Predicting the Morphology of Perovskite Thin Films Produced by

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Predicting the Morphology of Perovskite Thin Films Produced by Sequential Deposition Method: a Crystal Growth Dynamics Study Hyomin Ko, Dong Hun Sin, Min Kim, and Kilwon Cho Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04507 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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Chemistry of Materials

Predicting the Morphology of Perovskite Thin Films Produced by Sequential Deposition Method: a Crystal Growth Dynamics Study

Hyomin Ko, Dong Hun Sin, Min Kim, and Kilwon Cho* Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea

H. Ko , D. H. Sin, Dr. M. Kim, Dr. S. B. Jo, G. Y. Bae, and Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology (POSTECH) Pohang, 790−784, Korea E-mail: [email protected]

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Abstract We performed a kinetic analysis of the sequential deposition method (SDM) to investigate how to form perovskite (CH3NH3PbI3) phases, and the effects of processing conditions on the final perovskite morphology. The reaction was found to consist of two periods with distinct kinetics. During the first period, perovskite crystals nucleated on the lead iodide (PbI2) surface, and the reaction proceeded until the surface was completely converted to perovskites. The reaction during this period determined the surface morphology of the perovskites. We were able to extract the value of the rate of the phase transformation during the first period by applying the Johnson-Mehl-Avrami-Kolmogorov model, in which the rate r is related to the average grain size

R by ∝ . In this way, r was used to predict the surface morphology of the perovskite under certain processing conditions. During the second period, the remaining lead iodide under the top perovskite layer was converted. Methylammonium iodide (CH3NH3I, MAI) molecules apparently diffused into the buried PbI2 through intergrain gaps of the top perovskite layers. Added MAI molecules reacted with PbI2, but also generated single-crystal perovskite nanorods, nanoplates, and nanocubes. The current study has furthered the understanding of detailed features of the SDM, enabled a reliable prediction of the final perovskite morphology resulting from specified processing conditions, and contributed to a reproducible fabrication of highquality perovskite films.


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1. Introduction Organic-inorganic trihalide perovskite materials (e.g., CH3NH3PbI3) have boosted the power conversion efficiency (PCE) of solution-processed thin-film solar cells to 20.8%.1-14 As light absorbers, these perovskites have remarkable properties, such as high absorption coefficients, long carrier-diffusion lengths, and ambipolar charge-transport capabilities. These properties can be exploited by perovskite films with large grains15-17, full surface coverage18, 19, low surface roughness20, and good connections between the grains21. Various methods including one-step22-24 and two-step3, 15, 20, 21,25 solution processing, vaporassisted solution processing26, and vacuum evaporation4,

27

have been developed to prepare

perovskite films that have good photovoltaic properties. The sequential deposition method (SDM) is a notably simple process for fabricating high-efficiency perovskite solar cells (Pe-SCs). When applying the SDM, PbI2 is first deposited on a substrate to form closely packed PbI2 grains with dimensions of 30 nm. The PbI2 is then converted to perovskites (e.g., CH3NH3PbI3) by being dipped sequentially into organic solutions (e.g., CH3NH3I (MAI) in isopropyl alcohol (IPA)).28 The reaction between the PbI2 and CH3NH3I solution precursors can be regarded as a pseudointercalation reaction:29 PbI2 has been shown to consist of a layer of hexagonally packed Pb atoms sandwiched between two layers of hexagonally packed I atoms, with the I-Pb-I sandwich constituting the repeating unit;30 since these units are weakly bonded to each other by van der Waals forces, electron-rich molecules can be easily intercalated into the gap between two layers of PbI2. During the sequential deposition, electron-rich I- ions that intercalate into the gap disrupt the packing between the PbI2 layers and hence disrupt the overall crystal structure, and the final perovskites are formed by ionic bonding between I-, CH3NH3+ and Pb2+ ions. The pre-deposited PbI2 provides a template for the final perovskite morphology, so this method can be used to control and reproduce the final perovskite morphology better than other deposition methods. Two precursors, MAI and PbI2, have significant effects on the final perovskite morphology obtained using the SDM. The concentration of MAI ([MAI]) has been shown to be directly correlated with the number of nucleation sites for perovskite crystals, with high [MAI] inducing more nuclei and smaller grains.31 Some impurities in the MAI solution can also affect perovskite morphology. Hypophosphorous acid (HPA), which is a stabilizer in hydroiodic acid (HI)



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solutions, has been found to form MAH2PO2, i.e., H3PO2 in the MAI solution in IPA, resulting in these impurities broadening the intergrain gaps of perovskites.32 The other precursor, PbI2, also has impacts on the morphology. Amorphous PbI2 films fabricated from the strongly coordinating solvent dimethylsulfoxide (DMSO) have been shown to be directly correlated to a homogeneous and dense perovskite morphology.20 Several post-treatments, such as thermal33 and solvent34 annealing treatments, have been applied in order to increase the crystallinity of the obtained PbI2 films, and these treatments were found to affect the perovskite morphology. The purity of the PbI2 precursor also surely has an effect on the perovskite morphology.35 However, the mechanism by which these processing conditions affect the final perovskite morphology is not understood, and this lack of comprehension of the formation mechanism of perovskites has led to insufficient reproducibility for the reaction. A few previous studies tried to explain the effect. A thermodynamic reaction model equation involving the Gibbs free energy was suggested to explain the relationship between the [MAI] and the final perovskite grain size.36 Crystal growth mechanism based on the Ostwald ripening is also suggested to explain the effect of [MAI]37. In addition, it was also suggested that two reaction mechanisms occurs during the SDM: direct conversion and dissolution and recrystallization.38 Nevertheless, these models are still insufficient at explaining the overall process of perovskite crystallization during sequential deposition. Here we report the first detailed investigation of the kinetic analysis of the conversion process during the SDM. We analyzed intermediates collected at different reaction times to determine how the perovskite grain grows, and to determine the effect of processing conditions on perovskite morphology. Two reaction periods with different kinetics were identified. The process during the first period (Period I) was found to affect the surface morphology of perovskite. The results of Period I were explained using the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic model, which correlates the solid-solid phase transformation kinetics to the nucleation and growth processes.39-41 The overall reaction rate during Period I was a key factor determining surface perovskite morphology, so the correlation between the rate of the phase transformation r and final average grain size R was quantitatively investigated. Further conversion of residual PbI2 occurred during the second period (PeriodⅡ). This conversion did not initially affect the surface morphology of perovskites; however, single-crystalline perovskite


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nanostructures (e.g., nanorods, nanoplates, nanorods) did form during PeriodⅡ, so the surface morphology did change as the reaction time increased. Our investigations also showed the conversion during Period II to be affected by several processing conditions, and allowed us to predict the final morphology of perovskite films for certain processing conditions. These results provided basic information about the perovskite formation process during the sequential deposition, and suggested the rate r to be a useful parameter to predict final perovskite morphology under certain processing condition. Finally, this study can be used to guide the reproducible fabrication of an optimal morphology for perovskite films used in thin film solar cells.

2. Results & Discussion 2.1. Two kinetic regions: Period Ⅰ and Period Ⅱ Pristine PbI2 and intermediates that had reacted with MAI for a specified amount of time were prepared by the freezing-out method (Figure S1). For all samples, PbI2 films were coated on the substrate poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), which is widely used as the hole-transport layer of perovskite solar cells. Then a volume of 80 µl of an MAI solution with one of two concentrations (5 or 10 mg ml-1, i.e., 0.031 M or 0.062 M) was dripped onto the PbI2. At various time periods later, from several seconds to a few minutes, an excess of IPA was poured onto the samples to stop the conversion reaction. Scanning electron microscopy (SEM) images (Figure 1) and X-ray diffraction (XRD) spectra (Figure S2) were obtained at each of these stages. Peaks at 12.9° and 14.1° in the XRD spectra indicated the (001) peak of PbI2 and the (110) peak of CH3NH3PbI3 perovskite, respectively. Untreated PbI2 formed densely packed crystals with dimensions of ~ 30 nm (0 s in Figure 1a). When 5 mg ml-1 MAI was used, perovskite grains formed, and their number and size increased as the reaction proceeded. The grains stopped growing by 90 s of reaction; in a top view, all of the PbI2 grains seemed to have been converted to perovskites. However, the presence of XRD peaks of PbI2 indicated that some PbI2 remained, which were revealed by cross-sectional SEM images to be under the large perovskite grains (Figure S3). The perovskite grains were dense enough to prevent the penetration of MAI molecules, so the reaction noticeably slowed after the



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saturation time (~90 s). In contrast, when 10 mg ml-1 MAI was used, many small perovskite grains formed homogeneously initially; this morphology was maintained for a while, but then large single-crystal perovskite nanorods, nanoplates, and nanocubes started to form and grow (Figure 1c), and the PbI2 peak vanished (Figure S2). These features of the conversion have been reported in previous research, which indicated that single perovskite crystals may form by dissolution and recrystallization.38 These features of the reaction suggest that the conversion reaction during the sequential deposition can be divided into two periods: one in which a direct reaction of PbI2 with MAI occurred (Period I); and the other involving the reaction of MAI with buried PbI2 (Period II). We determined conversion levels at various reaction times for each condition to gain insights into the kinetics of the two reaction periods. We used two methods to determine these conversion levels: X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD results yielded the conversion level based on the relative intensity42 of the (110) peak of CH3NH3PbI3 compared to that of the (001) peak of PbI2 according to the equation =

∙

∙

,

(1)

Where Y is the conversion level, is the area under the perovskite peak, 2 is the area under the PbI2 peak, and C is a correction factor that includes information about structure factors and the molecular weights of the two materials to revise the determined molar ratio between them at each intermediate stage. However, the structure factors of the two peaks cannot be compared using the existing X-ray diffraction database, so we inferred the values by comparing the peak intensities of two different films of pure PbI2 and pure CH3NH3PbI3 that had the same molar amounts (Figure S4a). From the results, C was determined to be ≈ 8.68. The conversion level was also inferred by applying the ‘grain counting method’ (Figure S4b), i.e., assessing the area of every perovskite grain on the SEM images, and representing the size of each grain as the square root of its area. Various morphological characteristics, the number of grains, the average grain size, and the ‘perovskite coverage’ were extracted when applying this counting method. Perovskite coverage was calculated as the ratio of perovskite area to PbI2 area in top-view SEM images of intermediate steps, and is hence related to the direct reaction of PbI2


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on top surface with MAI. The XRD results for the dilute [MAI], i.e., 5 mg ml-1 (solid dots in Figure 1b), indicated that the conversion ceased at the saturation time of about 90 s when the perovskite coverage (empty dots in Figure 1b) reached 100%, and the time is the boundary for two reaction periods. However, when the [MAI] was increased, the reaction behavior changed. At the high [MAI] of 10 mg ml-1, the direct reaction between the two precursors finished quickly (Figure 1d), then the conversion of the buried PbI2 and the formation of single-crystal nanorods, nanoplates, and nanocubes occurred with a different kinetic tendency. As discussed in detail below, the two reaction periods were observed to have different consequences: Period I determined the initial surface morphology, whereas Period II determined the extent of the formation of nanocrystals. Both periods significantly affected the final morphology of the perovskite film. 2.2 Period I – Determination of the surface morphology of perovskites During Period I, the conversion level showed an asymptotically increasing trend similar to the conventional curve of a solid-solid phase transformation that follows the JMAK model39-41 = 1 − '(−)* + ,,

(2)

(the Avrami equation). Y is also the conversion level, k is rate constant, and n is avrami exponent in the equation. This model mainly describes the kinetics of isothermal phase transformations, especially solid-solid transitions that proceed by nucleation and growth. In the JMAK model, the overall rate r of the phase transformation is represented by the inverse of the half-time t1/2 ( = .

-

/

) of the reaction, i.e., at which time the overall conversion has reached

half of the final value. The reaction rate in this model is also affected by the nucleation rate N and growth rate G according to ∝ 01 2.39-41 In the sequential deposition for our case, the nucleation process matched the first appearance of a CH3NH3PbI3 perovskite crystal from the reaction of two precursors, and the final morphology was determined by the competition between N and G. The concentration of MAI ([MAI]) was found to be one of the most influential processing conditions during the course of the sequential deposition. SEM images of perovskites that were



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converted using MAI concentrations of 4 mg ml-1 (0.025 M), 5 mg ml-1 (0.0314 M), 6 mg ml-1 (0.0377 M), and 8 mg ml-1 (0.050 M) showed differences in perovskite morphologies (Figure 2a). First, the average grain size, R, of the perovskites decreased as the [MAI] was increased. A low [MAI] induced a low nucleation density, hence increasing R. The surface coverage increased as the [MAI] was increased. In the SDM, the surface coverage is proportional to the amount of PbI2, i.e. the thickness of PbI2, and inversely proportional to average grain size, R (Figure S5). In our SDM, the amount of PbI2 was fixed (~250nm), so the coverage increased as R decreased. We also observed the grain interconnectivity to decrease and surface roughness to increase as R increased, and both of these trends are undesirable. R can therefore be used as a representative factor to evaluate the overall morphology of perovskite produced using the SDM. Intermediates collected at different times of reaction for the four MAI concentrations were analyzed by XRD and SEM, and the time-dependent variations of conversion level, perovskite coverage, # of grains, and grain size were obtained (Figure 2). For each MAI concentration, the changes over time (Figure 2b, top) in the conversion levels, calculated using eq. (1) from the X-ray diffraction data (Figure S7), showed that all had similar shapes regardless of [MAI] and that the reaction could be divided into Period I, during which the conversion level increased sigmoidally as a function of reaction time, and Period II, during which the conversion level no longer increased with increasing reaction time. The end of the Period I reaction time in each case was consistent with the time at which each perovskite coverage had reached 100% (Figure 2b). The duration of Period I decreased as the [MAI] was increased; i.e., the reaction rate during Period I increased as the [MAI] was increased. The rate r = 1/t1/2 of the phase transformation for each condition was extracted from the graphs (Table 1), and r was observed to increase as the [MAI] was increased. N and G, which are components of r, were also estimated from the results of ‘grain counting’ at each condition (Figure 2c). The number of grains linearly increased with reaction time during Period I, and the slope (Figure 2c, top) can be regarded as N. The average grain size for each condition also increased over time, but at a decreasing rate. (Figure 2c, bottom) The decreasing rate of increase was due to individual grains ceasing to grow when two or more of them grew into each other, i.e., the proportion of grains that had stopped growing increased as the reaction proceeded. Consequently, G corresponded to the slope of a linear fit to the data at an early stage of the


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graph of grain size vs. time. N was observed to increase as the [MAI] increased. However, the values of G were relatively similar at all MAI concentrations tested (inset of Figure 2c, bottom). Therefore, the rate constant of the overall reaction, i.e., r, inferred to be affected by N rather than by G. Previous reports have mentioned the effect of [MAI] on the nucleation process using following terms such as ‘nucleation density’31 and ‘nucleation probability’20. However, those terms didn’t have kinetic insight including both nucleation and growth process of perovskite crystals. Following theoretical explanations can support this. In classical solidification theory43, the nucleation rate N is expressed as, 0 = 3 exp 7−

∆9 ∗ ;

= exp 7− ;

Predicting the Morphology of Perovskite Thin Films Produced by

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