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A Facile Solvo-Thermal Growth of Single Crystal Mixed Halide Perovskite CH3NH3Pb(Br1-xClx)3 Taiyang Zhanga, Mengjin Yangb, Eric E. Bensonb, Zijian Lic, Jao van de Lagemaatb, Joseph M. Lutherb, Yanfa Yand, Kai Zhub* and Yixin Zhaoa* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x We demonstrate a facile synthetic approach for preparing mixed halide perovskite (CH3NH3)Pb(Br1-xClx)3 single crystals by solvo-thermal growth of stoichiometric PbBr2 and [(1-y)CH3NH3Br+yCH3NH3Cl] DMF precursor solutions. The band gap of (CH3NH3)Pb(Br1-xClx)3 single crystals increased and the unit cell dimensions decreased with increased Cl content x, consistent with previous theoretical predictions. Interestingly, the Cl/Br ratio in the (CH3NH3)Pb(Br1-xClx)3 single crystals is larger than that of the precursor solution, suggesting an unusual crystal growth mechanism. Solution processed halide perovskites have emerged as a promising material for optoelectronic devices, especially photovoltaics.1-3 Strong light absorption, high charge carrier mobility, and a general tolerance to defects has led to much interest in CH3NH3PbX3 (X=I, Br, and Cl) perovskites, where the power conversion efficiency has rapidly climbed to above 20% which rivals all other thin film technologies.4-11 While the CH3NH3PbX3 perovskites have undergone rapid device optimization in terms of the efficiency, the fundamental understanding of their physical, chemical, and optoelectronic properties are still lacking. In almost all reports of film formation, solution deposition routes are used which involve precursors of ammonium halide salts and metal halide salts which under proper conditions, crystallize into the perovskite phase. Variations of the material properties (e.g., composition, crystal structure, and film morphology) of the perovskite films can occur when prepared via conventional deposition approaches (i.e. slight changes in deposition conditions can lead to significant variations of the crystal sizes, mobility, carrier lifetimes and even the amount of pure phase perovskite vs. PbX2). Thus to accurately model devices, or to learn more about the intrinsic properties of the perovskite material, it could be extremely useful to study properties of phase-pure bulk lead halide perovskites, especially single crystals. CH3NH3PbX3 (X=I, Br) single crystals have been previously prepared by conventional single crystal growth techniques, which is unfortunately complicated (to some degree) for non-specialists.12, 13 One of attractive features of lead halide perovskite is the cross substitution of the halides, which can tune the electronic structure (e.g., optical bandgap).3, 14-16 The photovoltaic properties of the mixed halide CH3NH3PbI3-xBrx were initially demonstrated by This journal is © The Royal Society of Chemistry [year]
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Seok and his coworkers.17 Other forms of mixed halide perovskites have received less attention. Developing mixed halide perovskite with high bandgap layers will be useful for multijunction solar cells, improving the stability of iodide based perovskite crystals, and even for light emitting applications where visible light can be made. A previous theoretical study has suggested that the solubility of Cl in CH3NH3PbI3 is low, but is high in CH3NH3PbBr3.18 The Cl alloying and their effect on perovskite halides, especially in the case of CH3NH3PbI3-xClx has been a subject of debate from experimental investigations owing to different observations regarding their exact crystal phase and chemical composition.19-23 Similar debate/question has been raised for thin-film (CH3NH3)PbBr3-xClx,24 although this compound is rarely investigated. Here we report a facile preparation of CH3NH3Pb(Br1-xClx)3 perovskite single crystals from DMF precursor solutions containing stoichiometric PbBr2 and [(1-y) CH3NH3Br+ yCH3NH3Cl]. The chemical and physical properties of these CH3NH3Pb(Br1-xClx)3 perovskites are experimentally examined and compared to theoretical studies. The typical method for depositing CH3NH3PbBr3 single crystals is to dissolve a high concentration of PbBr2 and CH3NH3Br in a hot aqueous solution and then cool down.14 Recently CH3NH3PbBr3 crystallization has also been observed by
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Figure 1. (Upper) The three typical growth stages (5min, 40min, 5hr) of red single crystal in 35 wt% CH3NH3PbBr3 DMF solution at 50ºC. (Lower) Photos of the single crystals of CH3NH3Pb(Br1-xClx)3, x=0, 0.15, 0.25 from left to right.
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following precursor solutions: 0.183 g PbBr2 and 0.056*(1-y) g CH3NH3Br and 0.034*y g CH3NH3Cl was dissolved in 0.443 g DMF to form a CH3NH3PbBr3-yCly precursor solution. The solution was kept at 50ºC without stirring in a sealed vial until CH3NH3Pb(Br1-xClx)3 single crystals formed, which is similar to the growth of CH3NH3PbBr3 single crystals. These yellow-to-red mixed halide CH3NH3Pb(Br1-xClx)3 single crystals are shown in Figure 1; the color can be tuned by adjusting the Cl/Br ratio. It is worth noting that we were unsuccessful in growing single crystals of CH3NH3Pb(Br1-xIx)3 using the same approach with mild heating.
Figure 2. The single crystal XRD cell dimensions for CH3NH3Pb(Br1xClx)3 (x=0, 0.15, 0.25) as a function of chloride inclusion. 70
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Figure 2 shows the single crystal XRD cell dimensions for CH3NH3Pb(Br1-xClx)3 as a function of chloride inclusion. The details of the room temperature single crystal diffraction data of the CH3NH3Pb(Br1-xClx)3 perovskites can be found in Table S2 in the Supporting Information. The amount of chloride inclusion was measured by the decrease in cell dimensions in comparison to the pure CH3NH3PbBr3 and known14 CH3NH3PbCl3 cell parameters. As the ratio of chloride increases, the cell dimensions decreases from 5.9312(3) Å for the pure bromide, to 5.8959(4) and 5.8638(7) for the 0.15 and 0.25 inclusion of Cl respectively. In all cases the methylammonium cation was disordered on a site of symmetry and the corresponding electron density was removed using the SQUEEZE routine within PLATON.29 This suggests that the CH3NH3+ cation can freely rotate in the crystal lattice, which is consistent with previous NMR studies.30 Our previous theoretical study has suggested that the energy barrier for CH3NH3+ rotation in CH3NH3PbI3 is about 20 meV.31 Previously reported Br/I ratios in the spin-coating prepared CH3NH3Pb(Br1-xIx)3 films were the same as that of the mixed halide precursor solutions.17 The elemental analysis demonstrated that the Cl/Br ratio of these spin-coating prepared CH3NH3Pb(Br1-xClx)3 films in this study were consistent with the Cl/Br ratio in their corresponding initial precursor solutions. However, the Cl/Br ratios in the CH3NH3Pb(Br1-xClx)3 single crystals are significantly different from their precursor solutions. For example, when the initial Cl/(Br+Cl) ratios in precursor This journal is © The Royal Society of Chemistry [year]
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evaporating of high concentration PbBr2 and CH3NH3Br DMF/Alcohol mixture solution.25 In these reports, the CH3NH3PbBr3 was prepared by the classical crystal growth approach (i.e., cooling high temperature saturated precursor solution or evaporating solvent from saturated precursor solution). This crystal growth is an exothermic reaction. However, we found that a mild heating of CH3NH3PbBr3 DMF solution (>35wt %) at about 50ºC would form red single crystals after several hours without evaporation of solvent. Figure 1 shows the growth of these red colored single crystals. Some small red seed crystals first formed and then grew into large crystals up to about 5-mm in one direction. To exclude the possibility of DMF evaporation during crystal growth, we have weighed the precursor solution bottle before and after the growth of single crystals. We find that the weight of the sealed precursor solution bottle do not change during crystal growth, suggesting that the formation of the single crystals is not resulting from the evaporation of DMF, as is often the case using other crystal growth methods. More interestingly, these red single crystals can be re-dissolved into DMF once the single crystals are kept in the precursor solution at room temperature overnight. This unusual crystal growth suggests that crystallization of CH3NH3PbBr3 in DMF could be an endothermic reaction, which is different from the commonly reported crystal growth processes (e.g., cooling saturated hot precursor solution). However, it is noteworthy that CH3NH3PbI3 or CH3NH3PbCl3 single crystals cannot be formed by using a similar DMF solution. This unusual endothermic crystal formation of CH3NH3PbBr3 suggests a stronger interaction between CH3NH3Br and PbBr2 at higher temperature, which could account for the improved thermal and moisture stability of CH3NH3PbBr3 than that of CH3NH3PbI3 or CH3NH3PbCl3.26 It is worth noting that these crystals (Figure 1), when separated from the solution, are stable in ambient condition for at least several months without degradation. To understand this phenomena for the different halides, we performed a theoretical investigation on the crystallization using VASP code with the standard frozen-core projector augmentedwave (PAW) method.27, 28 Our theoretical study reveals that the growth of CH3NH3Pb(Br1-xClx)3 perovskite from precursor solutions containing stoichiometric PbBr2 and [(1-y) CH3NH3Br+ yCH3NH3Cl] is energetically favorable, but it is energetically unfavorable for the growth of CH3NH3Pb(I1-xClx)3 from stoichiometric PbI2 and [(1-y) CH3NH3I+ yCH3NH3Cl] solution. It is seen that the reaction of CH3NH3Br+CH3NH3Cl+PbBr2 CH3NH3Br2Cl+CH3NH3Br gains an energy of 0.194 eV, whereas the reaction of CH3NH3I+CH3NH3Cl+PbI2 CH3NH3I2Cl+CH3NH3I losses an energy of 0.196 eV. These results support our experimental observation: CH3NH3Pb(Br1xClx)3 perovskite single crystals can form from precursor solutions containing stoichiometric PbBr2 and [(1-y) CH3NH3Br+ yCH3NH3Cl]. The cut-off energy for basis functions was 400 eV. The general gradient approximation (GGA) was used for exchange-correlation. The phase was considered for the perovskites. For our calculations, only x=1/3 is considered. The calculated energies for various systems are listed in Table S1 in the Supporting Information. The mixed halide perovskite single crystals of CH3NH3Pb(Br1xClx)3 with different Cl/Br ratios were grown by using the
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solution of [PbBr2 + (1-y) CH3NH3Br+ yCH3NH3Cl] is 0.08 and 0.17, the Cl/(Cl+Br) ratios of the CH3NH3Pb(Br1-xClx)3 single crystals obtained by ICP and single crystal analysis are about 0.15 and 0.25, respectively. The changed Cl/Br ratio during the single crystal growth suggests that Cl and Br have different affinity to form CH3NH3Pb(Br1-xClx)3 single crystal, which is different from the quick film formation via rapid heating of the spin-coated samples to evaporate the DMF solvent. Figure 3A shows the XRD patterns of grinded powders of three typical CH3NH3Pb(Br1-xClx)3 (x=0, 0.15 and 0.25) single crystals. With increase of Cl inclusion, the characteristic perovskite peaks shift to higher diffraction angles. This suggests the shrinking of
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Figure 3. The XRD (A) and normalized UV-vis absorption spectra (B) of the CH3NH3Pb(Br1-xClx)3 (x=0, 0.15, 0.25) single crystals. 70
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crystal lattices as more Cl is incorporated in the crystals, which is consistent with the single crystal studies. The simulated XRD patterns of these CH3NH3Pb(Br1-xClx)3 (x=0, 0.15 and 0.25) single crystals based on single crystal diffraction data are listed in Figure S1, which is consistent with Fig 3A. The reflectance measurement on these single crystal CH3NH3Pb(Br1-xClx)3 (x=0, 0.15, 0.25) powder exhibits a blue shift in UV-vis absorbance with increased Cl inclusion as shown in Figure 3B. This demonstrates that the inclusion of Cl broadens the bandgap.18 The bandgap of the CH3NH3Pb(Br0.75Cl0.25)3 is about 2.6 eV, which is about 0.3 eV higher than that of CH3NH3PbBr3 (~2.3 eV). The This journal is © The Royal Society of Chemistry [year]
DOI: 10.1039/C5CC01835H
UV-vis spectra and XRD patterns of thin films of CH3NH3Pb(Br1-xClx)3 perovskites prepared from the same precursor solutions of [PbBr2 + (1-y) CH3NH3Br+ yCH3NH3Cl] (y=0, 0.25 and 0.5) by spin coating are shown in Figure S2 and S3. The bandgap shift is only about 0.1 eV, which is significantly smaller than that for single crystals grown from the same precursor solutions. This observation is consistent with the different Cl/Br ratios observed between the single crystals and spin-coat films from the same precursor solutions. In growth of CH3NH3Pb(Br1-xClx)3 single crystals using the precursor solution of mixed PbBr2 and (1-y)CH3NH3Br with yCH3NH3Cl, the CH3NH3Pb(Br1-xClx)3 single crystal can only be obtained when y is no larger than 0.5. As mentioned above, the presence of CH3NH3I could undermine the growth of perovskite single crystal with this method. These observations suggest that the formation of CH3NH3Pb(Br1-xClx)3 single crystals by heating DMF may be related to the strong crystal forming affinity between Br- and Pb2+ at higher temperature, while the Cl- and Icould inhibit the formation of mixed halide perovskite single crystals. In addition to using a proper amount of CH3NH3Cl, another key factor for successful preparation of these single crystals of CH3NH3Pb(Br1-xClx)3 mixed halide perovskites is that the total molar amount of CH3NH3+ (from the sum of CH3NH3Br and CH3NH3Cl) must be set accurately to 1:1 ratio to the amount of PbBr2. If the CH3NH3Br:PbBr2 ratio is first fixed to 1:1, it would be unsuccessful to obtain CH3NH3Pb(Br1-xClx)3 single crystals with additional molar ratios of CH3NH3Cl. This result is consistent with a previous report that additional CH3NH3Cl significantly affects the crystallization of CH3NH3Br3 by slow DMF evaporation.25 More interestingly, we find if the molar ratio of CH3NH3Br to PbBr2 is larger than 1.2, we are not able to obtain any CH3NH3PbBr3 single crystals. It seems that excess molar ratios of both CH3NH3Br and/or CH3NH3Cl could significantly inhibit the crystallization of CH3NH3PbBr3, which is similar to our previous observation of retarded crystallization of CH3NH3PbI3 in presence of extra CH3NH3Cl.21 In summary, we have developed a facile growth of mixed halide CH3NH3Pb(Br1-xClx)3 single crystals by heating their DMF precursor solution. Single and powder crystal diffraction measurements have confirmed that Cl partially replaces Br in CH3NH3Pb(Br1-xClx)3 perovskite. These CH3NH3Pb(Br1-xClx)3 perovskites, with different Cl inclusion, exhibit different electronic structures and the bandgaps broaden with Cl inclusion. This unusual growth of CH3NH3Pb(Br1-xClx)3 perovskite single crystals could not only provide a facile method for growing CH3NH3Pb(Br1-xClx)3 perovskite single crystals but also enable various fundamental studies to understand perovskite crystal structure and crystal growth mechanism.
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TZ and YZ are thankful for the support of the NSFC (Grant 51372151 and 21303103). MY, JML, and KZ acknowledge the support by the U.S. Department of Energy (DOE) SunShot Initiative under the Next Generation Photovoltaics 3 program (DE-FOA-0000990). EEB and JvdL acknowledges the support on the single crystal diffraction data analysis by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences (DOE). The work at the National Renewable Energy Laboratory is supported by the U.S. Journal Name, [year], [vol], 00–00 | 3
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Department of Energy under Contract No. DE-AC36-08GO28308.
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School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China E-mail: [email protected] b Chemistry and Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401 USA. E-mail: [email protected] c School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China d Department of Physics & Astronomy, and Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA
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ChemComm Accepted Manuscript
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ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: T. Zhang, M. Yang, E. E. Benson, Z. Li, J. van de Lagemaat, J. M. Luther, Y. Yan, K. Zhu and Y. Zhao, Chem. Commun.,
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
A Facile Solvo-Thermal Growth of Single Crystal Mixed Halide Perovskite CH3NH3Pb(Br1-xClx)3 Taiyang Zhanga, Mengjin Yangb, Eric E. Bensonb, Zijian Lic, Jao van de Lagemaatb, Joseph M. Lutherb, Yanfa Yand, Kai Zhub* and Yixin Zhaoa* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x We demonstrate a facile synthetic approach for preparing mixed halide perovskite (CH3NH3)Pb(Br1-xClx)3 single crystals by solvo-thermal growth of stoichiometric PbBr2 and [(1-y)CH3NH3Br+yCH3NH3Cl] DMF precursor solutions. The band gap of (CH3NH3)Pb(Br1-xClx)3 single crystals increased and the unit cell dimensions decreased with increased Cl content x, consistent with previous theoretical predictions. Interestingly, the Cl/Br ratio in the (CH3NH3)Pb(Br1-xClx)3 single crystals is larger than that of the precursor solution, suggesting an unusual crystal growth mechanism. Solution processed halide perovskites have emerged as a promising material for optoelectronic devices, especially photovoltaics.1-3 Strong light absorption, high charge carrier mobility, and a general tolerance to defects has led to much interest in CH3NH3PbX3 (X=I, Br, and Cl) perovskites, where the power conversion efficiency has rapidly climbed to above 20% which rivals all other thin film technologies.4-11 While the CH3NH3PbX3 perovskites have undergone rapid device optimization in terms of the efficiency, the fundamental understanding of their physical, chemical, and optoelectronic properties are still lacking. In almost all reports of film formation, solution deposition routes are used which involve precursors of ammonium halide salts and metal halide salts which under proper conditions, crystallize into the perovskite phase. Variations of the material properties (e.g., composition, crystal structure, and film morphology) of the perovskite films can occur when prepared via conventional deposition approaches (i.e. slight changes in deposition conditions can lead to significant variations of the crystal sizes, mobility, carrier lifetimes and even the amount of pure phase perovskite vs. PbX2). Thus to accurately model devices, or to learn more about the intrinsic properties of the perovskite material, it could be extremely useful to study properties of phase-pure bulk lead halide perovskites, especially single crystals. CH3NH3PbX3 (X=I, Br) single crystals have been previously prepared by conventional single crystal growth techniques, which is unfortunately complicated (to some degree) for non-specialists.12, 13 One of attractive features of lead halide perovskite is the cross substitution of the halides, which can tune the electronic structure (e.g., optical bandgap).3, 14-16 The photovoltaic properties of the mixed halide CH3NH3PbI3-xBrx were initially demonstrated by This journal is © The Royal Society of Chemistry [year]
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Seok and his coworkers.17 Other forms of mixed halide perovskites have received less attention. Developing mixed halide perovskite with high bandgap layers will be useful for multijunction solar cells, improving the stability of iodide based perovskite crystals, and even for light emitting applications where visible light can be made. A previous theoretical study has suggested that the solubility of Cl in CH3NH3PbI3 is low, but is high in CH3NH3PbBr3.18 The Cl alloying and their effect on perovskite halides, especially in the case of CH3NH3PbI3-xClx has been a subject of debate from experimental investigations owing to different observations regarding their exact crystal phase and chemical composition.19-23 Similar debate/question has been raised for thin-film (CH3NH3)PbBr3-xClx,24 although this compound is rarely investigated. Here we report a facile preparation of CH3NH3Pb(Br1-xClx)3 perovskite single crystals from DMF precursor solutions containing stoichiometric PbBr2 and [(1-y) CH3NH3Br+ yCH3NH3Cl]. The chemical and physical properties of these CH3NH3Pb(Br1-xClx)3 perovskites are experimentally examined and compared to theoretical studies. The typical method for depositing CH3NH3PbBr3 single crystals is to dissolve a high concentration of PbBr2 and CH3NH3Br in a hot aqueous solution and then cool down.14 Recently CH3NH3PbBr3 crystallization has also been observed by
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Figure 1. (Upper) The three typical growth stages (5min, 40min, 5hr) of red single crystal in 35 wt% CH3NH3PbBr3 DMF solution at 50ºC. (Lower) Photos of the single crystals of CH3NH3Pb(Br1-xClx)3, x=0, 0.15, 0.25 from left to right.
[journal], [year], [vol], 00–00 | 1
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following precursor solutions: 0.183 g PbBr2 and 0.056*(1-y) g CH3NH3Br and 0.034*y g CH3NH3Cl was dissolved in 0.443 g DMF to form a CH3NH3PbBr3-yCly precursor solution. The solution was kept at 50ºC without stirring in a sealed vial until CH3NH3Pb(Br1-xClx)3 single crystals formed, which is similar to the growth of CH3NH3PbBr3 single crystals. These yellow-to-red mixed halide CH3NH3Pb(Br1-xClx)3 single crystals are shown in Figure 1; the color can be tuned by adjusting the Cl/Br ratio. It is worth noting that we were unsuccessful in growing single crystals of CH3NH3Pb(Br1-xIx)3 using the same approach with mild heating.
Figure 2. The single crystal XRD cell dimensions for CH3NH3Pb(Br1xClx)3 (x=0, 0.15, 0.25) as a function of chloride inclusion. 70
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Figure 2 shows the single crystal XRD cell dimensions for CH3NH3Pb(Br1-xClx)3 as a function of chloride inclusion. The details of the room temperature single crystal diffraction data of the CH3NH3Pb(Br1-xClx)3 perovskites can be found in Table S2 in the Supporting Information. The amount of chloride inclusion was measured by the decrease in cell dimensions in comparison to the pure CH3NH3PbBr3 and known14 CH3NH3PbCl3 cell parameters. As the ratio of chloride increases, the cell dimensions decreases from 5.9312(3) Å for the pure bromide, to 5.8959(4) and 5.8638(7) for the 0.15 and 0.25 inclusion of Cl respectively. In all cases the methylammonium cation was disordered on a site of symmetry and the corresponding electron density was removed using the SQUEEZE routine within PLATON.29 This suggests that the CH3NH3+ cation can freely rotate in the crystal lattice, which is consistent with previous NMR studies.30 Our previous theoretical study has suggested that the energy barrier for CH3NH3+ rotation in CH3NH3PbI3 is about 20 meV.31 Previously reported Br/I ratios in the spin-coating prepared CH3NH3Pb(Br1-xIx)3 films were the same as that of the mixed halide precursor solutions.17 The elemental analysis demonstrated that the Cl/Br ratio of these spin-coating prepared CH3NH3Pb(Br1-xClx)3 films in this study were consistent with the Cl/Br ratio in their corresponding initial precursor solutions. However, the Cl/Br ratios in the CH3NH3Pb(Br1-xClx)3 single crystals are significantly different from their precursor solutions. For example, when the initial Cl/(Br+Cl) ratios in precursor This journal is © The Royal Society of Chemistry [year]
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evaporating of high concentration PbBr2 and CH3NH3Br DMF/Alcohol mixture solution.25 In these reports, the CH3NH3PbBr3 was prepared by the classical crystal growth approach (i.e., cooling high temperature saturated precursor solution or evaporating solvent from saturated precursor solution). This crystal growth is an exothermic reaction. However, we found that a mild heating of CH3NH3PbBr3 DMF solution (>35wt %) at about 50ºC would form red single crystals after several hours without evaporation of solvent. Figure 1 shows the growth of these red colored single crystals. Some small red seed crystals first formed and then grew into large crystals up to about 5-mm in one direction. To exclude the possibility of DMF evaporation during crystal growth, we have weighed the precursor solution bottle before and after the growth of single crystals. We find that the weight of the sealed precursor solution bottle do not change during crystal growth, suggesting that the formation of the single crystals is not resulting from the evaporation of DMF, as is often the case using other crystal growth methods. More interestingly, these red single crystals can be re-dissolved into DMF once the single crystals are kept in the precursor solution at room temperature overnight. This unusual crystal growth suggests that crystallization of CH3NH3PbBr3 in DMF could be an endothermic reaction, which is different from the commonly reported crystal growth processes (e.g., cooling saturated hot precursor solution). However, it is noteworthy that CH3NH3PbI3 or CH3NH3PbCl3 single crystals cannot be formed by using a similar DMF solution. This unusual endothermic crystal formation of CH3NH3PbBr3 suggests a stronger interaction between CH3NH3Br and PbBr2 at higher temperature, which could account for the improved thermal and moisture stability of CH3NH3PbBr3 than that of CH3NH3PbI3 or CH3NH3PbCl3.26 It is worth noting that these crystals (Figure 1), when separated from the solution, are stable in ambient condition for at least several months without degradation. To understand this phenomena for the different halides, we performed a theoretical investigation on the crystallization using VASP code with the standard frozen-core projector augmentedwave (PAW) method.27, 28 Our theoretical study reveals that the growth of CH3NH3Pb(Br1-xClx)3 perovskite from precursor solutions containing stoichiometric PbBr2 and [(1-y) CH3NH3Br+ yCH3NH3Cl] is energetically favorable, but it is energetically unfavorable for the growth of CH3NH3Pb(I1-xClx)3 from stoichiometric PbI2 and [(1-y) CH3NH3I+ yCH3NH3Cl] solution. It is seen that the reaction of CH3NH3Br+CH3NH3Cl+PbBr2 CH3NH3Br2Cl+CH3NH3Br gains an energy of 0.194 eV, whereas the reaction of CH3NH3I+CH3NH3Cl+PbI2 CH3NH3I2Cl+CH3NH3I losses an energy of 0.196 eV. These results support our experimental observation: CH3NH3Pb(Br1xClx)3 perovskite single crystals can form from precursor solutions containing stoichiometric PbBr2 and [(1-y) CH3NH3Br+ yCH3NH3Cl]. The cut-off energy for basis functions was 400 eV. The general gradient approximation (GGA) was used for exchange-correlation. The phase was considered for the perovskites. For our calculations, only x=1/3 is considered. The calculated energies for various systems are listed in Table S1 in the Supporting Information. The mixed halide perovskite single crystals of CH3NH3Pb(Br1xClx)3 with different Cl/Br ratios were grown by using the
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DOI: 10.1039/C5CC01835H
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solution of [PbBr2 + (1-y) CH3NH3Br+ yCH3NH3Cl] is 0.08 and 0.17, the Cl/(Cl+Br) ratios of the CH3NH3Pb(Br1-xClx)3 single crystals obtained by ICP and single crystal analysis are about 0.15 and 0.25, respectively. The changed Cl/Br ratio during the single crystal growth suggests that Cl and Br have different affinity to form CH3NH3Pb(Br1-xClx)3 single crystal, which is different from the quick film formation via rapid heating of the spin-coated samples to evaporate the DMF solvent. Figure 3A shows the XRD patterns of grinded powders of three typical CH3NH3Pb(Br1-xClx)3 (x=0, 0.15 and 0.25) single crystals. With increase of Cl inclusion, the characteristic perovskite peaks shift to higher diffraction angles. This suggests the shrinking of
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Figure 3. The XRD (A) and normalized UV-vis absorption spectra (B) of the CH3NH3Pb(Br1-xClx)3 (x=0, 0.15, 0.25) single crystals. 70
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crystal lattices as more Cl is incorporated in the crystals, which is consistent with the single crystal studies. The simulated XRD patterns of these CH3NH3Pb(Br1-xClx)3 (x=0, 0.15 and 0.25) single crystals based on single crystal diffraction data are listed in Figure S1, which is consistent with Fig 3A. The reflectance measurement on these single crystal CH3NH3Pb(Br1-xClx)3 (x=0, 0.15, 0.25) powder exhibits a blue shift in UV-vis absorbance with increased Cl inclusion as shown in Figure 3B. This demonstrates that the inclusion of Cl broadens the bandgap.18 The bandgap of the CH3NH3Pb(Br0.75Cl0.25)3 is about 2.6 eV, which is about 0.3 eV higher than that of CH3NH3PbBr3 (~2.3 eV). The This journal is © The Royal Society of Chemistry [year]
DOI: 10.1039/C5CC01835H
UV-vis spectra and XRD patterns of thin films of CH3NH3Pb(Br1-xClx)3 perovskites prepared from the same precursor solutions of [PbBr2 + (1-y) CH3NH3Br+ yCH3NH3Cl] (y=0, 0.25 and 0.5) by spin coating are shown in Figure S2 and S3. The bandgap shift is only about 0.1 eV, which is significantly smaller than that for single crystals grown from the same precursor solutions. This observation is consistent with the different Cl/Br ratios observed between the single crystals and spin-coat films from the same precursor solutions. In growth of CH3NH3Pb(Br1-xClx)3 single crystals using the precursor solution of mixed PbBr2 and (1-y)CH3NH3Br with yCH3NH3Cl, the CH3NH3Pb(Br1-xClx)3 single crystal can only be obtained when y is no larger than 0.5. As mentioned above, the presence of CH3NH3I could undermine the growth of perovskite single crystal with this method. These observations suggest that the formation of CH3NH3Pb(Br1-xClx)3 single crystals by heating DMF may be related to the strong crystal forming affinity between Br- and Pb2+ at higher temperature, while the Cl- and Icould inhibit the formation of mixed halide perovskite single crystals. In addition to using a proper amount of CH3NH3Cl, another key factor for successful preparation of these single crystals of CH3NH3Pb(Br1-xClx)3 mixed halide perovskites is that the total molar amount of CH3NH3+ (from the sum of CH3NH3Br and CH3NH3Cl) must be set accurately to 1:1 ratio to the amount of PbBr2. If the CH3NH3Br:PbBr2 ratio is first fixed to 1:1, it would be unsuccessful to obtain CH3NH3Pb(Br1-xClx)3 single crystals with additional molar ratios of CH3NH3Cl. This result is consistent with a previous report that additional CH3NH3Cl significantly affects the crystallization of CH3NH3Br3 by slow DMF evaporation.25 More interestingly, we find if the molar ratio of CH3NH3Br to PbBr2 is larger than 1.2, we are not able to obtain any CH3NH3PbBr3 single crystals. It seems that excess molar ratios of both CH3NH3Br and/or CH3NH3Cl could significantly inhibit the crystallization of CH3NH3PbBr3, which is similar to our previous observation of retarded crystallization of CH3NH3PbI3 in presence of extra CH3NH3Cl.21 In summary, we have developed a facile growth of mixed halide CH3NH3Pb(Br1-xClx)3 single crystals by heating their DMF precursor solution. Single and powder crystal diffraction measurements have confirmed that Cl partially replaces Br in CH3NH3Pb(Br1-xClx)3 perovskite. These CH3NH3Pb(Br1-xClx)3 perovskites, with different Cl inclusion, exhibit different electronic structures and the bandgaps broaden with Cl inclusion. This unusual growth of CH3NH3Pb(Br1-xClx)3 perovskite single crystals could not only provide a facile method for growing CH3NH3Pb(Br1-xClx)3 perovskite single crystals but also enable various fundamental studies to understand perovskite crystal structure and crystal growth mechanism.
Acknowledgements
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TZ and YZ are thankful for the support of the NSFC (Grant 51372151 and 21303103). MY, JML, and KZ acknowledge the support by the U.S. Department of Energy (DOE) SunShot Initiative under the Next Generation Photovoltaics 3 program (DE-FOA-0000990). EEB and JvdL acknowledges the support on the single crystal diffraction data analysis by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences (DOE). The work at the National Renewable Energy Laboratory is supported by the U.S. Journal Name, [year], [vol], 00–00 | 3
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Department of Energy under Contract No. DE-AC36-08GO28308.
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Notes and references
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School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China E-mail: [email protected] b Chemistry and Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401 USA. E-mail: [email protected] c School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China d Department of Physics & Astronomy, and Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA
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