Angewandte A Journal of the Gesellschaft Deutscher Chemiker International Edition Chemie www.angewandte.org Accepted Article Title: Methylammonium Iodide Effect on the Supersaturation and Interfacial Energy of the Crystallization of Methylammonium Lead Triiodide Single Crystals Authors: Bichen Li, Furkan Isikgor, Hikmet Coskun, and Jianyong Ouyang This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710234 Angew. Chem. 10.1002/ange.201710234 Link to VoR: http://dx.doi.org/10.1002/anie.201710234 http://dx.doi.org/10.1002/ange.201710234 10.1002/anie.201710234 Angewandte Chemie International Edition COMMUNICATION Methylammonium Iodide Effect on the Supersaturation and Interfacial Energy of the Crystallization of Methylammonium Lead Triiodide Single Crystals Abstract: It is very important to study the crystallization of hybrid organic-inorganic perovskites because their thin films are usually prepared from solution. But the investigation on the growth of perovskite films is limited by their polycrystallinity. In this work, methylammonium lead triiodide single crystals grown from solutions with different methylammonium iodide (MAI):lead iodide (PbI2) ratios were investigated. We observed a V-shaped dependence of the crystallization onset temperature on the MAI:PbI2 ratio. This is attributed to the MAI effects on the supersaturation of precursors and the interfacial energy of the crystal growth. At low MAI:PbI2 ratio (<1.7), more MAI leads to the supersaturation of the precursors at lower temperature. At high MAI:PbI2 ratio, the crystal growing plans change from (100)-plane dominated to (001)-plane dominated. The latter have higher interfacial energy than the former, leading to higher crystallization onset temperature. Hybrid organic-inorganic perovskites (HOIPs) have attracted great attention due to their interesting structure and electronic properties. Film-based devices like perovskite solar cells (PSCs) show excellent performance.The power conversion efficiencies (PCEs) rapidly climbed from 3.8% to over 22% in just 6 years.[1–4] The performance of PSCs strongly depends on the quality of the HOIP thin film. The popular way to prepare a perovskite thin film is through the solution processing of precursors. The composition and properties of the precursor solution including the ratio of the precursors are critical for the quality of the perovskite film. For example, a small amount of excess PbI2 can improve the PCE because it can act as a passivation layer between the grain boundaries to inhibit recombination centers.[5–7] In contrast, excess MAI helps the formation of smooth, continuous and pinhole free HOIP films.[8,9] In addition, excess MAI can lead to better crystal quality and higher PL yields at the grains and grain boundaries, giving rise to high open circuit voltage. [10,11] However, the study on the precursor stoichiometry effect has been focused on the HOIP polycrystalline film. Coordination complexes in precursor solution may remain in the HOIP films due to rapid evaporation of solvent.[12-15] They may change the crystallization of HOIP by means of formation of unfavourable intermediate phases at the early stage. [*] B. Li, F. Isikgor, H. Coskun, Prof. J. Ouyang Materials Science and Engineering National University of Singapore 7 Engineering Drive 1, Singapore, 117574 E-mail: [email protected] These problems can be addressed by studying HOIP single crystals, because the perfect crystallographic structure of single crystals can exclude the effect of any residual such as excess precursor or intermediate phase on its properties. In addition, HOIP single crystals have much better stability than HOIP thin films in ambient conditions. Herein, MAPbI3 single crystals were grown from precursor solutions with different MAI:PbI 2 ratios. The MAI:PbI2 ratio affects the crystallization onset temperature and surface planes of the MAPbI3 single crystals. These are ascribed to the MAI effects on the supersaturation of the precursors and the interfacial energy of the growing planes. MAPbI3 single crystals were grown by the inverse temperature crystallization (ITC) method.[16,17] When the precursors become oversaturated at a certain temperature, crystallization starts and a black tiny crystal can be observed. The temperature for the observation of the first tiny crystal is defined as the crystallization onset temperature in this paper. As shown in Figure 1, the onset temperature depends on the MAI:PbI2 molar ratio. It is 159 oC for the solution with equimolar MAI and PbI2, and it decreases to 112 o C with the increasing MAI concentration until the ratio reaches 1.7. The temperature then increases to 138 oC at the ratio of 2.5. At the MAI:PbI2 ratio of 3, no crystals could be observed even when the temperature was raised to 170 oC. These results suggest the formation of perovskites needs to overcome higher energy barrier at the presence of much excess of MAI. This observation is consistent with the activation energy for the MAPbI 3 film formation as calculated by Khlyabich et al. Their calculations indicated a higher activation energy for the MAI:PbI 2 ratio of 3 than for the MAI:PbI2 ratio of 1. Apart from growing single crystal with excess MAI, we also tried to grow single crystals in solutions with excess PbI2. At the PbI2:MAI molar ratio of 1.2, yellow precipitates instead of black MAPbI 3 crystals were observed due to the poor solubility of PbI2 in γbutyrolactone (GBL). Therefore, we focus on the precursor solutions with excess MAI. The V-shaped dependence of the crystallization onset temperature on the MAI:PbI2 ratio indicates a complicated nucleation and growth process of the crystals. In terms of thermodynamics, the onset temperature is related to the nucleation process of the single crystal, which is dependent on the change of the total free-energy (G) of precursor solution together with the nucleus, ∆G(r) = (− 4πr3 3VM ) RTln(S) + 4πr 2 γCL (1) where CL is the interfacial energy between liquid and nucleus, VM is the molar volume of the nucleus, R is the gas constant, T is absolute temperature and S is the supersaturation ratio defined as S=C/Cs with C being the solute concentration and Cs the solubility limit (C>Cs). The supersaturation causes the nucleus This article is protected by copyright. All rights reserved. Accepted Manuscript Bichen Li, Furkan H. Isikgor, Hikmet Coskun and Jianyong Ouyang* 10.1002/anie.201710234 Angewandte Chemie International Edition COMMUNICATION 20 1:1-25 oC 1:1-50 oC 1:1-75 oC 1:1-90 oC Onset temperature Intensity (%) 160 150 140 130 15 1.6 10 1:1 1.2:1 1.5:1 1.7:1 2:1 1.4 1.3 1.2 1.1 5 120 1.0 2.0 1.5 2.0 2.5 MAI:PbI2 molar ratio Figure 1. Variation of the crystallization onset temperature of MAPbI3 with the MAI:PbI2 molar ratio formation, while the interfacial energy acts as energy barrier for the nucleation. In order to understand the MAI effect on the supersaturation, the precursor solutions with different MAI:PbI2 ratios were studied by the absorption spectroscopy. As reported by Yan et al, the HOIP precursor solutions are colloidal dispersions made of lead polyhalide frameworks, such as iodoplumbates PbIn(n-2)-. The iodoplumbates with different n values have absorption bands in different wavelengths. As shown in Figure S1, the three absorption bands at 276, 320 and 370 nm are due to PbI 2, PbI3and PbI42-, respectively. At the presence of more MAI, more iodoplumbates with higher n values can be observed. The colloids in the precursor solution were studied by and dynamic light scattering (DLS) and absorption spectroscopy (Figure 2 and Figure S2). The solutions were gradually heated from 25 to 90 oC. It is worth to note that there is no nucleation in this temperature range because the temperatures are lower than the lowest onset temperature of nucleation. This helps understand the change in the precursor species prior to the nucleus formation. In all the precursor solutions, there are mainly two DLS peaks at around 1 nm and several hundreds of nanometers. This is consistent with the observations by Tsai et al. The colloidal size changes with the MAI:PbI 2 ratio and the temperature. The size of the larger colloids increases from ~400 to ~1100 nm when the MAI:PbI2 ratio increases from 1 to 2, that is, more MAI may cause the formation of high-order iodoplumates. When the temperature is raised, the size of the large colloids decreases, which indicates the dissolution of colloids. The small colloids with the size of around 1 nm become smaller at high temperature as well. Although size change of the colloids in HOIP precursor solution was observed during aging, its fundamental mechanism is different from ours. The size change in our experiments is caused by temperature. The colloids sizes change to the original values when the solution at 90 oC is cooled back to 25 oC (Figure S2(e)), which also indicates the size change is reversible. The two types of colloids are much larger than the size of individual I- or Pb2+ ion whose ionic radii are 0.2 and 0.13 nm, respectively. It is interesting to note these ion sizes are quite close to the temperature-caused size change of the colloid of around 1 nm as shown in Figure 2(b). Thus, we can infer that the decrease in the colloid size arises from the release of free ions into the solution. This is consistent with a recent study that proposed the release of free ions occurs prior to the HOIP crystallization. Absorbance (a.u.) 1.0 1.5 0.9 1 10 100 1000 20 Size (d.nm) (c) RT 70 oC 100 oC 130 oC PbI2 1.0 PbI-3 PbI24 0.5 0.0 300 350 400 450 30 40 4 50 60 70 Temperature (oC) 80 (d) 90 100 RT 70 oC 100 oC 130 oC 3 PbI2 2 PbI3- PbI24 1 0 300 Wavelength (nm) 350 400 450 Wavelength (nm) Figure 2. (a) Size distribution of colloids in perovskite precursor solutions with the MAI:PbI2 ratio of 1 from 25 to 90 oC. (b) Variation of the colloids of ~1 nm with temperature. (c) Absorption spectra of a precursor solution with the MAI:PbI2 ratio of 1.7 at different temperatures and its normalized spectra with the absorbance normalized to that at 370 nm (d). The conversion from high-order to low-order iodoplumbates and release of free ions are further verified by the absorption spectroscopy (Figure 2(c)). It is evidently shown that the iodoplumbates decrease with the elevating temperature, indicating the conversion of iodoplumbates to free ions. In addition, the normalized spectra show that the PbI 3- and PbI2 amounts relative to PbI42- increase as well. This evidences the release of free ions from the iodoplumbates. This process is reversible as revealed by the recovery of the absorption spectrum (Figure S3). The chemical reactions in equation (2) present the conversion from iodoplumbates to I- and Pb2+. PbI42− ↔ PbI3− + I− PbI3− ↔ PbI2 + I− PbI2 ↔ PbI+ + I− PbI+ ↔ Pb2+ + I− (2) The iodoplumbate formation and the ion release shown above depend on the interaction between iodoplumbates and solvent molecule as well. The Gibbs free energy of formation of the hexacoordinated lead complex with GBL is much higher than that with DMSO or DMF and slightly higher than some iodoplumbates like PbIM5+ or PbI2M4 with M representing solvent molecule. Free ions are favoured at high temperature in GBL but not in DMF or DMSO. Hence, the dissociation of GBL-based plumbates at elevated temperature is necessary to commence retrograde solubility in the ITC method. As a result of the release of free ions, the solution becomes supersaturated, and the nucleation commences. This nucleation can be described by the La Mer mode.[25,26] The concentration of monomers increases above the saturation concentration when nucleation starts. Surface growth occurs by ions diffusion towards the clusters followed by reaction of the ions at the surface. In the case of excess MAI in the This article is protected by copyright. All rights reserved. Accepted Manuscript 0 110 100 (b) 1.5 Normalized absorbance (a.u.) Onset temperature (oC) 1.7 (a) Size (d.nm) 170 10.1002/anie.201710234 Angewandte Chemie International Edition Figure 3. Photos of MAPbI3 crystals grown from precursor solutions of different MAI:PbI2 ratios annotated as (a) type-I, (b) type-II, and (c) type-III. precursors, more concentrated colloids and high-order iodoplumbates accelerate the dissolution of colloids and release of free ions, which is understandable in terms of the equilibrium of the chemical reactions. The accelerated process of reaching super-saturation leads to higher S in equation (1), and thus lowers the onset temperature of the nucleation. As shown in the Figure 3, the MAI:PbI2 ratio affects the shape of the crystals and the crystallographic planes of the surfaces. The samples are single crystals rather than polycrystalline materials, which is supported by the SEM images (Figure S4). Apart from the surface, the crystals were cleaved, and no boundary was observed on the cleaved faces. The crystals can be categorized into three types in terms of their shapes. Crystals grown from the precursor solutions with the ratios of 1, 1.2 and 1.5 have the same shape, and they are annotated as type-I. The crystal shape becomes very different for the ratios of 1.7 and 2. They are annotated as type-II and type-III, respectively. The face indexes of the crystal surfaces were determined by the powder XRD (Figure S5 and Table S1). The shapes of the crystals are consistent with the face indexing results. The type-II crystal has three obvious surfaces that are perpendicular to each other, which should be the faces perpendicular to the crystallographic a, b, and c axes. But such three perpendicular faces cannot be found in type-I and type-III crystals. The large surface of the type-III crystal can be identified as the (001) plane. The crystal surfaces can be considered as the growing planes of the crystals. Thus, the main growing plane is (100) plane for the type-I crystal, which is the largest surface on top of the crystal and consistent with the single crystals reported by other groups.[27,28] This is evidenced by the strong diffraction from (200) and (400) as well. But the (001) plane becomes an important plane for the type-II crystal as shown by the large lateral surface in Figure 3(b). It is interesting to note that the (001) plane becomes the dominant growing plane at high MAI:PbI 2 ratio. We try to understand the interfacial energy through the estimation of the surface energy. Figure S6 and Figure S7 shows the atoms on the different crystal planes and the creation of corresponding planes, respectively. The surface energies are mainly due to the dangling bonds at the surfaces. The numbers of the broken bonds, surface areas and estimated surface energy are listed in Table S2. The estimated surface energies were close to previous report on the calculation of surface energy in orthorhombic crystal. Among the crystal planes, the (100) plane has the lowest surface energy. This plane is the main growth plane for the type-I crystal. With the increasing amount of MAI in the precursor solution, (001) planes can be observed for the typeII crystal. Since the (001) plane has higher surface energy than (100), the interfacial energy contribution to the change of the Gibbs free energy increases. This becomes more significant when the MAI:PbI2 ratio increases to 2. It is worthy to mention that the type-III crystal is not perfect single crystal even though no grain boundaries were found. Faceting occurs on the surface of the crystal. This may be attributed to the very high interfacial energy between the nuclei and the solution. To reduce the energy of the system, decomposition of the boundary plane into planes with low energies is expected to create small areas of connecting planes. This is consistent with the existence of relatively low-energy plane such as (110). The interfacial energy does not change too much when the MAI:PbI2 ratio is less than 1.7, which is supported by the same shape of the crystals. In terms of equation (1), when the interfacial energy remains unchanged, higher S leads to lower onset temperature of crystallization. As a result, the onset temperature will decrease until the MAI:PbI2 ratio reaches 1.7. In this case, the supersaturation process is the dominant factor for the crystallization. However, when the ratio is higher than 1.7, S may further increase but the interfacial energy becomes the dominant factor. This is evidenced by the shape change of the crystals and the appearance of planes with higher surface energy. As surface energy is related to interfacial energy, the increase in the surface energy will elevate the onset temperature. The results on single crystal growth can help understand the formation of HOIP thin films. However, the temperature for film formation is usually different from the crystallization onset temperature, and the crystallization is caused mainly by the solvent vaporization. In order to understand whether the temperature and solvent vaporization affect the growing planes of the single crystals, we prepare samples of numerous tiny crystals by drop casting method. Two temperatures, the crystallization onset temperature and 100 oC, were investigated for each precursor solution. The SEM images of the samples are shown in Figure S8. The crystals grown at the two temperatures exhibit the same shapes. Thus, the temperature during the crystallization may have negligible effect on the surface planes of the crystals. The surface planes depend primarily on the MAI:PbI2 ratio in the precursor solution. In addition, the shapes of the tiny crystals prepared by drop casting are consistent with the single crystals by the ITC method, though their ways to the crystallization are different. Therefore, our study on the effect of excess MAI on the crystallization of perovskite single crystal helps understand the crystallization of perovskite thin films. In conclusion, we have reported the effects of MAI on the crystallization onset temperature, the growing planes and the shape of perovskite single crystals . When the MAI:PbI 2 ratio is low, the MAI effect on the supersaturation of the perovskite precursor is the dominant factor for the crystallization. More MAI results in crystal growth at lower temperature. At high MAI:PbI 2 ratio, the interfacial energy becomes the dominant factor. More MAI leads to the increase in the interfacial energy. Accordingly, the crystallization onset temperature increases with the increasing MAI:PbI2 ratio. Acknowledgements This article is protected by copyright. All rights reserved. Accepted Manuscript COMMUNICATION 10.1002/anie.201710234 Angewandte Chemie International Edition This research work was financially supported by a research grant from the Ministry of Education, Singapore (R284-000-147-112).   Conflict of interest The authors declare no conflict of interest. Keywords: crystallization • interfacial energy • perovskites • HOIP • growing plane                J.-P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T. Jesper Jacobsson, M. Grätzel and A. Hagfeldt, Energy Environ. Sci., 2017, 10, 710. M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu, S. M. Zakeeruddin, J.-P. Correa-Baena, W. R. Tress, A. Abate, A. Hagfeldt and M. 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Isikgor, Hikmet Coskun, Jianyong Ouyang* 1.0 1.5 2.0 2.5 MAI:PbI2 molar ratio This article is protected by copyright. All rights reserved. Accepted Manuscript 170 Onset temperature (oC) The methylammonium iodide (MAI):PbI2 ratio significantly affect the supersaturation of precursors and the interfacial energy of the crystal growth. As a result, it has effect on both the crystallization onset temperature and the shape of the perovskite single crystal.