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Anomalous Growth and Coalescence Dynamics of Hybrid Perovskite Nanoparticles Observed by Liquid-Cell Transmission Electron Microscopy Fuyu Qin,† Zhiwei Wang,*,† and Zhong Lin Wang*,†,‡ †
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing 100083, P. R. China ‡ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States S Supporting Information *
ABSTRACT: We report on in situ observations of nucleation, growth, and aggregation of hybrid organic−inorganic perovskites by liquid-cell transmission electron microscopy. Direct crystallization of hybrid CH3NH3PbI3 nanoparticles is achieved through an electron beam-assisted solvent evaporation approach. Time-lapse liquid-cell TEM imaging of the nanoparticles reveals a growth trend which is not entirely consistent with the classical Lifshitz−Slyozov−Wagner growth model. Significantly complex dynamical behaviors are observed during the coalescence process of CH3NH3PbI3 nanoparticles. We propose that the chemical instability inherent in the hybrid perovskite iodides should be considered to understand this phenomenon in addition to the oriented attachment mechanism. This study provides a useful reference for understanding the intriguing chemical and physical properties of hybrid organic− inorganic perovskites. KEYWORDS: hybrid organic−inorganic perovskites, liquid-cell transmission electron microscopy, crystallization, growth kinetics, aggregation and coalescence
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manufacturing in industries. Such research will also promote our deep understanding of how the growth morphology, chemistry, and defects influence the photovoltaic properties of light-absorption materials. Transmission electron microscopy (TEM) has been widely used in imaging solid materials, providing a wide spectrum of chemical and structural information on the dry samples. Nowadays, dynamic observations of liquid samples have become possible through the liquid-cell transmission electron microscopy (LCTEM) imaging technique, which is implemented by using electron beam transparent silicon nitride films to isolate the liquid from column vacuum.13−16 With the LCTEM approach, a large success has been achieved in observing the crystallization process of various nanomaterials from solutions, especially metal nanoparticles.17−19 By taking advantage of the in situ liquid-cell TEM technique, we performed a systematic study of nucleation and growth kinetics of CH3NH3PbI3 nanoparticles from solutions. Significant
ybrid organic−inorganic perovskites (HOIPs) have been demonstrated as highly efficient sensitizers for fabricating photovoltaic solar cells due to their fascinating chemical and physical properties, such as suitable and direct band gap, large absorption coefficients, and longrange ambipolar charge transport character.1−3 The conversion efficiency of HOIP solar cells has increased, at an unprecedented speed, from 3.8% to 22.1% over the past half a decade, which already surpassed multicrsytalline silicon cells (21.3%) and approached to the best chalcogenide thin-film cells without concentrator (CuInGaSe, 22.3%).4−7 The distinct advantage of HOIP photovoltaic devices is that high-performance absorption layers can be prepared using low-temperature solution-processing approach, resulting in a facile and low-cost fabrication. However, the growth of superior absorption films from the solution-based method is certainly not trivial as synthetic conditions can significantly influence the film quality. The absorption layers with poor morphology, high defect concentration, and compositional segregation could significantly degrade the device performance.8−12 It is therefore crucial to carry out the fundamental investigation of crystallization mechanism to enable the controllable syntheses of high-quality HOIP films, a precondition for large-scale device © 2016 American Chemical Society
Received: June 27, 2016 Accepted: September 20, 2016 Published: September 20, 2016 9787
DOI: 10.1021/acsnano.6b04234 ACS Nano 2016, 10, 9787−9793
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Figure 1. (a and b) Two representative frames (Frames 1 and 25, respectively) from time-lapse imaging of CH3NH3PbI3 precursor solution at a acquisition speed of 0.5 s/frame and a dose rate of 86 e−/(Å2 s). (c and d) Schematic representation of the precipitation process of hybrid organic−inorganic perovskites via solvent evaporation approach assisted by electron beam.
the crystallization approach presented here differs from that widely used in previous LCTEM studies, where the nucleation of solvent-insoluble metal nanoparticles occurs by the way of electron beam-promoted reduction reactions.18,19 In addition, we also determined atomic-scale structures of HOIPs grown on silicon nitride window chips by the ex situ high-resolution TEM imaging technique (see Supporting Information), which does not show obvious differences from those of HOIP solar cells fabricated upon TiO2, ZnO, or SnO2 substrates (electron transport layers).1,23 This indicates that the in situ LC-TEM studies can be used as a valuable reference for understanding growth dynamics of practical photovoltaic devices. Although the important role of electron beam was observed in the precipitation of CH3NH3PbI3 nanoparticles, beam irradiation is not a mandatory condition for HOIP crystallization. Despite its low volatility, DMF could still dry up naturally after a long time stay at room temperature. Experimentally, we have tried to minimize the aging effect by performing a prompt investigation once loading the samples into TEM columns. Even so, we still find that significantly large CH3NH3PbI3 nanoparticles already arise in some regions upon the first exposure of beam irradiation. This phenomenon may be pertinent to the following two factors. One is that the amorphous Si3N4 substrate is not perfectly flat at least at atomic-scale level, which may promote inhomogeneous nucleation rates. The other is that the liquid layer does not feature uniform thickness because of its “bowl-like” shapes caused by the pressure difference existing between the two sides of sealing windows. Solutions near to the edges of viewing windows may have more chance to reach supersaturation in a short drying period because of their small thickness. Considering that perovskite CH3NH3PbI3 contains irradiation-sensitive organic compositions, we carefully investigated the effect of electron beam damage to determine the upper limit of dose rates suitable for the nucleation and growth of
dynamical complexities were found during the coalescence of the HOIP nanoparticles, which is attributed into the inherent chemical characteristics of hybrid perovskite iodides.
RESULTS AND DISCUSSION The CH3NH3PbI3 solution samples were prepared based on a single-step synthetic approach in dimethylformamide (DMF) solvents. In situ TEM investigation of crystallization process was performed using a Protochips liquid-cell TEM holder and amorphous silicon nitride E-chips with 550 μm × 20 μm viewing window. We explored the crystallization of CH3NH3PbI3 nanoparticles from DMF solutions by timelapse LCTEM imaging. Figure 1a,b shows two individual frames from sequential TEM imaging of the HOIP samples at a dose rate of 86 e−/(Å2 s) per frame. The initial frame (Figure 1a) displays a rather uniform image contrast, indicating no precipitates with detectable size from the precursor solution. In Figure 1b, taken 12.5 s after Figure 1a, a large number of nanoparticles with various sizes are clearly present in the beam illumination region. Since CH3NH3PbI3 is dissolvable in DMF, the nucleation eventually occurs from supersaturating solutions formed by the way of solvent evaporation. Electron beam plays an important role in promoting the solvent evaporation. An evidence is that when very low dose rate is used for imaging, no precipitation can be found throughout the whole irradiation period (Movie S1). The influence of electron beam on the DMF evaporation mainly concerns local heating and radiolysis. Quantitative measurements of temperature elevation due to beam heating are not yet accessible in liquid environments, but it has been estimated that the effect is rather limited when imaging with mild electron beams.20 The radiolysis effect could lead to the decomposition of DMF, yielding some gaseous products including carbon monoxide, hydrogen, and methane.21,22 A schematic illumination of the precipitation process of the HOIP nanoparticles is given in Figure 1c,d. Obviously, 9788
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Figure 2. (a and b) Two typical frames (Frames 1 and 50, respectively) from sequential imaging of CH3NH3PbI3 nanoparticles at 0.5s/frame and 125 e−/(Å2 s). (c−e) The atomic model of cubic CH3NH3PbI3 structure (space group: Pm-3m): (c) a perspective view; (d and e) the projections along ⟨100⟩ and ⟨111⟩, respectively. I: purple; Pb: gray; C: yellow; N: blue. H is not included.
Figure 3. Nucleation and growth kinetics of CH3NH3PbI3 nanoparticles. The video images were recorded at 0.5 s/frame and 50 e−/(Å2 s). (a− c) Frames 1, 9, and 50, respectively. (d) The number of nanoparticles as a function of acquisition time. (e) Plots the averaged radius of nanoparticles as a function of time. (f) Logarithmic relationship of the averaged size versus time based on the data in (e).
continuously with electron irradiation, indicating that the electron beam with such dose level can produce an obvious damage to CH3NH3PbI3 nanoparticles. Our statistical inves-
nanoparticles. Figure 2a,b shows two representative frames in the video images (Movie S2) recorded with a dose rate of 125 e−/(Å2 s). It is seen that most of the nanoparticles shrink 9789
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Figure 4. Aggregation and coalescence in a region featuring high density of CH3NH3PbI3 precipitates. (a and b) Frames 2 and 50, respectively, extracted from time-lapse imaging at 0.5 s/frame and 80 e−/(Å2 s). Arrow A denotes the shrinking/disappearance of a nanoparticle and circles B1−B3 the aggregation and coalescence of nanoparticles. (c) The number of nanoparticles as a function of time. (d−i) An enlarged view of the squared region “C” in (a) and (b) for Frames 2, 11, 15, 19, 32, and 50, respectively.
tigation shows that the dose rate below 90 e−/(Å2 s) is generally safe for the crystallization study. Compared with previously reported (metal−organic) zeolitic imidazolate frameworks (ZIFs) which suffer significant electron damage even at the dose rate of about 5 e−/(Å2 s),24 the HOIP samples display apparently higher stability under TEM imaging. The difference may be due to the variation of their structural configurations. For the cubic structure of HOIPs (space group: Pm-3m), as shown in Figure 2c−e, the organic cations lie at the body center of unit cells, surrounded by 12 nearest halide anions (edge centers) and 8 second-nearest metal cations (vertices), basically. This structural configuration follows the basic inorganic closely packed framework and is rather compact. Especially, none of crystallographic planes in this structure are constructed by purely radiation-sensitive organic cations, so the beam damage (if it occurred) would most likely start from the nanoparticle surfaces, layer by layer, to the interiors. This should lead to a gradual size decrease, consistent with our experimental observations (Movies S2). The tetrahedral structure (I4/mcm)the preferable configuration of CH3NH3PbI3 at room temperaturecan be regarded as a slight deformation from the cubic counterpart and thus should have similar endurance ability to electron irradiation. Comparatively, the metal−organic frameworks exhibit much looser structural characteristics (Figure S2), where very large spaces can be seen between metal atoms and organic groups. In addition, there exist some low-index crystallographic planes consisting of the merely organic groups. When ZIFs nanoparticles are channeled through by electron beam along these radiation-sensitive crystallographic planes, a part of the
nanoparticles could be cut off under long violent electron irradiation.24 The high tolerance of the perovskite structure to electron irradiation allows us to investigate quantitatively the nucleation and growth kinetics of perovskite nanoparticles. Figure 3a−c shows three typical frames from a time-lapsed series of images (Movie S3) recorded with electron dose of 55 e−/(Å2 s). Several large nanoparticles (one of which was marked with the blue arrow in Figure 3a) was found to already present in the initial frame, which may be formed by the natural solvent evaporation as mentioned above. Figure 3d plotted the number of nanoparticles (Np) as a function of time (t) for all the frames recorded in this movie. As the nanoparticles with radius R smaller than 2 nm is difficult to detect for the magnifications used, we set R = 2 nm as a threshold for the nanoparticle counting. R is calculated based on its projection area S, i.e., R = (S/π)1/2. It is seen that Np exhibits a dramatic increase within the first ∼7 s and then rises very slowly or becomes constant. This indicates a rapid nucleation of CH3NH3PbI3 nanoparticles from supersaturating solutions, followed by a steady growth to the detectable size. Figure 3e shows an evolution of the average-number radius (Ra) as a function of time. A tendency of size decrease is clearly present in the first ∼3 s. This represents a marked size-averaging effect between the emerging precipitates and the several pre-existing large nanoparticles. Figure 3f plots the logarithmic relationship of the nanoparticle sizes with acquisition time based on the data in Figure 3e. It shows that a roughly linear relationship arises after the rapid nucleation has been completed (7 s). However, the curve fitting displays an approximate relationship of Ra ∼ t1/5, indicating that the CH3NH3PbI3 growth does not completely obey either 9790
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ACS Nano diffusion-limited (Ra ∼ t1/3) or reaction-limited (Ra ∼ t1/2) growth mechanism predicted by Lifshitz−Slyozov−Wagner (LSW) theory for the current synthetic conditions.25−27 Note that no obvious aggregation or coalescence is present in this sample region throughout the time-lapse video image sequence. The comparatively smaller exponent n in the Ra ∼ tn may indicate the inherent growth dynamical characteristics of hybrid perovskite nanoparticles. Real-time observations of HOIP growths were performed in multiple solution regions. Figure 4a,b shows two typical frames from sequential imaging (Movie S4) in another region at the dose rate of 80 e−/(Å2 s). This region features obviously higher density of CH3NH3PbI3 nanoparticles. As the dose rate used here is very similar to that for Figure 1, the density variation may be attributed into the effect of inhomogeneous Si3N4 substrates on the nucleation. The shrinking/disappearance of nanoparticles sometimes occurs; an example is shown in Figure 4a (see the arrow A). However, the really predominated phenomenon in this region is the high-frequency aggregation and coalescence of nanoparticles (see the exemplary circled regions B1−B3 in Figure 4a,b). It is the nanoparticle coalescence that mainly causes the decreases of Np with time (Figure 4C). The Brownian-type motion is evident during the growth and aggregation process of HOIP nanoparticles. Figure 4d−i shows several individual image frames with an enlarged view of the squared region “C” in Figure 4a,b. It is seen that at the beginning the nanoparticle “1” has been very closer to the nanoparticle “4” (Figure 4d), but it finally combines with another two (“2” and “3” in Figure 4d) to form a new elongated nanoparticle (“1 + 2 + 3” in Figure 4i) after high-speed, random movements. A further data analysis reveals the presence of significantly dynamical behaviors in the coalescence process of HOIP nanoparticles. In Figure 4e, 4.5 s after Figure 4d, we see that the nanoparticle “5” is divided into two parts: the larger “5L” and the smaller “5S”. Then, the part “5L” evolves into a new nanoparticle by combining with its neighbor “4” at the late stage (“4 + 5L” in Figure 4i). However, prior to the final coalescence, they have moved closer again and seemingly tried to get back to the original state (see Figure 4f). This clearly manifests that the actual coalescence process is rather complicated and roundabout for HOIP nanoparticles. It is well established that translational and rotational alignments are often involved in the aggregation/coalescence of inorganic nanoparticles in liquid solutions, assisted by longrange or short-range attractive forces.28 This mechanism indicates intricate dynamical characteristics of the nanoparticle aggregation process, but it is unable to entirely explain the so intense dynamical complexity present in the coalescence of CH3NH3PbI3 nanoparticles. The chemical characteristics of HOIPs may have to be considered to understand this unusual phenomenon. Since the HOIP nanoparticles were synthesized by low-temperature solution route on the uneven amorphous substrates, the kinetic effect should play an important role in defining the nanoparticle morphologies as well as compositions during the earlier stage of crystallization.8,29−31 Furthermore, the 200 kV electron beam could also remove some organic species from the synthetic nanoparticles, creating some point effects (chemical vacancies) within the nanoparticles.22,32 Both the factors unavoidably influence chemical compositions and may result in the formation of some thermodynamically unstable, off-stoichiometry HOIP nanoparticles. These unstable nanoparticles are likely to become dissociable under external excitations. If a nanoparticle (or a dissociated product) has
favorable composition to any of its neighbors (e.g., one is I-rich and the other I-deficient), then the coalescence may occur according to the oriented-attachment mechanism. In brief, the key point we propose here is that the chemical compatibility should be also important for the coalescence of HOIP nanoparticles.
CONCLUSIONS We performed a systematic and dynamical investigation of crystallization process of hybrid organic−inorganic perovskite nanoparticles by LCTEM. We demonstrate that the simple electron beam-promoted solvent evaporation is an effective approach for the direct precipitation of CH3NH3PbI3 from precursor solutions. The HOIP nanoparticles exhibit rather high endurance ability to electron irradiation owing to their compact structural configurations, but the dose rate smaller than 90 e−/(Å2 s) is still required to avoid or minimize the beam damage. Quantitative analysis of nanoparticle evolution in solution shows that the averaged nanoparticle radius (Ra) and time (t) approximately follows a relationship of Ra ∼ t1/5, thus not fully complying with either diffusion or reaction-limited growth model as predicted by the LSW theory. The frequent coalescence was observed in the solution regions containing high density of precipitates, which displays unusually complex dynamical behaviors. An integrated consideration of both crystal orientations and chemical characteristics is proposed for understanding this interesting phenomenon. This work provides a way to investigate the nucleation and growth characteristics of organic−inorganic hybrid materials through liquid-cell TEM techniques. With the approach developed here, future comparative studies of hybrid organic−inorganic perovskites made of various anions (e.g., Cl and Br) and cations (e.g., formamidinium and Cs) may be carried out to promote the mechanistic understanding and aid in the production of perovskite absorptions with long-time chemical and structural stabilities.
MATERIALS AND METHODS Preparation of CH3NH3PbI3 Solutions. To study the crystallization mechanism of hybrid organic−inorganic perovskites, we prepared CH3NH3PbI3 solution samples based on a single-step synthetic approach. 0.1 mM CH3NH3PbI3 precursor solutions were produced by dissolving a mixture of methylammonium iodide (CH3NH3I, 99.5%) and lead diiodide (PbI2, 99.99%) with 1.1:1 mole ratio in dimethylformamide (DMF) solvents. The solutions were then heated to 70 °C for 30 min. Liquid-Cell Transmission Electron Microscopy Characterizations. LCTEM investigation was carried out using a Protochips LCTEM holder and amorphous silicon nitride Echips with 550 μm × 20 μm viewing window and 500 nm spacer. The thickness of the silicon nitride films is 50 nm for each. Prior to loading the CH3NH3PbI3 solution, the E-chips were first processed with plasma cleaner (300w, 9:1 Ar/O ratio) to obtain hydrophilic surface. TEM imaging was performed in a 200 kV FEI F20 transmission electron microscope with a field emission gun. Time-lapse video image sequences were recorded with an exposure time of 0.5 s/frame on a Gatan 832 CCD camera. 9791
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04234. Ex situ chemical and structural characterizations; structural stability analysis; LCTEM movies (PDF) Movie S1: Time-lapse imaging of CH3NH3PbI3 solution at an acquisition speed of 0.5 s/frame and a dose rate of 3 e−/(Å2 s) (AVI) Movie S2: Full dataset of Figure 2. The image series was recorded at 0.5 s/frame and 125 e−/(Å2 s) (AVI) Movie S3: Full dataset of growth process of CH3NH3PbI3 nanoparticles depicted in Figure 3 (AVI) Movie S4: Full dataset of Figure 4, indicating the aggregation and coalescence in a region featuring high density of CH3NH3PbI3 precipitates (AVI)
AUTHOR INFORMATION Corresponding Authors
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS This research was supported by the “Thousands Talents” program for pioneer researcher and his innovation team and National Natural Science Foundation of China (grant no. 51571035). REFERENCES (1) Brenner, T. M.; Egger, D. A.; Kronik, L.; Hodes, G.; Cahen, D. Hybrid Organic-Inorganic Perovskites: Low-Cost Semiconductors with Intriguing Charge-Transport Properties. Nat. Rev. Mater. 2016, 1, 15007. (2) Sun, S. B.; Yuan, D.; Xu, Y.; Wang, A. F.; Deng, Z. T. LigandMediated Synthesis of Shape-Controlled Cesium Lead Halide Perovskite Nanocrystals via Reprecipitation Process at Room Temperature. ACS Nano 2016, 10, 3648−3657. (3) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647. (4) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (5) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphrey-Baker, R.; Yum, J.-H.; Moser, J. E.; Gratzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (6) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476−480. (7) Best Research-Cell Efficiencies from NREL Website, rev. 08-122016. http://www.nrel.gov/ncpv/images/efficiency_chart.jpg. (8) Salim, T.; Sun, S.; Abe, Y.; Krishna, A.; Grimsdale, A. C.; Lam, Y. M. Perovskite-Based Solar Cells: Impact of Morphology and Device Architecture on Device Performance. J. Mater. Chem. A 2015, 3, 8943−8969. (9) Song, T. B.; Chen, Q.; Zhou, H.; Jiang, C.; Wang, H. H.; Yang, Y. M.; Liu, Y.; You, J.; Yang, Y. Perovskite Solar Cells: Film Formation and Properties. J. Mater. Chem. A 2015, 3, 9032−9050. (10) Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-Temperature Solution Processing Techniques. Nat. Photonics 2013, 8, 133−138. 9792
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