Letter pubs.acs.org/NanoLett
Crystallization Dynamics of Organolead Halide Perovskite by RealTime X‑ray Diffraction Tetsuhiko Miyadera,*,†,‡ Yosei Shibata,† Tomoyuki Koganezawa,§ Takurou N. Murakami,† Takeshi Sugita,† Nobutaka Tanigaki,† and Masayuki Chikamatsu† †
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ‡
S Supporting Information *
ABSTRACT: We analyzed the crystallization process of the CH3NH3PbI3 perovskite by observing real-time X-ray diffraction immediately after combining a PbI2 thin film with a CH3NH3I solution. A detailed analysis of the transformation kinetics demonstrated the fractal diffusion of the CH3NH3I solution into the PbI2 film. Moreover, the perovskite crystal was found to be initially oriented based on the PbI2 crystal orientation but to gradually transition to a random orientation. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells. KEYWORDS: organolead halide perovskite, solar cell, crystallization, synchrotron-radiation X-ray diffraction, grazing incident wide-angle X-ray scattering, anomalous diffusion
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ray diffraction (XRD) observations of the crystallization process during the annealing of spin-coated film have also been published.6−8 These studies analyzed the film structure after various elapsed time intervals following mixing. The most important phenomena in such cases is the time evolution of the crystallization just after mixing of the PbI2 and CH3NH3I, and so the real-time analysis of such phenomena is meaningful. Real-time XRD analysis during vacuum deposition of the perovskite has also been reported, during which the phase transition of the perovskite films throughout the deposition was assessed.9 In the present study, we focused on the analysis of the dynamics of perovskite crystallization during solution processing. As such, we acquired real-time grazing incident wide-angle X-ray scattering (GIWAXS) data for samples during the crystal transformation process by dropping a CH3NH3I solution onto PbI2 films during trials within the SPring-8 BL46XU synchrotron radiation facility (Figure 1). The source material, PbI2, was purchased from Aldrich and CH3NH3I was synthesized in our lab. PbI2 films were prepared using two methods: vacuum deposition and spin coating. The vacuum-deposited PbI2 was fabricated on glass substrates by laser deposition. The spin-coated PbI2 was produced on glass substrates coated with a TiO2 mesoporous layer by spin-coating a solution of PbI2 in N,N-dimethylformamide (462 mg/mL).
etermining crystallization mechanisms is a crucial issue in the field of semiconductor processing. Organolead halide perovskite solar cells are developing very rapidly and have attracted much attention because of their high power conversion efficiencies, typically above 20%.1−4 The study of the crystallization processes of the perovskites is important because their performance in solar cells is strongly affected by their crystal structure and morphology. Many studies concerning the fabrication of perovskite films have been reported based on both solution processing and vacuum deposition.5−22 These reports have primarily focused on differences in the crystallinity and morphology of the perovskite films and their effects on device characteristics, both of which result from variations in the fabrication process. Only a few studies, however, have dealt with the fundamental crystallization mechanism of perovskite films. Clarification of the fundamental mechanism by detailed observations of the crystallization process is essential for the establishment of optimum fabrication processes, especially because perovskite solar cells suffer from poor reproducibility and their development to date has largely relied on trial and error. Consequently, the establishment of a fundamental mechanism is a very important step in fabricating high efficiency devices with good reproducibility. In a previous report on ex situ observations of perovskite crystallization, Im et al. analyzed changes in the surface morphology of samples during the solution-dipping process by scanning electron microscopy (SEM).5 In addition, in situ X© XXXX American Chemical Society
Received: June 17, 2015 Revised: July 19, 2015
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DOI: 10.1021/acs.nanolett.5b02402 Nano Lett. XXXX, XXX, XXX−XXX
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the reaction rate at varying temperatures from approximately ambient (30 °C) to a normal processing temperature of 70 °C. The diffraction intensity was assessed using the 1 0 1 reflection of PbI2 for vacuum-deposited samples and the 0 0 1 reflection for spin-coated samples, since these gave high diffraction intensities. In the case of perovskite, the 1 1 0 reflection was used for both samples. It should be noted that the diffraction patterns also contained scattering from the solution that disturbed the diffraction signals, especially at 70 °C because the solution rapidly evaporated at this temperature and had to be reapplied repeatedly at intervals of several seconds. During the analyses below, a background signal was subtracted to eliminate this solvent effect. The time evolution of the PbI2 signal, I(t), is plotted in Figure 3a−c, from which it is evident that the majority of the decay curves were well fitted by the two components of the following compressed/stretched exponential function:
Figure 1. Schematic illustration of the experimental setup used to observe the crystallization dynamics of an organolead halide perovskite in real-time. Real-time GIWAXS data were acquired using the Spring-8 BL46XU synchrotron radiation facility.
Each PbI2 film was mounted on the gonio stage (HUBER Xray diffractometer) of the SPring-8 beamline BL46XU to allow for in situ XRD analysis. GIWAXS data were acquired using 12.398 keV X-rays (λ = 1.000 Å) at an incident angle of 0.25° and diffracted X-rays were captured at 0.1 s intervals by a twodimensional detector (PILATUS 300K) located at a distance of L = 174.5 mm from the sample. After sample alignment, in situ measurements were performed as a solution of CH3NH3I in 2propanol (10 mg/mL) was dropped onto the PbI2 film. Solution droplets were added at sufficient intervals so as to maintain coverage of the substrate by the solution throughout the reaction process. At room temperature, the droplets would remain on the surface for suitable lengths of time, although fresh solution had to be added every few seconds at 70 °C to prevent drying of the film. The diffraction pattern obtained before the reaction represents a typical PbI2 crystal pattern, whereas patterns attributed to the perovskite appear after applying the CH3NH3I solution.23 PbI2 films were prepared by vacuum deposition without a mesoporous TiO2 layer and by spin coating with mesoporous TiO2. In the case of the PbI2 films prepared by vacuum deposition, a spot pattern was observed (Figure 2a), indicating a crystal with its [0 0 1] axis along the normal direction.24 The Miller indices demonstrating hexagonal PbI2 crystal structure are shown in Figure 2a, whereas the perovskite indices indicate a tetragonal crystal structure and are shown in Figure 2c. In contrast, PbI2 films prepared by spin coating exhibited a polycrystalline structure that generated concentric semicircle diffraction patterns (see Supporting Information). Changes in the diffraction patterns over time were analyzed in detail, and the diffraction intensity of PbI2 was found to be reduced immediately after application of the CH3NH3I solution, at which point the perovskite peak intensity increased. We conducted a series of experiments by varying the PbI2 film thickness from 50 to 100 to 200 nm with subsequent analysis of
I(t ) = A1exp{− (t /t1)β1 } + A 2 exp{− (t /t 2)β2 } + C
Here, the term exp{−(t/τ)β} represents the stretched exponential function for 0 < β < 1 and the compressed exponential function for 1 < β. The fitted functions are indicated by broken lines in Figure 3a−c. In addition, the time evolutions of the perovskite formation signals are presented in Figure 3d−f. Here, the rising perovskite plots are compared with the fitting results for the PbI2 decay curves by vertically flipping and magnifying the fitting functions (indicated by broken lines in Figures 3d−f). This comparison shows that the time constants for the decay of the PbI2 and increase in the perovskite are in good agreement, especially for the results obtained at 30 °C. Transformations that can be represented using compressed/stretched exponentials are commonly associated with anomalous diffusion into soils and other soft materials.25,26 This diffusion may be explained by considering that the diffusion throughout the media generates fractal structures. The fractal element in the present case is the fractallike diffusion of the CH3NH3I into the polycrystalline PbI2 morphology, as shown in Figure 4b. The existence of dual flow paths can be explained by considering that the CH3NH3I initially diffuses into a pure PbI2 film and then diffuses throughout a mixed structure composed of PbI2 and the newly generated perovskite. The reaction rate can be estimated by analyzing the time constant of the diffusion (τ). This analysis demonstrated the expected trend, in which the reaction rate is increased in the case of thinner films and at elevated temperatures, albeit with some minor exceptions (Figure 3a). The rate of perovskite formation from the spin-coated PbI2 was much faster than that
Figure 2. Time dependent change of GIWAXS patterns represented in reciprocal lattice space: (a) the initial 100 nm PbI2 film and the film (b) 1.1 s and (c) 75.9 s after adding the CH3NH3I solution. B
DOI: 10.1021/acs.nanolett.5b02402 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. Time evolution of the intensity of scattered X-rays for (a) PbI2 101 at 25 °C, (b) PbI2 101 at 70 °C, (c) spin-coated PbI2 001, (d) perovskite 110 at 25 °C, (e) perovskite 110 at 70 °C, and (f) perovskite 110 fabricated from spin-coated PbI2. The broken lines in (a), (b), and (c) indicate the fitted experimental data based on the compressed/stretched function. The broken lines in (d), (e), and (f) show the vertically flipped fitted function plots from (a), (b), and (c), which have been adjusted along the vertical direction to assist in making comparisons.
approximately 10-fold increase in the rate at the higher temperature. In contrast, the reaction rates of the spin-coated PbI2, as calculated from single film thickness data, were 2.0 and 8.2 μm/s at 30 and 70 °C. In the case of the spin-coated PbI2, the decay curve was also found to contain only one component. The reaction times reported by other groups vary significantly, ranging from 20 s to several tens of minutes. Wakamiya et al., for example, reported a very rapid reaction that was complete within several tens of seconds.10 They attributed this fast reaction to the presence of voids in the PbI2 films, generated by solvent evaporation, that allowed the CH3NH3I solution to rapidly penetrate the film. Im et al. analyzed the surface morphology changes of films and determined that the reaction was complete within 20 s,5 whereas Wu et al. observed a reaction time of several minutes and showed that the reaction rate was reduced when residual solvent used for the initial fabrication of the PbI2 remained in the film.11 Jeon et al. also reported that the addition of different solvents during the CH3NH3I spin coating process can vary the crystallinity of the resulting film.12 On the basis of these results, it appears that the reaction speed will vary depending on both the void density and the effect of various solvents. Thus, for each reported fabrication process, the effects of the PbI2 film morphology, including grain boundaries and voids, as well as
Figure 4. (a) Time constant values (τ) obtained by fitting for various conditions. (b) Schematic illustration of the diffusion of CH3NH3I in the medium showing fractal features.
from vacuum-deposited PbI2 and the reaction was found to be complete within several seconds, even though the spin-coated film was thicker (400 nm). The experimental conditions used herein are widely applied as the so-called two step method and, as noted, our results indicate that the reaction was complete within several seconds under such conditions. In addition, the perovskite formation rates obtained from the vacuum-deposited PbI2 (as calculated to determine the film thickness dependence of the time constant of the slower component) were 1.4 and 18 nm/s at 30 and 70 °C, respectively, representing an
Figure 5. Time evolution of the angle-dependent diffraction intensity for a PbI2 film reacted at 70 °C. (b) Crystal structure of PbI2. (c and d) Crystal structures of perovskite oriented at 35° and 90°, respectively. C
DOI: 10.1021/acs.nanolett.5b02402 Nano Lett. XXXX, XXX, XXX−XXX
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of a fluid state for the perovskite. The reported effects of thermal and solvent annealing could originate from fluid features of the perovskite such as these.13 The reaction dynamics of organolead halide perovskites determined during this study will assist in establishing more finely controlled fabrication processes. In such processes, the specific phenomena reported herein, such as the anomalous diffusion of the CH3NH3I solution and orientational fluctuations, should be controlled to permit the fabrication of highquality perovskite films with good reproducibility. Further investigation is required to obtain a full understanding of all aspects of the fabrication process, such as the manner in which the reaction dynamics are influenced by the reaction conditions. The results of this study therefore can be considered to add to the fundamental knowledge necessary to elucidate the perovskite fabrication process. In conclusion, the reaction dynamics of the organolead halide perovskite fabrication process was analyzed by real-time X-ray diffraction. A specific phenomenon, namely anomalous diffusion was observed in the reaction process, which was represented with stretched/compressed exponential function. This is originated from the diffusion into fractal media, where the CH3NH3I diffuse anomalously into the PbI2 films with fractal morphology. Moreover, the perovskite crystal was observed to be oriented according to the initial PbI6 octahedral framework at the beginning of the reaction and then the crystal changed fluidly into random orientation. The particular dynamics in perovskite formation process was revealed. Controlling such features of crystallization is important to establish the fabrication process with high reproducibility.
the effect of residual and postadded solvent, should be investigated in detail to allow a full understanding of the reaction dynamics. The reaction dynamics analysis in the present study focused on samples with uncomplicated morphologies because the vacuum-deposited PbI2 films used as the initial materials were free from the effect of solvents or mesoporous layers. The results of this study therefore should contribute to the overall understanding of the crystallization process in various situations. The CH3NH3I diffusion through the fractal film morphology can be assessed based on the experimental time constants, allowing an analysis of the effects of the internal void density and the presence of solvent with regard to acceleration or deceleration of the reaction speed. We subsequently analyzed orientational changes during the crystallization process in the case of the vacuum-deposited PbI2 films, which initially exhibited orientation in a specific direction. During the initial stage of crystal transformation, the newly generated perovskite crystal was highly oriented, with the diffraction pattern appearing in a specific direction. Following this stage, the diffraction pattern gradually transitioned to a ring, indicating the random orientation of the perovskite. Intense 1 1 0 diffraction peaks generated by the perovskite were initially present at polar angles of 90° and 35°, taking the incident X-ray direction as the axis and defining 0° as the horizontal plane direction. This result indicated that the cubic units in the perovskite crystal were oriented in the normal direction and also tilted at an angle of 35°. After a short time, typically 20 s in the case of the 100 nm PbI2 films, this orientation exhibited a gradual change, becoming totally random after 500 s. The PbI2 crystal lattice had a hexagonal structure with octahedral PbI6 elements (Figure 5b) with a diagonal orientation at 35°. The 35°-oriented crystal observed in this study represents the perovskite-crystal growth mode in the initial direction of the PbI6 octahedral structure. That is, during the initial stage of the crystal reaction, there are two possible growth orientation modes; one that maintains the orientation of the initial materials and another that represents the ordinary crystal orientation in the vertical direction. The subsequent transition to a random orientation gradually takes place even after the perovskite formation and the crystal structure thus continuously changes after the perovskite formation reaction is complete. These dynamic changes in the crystal orientation of the perovskite represent a novel observation because such phenomena have never before been reported. These results, thus, will provide useful information with regard to elucidating perovskite formation dynamics. The films were observed to swell as perovskite was formed from the initial PbI2 because of the difference in the lattice constants (see atomic force microscopy images in Supporting Information). The typical grain size of PbI2 is less than 100 nm, whereas that of perovskite film is more than 500 nm not depending on the fabrication condition in this study. The SEM observations of Im et al. also identified swelling associated with the crystal growth process.5 The perovskite grains were consequently much larger than the initial PbI2 grains. When we consider this grain swelling in addition to the two stages of reaction rates and the accompanying orientational changes, it is evident that the final, fixed morphology of the perovskite is not generated immediately. Rather, the film goes through a transitional state in which its grain morphology and crystal orientation continuously change. Changes in the lattice constant were also observed during the reaction process (see Supporting Information), which further supports the existence
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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/acs.nanolett.5b02402.
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Details of experiment and analysis, GIWAXS data (still image), and film morphology. Details of methods and supporting figures. (PDF) GIWAXS data (AVI) GIWAXS data (AVI) GIWAXS data (AVI) GIWAXS data (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank A. Wakamiya in Kyoto Univ. for helpful discussion. The GIWAXS measurement was performed at SPring-8 BL46XU with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, proposal nos. 2014B1614 and 2015A1689). This work is financially supported by Japan Science and Technology Agency (JST) through its funding program for Precursory Research for Embryonic Science and Technology (PRESTO). D
DOI: 10.1021/acs.nanolett.5b02402 Nano Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.nanolett.5b02402 Nano Lett. XXXX, XXX, XXX−XXX
