This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see: https://dx.doi.org/10.1021/jacs.8b11210. Unraveling the impact of halide mixing on perovskite stability Jeremy Hieulle1,‡, Xiaoming Wang2,‡, Collin Stecker1, Dae-Yong Son1, Longbin Qiu1, Robin Ohmann1,†, Luis K. Ono1, Aitor Mugarza3,4, Yanfa Yan2,*, and Yabing Qi1,* 1 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. 2 Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The Uni- versity of Toledo, Toledo, Ohio 43606, United States. 3 Catalan Institute of Nanoscience and Nanotechnology (ICN2), Consejo Superior de Investigaciones Científicas (CSIC) and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain. 4 ICREA–Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain. 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 in the mixed CH3NH3PbBr3-yIy and CH3NH3PbBr3-zClz perovskite lattices and correlate it with the enhanced stability we find for the latter. By combining scanning tunneling microscopy (STM) and density functional theory (DFT), we predict that for low ratios, iodine and chlorine incorporation have opposite effects on the electronic properties and stability of the CH3NH3PbBr3 perovskite material. In addition, we determine an ideal halide ratio for stability increase without detrimental bandgap modification, providing an important direction for the fabrication of stable perovskite devices. The increased material stability in- duced by chlorine incorporation is verified by performing photoelectron spectroscopy on a device architecture. Our findings provide an answer to the current debate on halide incorporation and demonstrate their direct influence on device stability. a smaller ion (Cl < Br) can be addressed. In this work, combin- INTRODUCTION ing scanning tunneling microscopy (STM), density functional Metal halide perovskite materials have been the focus of in- theory (DFT) and UV/x-ray photoelectron spectroscopy tense investigation due to their low cost and superior photovol- (UPS/XPS), we reveal the exact location of I and Cl anions in taic performance, with record power conversion efficiencies ex- the perovskite structure, and demonstrate the impact of halide- ceeding 23.3%1. Despite numerous advantages, their poor sta- incorporation on the material electronic properties and stability. bility hinders commercialization of perovskite-based devices. Furthermore, we determine the ideal Cl-incorporation ratio for To increase perovskite stability various strategies have been en- stability increase without detrimental bandgap modification. visaged2,3. Mixing different halides (I, Br, Cl) has been shown both experimentally and theoretically to have a strong impact RESULTS AND DISCUSSION on the device performance and stability4-9. For example, a small CH3NH3PbBr3 perovskite thin films were prepared on a clean addition of bromine10 or chlorine11 confers increased device Au(111) surface22 via molecular beam epitaxy in an ultrahigh lifetime compared to pure iodide (CH3NH3PbI3) based devices. vacuum environment. The thickness of the ultra-thin film was Addition of Cl to CH3NH3PbBr3 films improves carrier recom- approximately 4 ± 1 nm. Scanning tunneling microscopy (STM) bination lifetime12 and increases open circuit voltage13,14. How- images reveal a pair motif similar to the characteristic surface ever, the role of Cl and the extent of its incorporation into the reconstruction previously observed for CH3NH3PbBr3 single crystal lattice remain a topic of debate, with conflicting results crystals23-25 showing the (010) surface termination (Figure 1a). reported in the literature15-21. Here, we choose to mix The bright spots in the STM images correspond to bromine ions CH3NH3PbBr3 perovskite with two different halides, I and Cl, of the top layer perovskite structure. For pure CH3NH3PbBr3 all that have different ionic radii and bond strengths with Pb. In this the bromine ions have the same height and width in the STM way, the effect of the incorporation of a larger ion (I > Br) and topographs. Figure 1. Halide substitution at the perovskite surface. a-c Scanning tunneling microscopy images of CH3NH3PbBr3 (a), CH3NH3PbBr3- yIy (b), and CH3NH3PbBr3-zClz (c) 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 following parameters: A = B = 7.4 Å, and was found to not be altered by the deposition of PbI2 or PbCl2 molecules. The size 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 depo- sition of PbI2 or PbCl2 molecules (Only the PbI2 case is presented for clarity, but PbCl2 is following the same mechanism). Image parameters: (a) Bias=+1.3 V, Current=80 pA; (b) +2.0V, 120pA; (c) -1.9V, 19pA; Image size: (a-c) 2.3 nm x 2.3 nm. Thereafter, PbI2 or PbCl2 was deposited on the pure the (010) top layer surface of CH3NH3PbBr3 perovskite was re- CH3NH3PbBr3 film kept at room temperature (see Methods). produced using DFT and taken as a reference (Figure 1d). Distinct protrusions with different apparent height and width Thereafter the impact of PbI2 and PbCl2 deposition on the appear in the perovskite films (Figures 1b, 1c). After deposition atomic structure of the perovskites was further addressed by of PbI2, strong bright spots appear with an estimated height of DFT. Different scenarios were considered such as, the adsorp- 40 ± 10 pm (Figure 2f) higher than the surrounding Br ions and tion of the entire PbI2/PbCl2 molecules, as well as the adsorption with larger diameters (Figure 1b). In contrast, after PbCl2 dep- or substitution of Pb, I, and Cl at the CH3NH3PbBr3 surface. The osition, slightly darker spots are evident, with smaller diameters best match between the STM experimental results and DFT and a lower apparent height (Figure 1c). These dark spots ap- simulation is found for the dissociation of the PbI2 (or PbCl2) pear 20 ± 10 pm lower than the neighboring bromine ions. molecule, followed by the substitution of Br by I (or Cl). The To unravel the origin of the dark and bright protrusions ob- schematic drawing for the iodine case is shown in Figure 1g as served at the surface of the perovskite films, we performed DFT an example. We thus assign the bright and dark protrusions ob- calculations. The atomic structures, as well as the projected den- served in our STM experiment respectively to I and Cl ions, that sity of states (PDOS), of the perovskite films were determined substitute Br ions at the surface of the perovskite film (Figures using the VASP code (see Methods). First the pair structure of 1e, 1f). These scenarios are the most energetically favorable ones, and the corresponding simulated STM images best repro- given in Figures 2d-f. While for the pristine perovskite only a duce our experimental data (supplementary note 1). Thus, by single peak is present corresponding to Br ions, additional peaks depositing PbI2 or PbCl2 on a pristine CH3NH3PbBr3 film, two show up for the mixed halide perovskites corresponding to I and different mixed-halide perovskites, CH3NH3PbBr3-yIy and Cl ions, respectively. Surprisingly, the substitution of Br by I or CH3NH3PbBr3-zClz, were formed. Cl does not significantly change the lattice unit cell (i.e., lateral Large-scale STM images of the surface topography show that structure) of the perovskite surface. However, the vertical posi- the additional bright and dark spots observed after PbI2 and tions of the I and Cl were found to be different than the Br ions PbCl2 deposition are randomly distributed at the surface of the in pristine CH3NH3PbBr3. The iodine ions are slightly upshifted perovskite films without any long-range order (Figures 2b, 2c). with respect to the Br plane (Figure 2h). Additionally, the iodine These findings are supported by the Fast Fourier Transform ions have a larger ionic radius than bromine. Both the upshift (FFT) of the STM images, which do not show any additional and larger radius contribute to the observed topographic height peak at low k-values for the mixed perovskite compared to pris- increase. In contrast, the chlorine ions are downshifted and have tine CH3NH3PbBr3. The distribution of the different apparent a smaller ionic radius compared to Br (Figure 2i) explaining heights that are measured at the surface of the three films is their lower apparent height. Figure 2. Disordered atomic structure of mixed halide perovskites. (a-c) Large area STM images (10 nm x 10 nm) of the (010) surface of pristine CH3NH3PbBr3 (a), and mixed CH3NH3PbBr3-yIy (b), CH3NH3PbBr3-zClz (c) perovskites. Scan parameters: (a) Bias=+1.3 V, Cur- rent=80 pA; (b) +2.0 V, 120 pA; (c) -2.0 V, 100 pA. Inset: Fast Fourier Transform (FFT) 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 to 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 halides with respect to the top CH3NH3Br surface. The dashed black lines are a guide for the eyes to show the respective positions and heights of the Br, I and Cl ions. Color code: Pb (gray), Br (brown), I (purple), Cl (green), C (black), N (light blue), H (white). Besides the single bright and dark protrusions, which are at- tributed 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 bromines of the perovskite surface. Two types of pairs are found on the sur- face. Iodine and chlorine pairs can be formed either by substi- tution of two bromine ions within the same pair, or by the sub- stitution of two neighboring Br ions within two adjacent pairs (Figure S3). DFT calculations shows that the chemical nature of the halides strongly influences the stability of the pair. The energy barrier for the formation of two isolated Cl-Br pairs is lower than the energy formation of a single Cl-Cl pair (E2(Cl-Br) < ECl-Cl). These results suggest that at low coverage the chlorine ions prefer to form isolated Cl-Br pairs rather than clustering together to form Cl-Cl pairs or phase segregating. The same trend was found for the iodine case, i.e., E2(I-Br) < EI-I (Table S1). This is the reason why the substituting halide ions were found to be randomly distributed at the surface of the perovskite, lead- ing to the disordered atomic structure observed in the experi- ment. Ultraviolet photoemission spectroscopy (UPS) experiments were performed on the mixed perovskites to correlate their atomic structures with their electronic properties (Figure 3). The valence band (VB) spectra of the perovskite films are com- posed of 6 main peaks (A-F) as shown in Figure 3. A strong modification of the valence band lineshape can be observed when substituting bromine ions at the surface with iodine or chlorine. Based on DFT calculations, we found that the VB of pristine CH3NH3PbBr3 is composed mainly of Br px, py, and pz hybridized states with a small contribution from the Pb s and p orbitals (Figure 3b). In addition to these states, contributions of I and Cl appear in the VB of mixed perovskites. The substituting halides are responsible for the valence band modification as ob- served in the experiment. A higher binding energy was found for Cl as compared to Br and I (Figure 3b), in line with the higher stability of the chlorine-mixed perovskite (CH3NH3PbBr3-zClz) as discussed later. Additionally, the im- pact of halide substitution on the material work-function was revealed by UPS. A work-function of 4.77 eV was measured for Figure 3. Valence band modification in mixed halide perovskite. pristine CH3NH3PbBr3, similar to the values reported in litera- a) Experimental ultra-violet photoelectron spectroscopy (UPS) ture26-28, while a decrease of 0.2 eV of the work function (ϕ) was spectra of pure CH3NH3PbBr3 (orange) and mixed CH3NH3PbBr3- observed after chlorine substitution (ϕ = 4.57 eV). Interestingly yIy, (purple); CH3NH3PbBr3-zClz, (green) perovskites. b) Calculated after iodine incorporation in the perovskite film, the work func- PDOS of the pristine and mixed-halide perovskites. The Thick tion was 4.79 eV, closer to the original value of pristine curves represents the summation of all contributions of each orbital CH3NH3PbBr3 (Figure S5). We found that the work-function indicated by differently colored lines. The PDOS were obtained for changes originate from a downshift of the vacuum level induced the full halide substitution of the perovskite surface. by a modification of the surface dipole moment after halide in- corporation (Figure S6). A larger dipole moment amplitude re- To further understand the impact of halide substitution on the sults in a stronger downshift of the vacuum level, and thus a electronic properties of perovskite materials and its correlation smaller work function (ϕI>ϕBr>ϕCl). The possibility of adjusting with the stability, we evaluated the changes in the decomposi- the work-function as well as the vacuum level position through tion energy (see Methods) and bandgap of the CH3NH3PbBr3- halide substitution is of prime interest for engineering energy yIy and CH3NH3PbBr3-zClz films for different iodine and chlo- level alignment in perovskite-based devices. rine ratios. As depicted in Figure 4a, the substitution of bromine ions by iodine at the surface of the perovskite induces a decrease of the decomposition energy (purple curve) and a small but de- tectable reduction of the band gap (orange curve). In contrast, Figure 4b shows that the incorporation of a small amount of chlorine (below 25%, green curve) leads to an increase of the decomposition energy, with no substantial bandgap changes. Figure 4. Stability versus bandgap change in mixed perovskite. (a-b) Decomposition energy difference between mixed-halide and pristine perovskite determined by DFT calculations (see methods). a) CH3NH3PbBr3-yIy with increasing iodine content (y) at the surface. b) CH3NH3PbBr3-zClz with increasing Cl content (z) at the surface. The calculations were performed on a 6 layers 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 perov- skite. Color code: Br(brown), I(purple), Cl(green), C(black), N(light blue), H(white). An increase of the decomposition energy after chlorine sub- First, the evolution of the Pb 4f peak was monitored for the pure stitution suggests higher stability of the perovskite film with re- CH3NH3PbBr3 film without the incorporation of Cl. Figure 5a spect to external stimuli such as temperature. This stability in- shows that initially (at T = 0h), the spectrum is composed of crease is related to the higher bond strength of Cl-Pb compared only two peaks at 137.9 and 142.8 eV, attributed to the spin- to Br-Pb and I-Pb 29,30. Beyond the substitution ratio of 25%, the orbit split Pb 4f7/2 and Pb 4f5/2 peaks, later referred to as Pb(2+). strain induced by the incorporation of the smaller Cl ions with After 4h, additional peaks appear at low binding energies asso- respect to Br counterbalances the benefit of the stronger Cl-Pb ciated with the lower oxidation Pb(0) state. Such a state is asso- bond strength and results in a lowering of the material stability ciated with deterioration of the perovskite film31. Therefore, in (decomposition energy). Thus, the stability in mixed-halide per- the present study we use the Pb(0) peak evolution as an indica- ovskite results from the interplay between the Pb-halide bond tor of the perovskite degradation. A progressive and strong in- strength that stabilizes the material and the strain induced by the crease of the Pb(0) peak intensity is observed as a function of halide substitution that lowers decomposition energy. Based on time, indicating that the pure CH3NH3PbBr3 film undergoes a our results, we suggest that as a general trend, substitution of fast degradation. In contrast, much slower degradation (i.e., a the original surface halide atoms by a different halide with a significantly reduced Pb(0) peak intensity after a given time) smaller ions size and a higher Pb-halide bond strength leads to was observed for the samples with the additional PbCl2 evapo- an increase of the decomposition energy of the system, which ration (Figures 5b, 5c). The degradation process is further in turn increases the material stability. Our DFT finding shows slowed down as the amount of chlorine substitution increases. a stability increase without a strong bandgap modification. Alt- When 12% of the Br ions are substituted by Cl at the perovskite hough here only the substitution at the surface is considered, surface, the Pb(0) peak emerged after 28h, while it only appears these results indicate that there may exist an optimal bulk Cl after 116h for a 18% substitution ratio (Figure 5d). For compar- substitution ratio that would provide higher stability without ison, the substitution at the perovskite surface in the STM image substantially changing the bandgap of the perovskite material. of Figure 2c is about 26%. Substitution ratios were determine To further confirm these findings experimentally, we have by measuring the Cl:Br chemical ratio in XPS. A quantitative studied the stabilization effect of chlorine on an actual solar cell analysis of the remaining Pb(2+) species (i.e., undegraded Pb device architecture. First, a 300 nm CH3NH3PbBr3 film was ions on the surface) after a given time is presented in Figure 5e. spun on a TiO2/FTO/Glass substrate (see Methods), which re- The results demonstrate experimentally that the incorporation sembles the half (up to the perovskite layer) of a standard per- of Cl improves stability of perovskite films. It has been reported ovskite solar cell structure. Thereafter, the half-cell architecture that CH3NH3PbBr3 shows significantly better stability than was transferred into the UHV system, where chlorine was in- CH3NH3PbI332. Here in this study, we have shown that with the corporated into the perovskite film by further PbCl2 vacuum surface Br ions of CH3NH3PbBr3 partially substituted by Cl ions, evaporation. The degradation was then investigated by x-ray the half-cell sample stability gets further enhancement. Such a photoelectron spectroscopy (XPS) for the half-cell samples result is expected to have strong implications for device appli- with different Cl substitution ratios at the surface (Figure 5). cations. 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. research, to address the perovskite stability issue, which re- mains one of the main challenge for industrial application. CONCLUSION We have determined for the first time the surface atomic METHODS structure of CH3NH3PbBr3-yIy and CH3NH3PbBr3-zClz mixed- halide perovskite in real space, and correlate it to a correspond- Surface characterization. CH3NH3PbBr3 perovskite films ing enhancement or degradation of the stability of the com- were grown on Au(111) single crystal using molecular beam pound. Using scanning tunneling microscopy combined with epitaxy22. First, the gold sample was cleaned in an ultrahigh density functional theory we find that Br ions are replaced by vacuum environment (~1 x 10-9 torr) with several cycles of Ne+ the halide ions. The substituted halides are randomly distributed sputtering followed by subsequent annealing at 773 K for 5 min. at the surface, showing a disordered atomic structure with no Thereafter, CH3NH3PbBr3 was obtained by the co-deposition of phase segregation. For the low substitution ratio (below 25%), CH3NH3Br and PbBr2 molecules at 361 K and 498 K respec- Cl and I affect in opposing ways to the work function and sta- tively for 10 min. During deposition, the gold surface was kept bility of the mixed perovskite. Our studies on device architec- at 130 K to ensure the adhesion of the methyl-ammonium com- tures demonstrate that the stability is increased after incorporat- pound (MA). Mixed perovskites were obtained by sublimating ing 12~18% of Cl, which agrees with the calculated increase of PbI2 and PbCl2 on the CH3NH3PbBr3 film from Knudsen cells the decomposition energy. Our results suggest that there is an held at 525 K and 560 K respectively. During sublimation, the optimal ratio of chlorine substitution which provides a maxi- sample was kept at room temperature. Low-temperature scan- mum increase in stability without significant bandgap modifi- ning tunneling microscopy (LT-STM) was used to characterize cation. This balance is an important direction for future device the atomic-scale structures of the different perovskite ultra-thin films. The STM measurements were performed at 5 K. A cut Pt/Ir tip was used to acquire the STM images. The bias voltage This material is available free of charge via the Internet at was applied to the sample. X-ray photoelectron spectroscopy http://pubs.acs.org. (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed in the same ultra-high vacuum (UHV) setup as the AUTHOR INFORMATION STM measurements, preventing any contamination from air. Corresponding Author Density functional theory. The calculations were performed * Yabing.Qi@OIST.jp, using the VASP code33-35 with the projector augmented wave * Yanfa.Yan@utoledo.edu (PAW)36 potential and Perdew−Burke−Ernzerhof (PBE)37 ex- Present Addresses change-correlation functional. For the unit cell, Brillouin zone †Department Physik, Universität Siegen, 57068 Siegen, Germany. (BZ) sampling was done using a 4 × 4 × 4 Monkhorst−Pack grid. The CH3NH3PbBr3 (010) surfaces were modeled using (2 × 2) ORCID IDs slabs containing six atomic layers. The size of the supercell was Jeremy Hieulle: 0000-0003-4891-4007 (16.24 × 16.54 × 35.52) Å. A large vacuum thickness of 20 Å Xiaoming Wang: N/A along the direction normal to the surface was employed to sep- Collin Stecker: 0000-0003-4247-5699 arate surfaces from their periodic images. The bottom four lay- Dae-Yong Son: 0000-0001-6898-5326 ers of atoms were kept fixed at their optimized bulk positions. Longbin Qiu: 0000-0002-7696-4901 All other atoms were fully relaxed until the forces on each atom Robin Ohmann: 0000-0003-2483-1282 were smaller than 0.01 eV/Å. The nonlocal van der Waals in- Luis K. Ono: 0000-0003-3176-1876 teractions were evaluated with the optB86-vdW functional38. Aitor Mugarza: 0000-0002-2698-885X The Gamma-centered Monkhorst−Pack sampling of 3 × 3 × 1 Yanfa Yan: 0000-0003-3977-5789 was adopted to study the electronic properties of the surfaces of Yabing Qi: 0000-0002-4876-8049 CH3NH3PbBr3 (MAPbBr3). From the electronic ground-state Author Contributions structure, STM images were calculated with the p4vasp code. The manuscript was written through contributions of all authors. To evaluate the stability of the mixed perovskite, we consider All authors have given approval to the final version of the manu- the following reaction: script. ‡These authors contributed equally. 2MAPbBr) + 𝑦PbX - → 2MAPbBr)/0 X 0 + 𝑦PbBr- Notes The authors declare no competing financial interest. Where X is Cl or I, and y/3 is its ratio. The decomposition en- ergy of the mixed perovskite 2MAPbBr)/0 X0 is calculated as: ACKNOWLEDGMENT 𝑦 This work was supported by funding from the Energy Materials and 𝐸234 = 𝐸6MAPbBr)/0 X 0 7 − 𝐸(MAPbBr) ) + [𝐸(PbBr- ) − 𝐸(PbX - )] Surface Sciences Unit of the Okinawa Institute of Science and 2 Technology Graduate University, the OIST R&D Cluster Research The stability of the perovskite is characterized by the decompo- Program, the OIST Proof of Concept (POC) Program, and JSPS sition energy (Edec). The larger the decomposition energy, the KAKENHI Grant Number JP18K05266. The work at The Univer- more stable the material is. sity of Toledo was supported by the Center for Hybrid Organic In- organic Semiconductors for Energy (CHOISE), an Energy Frontier Device fabrication. The patterned FTO glass (Pilkington, Research Center funded by the Office of Basic Energy Sciences, TEC-8, 8Ω/sq) was cleaned with detergent and DI water and Office of Science within the US Department of Energy. The theo- sonicated with ethanol in an ultrasonic bath for 20 min. UV- retical calculation part used the resources of the National Energy Research Scientific Computing Center, which is supported by the Ozone was treated for 15 min prior to use. The mesoporous Office of Science of the U.S. Department of Energy under Contract TiO2 (ca. 50 nm) layer was deposited by spin coating at 2000 No. DE-AC02-05CH11231. rpm for 20 s (acceleration was 500 rpm/s) using the diluted TiO2 solution in 1-butanol39. The deposited TiO2 film was annealed ABBREVIATIONS at 550 °C for 30 min. The CH3NH3PbBr3 film was prepared via adduct method40. 0.367g of PbBr2 (TCI) and 0.112 CH3NH3Br STM, scanning tunneling microscope; DFT, density functional the- (Dyesol) were dissolved in 0.6 mL of DMF with 75 μL of ory; XPS, x-ray photoelectron spectroscopy; UPS, ultraviolet pho- toemission spectroscopy; FFT, fast Fourier transform; MA, me- DMSO at 60 °C. The CH3NH3Br3 solution was spin-coated on thylammonium CH3NH3. the mp-TiO2 substrate at 4000 rpm for 20 and 0.3 mL of diethyl ether was poured when 10 s was remained. Then, it was dried REFERENCES on a hot plate at 100 °C for 20 min. (1) National Renewable Energy Laboratory (NREL). https://www.nrel.gov/pv/national-center-for-photovoltaics.html (2018). ASSOCIATED CONTENT (2) Noh, J.H.; Im, S.H.; Heo, J.H.; Mandal, T.N.; Seok, S.I. Chemical management for colorful, efficient, and stable inorganic-organic hybrid Supporting Information: nanostructured solar cells. Nano Letters 2013, 13, 1764. 1. Adsorption vs substitution in mixed-halide perovskite (3) Bush, K. A.; Palmstrom, A.F.; Yu, Z.J.; Boccard, M.; Cheacharoen, 2. Halide pairs and clusters at the surface of mixed perovskites R.; Mailoa, J.P.; McMeekin, D.P.; Hoye, R.L.Z.; Bailie, C.D.; Leijtens, 3. Determination of the valence band maximum (VBM) T.; Peters, I.M.; Minichetti, M.C.; Rolston, N.; Prasanna, R.; Sofia, S.; 4. Electrostatic potential and work-function changes Harwood, D.; Ma, W.; Moghadam, F.; Snaith, H.J.; Buonassisi, T.; 5. Device architecture Holman, Z.C.; Bent, S.F.; McGehee, M.D. 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nat. En- crystal mixed halide perovskite CH3NH3Pb(Br1-xClx)3. Chem. Commun. ergy. 2017, 2, 17009. 2015, 51, 7820. (4) Jiang, F.; Rong, Y.; Liu, H; Liu, T.; Mao, L.; Meng, W.; Qin, F.; (22) Ono, L.K.; Wang, S.; Kato, Y.; Raga, S.R.; Qi, Y.B. Fabrication Jiang, Y.; Luo, B.; Xiong, S.; Tong, J.; Liu, Y.; Li, Z.; Han, H.; Zhou, of semi-transparent perovskite films with centimeter-scale superior Y. Synergistic effect of PbI2 passivation and chlorine inclusion yielding uniformity by the hybrid deposition method. Energy Environ. Sci. 2014, high open-circuit voltage exceeding 1.15 V in both mesoscopic and in- 7, 3989. verted planar CH3NH3PbI3(Cl)-based perovskite solar cells. Adv. Funct. (23) Ohmann, R.; Ono, L.K.; Kim, H.-S.; Lin, H.; Lee, M.V.; Li, Y.; Mater. 2016, 26, 8119. Park, N.-G.; Qi, Y.B. Real-space imaging of the atomic structure of (5) Liu, J.; Prezhdo, O.V. Chlorine doping reduces electron-hole re- organic-inorganic perovskite. J. Am. Chem. Soc. 2015,137, 16049. combination in lead iodide perovskites: time-domain ab initio analysis. (24) Hieulle, J.; Stecker, C.; Ohmann, R.; Ono, L.K.; Qi, Y.B. Scanning J. Phys. Chem. Lett. 2015, 6, 22, 4463. probe microscopy applied to organic-inorganic halide perovskite mate- (6) Yu, H.; Wang, F.; Xie, F.; Li, W.; Chen, J.; Zhao, N. The role of rials and solar cells. Small Methods 2018, 2, 1700295. chlorine in the formation process of “CH3NH3PbI3-xClx” perovskite. (25) She, L.; Liu, M.; Zhong, D. Atomic structures of Adv. Funct. Mater. 2014, 24, 7102. CH3NH3PbI3(001) surfaces. ACS Nano 2016, 10, 1126. (7) Chen, Q.; Zhou, H.; Fang, Y.; Stieg, A.Z.; Song, T.-B.; Wang, H.- (26) Kedem, N.; Kulbak, M.; Brenner, T.M.; Hodes, G.; Cahen, D. H.; Xu, X.; Liu, Y.; Lu, S.; You, J.; Sun, P.; McKay, J.; Goorsky, M.S.; Type-inversion as a working mechanism of high voltage MAP- Yang, Y. The optoelectronic role of chlorine in CH3NH3PbI3(Cl)-based bBr3(Cl)-based halide perovskite solar cells. Phys. Chem. Chem. Phys. perovskite solar cells. Nat. Commun. 2015, 6, 7269. 2017, 19, 5753. (8) Quarti, C.; Mosconi, E.; Umari, P.; De Angelis, F. Chlorine (27) Olthof, S.; Meerholz, K. Substrate-dependent electronic structure incorporation in the CH3NH3PbI3 perovskite: small concentration, big and film formation of MAPbI3 perovskites. Sci. Rep. 2017, 7, 40267. effect. Inorg. Chem. 2017, 56, 74. (28) Comin, R.; Walters, G.; Thibau, E.S.; Voznyy, O.; Lu, Z.-H.; Sar- (9) Colella, S.; Mosconi, E.; Pellegrino, G.; Alberti, A.; Guerra, V.L.P.; gent, E.H. Structural, optical, and electronic studies of wide-bandgap Masi, S.; Listorti, A.; Rizzo, A.; Condorelli, G.G.; De Angelis, F.; Gigli, lead halide perovskites. J. Mater. Chem. C 2015, 3, 8839. G. Elusive presence of chloride in mixed halide perovskite solar cells. J. Phys. Chem. Lett. 2014, 5, 20, 3532. (29) Darwent, B. deB. National Standard Reference Data Series. Na- tional Bureau of Standards, Washington, D.C. 1970, 31. (10) Jeon, N.J.; Noh, J.H.; Kim, Y.C.; Yang, W.S.; Ryu, S.; Seok, S.I. Solvent engineering for high-performance inorganic-organic hybrid (30) Cottrell, T.L. The Strengths of Chemical Bonds, 2nd Edition, But- perovskite solar cells. Nat. Mater. 2014, 13, 897. terworth, London 1958. (11) Lee, M.M.; Teuscher, J.; Miyasaka, T.; Murakami, T.N.; Snaith, (31) Juarez-Perez, E.J.; Ono, L.K.; Maeda, M.; Jiang, Y.; Hawash, Z.; H.J. Efficient hybrid solar cells based on meso-superstructured organo- Qi, Y.B. Photodecomposition and thermal decomposition in me- metal halide perovskites. Science 2012, 338, 643. thylammonium halide lead perovskites and inferred design principles to increase photovoltaic device stability. J. Mater. Chem. A 2018, 6, (12) Zhang, M.; Yu, H.; Lyu, M.; Wang, Q.; Yun, J.-H.; Wang, L. Com- 9604. position-dependent photoluminescence intensity and prolonged recom- bination lifetime of perovskite CH3NH3PbBr3-xClx films. Chem. Com- (32) Pont, S; Bryant, D.; Lin, C.-T.; Aristidou, N.; Wheeler, S.; Ma, X.; mun. 2014, 50, 11727. Godin, R.; Haque, S.A.; Durrant, J.R. Tuning CH3NH3Pb(I1-xBrx)3 per- ovskite oxygen stability in thin films and solar cells. J. Mater. Chem. A (13) Edri, E.; Kirmayer, S.; Cahen, D.; Hodes, G. High open-circuit 2017, 5, 9553. voltage solar cells based on organic-inorganic lead bromide perovskite. J. Phys. Chem. Lett. 2013, 4, 897. (33) Kresse, G.; Hafner, J. Ab initio. Phys. Rev. B 1993, 47, 558. (14) Edri, E.; Kirmayer, S.; Kulbak, M.; Hodes, G.; Cahen, D. Chloride (34) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy cal- inclusion and hole transport material doping to improve methyl ammo- culations for metals and semiconductors using a plane-wave basis set. nium lead bromide perovskite-based high open-circuit voltage solar Comput. Mater. Sci. 1996, 6, 15. cells. J. Phys. Chem. Lett. 2014, 5, 429. (35) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio (15) Ono, L.K.; Juarez-Perez, E.J.; Qi, Y.B. Progress on perovskite ma- total-energy calculations using a plane-wave basis set. Phys. Rev. B terials and solar cells with mixed cations and halide anions. ACS Appl. 1996, 54, 11169. Mater. Interfaces 2017, 9, 30197. (36) Blöchl, P.E. Projector augmented-wave method. Phys. Rev. B (16) Zhou, Y.; Zhou, Z.; Chen, M.; Zong, Y.; Huang, J.; Pang, S.; 1994, 50, 17953. Padture, N.P. Doping and alloying for improved perovskite solar cells. (37) Perdew, J.P.; Burke, K.; Ernzerhof, M. Generalized gradient ap- J. Mater. Chem. A 2016, 4, 17623. proximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (17) Yin, W.-J.; Yang, J.-H.; Kang, J.; Yan, Y.; Wei, S.-H. Halide per- (48) Klimeš, J.; Bowler, D.R.; Michaelides, A. Van der Waals density ovskite materials for solar cells: a theoretical review. J. Mater. Chem. functionals applied to solids. Phys. Rev. B 2011, 83, 195131. A 2015, 3, 8926. (39) Son, D.-Y.; Lee, J.W.; Choi, Y.J.; Jang, I.H.; Lee, S.; Yoo, P.J.; (18) Ye, M.; Hong, X.; Zhang, F.; Liu, X. Recent advancements in per- Shin, H.; Ahn, N.; Choi, M.; Kim, D.; Park, N.-G. Self-formed grain ovskite solar cells: flexibility, stability and large scale. J. Mater. Chem. boundary healing layer for highly efficient CH3NH3PbI3 perovskite so- A 2016, 4, 6755. lar cells. Nat. Energy. 2016, 1, 16081. (19) Pool, V.L.; Gold-Parker, A.; McGehee, M.D.; Toney, M.F. Chlo- (40) Ahn, N.; Son, D.-Y.; Jang, I.H.; Kang, S.M.; Choi, M.; Park, N.- rine in PbCl2-derived hybrid-perovskite solar absorbers. Chem. Mater. G. Highly reproducible perovskite solar cells with average efficiency 2015, 27, 7240. of 18.3% and best efficiency of 19.7% fabricated via Lewis base adduct (20) Luo, S.; Daoud, W.A. Crystal structure formation of CH3NH3PbI3- of lead(II) iodide. J. Am. Chem. Soc. 2015, 137, 8696. xClx perovskite. Materials 2016, 9, 123. (21) Zhang, T.; Yang, M.; Benson, E.E.; Li, Z.; Lagemaat, J.; Luther, J.M.; Yan, Y; Zhu, K; Zhao, Y. A facile solvothermal growth of single