Surface Effects and Adsorption of Methoxy Anchors on Hybrid Lead

Article pubs.acs.org/JPCC

Surface Effects and Adsorption of Methoxy Anchors on Hybrid Lead Iodide Perovskites: Insights for Spiro-MeOTAD Attachment Alberto Torres and Luis G. C. Rego* Department of Physics, Universidade Federal de Santa Catarina, Florianópolis, SC CEP 88040-900, Brazil S Supporting Information *

ABSTRACT: Despite the intense study that has been carried out on hybrid organic−inorganic perovskites, motivated by the breakthrough of perovskite solar cells, the surface properties of the methylammonium (MA) lead halide perovskites still remain vastly unexplored. The polar structure of the hybrid perovskites suggests that their surface properties are important to the operation of the photovoltaic devices. To gain insight into these properties, we have investigated the (001) the surfaces of the hybrid CH3NH3PbI3 perovskite by a combination of ab initio methods that include the surface relaxation and the molecular dynamics simulations. We also investigated the binding modes of the methoxybenzene (anisole molecule, CH3OC6H5) on the perovskite surface, with the goal of clarifying the perovskite/spiroMeOTAD attachment mechanisms. It is found that the methoxybenzene can adsorb on the (001) octahedral surface of the MAPbI3 perovskite depleted of the methylammonium cation, where the methoxy group finds a stable minimum of ∼38.6 kJ/mol in the interstice of the corner-sharing PbI6 octahedra. However, there is no stable attachment site for the methoxybenzene on the PbI2 flat (001) surface of the perovskite. The attachment mechanism is governed by the electrostatic interaction of the methoxy group with the surface, which is repulsive with the iodides and attractive with the Pb(II) ion.

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INTRODUCTION The bulk properties of the methylammonium (MA) lead halides have been extensively investigated from the experimental1−7 and theoretical8−15 viewpoints in an effort to uncover the underlying mechanisms that are responsible for the extremely good performance of this materials in photovoltaic applications as well as to provide guidance for the design of improved organometallic perovskite-based solar cells. Although the hybrid organic−inorganic16,17 perovskites have been studied for several decades, the performance of the hybrid methylammonium lead halide perovskites in photovoltaic applications was unknown until very recently and has taken aback the scientific community. Within 5 years, the efficiency of perovskite based solar cells has increased steadily from 3.8%18 to a certified efficiency of 18%19 and with expectations for further improvements. Equally surprising is that, accompanying the efficiency evolution of the devices, the paradigms for perovskite-based solar cell fabrication have changed several times during this period,4,19−24 since the material was initially employed only as a light absorber in dye-sensitized solar cells and evolved to play the dual role of light absorber and ambipolar transport material in solid state solar cells. Mesostructured as well as thin-film architectures have been explored, and flexible devices were also fabricated.23 There are, however, important challenges that need to be overcome such as the issues of stability of operation under environmental conditions, most notably humidity and temperature.24−26 During the past 3 years, the electronic properties of the methylammonium lead halide perovskite family, CH3NH3PbX3, © 2014 American Chemical Society

with X = Cl, Br, or I, have been extensively investigated in the bulk by state of the art first-principles methods.8−11,13,15,27−29 Calculations for the band gap11,15 and carrier mobility8,13 in the bulk agree with the experimental measurements, providing a basic understanding for the observed high conversion efficiencies. However, to gain more understanding of these materials and to advance further the design and the efficiency of perovskite solar cells, it is essential to know their surface properties from the atomistic point of view. Several of the relevant effects for the device operation, such as charge separation,12,30 transport,5,31 and recombination,13,32,33 have been investigated. Despite the extensive investigation of the bulk properties of the hybrid perovskites, the surface properties of these materials still remain unexplored. This paper aims at paving some of this ground by investigating the representative (001) surfaces of the pseudocubic CH3NH3PbI3 (or MAPbI3) in vacuo, at static and dynamic situations, thus extending the comprehension gained by Haruyama et al.33 and Mosconi et al.30 on the CH3NH3PbI3 surfaces and its interfaces with TiO2, respectively. Furthermore, we have investigated the binding modes of the methoxybenzene (anisole molecule, CH3OC6H5) on the relevant (001) surfaces of the MAPbI3. This molecule is an excellent model for the anchoring groups employed in various polymeric hole conductor materials, notably the spiroMeOTAD. The polytriarylamine derivatives are bulky and steric, which hampers their description from the theoretical Received: October 21, 2014 Published: October 28, 2014 26947

dx.doi.org/10.1021/jp510595s | J. Phys. Chem. C 2014, 118, 26947−26954

The Journal of Physical Chemistry C

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viewpoint; nevertheless, these molecules are usually attached to the perovskite surface by the CH3O methoxy group. The work also describes the role of the electric polarization effects13,34 on the relaxation mechanisms on the surface of the hybrid metal halide perovskites.

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COMPUTATIONAL DETAILS The calculations in this paper were performed using the plane wave density functional theory (DFT) implemented in the code VASP,35 with the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhofer (PBE) for the exchange-correlation functional and the core−valence interaction treated by the projected augmented wave (PAW) method. The valence wave functions were expanded by plane waves with a cutoff energy of 500 eV. For the bulk calculations of the CH3NH3PbI3 perovskite the calculations used a 6 × 6 × 6 Monkhorst−Pack k-point grid. Geometries were relaxed until the residual forces became less than 0.02 eV/Å per atom. The effect of the spin−orbit coupling (SOC) in the geometry relaxation was tested, but it did not produce appreciable effects; it was noted elsewhere30 that the structural properties of the MAPbI3 are well reproduced without SOC. It is well-known that the CH3NH3PbI3 perovskite presents a gradual transition from the tetragonal to cubic phase as the temperature increases around 57 °C.7 Thus, starting from a cubic geometry, the bulk unit cell of the CH3NH3PbI3 was allowed to relax, giving rise to a pseudocubic structure with optimized lattice parameters a = b = 6.486 Å and c = 6.474 Å; the interatomic distances are presented in Table 1 of the Supporting Information. After simulated annealing and energy minimization procedures the methylammonium (MA) cation was found with its CN bond oriented approximately along the [111] direction, which allows the NH3+ group of the MA to make three hydrogen bonds (H-bond) with the nearby bridging halides. The partial charges at the atoms of the unit cell were calculated by the Bader36 and Mulliken methods; the results are presented in Table 2 of the Supporting Information. Despite the large differences produced by both methods in the net atomic charges, the net charges in the chemical groups are almost equal: −0.76 |e| (−0.76) for the PbI3−, +0.47 |e| (0.49) for the CH3 group, and +0.29 |e| (0.27) for the NH3 group according to the Bader (Mulliken) method. For the sake of comparison, under in-vacuo conditions the CH3NH3+ exhibits the charges +0.31 |e| in the CH3 group and +0.69 |e| in the NH3 group, which indicates an electronic charge transfer from the inorganic PbI3− to the MA cation. Band structure calculations were performed at the DFT level for the optimized pseudocubic MAPbI3 structure with and without spin−orbit coupling (SOC), red-dashed and black lines in the graph of Figure 1, respectively. By disregarding the SOC, a direct band gap of 1.62 eV is obtained whereas the inclusion of the SOC decreased the band gap to 0.51 eV, in agreement with reports of calculations performed at an equivalent theory level.11 The good accordance of the DFT calculations with the band gap obtained by experimental studies (ca. 1.6 eV) does not occur for other hybrid perovskites,11 though. An accurate theoretical prediction of 1.67 eV was obtained by De Angelis et al.11,15 using the spin−orbit coupling with GW corrections. Since the preliminary calculations showed that the structural properties of the MAPbI3 are not affected by the spin−orbit coupling, the study of the surface effects in CH3NH3PbI3 is performed without it.

Figure 1. Calculated electronic band structure for the bulk CH3NH3PbI3 perovskite. The solid black lines designate the results obtained without considering the spin−orbit coupling, whereas the data in red dashed lines were obtained with spin−orbit interaction. The arrows indicate the band gap.

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MAPbI3 SURFACES Structural Properties. MAPbI3 (001) surfaces were built from preoptimized pseudocubic unit cells of the bulk to form 2D slabs composed of several crystal planes perpendicular to the c-axis, as shown in Figure 2, where the relaxed structures are

Figure 2. Model unit cells used in the study of the four different surfaces formed by the CH3NH3PbI3 along the (001) direction. The colored ball-and-stick representation model depicts the relaxed structures whereas the orange lines represent the initial unrelaxed structure gained from the bulk structure. The four different surfaces are depending on the termination of the inorganic PbI6 structure and also on the orientation of the organic cation CH3NH3 with respect to the inorganic structure.

depicted by the ball-and-stick model, superimposed with the unrelaxed structure which is depicted by orange lines. We focus on the (001) surface of the CH3NH3PbI3 perovskite because it is one of the most stable surfaces of both cubic and tetragonal phases, as demonstrated by DFT-based stability calculations for various surfaces33 and evinced by X-ray diffraction measurements.7 This surface is expected to remain unchanged, without the need of reconstructions and defect formation.33 The MAPbI3 perovskite structure consists, along the [001] direction, of alternating planes of neutral MAI and PbI2 units. Moreover, since the [001] direction in the tetragonal phase corresponds to the [100] direction of the cubic phase, the (001) surfaces of the pseudocubic and tetragonal phases have the same topology. The results of the following study, therefore, apply for the three equivalent faces of the pseudocubic phase as well as for the tetragonal (001) MAPbI3 surfaces. Nevertheless, despite the pseudocubic symmetry, by building 2D slabs with the unit cells of the 26948



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the surface cation moves because of the relaxation of the inorganic network. ii. Surf-2. A surface constituted of corner sharing octahedra nesting an MA cation with its dipole moment (NH3 group) pointing initially inward the slab (bottom surface of the structures depicted in Figure 2a,d). This surface differs from Surf-1 only in the orientation of the methylammonium cation relative to the (001) inorganic surface. Contrary to the behavior described above, this surface shows little relaxation effects. In this case the NH3 moiety and the neighboring iodides are always making the maximum number of H-bonds, which renders a good stability to the inorganic network. Consequently, the methylammoniums do not align with each other. Amat et al.14 showed the relevance of H-bonds for gap tuning in organohalide perovskites, comparing the effects produced by methylammonium and formamidinium organic cations. We notice that the self-interaction between image cells in the plane of the slab can be responsible for the observed dipole alignment of the cation molecules on Surf-1. To verify this matter, we carried out a postrelaxation in a 2 × 2 × 7 extended supercell composed of 180 ions, using a 6 × 6 × 1 k-point grid. The energy of the extended supercell showed negligible decay beyond that obtained with the original slab. We also performed ab initio molecular dynamics (MD) simulations for the 1 × 1 × 7 S-slab. Starting with the relaxed configuration, the structure was thermalized during 1 ps, with a time step of 0.5 fs. After thermalization, representative configurations from an MD trajectory at 300 K were collected for 1 ps. The frames of the MD simulations are shown in Figure 3. In agreement with the results of the relaxation simulation, the

bulk, four different (001) surfaces arise, depending on the termination of the inorganic structure and the orientation of the MA cation along the [111] direction. In regard to the inorganic PbI3 network of the perovskite, two types of 1 × 1 × 7 surfaces were constructed, each one composed of seven planes, as depicted in Figure 2a,b: the first slab, Figure 2a, has both surfaces terminated with PbI 6 octahedra whereas the second, Figure 2b, is terminated on flat PbI2 planes of atoms, exposing the pentacoordinated lead atoms Pb5C. Because of the symmetry between top and bottom layers of the inorganic network, we denominate such structures as symmetric (S), although the orientation of the polar methylammonium (MA) is not symmetric. Prior to the relaxation of the model surface, the organic molecule can be oriented with its dipole moment pointing inward the slab or outward the slab. Our calculations for geometry relaxation and molecular dynamics demonstrate that there is a significant difference in the behavior of both surfaces. Because the symmetric (S) slabs are nonstoichiometric, a set of two 1 × 1 × 8 stoichiometric 8-layer asymmetric (A) slabs were also built to be used as reference. These are illustrated in Figure 2c,d. Each of the A-slabs are terminated on one side by PbI6 octahedra and on the opposite side by a plane of PbI2 atoms. Like in the Sslabs, each plane of the A-slabs has the dipole moment of the MA cation pointing inward and outward the slab on the opposite (001) surfaces. Therefore, there are four different (001) surfaces. For the calculations of energy relaxation and molecular dynamics the central three atomic layers of the inorganic network were kept rigid for the S-slabs as well as the central four atomic layers for the A-slabs; all the MA cations were free to move regardless of their position. The calculations carried out for the surfaces used a 6 × 6 × 1 k-point grid, without spin−orbit coupling. In Figure 2, the relaxed structures are depicted by the balland-stick model, superimposed with the unrelaxed structure, which is depicted by orange lines. The first result to be noticed is that equivalent surfaces on both the S- and A-slabs present the same relaxation pattern. Detailed views of the surface relaxation projected along the [100] and [010] directions are also included as Supporting Information. In the following, we analyze separately the relaxation effects for each of the four surfaces: i. Surf-1. A surface constituted of corner sharing octahedra nesting an MA cation with its dipole moment (NH3 group) pointing initially outward the slab (top surface of the structures depicted in Figure 2a,c). This surface exhibits the most pronounced relaxation effects. During the relaxation, the methylammonium molecule reorients itself to form three strong H-bonds with two apical iodides and a corner-sharing one, at the expense of missing a weak H-bond at the CH3 end of the cation. In addition, the cation molecule on this surface takes on a dipole alignment mode on the (001) plane. The inorganic component of the structure is also relaxed to accommodate the cation in the new orientation. An equivalent relaxation is observed for the same surface in the A-slab (Figure 2c). In both cases the new orientation of the cation molecule on Surf-1 increases the number of strong H-bonds between the NH3 group and the iodide and gives rise to a stabilizing electric coupling between the MA molecules on the surface. Frost and collaborators13 have estimated the dipole coupling between neighboring MA molecules in the MAPbI3 to be around 48 meV (4.6 kJ/mol). The MA cation just underneath the Surf-1, however, remains in its original configuration, indicating that

Figure 3. Snapshots of the molecular dynamics simulation carried out at 300 K on the symmetrical Surf-1/2 (Figure 2a); two viewpoints are presented.

MD evinces that Surf-2 (the bottom surface) is very stable. The dynamics of the methylammonium on this surface remains firmly connected to the inorganic structure by three strong Hbonds with the iodides of the neighboring octahedron facet. On the contrary, the two layers that comprise the opposite surface, Surf-1, are much less stable. During the 1 ps production run at 300 K the cation did not leave the octahedral interstice because of the H-bonds between the NH3 and the iodide; however, the dipolar alignment is completely lost, and the CH3 moiety is directed outward the surface plane. That behavior shows the dominance of the H-bond over the dipole−dipole interaction in this perovskites. In the layer underneath, the methylammonium 26949



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remains within the dodecahedron formed by the lead triiodides; nevertheless, the cation orientation changes: it no longer points along the [111] direction but rotates in the (001) plane. Another symmetric slab is shown in Figure 2b, also with two different surfaces. iii. Surf-3. A surface constituted by the PbI2 plane of atoms and the dipole moment of the MA cation pointing outward the slab (top surface of the structures depicted in Figure 2b,d). This surface is very stable and exhibits very little relaxation for both the organic and inorganic components because the triple Hbonds of the [111] oriented NH3 group give support to the superficial metal−halide plane. iv. Surf-4. A surface constituted by the PbI2 plane of atoms and the dipole moment of the MA cation pointing inward the slab (bottom surface of the structures depicted by Figure 2b,c). Upon relaxation the inorganic cage at the surface shrinks by 0.19 Å and the methylammonium rotates to lay itself in the (110) plane, giving rise to a partial dipolar alignment along the [100] direction. Molecular dynamics simulations were performed for Surf-3 and Surf-4 (001) surfaces of the 1 × 1 × 7 symmetric S-slab, following the procedures already described for the MD simulations on the other S-slab. The results are presented in Figure 4. In agreement with the surface relaxation results, the

Figure 5. (a) Chemical structure of the spiro-MeOTAD molecule. (b) Methoxybenzene (anisole molecule) is the model system for the anchor group of the spiro-MeOTAD and polytriarylamine materials: carbon (cyan), oxygen (red), hydrogen (white).

theoretical viewpoint. All, though, are supposed to attach to the perovskite surface by the CH3O methoxy group. Thus, to gain insights into the adsorption mechanisms of the triarylamine derivatives on the perovskite surface, we investigate the binding modes and energies of the methoxybenzene molecule, shown in Figure 5b, on the (001) surfaces of the MAPbI3 perovskite. Our goal is to find the most stable attachment modes and geometries. Such basic information is essential to understand the perovskite/HTM interface and its underlying charge transfer, notably for the case of spiro-MeOTAD. It is worth noting as well that reasonable energy conversion efficiencies (ca. 8%) are also obtained in perovskite solar cells without a HTM1,20,21 or using an inorganic HTM25 that provides a smaller conversion efficiency (ca. 6%) with improved stability. The spiro-MeOTAD hole conductor is an expensive part of perovskite solar cells. Although other organic hole conductor materials have been investigated, the alternatives produce inferior conversion efficiencies. Lindblad et al.38 carried out photoelectron spectroscopy on CH3NH3PbI3/TiO2 interfaces and reported a small understoichiometry in the I and N atoms, for samples grown by the two-step process.39 In regard to the understoichiometry in iodine, the experimental observation suggests the prevalence of the PbI2 metal−halide flat surface. The work also reports an understoichiometry in N, which indicates that some of the superficial CH3NH3+ is lost after the perovskite crystal formation. First we examine the adsorption of the methoxybenzene (anisole molecule) on the (001) octahedral surfaces of the pseudocubic MAPbI3 perovskite. To investigate the adsorption mechanisms of the anisole molecule on this surface a 2 × 2 × 7 S-slab supercell was built without the superficial methylammonium molecule, as shown in Figure 7, which is big enough to accommodate the anchoring molecule. The resulting system is composed of 164 atoms, and the ab initio calculations were performed on a 3 × 3 × 1 k-point grid. To map the binding energies (Ebind) on the surface, the anisole was translated above the surface with the carbon of the methoxy moiety 2.5 Å above the apical iodides of the corner-sharing PbI6 octahedra. During this process the surface atoms and the internal degrees of freedom of the anisole were not allowed to relax. The high symmetry points of the surface (labeled A, B, and C), shown in Figure 6, were used as references for the analysis whose results are presented in the upper graph of Figure 6a. The top of the PbI6 octahedron, site C, is the most unfavorable site for binding on the surface. From that site there are two distinct paths

Figure 4. Snapshots of the molecular dynamics simulation carried out at 300 K on the symmetrical Surf-3/4 (Figure 2b); two viewpoints are presented.

atomic displacements of the inorganic cage at the top surface (Surf-3) are small and the MA cation moves around its original orientation in the cage, which is not much deformed. On the opposite surface (Surf-4), however, the inorganic cage undergoes a larger deformation and the MA cation rotates loosely inside the cage. The agreement obtained between the calculations carried out for the 1 × 1 × 7 symmetric (S) and for the 1 × 1 × 8 asymmetric (A) slabs evince the convergence of the results and the independence of the (001) surfaces. Adsorption of Methoxybenzene. In the most efficient perovskite solar cells, including the record-breaking ones,19,23 the holes are collected by the organic hole transport material (HTM) spiro-MeOTAD, whose chemical structure is shown in Figure 5a. Other polymeric hole conductor materials have been tested,37 especially the polytriarylamine derivatives, which are bulky and steric and, therefore, difficult to describe from the 26950



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almost identical; they are described by the solid and dashed lines of the graph. From the central site A to site B, located above the corner-sharing iodine atom, there are also two different paths, which yield almost identical binding energies as well. The independence on the paths demonstrates that the orientation of subsurface MA cations does not influence the adsorption process. Finally, by returning to site C, the binding energy increases again. Then we placed the methoxybenzene above each of the high symmetry sites and varied its height with respect to the surface, that is, by varying the z coordinate that is calculated as z = zC − zI, where zC and zI designate the carbon of the methoxy moiety and the apical I− of the PbI6 octahedra, respectively. The ensuing binding energies are shown in the lower graph of Figure 6a for the anisole placed on site A (black circle), B (blue square), and C (red triangle). Although each of the three curves shows a minimum, the global minimum is located on site A, in the interstice of the corner-sharing PbI6 octahedra. Since no relaxation effects were allowed, these binding energies have to be understood as estimates to the real binding energies. The binding curve for site A exhibits the global minimum, which provides a firm binding of Ebind ≈ 0.4 eV (38.6 kJ/mol) for 0.5 Å < z < 1 Å. The second minimum, Ebind ≈ 0.24 eV (23.1 kJ/mol), occurs for the binding curve at site B. The third shallow minimum is found at site C, with Ebind ≈ 0.2 eV (19.3 kJ/mol). It is possible, therefore, to assert that the most probable binding mode for the methoxybenzene molecule is at site A, just above the octahedral peaks. Moreover, since the binding curves merge with each other, the adsorption path to the global minimum is either through C → B → A or directly from C → A, as depicted in Figure 6b. After identifying the position of the global minimum, a relaxation simulation is carried out using a 6 × 6 × 1 k-point grid, in which the surface atoms and the internal degrees of freedom of the methoxybenzene are allowed to relax. The ensuing structure is shown in Figure 7. Because of the

Figure 6. Position-dependent binding energy (Ebind) of the upright methoxybenzene molecule on the surface of the octahedra-terminated surface of the CH3NH3PbI3 perovskite (see Figure 7 for visual reference). (a) The top panel presents Ebind calculations performed with the methoxy group of the anisole molecule at a height of 2.5 Å above the top of the surface PbI6 octahedra, where it is translated through the path C→ A → B → C, as indicated by the solid and the dashed lines in the inset. The lower panel of (a) shows Ebind as a function of the height (z coordinate) at the symmetry points A (black circles), B (blue squares), and C (red triangles). (b) Schematic representation of the global (on point A) and local (B and C) minima of the adsorption potential. (c) Charge density isosurfaces of the methoxybenzene.

toward the center of the four corner-sharing octahedra, site A, due to the orientation of the MA cation along the [111] direction under the surface. Although the MA cations in a real situation can be randomly oriented, we chose to maintain this orientation to verify the influence of subsurface cations on the adsorption process. The binding energy along both paths are

Figure 7. View of the geometry relaxation on the (001) octahedral surface of the NC3CH3PbI3 perovskite upon the adsorption of the methoxybenzene at the interstice of the corner-sharing octahedra. The colored ball-and-stick structure describes the structure after the relaxation, whereas the orange lines represent the unrelaxed structure. The same surface is presented from two lateral viewpoints. 26951



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iodide perovskites with the goal of producing low-dimensional materials, that is, 2D-layers or 1D-chains, depending on the size of the incorporated hydrocarbons. The difference of binding ammonium cations, in regard to the present methoxy anchors, is that the former are not repelled by the I− and can penetrate the octahedral structure to make as much as three H-bonds with the nearby iodides.

interaction with the anisole, the surface octahedra are considerably distorted but the height of the binding site changes only by a little to z ≈ 0.35 Å. Based on the analysis of the partial charges (see Figure 6c), it is possible to ascribe the distortion of the superficial (001) PbI6 to a couple of factors: (1) the electrostatic repulsion between the negatively charged methoxy moiety (−0.44 |e|) with the apical iodides and (2) the attractive electrostatic interaction between the methoxy and the Pb2+. The Supporting Information presents calculation results of the atomic partial charges of the anisole molecule in vacuum and adsorbed on the PbI6-terminated MAPbI3 surface. We also investigated the binding energies for the methoxybenzene on the flat metal−halide (001) surface that is terminated on pentacoordinated lead atoms (Pb5C). The results are presented in Figure 8. The symmetry sites used for

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CONCLUSIONS The paper presented a study of relaxation effects on the (001) surfaces of the pseudocubic CH3NH3PbI3 hybrid perovskite and investigated the adsorption modes for methoxy-based anchors with the goal of providing insights for the perovskite/ HTM interface. The relaxation of four different surfaces was described. The initial surface morphologies were characterized by the termination of the inorganic PbI3 cage as well as by the orientation of the organic cation CH3NH3 with respect to the inorganic structure. It was observed that the surface relaxation is driven by a competition of three effects: shrinkage of the inorganic cage on the flat PbI2 (001) surfaces; the need to maximizeas much as threehydrogen bonds between the NH3 group of the MA cation and the bridging iodides; dipole alignment of the CH3NH3+ molecules on the superficial layers. The adsorption of methoxybenzene was studied to provide information on the attachment mechanism and binding modes of bigger hole transport compounds to the surface of the perovskites for the typical spiro-MeOTAD and for polytriarylamine derivatives in general. The results demonstrated that adsorption can occur with stability only on the (001) surface terminated on PbI6 octahedra, where the methoxy group finds a stable minimum in the interstice of four corner-sharing octahedra, corresponding to an adsorption energy of ∼38 kJ/ mol, just above (ca. 0.35 Å) the surface of the perovskite. The methoxybenzene can reach this adsorption site directly or by passing through intermediary local minima which avoid the direct proximity with the iodine ions. The flat PbI2 (001) surface, however, presents no stable minimum for the adsorption of the methoxybenzene and should not provide attachment for any HTM with a methoxy anchor. It is observed that the adsorption of the methoxybenzene is not influenced by the orientation of the CH3NH3 cations underneath the surface. The attachment mechanism is governed by the electrostatic interaction of the methoxy group with the surface, which is repulsive with the iodides and attractive with the Pb(II) ion.

Figure 8. Position-dependent binding energy (Ebind) of the upright methoxybenzene molecule on the surface terminated with the pentacoordinated Pb5C of the CH3NH3PbI3 perovskite (Figure 2b). (a) The top panel presents Ebind calculations performed with the methoxy group of the anisole molecule at a height of 2.5 Å above the top of the surface PbI6 octahedra, where it is translated through the path C→ A → B → C, as indicated by arrows in (b). The lower panel of (a) shows Ebind as a function of the height (z coordinate) at the symmetry points A (black circles), B (blue squares), and C (red triangles). (b) Schematic representation of the symmetry points A, B, and C on the surface.

mapping the binding on this surface are the center of the rectangular PbI2 network (A), the bridging iodide site (B), and the Pb5C site (C). The calculated binding energies, displayed in the graphs of Figure 8a, show that there is no stable adsorption site for the methoxybenzene on this surface. It is observed a strong repulsion between the methoxy and the bridging iodide on site B, as expected from the results obtained for the octahedral surface. Moreover, the four iodine atoms surrounding the Pb5C hinder the binding on sites C and A as well. Therefore, we conclude that different anchoring groups should to be used for a close attachment of the polytriarylamine derivatives to the metal−halide (001) flat surfaces. The insights provided by this study are general and can be used to understand the adsorption of the methoxybenzene on other MAPbI3 surfaces as well as to guide the design of different polymeric HTM. Previous studies16,17,40 have investigated the incorporation of hydrocarbons linked to ammonium cations on hybrid lead

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ASSOCIATED CONTENT

S Supporting Information *

Interatomic distances and partial charges of the pseudocubic CH3NH3PbI3 bulk unit cell; additional details of the relaxed (001) surfaces Surf:1−4; partial charges at the atoms of the methoxybenzene in vacuum and adsorbed onto the PbI6− terminated MAPbI3 surface Surf-1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail lrego@fisica.ufsc.br (L.G.C.R.). Notes

The authors declare no competing financial interest. 26952



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ACKNOWLEDGMENTS The authors are indebted to the Brazilian National Counsel of Technological and Scientific Development (CNPq) and the Coordenaçaõ de Aperfeiçoamento de Pessoal de Nvel Superior (CAPES) for funding the project.

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