This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes. Article Cite This: J. Am. Chem. Soc. 2019, 141, 3515−3523 pubs.acs.org/JACS Unraveling the Impact of Halide Mixing on Perovskite Stability Jeremy Hieulle,†,⊥ Xiaoming Wang,‡,⊥ Collin Stecker,† Dae-Yong Son,† Longbin Qiu,† Robin Ohmann,†,# Luis K. Ono,† Aitor Mugarza,§,∥ Yanfa Yan,*,‡ and Yabing Qi*,† † Energy Materials and Surface Sciences Unit (EMSSU), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa 904-0495, Japan ‡ Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States § Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain ∥ ICREAInstitució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain * S Supporting Information ABSTRACT: Increasing the stability of perovskites is essential for their integration in commercial photovoltaic devices. Halide mixing is suggested as a powerful strategy toward stable perovskite materials. However, the stabilizing effect of the halides critically depends on their distribution in the mixed compound, a topic that is currently under intense debate. Here we successfully determine the exact location of the I and Cl anions in the CH3NH3PbBr3−yIy and CH3NH3PbBr3−zClz mixed halide perovskite lattices and correlate it with the enhanced stability we find for the latter. By combining scanning tunneling microscopy and density functional theory, we predict that, for low ratios, iodine and chlorine incorporation have different effects on the electronic properties and stability of the CH3NH3PbBr3 perovskite material. In addition, we determine the optimal Cl incorporation ratio for stability increase without detrimental band gap modification, providing an important direction for the fabrication of stable perovskite devices. The increased material stability induced by chlorine incorporation is verified by performing photoelectron spectroscopy on a half-cell device architecture. Our findings provide an answer to the current debate on halide incorporation and demonstrate their direct influence on device stability. ■ INTRODUCTION Metal halide perovskite materials have been the focus of (DFT), and UV/X-ray photoelectron spectroscopy (UPS/ XPS), we reveal the exact location of I and Cl anions in the intense investigation due to their low cost and superior perovskite structure at the atomic scale, and demonstrate the photovoltaic performance, with record power conversion impact of halide incorporation on the material electronic efficiencies reaching 23.7%.1 Despite numerous advantages, properties and stability. Furthermore, we determine the their poor stability hinders commercialization of perovskite- optimal Cl incorporation ratio for stability increase without based devices. To increase perovskite stability various detrimental band gap modification. strategies have been envisaged.2,3 Mixing different halides (I, Despite the great capability of STM and UPS as Br, Cl) has been shown both experimentally and theoretically demonstrated in the current study, it is important to point to have a strong impact on the device performance and out that there are some limitations associated with such surface stability.4−9 For example, a small addition of bromine10 or sensitive characterization techniques. Combined with suitable chlorine11 confers increased device stability compared to pure DFT calculations, STM can be used to investigate mainly the iodide-based (CH3NH3PbI3) devices. Addition of Cl to surface and possibly also the subsurface structures of CH3NH3PbBr3 films improves carrier recombination lifetime12 perovskite materials. The probing depth of photoelectron and increases open circuit voltage.13,14 However, the role of Cl spectroscopy (PES) depends on the energy of photoelectrons, and the extent of its incorporation into the crystal lattice but in general it is still considered to be surface sensitive as it remain a topic of debate, with conflicting results reported in can only probe up to approximately 5 nm of the top surface of the literature.15−21 the sample. Therefore, our current surface science study is not Here, we choose to mix CH3NH3PbBr3 perovskite with two an attempt to provide a full picture of what it is happening in different halides, I and Cl, which have different ionic radii and the bulk material of a solar cell device. On the other hand, bond strengths with Pb. In this way, the effect of the although our results may not accurately represent the interior incorporation of a larger ion (I > Br) and a smaller ion (Cl < Br) can be addressed. In this work, combining scanning Received: October 22, 2018 tunneling microscopy (STM), density functional theory Published: January 15, 2019 © 2019 American Chemical Society 3515 DOI: 10.1021/jacs.8b11210 J. Am. Chem. Soc. 2019, 141, 3515−3523 Journal of the American Chemical Society Article Figure 1. Halide substitution at the perovskite surface. Scanning tunneling microscopy images of (a) CH3NH3PbBr3, (b) CH3NH3PbBr3−yIy, and (c) CH3NH3PbBr3−zClz perovskite surfaces. (d−f) Calculated (010) surface of the mixed-halide organic−inorganic perovskites. Inset: corresponding surface structure and unit cell. The lattice unit cell of pristine CH3NH3PbBr3 is square with the parameters A = B = 7.4 Å, and was found to not be altered by the deposition of PbI2 or PbCl2 molecules. The sizes of Br, I, and Cl ions were multiplied by a factor of 4.5 for better visibility. (g) Scheme of the substitution mechanism occurring at the surface of CH3NH3PbBr3 perovskite after deposition of PbI2 or PbCl2 molecules (only the PbI2 case is presented for clarity, but PbCl2 is following the same mechanism). Image parameters: (a) sample bias = +1.3 V, tunneling current = 80 pA; (b) +2.0 V, 120 pA; (c) −1.9 V, 19 pA; image size, (a−c) 2.3 nm × 2.3 nm. of the perovskite films as in real solar cell absorbers, our study does offer valuable insights on other aspects such as surface/ ■ RESULTS AND DISCUSSION CH3NH3PbBr3 perovskite thin films were prepared on a clean interface properties. Several studies have reported that surface/ Au(111) surface28 via dual-source co-evaporation in an interface properties themselves play an important role in ultrahigh vacuum environment (see the Methods section). The thickness of the ultrathin film was approximately 4 ± 1 determining perovskite solar cell performance.22,23 For nm. Scanning tunneling microscopy images reveal a pair motif example, it has been shown that the interface between the similar to the characteristic surface reconstruction previously perovskite film and the hole transport layer (HTL) is crucial observed for CH3NH3PbBr3 single crystals29−31 showing the (010) surface termination (Figure 1a). The bright spots in the for achieving high solar cell device performance.24−27 The aim STM images correspond to bromine ions of the top layer of our current study combining the use of STM, DFT perovskite structure. For pure CH3NH3PbBr3 all the bromine calculations, and PES is therefore to obtain a better ions have the same height and width in the STM images. understanding of these interfacial properties by examining Thereafter, PbI2 or PbCl2 was deposited on the pure CH3NH3PbBr3 film kept at room temperature (see the their electronic structure and morphology with an unprece- Methods section). Distinct protrusions with different apparent dented high spatial resolution. height and width appear on the perovskite film surface (Figure 3516 DOI: 10.1021/jacs.8b11210 J. Am. Chem. Soc. 2019, 141, 3515−3523 Journal of the American Chemical Society Article Figure 2. Atomic-resolution surface structures of mixed-halide perovskites. Large area STM images (10 nm × 10 nm) of the (010) surface of (a) the pristine CH3NH3PbBr3, (b) CH3NH3PbBr3−yIy, and (c) CH3NH3PbBr3−zClz mixed-halide perovskites. Scan parameters: (a) sample bias = +1.3 V, tunneling current = 80 pA; (b) +2.0 V, 120 pA; (c) −2.0 V, 100 pA. Inset: fast Fourier transform (FFT) results obtained from the topographic STM images showing no additional peak at low k-values for the mixed-halide perovskite, suggesting that there is no long-range order of the additional iodine and chlorine ions at the surface. (d−f) Histogram of the apparent height distribution of local maxima (ions). The main peaks correspond to the Br ions, while the small additional peaks are associated with iodine and chlorine ions (e and f, respectively). Inset: typical profiles obtained for the different halides (Br, I, Cl) at the perovskite surface. (g−i) Relative positions of the halide ions with respect to the top CH3NH3Br structure. The dash lines in side view indicate the differences in the height calculated by DFT for the I ion (+0.35 Å in part h) and Cl ion (−0.24 Å in part i) with respect to the Br ion. The values calculated by DFT are in good agreement with the experimentally measured differences in the STM apparent height (in parts d−f). Color code: Pb (gray), Br (brown), I (purple), Cl (green), C (black), N (blue), H (light pink). 1b,c). After deposition of PbI2, strong bright spots appear with projected density of states (PDOS), of the perovskite films an estimated height of 40 ± 10 pm higher than the were determined using the Vienna ab initio simulation package surrounding Br ions and with larger diameters (Figure 1b). (VASP) code (see the Methods section). First the pair In contrast, after PbCl2 deposition, slightly darker spots are structure of the (010) top layer surface of CH3NH3PbBr3 evident, with smaller diameters and a lower apparent height perovskite was reproduced using DFT and taken as a reference (Figure 1c). These dark spots appear 20 ± 10 pm lower than (Figure 1d). Thereafter the impact of PbI2 and PbCl2 the neighboring bromine ions. deposition on the atomic structure of the perovskites was To unravel the origin of the dark and bright protrusions further addressed by DFT. Different scenarios were consid- observed on the surface of the perovskite films, we performed ered, such as the adsorption of the entire PbI2/PbCl2 DFT calculations. The atomic structures, as well as the molecules (Figure S1), as well as the adsorption (Figure S2) 3517 DOI: 10.1021/jacs.8b11210 J. Am. Chem. Soc. 2019, 141, 3515−3523 Journal of the American Chemical Society Article or substitution of Pb, I, and Cl on the CH3NH3PbBr3 surface perovskite, leading to the disordered surface structure observed (Figure 1). The best match between the STM experimental in the experiment. results and DFT simulation is found for the dissociation of the Ultraviolet photoemission spectroscopy (UPS) experiments PbI2 (or PbCl2) molecule, followed by the substitution of Br were performed on the mixed-halide perovskites to correlate by I (or Cl). The schematic drawing for the iodine case is their atomic structures with their electronic properties (Figure shown in Figure 1g as an example. We thus assign the bright 3). The valence band (VB) spectra of the perovskite films are and dark protrusions observed in our STM experiment, respectively, to I and Cl ions, which substitute Br ions at the surface of the perovskite film (Figure 1e,f). These scenarios are the most energetically favorable ones, and the corresponding simulated STM images best reproduce our experimental data (supplementary note 1, in the SI). Thus, by depositing PbI2 or PbCl2 on a pristine CH3NH3PbBr3 film, two different mixed- halide perovskites, CH 3 NH 3 PbBr 3−y I y and CH 3 NH 3 - PbBr3−zClz, were formed. Large-scale STM images (Figure 2a−c) of the surface topography show that the additional bright and dark spots observed after PbI2 and PbCl2 deposition are randomly distributed at the surface of the perovskite films without any long-range order (Figure 2b,c). These findings are supported by the fast Fourier transform (FFT) analyses of the STM images, which do not show any additional peak at low k-values for the mixed-halide perovskites compared to pristine CH3NH3PbBr3 (insets in Figure 2a−c). The distribution histogram of the different apparent heights that are measured at the surface of the three films is given in Figure 2d−f. While for the pristine perovskite only a single peak is present corresponding to Br ions, additional peaks show up for the mixed-halide perovskites corresponding to I and Cl ions, respectively. Surprisingly, the substitution of Br by I or Cl does not significantly change the lattice unit cell (i.e., lateral structure) of the perovskite surface. However, the vertical positions of the I and Cl were found to be different from the Br ions in pristine CH3NH3PbBr3 (Figure 2g−i). The iodine ions are slightly upshifted with respect to the Br plane (Figure 2h). Additionally, the iodine ions have a larger ionic radius than bromine. Both the upshift and larger radius contribute to the observed topographic height increase. In contrast, the chlorine ions are downshifted and have a smaller ionic radius compared to Br (Figure 2i) explaining their lower apparent height. These experimentally observed STM apparent height differences are further confirmed by the DFT calculations (Figure 2h,i and Table S1). Besides the single bright and dark protrusions, which are attributed to the substitution of Br with I or Cl, we observed pairs and clusters of bright and dark protrusions (Figure S3). They are attributed to I or Cl ions that substitute multiple Figure 3. Valence band modification in mixed-halide perovskite. (a) Experimental ultraviolet photoemission spectroscopy (UPS) spectra nearby bromine ions of the perovskite surface (Figure S4). of pure CH3NH3PbBr3 (orange) and mixed CH3NH3PbBr3−yIy Two types of pairs are found on the surface. Iodine and (purple), CH3NH3PbBr3−zClz (green) perovskites. (b) Calculated chlorine pairs can be formed either by substitution of two PDOS of the pristine and mixed-halide perovskites. The thick curves bromine ions within the same pair, or by the substitution of represent the summation of all contributions of each orbital indicated two neighboring Br ions within two adjacent pairs (Figure S3). by differently colored lines. The PDOS were obtained for the full DFT calculations show that the chemical nature of the halides halide substitution of the perovskite surface. strongly influences the stability of the pair. The energy barrier for the formation of two isolated Cl−Br pairs is lower than the composed of 6 main peaks (A−F) as shown in Figure 3a. A energy formation of a single Cl−Cl pair (E2(Cl−Br) < ECl−Cl). strong modification of the valence band line shape can be These results suggest that at low coverage the chlorine ions observed when substituting bromine ions at the surface with prefer to form isolated Cl−Br pairs rather than clustering iodine or chlorine. Based on DFT calculations, we found that together to form Cl−Cl pairs or phase segregating. The same the VB of pristine CH3NH3PbBr3 is composed mainly of Br px, trend was found for the iodine case, i.e., E2(I−Br) < EI−I (Table py, and pz hybridized states with a small contribution from the S1). This is the reason why the substituting halide ions were Pb s and p orbitals (Figure 3b). In addition to these states, mainly found to be randomly distributed at the surface of the contributions of I and Cl appear in the VB of mixed-halide 3518 DOI: 10.1021/jacs.8b11210 J. Am. Chem. Soc. 2019, 141, 3515−3523 Journal of the American Chemical Society Article Figure 4. Stability versus band gap change in mixed perovskite. Decomposition energy difference between mixed-halide and pristine perovskite determined by DFT calculations (see the Methods section): (a) CH3NH3PbBr3−yIy with increasing iodine content (y) at the surface, and (b) CH3NH3PbBr3−zClz with increasing Cl content (z) at the surface. The calculations were performed on a 6 layer slab with only the top layer being substituted. Bottom panels show the atomic structure of the pristine, half-mixed, and completely substituted top surface of the perovskite. Color code: Br (brown), I (purple), Cl (green), C (black), N (blue), H (light pink). perovskites. The substituting halides are responsible for the respect to external stimuli such as temperature, X-ray beam valence band modification as observed in the experiment. A damage, etc. This stability increase is related to the stronger higher binding energy was found for Cl as compared to Br and bond strength of Cl−Pb compared to Br−Pb and I−Pb.35,36 I (Figure 3b), in line with the higher stability of the chlorine- Beyond the substitution ratio of 25%, the strain induced by the mixed perovskite (CH3NH3PbBr3−zClz) as discussed later. incorporation of the smaller Cl ions with respect to Br Additionally, the impact of halide substitution on the material counterbalances the benefit of the stronger Cl−Pb bond work function was revealed by UPS. A work function (ϕ) of strength and results in a lowering of the material stability 4.77 eV was measured for pristine CH3NH3PbBr3, similar to (decomposition energy). Thus, the stability in mixed-halide the values reported in the literature,32−34 while a decrease of perovskite results from the interplay between the Pb−halide 0.2 eV of the work function was observed after chlorine bond strength that stabilizes the material and the strain substitution (ϕ = 4.57 eV). Interestingly with iodine induced by the halide substitution that lowers decomposition incorporation in the perovskite film, the work function was energy. Based on our results, we suggest that, as a general 4.79 eV, similar to the value of pristine CH3NH3PbBr3 (Figure trend, substitution of the original surface halide ions with a S5). We found that the work function changes originate from a smaller ion size and a stronger Pb-halide bond strength leads to downshift of the vacuum level induced by a modification of the an increase of the decomposition energy of the system, which surface dipole moment after halide incorporation (Figure S6). in turn increases the material stability. Our DFT finding shows A larger dipole moment amplitude results in a stronger a stability increase without a strong band gap modification. downshift of the vacuum level, and thus a smaller work Although here only the substitution at the surface is function (ϕI > ϕBr > ϕCl). The possibility of adjusting the work considered, these results indicate that there may exist an function as well as the vacuum level position through halide optimal bulk Cl substitution ratio that would provide higher substitution is of prime interest for engineering energy level stability without substantially changing the band gap of the alignment in perovskite-based devices. perovskite material. To further understand the impact of halide substitution on To further confirm these findings experimentally, we have the electronic properties of perovskite materials and its studied the stabilization effect of chlorine on an actual solar cell correlation with the stability, we evaluated the changes in the device architecture (Figure S7). First, a CH3NH3PbBr3 film decomposition energy (see the Methods section) and band gap with a thickness of approximately 300 nm was spun on a TiO2/ of the CH3NH3PbBr3−yIy and CH3NH3PbBr3−zClz films for FTO/glass substrate (FTO, fluorine-doped tin oxide; see the different iodine and chlorine ratios. As depicted in Figure 4a, Methods section), which resembles half (up to the perovskite the substitution of bromine ions by iodine at the surface of the layer) of a standard perovskite solar cell structure. Thereafter, perovskite induces a decrease of the decomposition energy the half-cell architecture was transferred into the UHV system, (purple curve) and a small reduction of the band gap (orange where chlorine was incorporated into the perovskite film by curve). In contrast, Figure 4b shows that the incorporation of a PbCl2 vacuum evaporation. The degradation was then small amount of chlorine (below 25%, green curve) leads to an investigated by X-ray photoelectron spectroscopy (XPS) for increase of the decomposition energy, with no substantial band the half-cell samples with different Cl substitution ratios at the gap changes. surface (Figure 5). First, the evolution of the Pb 4f peak was An increase of the decomposition energy after chlorine monitored for the pure CH3NH3PbBr3 film without the substitution suggests higher stability of the perovskite film with incorporation of Cl. Figure 5a shows that initially (at T = 0 h), 3519 DOI: 10.1021/jacs.8b11210 J. Am. Chem. Soc. 2019, 141, 3515−3523 Journal of the American Chemical Society Article Figure 5. Beneficial aspects of halide mixing on perovskite stability. Time evolution of the Pb 4f peak obtained by X-ray photoelectron spectroscopy (XPS) on a device architecture (a) without addition of chlorine, (b) with 12% of Cl at the surface, and (c) with 18% Cl at the surface. (d) Appearance time of Pb(0) feature with respect to the Cl substitution ratio at the surface. (e) Time evolution of the percentage of Pb(2+) remaining for the different Cl substitution ratio at the surface. the spectrum is composed of only two peaks at 137.9 and Substitution ratios were determined by measuring the Cl:Br 142.8 eV, attributed to the spin−orbit split Pb 4f7/2 and Pb chemical ratio in XPS. A quantitative analysis of the remaining 4f5/2 peaks, later referred to as Pb(2+). After 4 h, additional Pb(2+) species (i.e., undegraded Pb ions on the surface) after peaks appear at lower binding energies associated with the a given time is presented in Figure 5e. The results demonstrate lower oxidation Pb(0) state. Such a state is associated with experimentally that the incorporation of Cl improves stability deterioration of the perovskite film.37 Therefore, in the present of perovskite films. It has been reported that CH3NH3PbBr3 study we use the Pb(0) peak evolution as an indicator of the shows significantly better stability than CH3NH3PbI3.38 Here perovskite degradation. A progressive and strong increase of in this study, we have shown that, with the surface Br ions of the Pb(0) peak intensity is observed as a function of time, CH3NH3PbBr3 partially substituted by Cl ions, the half-cell indicating that the pure CH3NH3PbBr3 film undergoes a fast sample stability gets further enhancement. Such a result is degradation. In contrast, much slower degradation (i.e., a significantly reduced Pb(0) peak intensity after a given time) expected to have strong implications for device applications. was observed for the samples with the additional PbCl2 Although the current study focuses on the CH3NH3PbBr3 case, evaporation (Figure 5b,c). The degradation process is further the stability increase induced by Cl incorporation is expected slowed down as the amount of chlorine substitution increases. to be even stronger in the CH3NH3PbI3 perovskite case, due to When 12% of the Br ions are substituted by Cl at the a larger difference in bond strength between Pb−Cl and Pb−I perovskite surface, the Pb(0) peak emerged after 28 h, while it when compared to Pb−Cl and Pb−Br. Finding the balance only appears after 116 h for a 18% substitution ratio (Figure between stability increase and band gap modification in Cl- 5d). For comparison, the surface substitution ratio at the incorporated CH3NH3PbI3 is expected to play an important perovskite surface in the STM image of Figure 2c is about 26%. role in optimizing CH3NH3PbI3 perovskite-based devices. 3520 DOI: 10.1021/jacs.8b11210 J. Am. Chem. Soc. 2019, 141, 3515−3523 Journal of the American Chemical Society Article ■ CONCLUSION We have determined for the first time the surface atomic surfaces of CH3NH3PbBr3 (MAPbBr3). From the electronic ground- state structure, STM images were calculated with the p4vasp code. To evaluate the stability of the mixed perovskite, we consider the structure of CH3 NH 3PbBr 3−y Iy and CH3 NH3PbBr 3−zClz following reaction: mixed-halide perovskites in real space and correlate it to the 2MAPbBr3 + y PbX 2 → 2MAPbBr3 − yX y + y PbBr2 stability of the corresponding compound. Using scanning tunneling microscopy combined with density functional theory where X is Cl or I, and y/3 is its ratio. The decomposition energy of we find that Br ions are substituted by I or Cl ions. The the mixed perovskite 2MAPbBr3−yXy is calculated as substituted halides are mainly randomly distributed at the y surface, showing a disordered surface structure with no phase Edec = E(MAPbBr3 − yX y) − E(MAPbBr3) + [E(PbBr2) − E(PbX 2)] 2 segregation. For the low substitution ratio (below 25%), I and The stability of the perovskite is characterized by the decomposition Cl substitutions have different effects on the work function and energy (Edec). When the decomposition energy is larger, the material stability of the mixed perovskites. Our studies on half-cell is more stable. device architectures demonstrate that the stability is increased Device Fabrication. The patterned FTO glass (Pilkington, TEC- after incorporating 12−18% of Cl, which agrees with the 8, 8Ω/sq) was cleaned with detergent and DI water and sonicated calculated increase of the decomposition energy. Our results with ethanol in an ultrasonic bath for 20 min. UV-ozone was treated suggest that there is an optimal ratio of chlorine substitution for 15 min prior to use. The TiO2 (ca. 50 nm) layer was deposited by which provides an increase in stability without significant band spin-coating at 2000 rpm for 20 s (acceleration was 500 rpm/s) using gap modification. This balance is an important direction for a diluted TiO2 solution in 1-butanol.48 The deposited TiO2 film was annealed at 550 °C for 30 min. The CH3NH3PbBr3 film was prepared future device research, to address the perovskite stability issue, via the adduct method.49 0.367 g of PbBr2 (TCI) and 0.112 g of which remains as one of the main challenges for industrial CH3NH3Br (Dyesol) were dissolved in 0.325 mL of DMF with 225 applications. μL of DMSO at 60 °C. The CH3NH3Br3 solution was spin-coated on ■ the TiO2 substrate at 4000 rpm for 20 s, and 0.3 mL of diethyl ether METHODS was poured when 10 s remained. Then, it was dried on a hot plate at 100 °C for 20 min. ■ Surface Characterization. CH3NH3PbBr3 perovskite films were grown on a clean Au(111) single crystal using dual-source co- ASSOCIATED CONTENT evaporation, following a previously reported method.28 Vacuum deposition techniques have been used to fabricate perovskite solar cell * S Supporting Information devices with similar power conversion efficiencies to those fabricated The Supporting Information is available free of charge on the by solution-based deposition methods.39−41 In addition, the use of ACS Publications website at DOI: 10.1021/jacs.8b11210. vacuum deposition techniques ensures the sample’s cleanliness and Adsorption versus substitution in mixed-halide perov- high purity (e.g., free from solvent molecules), which facilitates further atomically precise characterization with scanning tunneling skites, halide pairs and clusters on the surface of mixed microscopy. First, the gold sample was cleaned in an ultrahigh vacuum perovskites, determination of the valence band max- environment (∼1 × 10−9 Torr) with several cycles of Ne+ sputtering imum (VBM), electrostatic potential and work function followed by subsequent annealing at 773 K for 5 min. Thereafter, changes, and half-cell architecture (PDF). ■ CH3NH3PbBr3 was obtained by dual-source co-evaporation of CH3NH3Br (MABr) and PbBr2 molecules at 361 and 498 K, AUTHOR INFORMATION respectively, for 10 min. During deposition, the gold surface was kept at 130 K to ensure the adhesion of MABr, and then annealed at room Corresponding Authors temperature for at least 3 h. Mixed perovskites were obtained by *Yabing.Qi@OIST.jp sublimating PbI2 and PbCl2 on the CH3NH3PbBr3 film from Knudsen *Yanfa.Yan@utoledo.edu cells held at 525 and 560 K, respectively. During sublimation, the ORCID sample was kept at room temperature. Low-temperature scanning tunneling microscopy (LT-STM) was used to characterize the atomic- Jeremy Hieulle: 0000-0003-4891-4007 scale surface structures of the different perovskite ultrathin films. The Xiaoming Wang: 0000-0002-5438-1334 STM measurements were performed at 5 K. A cut Pt/Ir tip was used Robin Ohmann: 0000-0003-2483-1282 to acquire the STM images. The bias voltage was applied to the Aitor Mugarza: 0000-0002-2698-885X sample. X-ray photoelectron spectroscopy (XPS) and ultraviolet Yanfa Yan: 0000-0003-3977-5789 photoemission spectroscopy (UPS) were performed in the same ultrahigh vacuum (UHV) setup as the STM measurements, Yabing Qi: 0000-0002-4876-8049 preventing any contamination from air. Present Address # Density Functional Theory. The calculations were performed R.O.: Department Physik, Universität Siegen, 57068 Siegen, using the VASP code42−44 with the projector augmented wave Germany. (PAW)45 potential and Perdew−Burke−Ernzerhof (PBE)46 ex- Author Contributions change-correlation functional. For the unit cell, Brillouin zone (BZ) ⊥ sampling was done using a 4 × 4 × 4 Monkhorst−Pack grid. The J.H. and X.W. contributed equally. CH3NH3PbBr3 (010) surfaces were modeled using (2 × 2) slabs Notes containing six atomic layers. The size of the supercell was (16.24 × The authors declare no competing financial interest. 16.54 × 35.52) Å. A large vacuum thickness of 20 Å along the direction normal to the surface was employed to separate surfaces from their periodic images. The bottom four layers of atoms were kept fixed at their optimized bulk positions. All other atoms were fully ■ ACKNOWLEDGMENTS This work was supported by funding from the Energy relaxed until the forces on each atom were smaller than 0.01 eV/Å. Materials and Surface Sciences Unit of the Okinawa Institute The nonlocal van der Waals interactions were evaluated with the of Science and Technology Graduate University, the OIST optB86-vdW functional.47 A Γ-centered Monkhorst−Pack sampling R&D Cluster Research Program, the OIST Proof of Concept of 3 × 3 × 1 was adopted to study the electronic properties of the (POC) Program, and JSPS KAKENHI Grant JP18K05266. 3521 DOI: 10.1021/jacs.8b11210 J. Am. Chem. Soc. 2019, 141, 3515−3523 Journal of the American Chemical Society Article The work at The University of Toledo was supported by the (15) Ono, L. K.; Juarez-Perez, E. J.; Qi, Y. B. Progress on perovskite Center for Hybrid Organic Inorganic Semiconductors for materials and solar cells with mixed cations and halide anions. ACS Energy (CHOISE), an Energy Frontier Research Center Appl. Mater. 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