et al. reported that the MAPbI3 perovskite can work as stable hydrogen evolution reaction (HER) catalyst in the aqueous solution of hydrogen iodide. Since the dynamic equilibrium of I− ions may effectively prevent the decomposition of MAPbI3 perovskite, we think that

METHODS The first-principles DFT calculations were conducted with the Vienna Ab Initio Simulation Package (VASP).33,34 The core and valence electronic interactions were described with the frozen-core projector augmented-wave (PAW) potentials.35,36 The Kohn−Sham oneelectron states were expanded in plane waves with a kinetic energy of up to 400 eV. The exchange correlation energy was calculated with the Perdew−Burke−Ernzerhof (PBE) of generalized gradient approximation (GGA).37 The tolerance of 10−4 eV was adopted for energy convergence of electronic calculations. The CH3NH3PbBr3 (010) surfaces were modeled using (2 × 2) slabs containing seven 2063

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Figure 5. Optimized configurations of (a) O2, (b) H2O, and (c) CH3CN molecules and (d) OH ions adsorbed at the Br vacancy sites and (e) two O2, (f) two H2O, and (g) one CH3CN molecule and (h) OH and H ions adsorbed at the Br-MA double-vacancy sites. The red arrows show the direction of motion of the MA cations (or Br atoms in (h)). The O, Br, N, C, and H atoms are represented by red, brown, blue, gray, and white circles, respectively, and the two pink circles are two Br atoms of the second layer. atomic layers. The size of the supercell was (17.57 × 15.74 × 50.00) Å. A large vacuum thickness of 25 Å along the direction normal to the surface was employed to separate surfaces from their periodic images that are normal to the surface plane. The bottom four layers of atoms were kept fixed at their optimized bulk positions. All other atoms were fully relaxed in three dimensions until the atomic forces were smaller than 0.01 eV/Å. The nonlocal van der Waals interactions were evaluated with the vdw-DF functional of Langreth and Lundqvist.38,39 The Gamma-centered Monkhorst−Pack sampling of 3 × 3 × 1 was adopted to study the electronic properties of surfaces CH3NH3PbBr3. In non-self-consistent-field (NSCF) calculations, 12 K points were used to model the Brillouin zone (BZ) of the surface structures of CH3NH3PbBr3.40 The water solvent environment was simulated with an implicit solvation model implemented in VASPsol.41 The firstprinciples scanning tunneling microscopy (STM) calculations were performed with the bSKAN code.25−27,42 The STM tip was modeled with a W (111) pyramid made of seven atomic layers in which there was only one tungsten atom at the apex. All STM simulations presented in this manuscript were calculated with a scattering method developed to first order in the Green’s functions.43−45

ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (Grant No. 21673149), Collaborative Innovation Centre of Suzhou Nano Science & Technology, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. H.L. is grateful to the Natural Science Foundation of Jiangsu Province (BK20150305) and Soochow University (SDY2014A14) for funding. K.P. acknowledges a SASPRO Fellowship of the Slovak Academy of Sciences (Project No. 1239/02/01). REFERENCES (1) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (2) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506−514. (3) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647. (4) Boix, P. P.; Nonomura, K.; Mathews, N.; Mhaisalkar, S. G. Current Progress and Future Perspectives for Organic/Inorganic Perovskite Solar Cells. Mater. Today 2014, 17, 16−23. (5) Loi, M. A.; Hummelen, J. C. Hybrid Solar Cells: Perovskites under the Sun. Nat. Mater. 2013, 12, 1087−1089. (6) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 45). Prog. Photovoltaics 2015, 23, 1−9. (7) Shi, T.; Yin, W.-J.; Hong, F.; Zhu, K.; Yan, Y. Unipolar SelfDoping Behavior in Perovskite CH3NH3PbBr3. Appl. Phys. Lett. 2015, 106, 103902. (8) Xie, J.; Liu, Y.; Liu, J.; Lei, L.; Gao, Q.; Li, J.; Yang, S. Study on the Correlations Between the Structure and Photoelectric Properties of CH3NH3PbI3 Perovskite Light-Harvesting Material. J. Power Sources 2015, 285, 349−353. (9) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (10) Bi, D.; Yi, C.; Luo, J.; Décoppet, J.-D.; Zhang, F.; Zakeeruddin, S. M.; Li, X.; Hagfeldt, A.; Grätzel, M. Polymer-Templated Nucleation and Crystal Growth of Perovskite Films for Solar Cells with Efficiency Greater than 21%. Nat. Energy 2016, 1, 16142. (11) Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.;

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08260. Tip effect of the STM simulations of the VBr vacancy; test of vacuum thickness; effects of relaxed atomic layers on density of states (DOS); schematic diagram to illustrate the calculations of formation energies of surface defects and adsorption energies of atoms, ions and molecules; effects of different bias voltages on the same surfaces (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Youyong Li: 0000-0002-5248-2756 Notes

The authors declare no competing financial interest. 2064

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DOI: 10.1021/acsnano.6b08260 ACS Nano 2017, 11, 2060−2065