Fabrication and Growth Mechanism of Uniform Suspended Perovskite

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Fabrication and Growth Mechanism of Uniform Suspended Perovskite Thin Films Chong Geng, Fangfang Li, Yuemei Fan, Lijing Zhang, Shuangshuang Shi, Zi-Hui Zhang, Yonghui Zhang, Shu Xu, and Wengang Bi Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00149 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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Crystal Growth & Design

Fabrication and Growth Mechanism of Uniform Suspended Perovskite Thin Films Chong Geng1, Fangfang Li1, Yuemei Fan1, Lijing Zhang2, Shuangshuang Shi1, Zi-Hui Zhang1, Yonghui Zhang1, Shu Xu1*, and Wengang Bi1* 1

Tianjin Key Laboratory of Electronic Materials and Devices, School of Electronics and

Information Engineering, Hebei University of Technology, Tianjin 300401, China. 2

Department of Chemistry, Dalian University of Technology, Dalian 116024, Liaoning, China.

KEYWORDS: Crystal growth kinetics, Micro/Nanostructure-assisted crystallization, Suspended films, Perovskite materials

ABSTRACT: Crystallization and morphology control is a fundamental challenge in the preparation of perovskite thin films. In this article, we demonstrate the fabrication of large area and uniform CH3NH3PbBr3 suspended films on a periodic microstructure. Compared with the conventional perovskite film, the suspended perovskite film proves to have a better optical performance from both aspects of the simulation and the characterization. Studies on the growth mechanism reveal that both the capillary-induced adhesion and the solvent evaporation rate play important roles in controlling the morphology and formation of the suspended film. In particular, the capillary-induced adhesion supports the precursor-solution film on the nanobowl-like

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structures, while the dynamic-dripping (DD) method is utilized to accelerate the solvent evaporation rate and promotes the formation of the transition-state film.

In situ

photoluminescence characterization is employed to investigate the growth kinetics of the crystals in the transition-state film during the annealing process, and X-ray diffraction peak shift of the crystalline perovskite films presents the relationship between the film formation process and the lattice strain. This study provides insights into the influences of the interfacial forces and the evaporation dynamics on the kinetics of perovskite film formation, and draws the conclusion that both the DD approach and microstructural parameters are key factors in controlling the film morphologies and achieving high quality CH3NH3PbBr3 suspended films.

INTRODUCTION Organolead halide perovskite materials (OHPs) have evoked enormous attention over the past few years in optoelectronics field due to their excellent optoelectronic properties and solution processability for thin film device fabrication at room temperature.1-4 Thereinto, ‘one-step’ solution approach is considered to be the most economic method for the fabrication of OHPs films.5,6 A typical ‘one-step’ method involves three stages: the initial precursor solution state, the transition stage from a solution to a solid mixture of intermediates, and the final crystalline stage to form perovskite film.7 Firstly, OHPs precursor solutions are spin-coated on a substrate and form a precursor-solution film, which comprises the composition of complex intermediate, such as CH3NH3I-PbI2-DMF particles.8 Subsequently, with the evaporation of solvent, the liquid film is concentrated and these particles assemble into a solid transition-state film with complex adducts, in which coexist intermediate particles and perovskite nuclei. Finally, a thermal


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annealing process is executed to convert the intermediate particles into perovskite crystals, forming a crystalline perovskite film. However, during spinning the tardily evaporating solvents and slow precipitation of OHPs cause significant dewetting of the precursor solution and low nuclei density on the substrate, resulting in the subsequent exaggerated growth of the few nuclei and the lack of planar structures.9,10 Hence the obtained OHPs films usually present incomplete coverage and non-uniform thickness as the consequence. Therefore, in ‘one-step’ method, controlling the morphology and the uniformity of OHPs films becomes a critical issue. Scientists have paid great effort to solve this issue through various methods. An extensive method is the interfacial engineering approach,11-13 in which the dewetting of the precursor solution could be suppressed by adjusting the surface hydrophilic property of the underlying substrate. Another common method is the addition of antisolvent,14-16 which could rapidly reduce the solubility of OHPs in the precursor solution and thereby promote fast nucleation and growth of the crystals in the films. Recently, micro/nanostructure-assisted OHPs crystal growth has been proposed to control the morphology of the OHPs films. After infiltrating micro/nanostructures with the precursor solutions, the crystal growth could be bounded by the structural wells, accordingly leading to OHPs domains with controllable coverage. For instance, Hörantner17 and Zhang18 filled CH3NH3PbI3 (MAPbI3) precursor solution into the holes of a metal oxide mesoporous structure, leading to the MAPbI3 crystal growth into desired domain size. Lee19 et al. confined the crystallization of MAPbI3 with anodized aluminum oxide templates, and the continuous MAPbI3 film exhibited a preferred crystalline orientation. Zheng20 and Zhang21 improved the morphology and the crystallinity of the MAPbI3 films by introducing porous structures in the transporting layer. By the micro/nanostructure-assisted crystallization approach, the surface coverage of OHPs films could be significantly enhanced. Nevertheless, there still


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remain a prominent issue that after the complete evaporation of solvent, OHPs tends to fill up the pores, and usually forms an additional capping layer on top of the pores with uncontrollable thickness. This drawback limits the application of the micro/nanostructure-assisted growth method in controlling the morphology and the thickness of the film. In this report, we demonstrate the fabrication of a uniform and capping-free suspended CH3NH3PbBr3 (MAPbBr3) thin film on top of a microstructural SiO2 scaffold. The scaffold is developed by employing colloidal monolayer of 450 nm polystyrene (PS) nanospheres as sacrificial templates, which gives rise to SiO2 nanobowl micro-arrays on a substrate. Both optical simulation and characterization exhibit that the suspended film has potential advantage in improving the light efficiency of the optoelectronic devices. Further investigation reveals that dynamic-dripping (DD) method for spin-coating dominates the formation of suspended perovskite film, which is hard to achieve through traditional steady-dripping (SD) method. The effect of the microstructures on the state of the precursor-solution film is studied by the wettability of the precursor solution on the scaffolds and a flat Si substrate. The studies on the crystal growth kinetics of crystals in the suspended films are carried out by X-ray diffraction (XRD) and in situ photoluminescence (PL) characterization. The mechanism study provides a clear picture that the capillary-induced adhesion plays a role of supporting the precursor-solution film through the nanobowl nozzles, and the rapid evaporation rate is also critical for driving the intermediate particles to assemble into the solid transition-state film before the solution enters into the nanobowls, and helps the growth of large area and continuous perovskite thin film.


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EXPERIMENTAL SECTION Preparation of colloidal monolayer template Monodispersed PS colloidal spheres with a polydispersity less than 3% were synthesized using the emulsifier-free polymerization method.22 The PS nanosphere monolayer was prepared by the self-assembly approach at the air/water interface.23 The detailed process is as follows. PS colloidal suspensions were mixed with an equal volume of ethanol, and slowly dropped onto a glass slide which was cleaned by piranha solution and put in the center of a glass Petri dish with upper surface at the same level as the top of the water. Once the suspensions spread and contacted the water, PS spheres rapidly assembled into 2D monolayer. The floating PS spheres were consolidated into a large-area monolayer by adding a drop of 2 wt% sodium dodecyl sulfate solution. A Si substrate was inserted beneath the floating monolayer, and then lifted to transfer the monolayer from the water to the substrate. Deionized water (resistivity greater than 18.2 MΩ cm) was used in the experiments. Fabrication of silica nanobowl micro-arrays The silica nanobowl micro-arrays were fabricated by spin-coating a silica precursor solution onto the PS monolayer which was thermally annealed at 100 ℃ for several minutes. The silica precursor solution was prepared by mixing tetraethoxysilane, ethanol, and 0.1 M hydrochloride acid solution in a volume ratio of 2:10:1 and stirred at room temperature for 2 h prior to use.24 With the help of capillary force, the silica precursor was infiltrated in the interstitial spaces of PS spheres. The infiltrated samples were left to dry in ambient environment for the gelation of silica sol, and then soaked in the toluene solution to remove the PS template. Periodic silica nanobowl micro-arrays were obtained to serve as a scaffold for assisting the growth of perovskite.


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Preparation of MAPbBr3 films The perovskite precursor solution was prepared by mixing PbBr2 and CH3NH3Br in anhydrous N, N-dimethylformamide (DMF) at a 1:3 M ratio.25 Filling the silica nanobowl micro-arrays with perovskite was realized by spin-coating 30 wt% MAPbBr3 precursor solution onto the nanobowl micro-arrays at a spin speed of 6000 rpm for 90 s. Then, the samples were annealed at 80℃ on a hot plate for 30 min. The processes described above were carried out in a N2-filled glove box. CH3NH3Br was synthesized in our laboratory by following standard procedures.26 All the other chemicals were purchased from Sigma-Aldrich and used as received without further purification. Characterization Morphologies of the silica nanobowl micro-arrays and the perovskite films were observed by a field-emission scanning electron microscopy (SEM, FEI Nova Nano 450). Contact angle goniometer (Powereach JC2000D) equipped with CCD camera was used to measure the contact angles. XRD measurements were collected on the MAPbBr3 films infiltrated in the nanobowls by using a Rigaku Co. X-ray power diffractometer. Samples were stored in a N2-filled container to prevent air exposure before measurement. In situ PL spectra of the MAPbBr3 films was obtained by using a dedicated PL test system, which consists a commercial blue laser diode (440 nm, 1W·cm−2) used as an excitation source and a fiber-optic spectrometer (Ideaoptics FX2000) employed to collect the excitation light contribution. RESULTS AND DISCUSSION Morphological control of MAPbBr3 films on SiO2 scaffolds Figure 1 shows a schematic illustration for the fabrication process of the suspended MAPbBr3 perovskite films on SiO2 scaffolds. Firstly, PS colloidal nanosphere monolayer is transferred


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onto a Si substrate by the self-assembly method. Then the interstices of the PS monolayer are filled with SiO2 precursor solution by spin-coating (Step 1). After being dried in ambient environment, the SiO2 coated PS monolayer is immersed in toluene to remove the PS template, forming the SiO2 nanobowl micro-arrays (Step 2). Subsequently, MAPbBr3 precursor solution is casted onto a high-speed rotating SiO2 scaffold, forming a liquid film on the scaffold. With the solvent evaporating from the liquid film, a solid transition-state film with complex chemical composition is formed, which contains both the intermediate particles and perovskite nuclei (Step 3). Finally, after annealing in a glovebox at 80℃ for 30 min, a suspended MAPbBr3 thin film is obtained (Step 4).

Figure 1 Schematic processing flows for fabricating suspended MAPbBr3 perovskite films. (a) the self-assembled PS nanospheres monolayer; (b) PS monolayer filled with SiO2 precursor solution; (c) SiO2 scaffolds; (d) forming a liquid film on the scaffold, and particles in the liquid film assembling into the solid transition-state film; (e) a suspended MAPbBr3 polycrystalline thin film. For comparison, we prepare a planar perovskite film (Planar-P) on a flat Si substrate. Dropping precursor solutions directly on the substrate leads to fast growth of large perovskite


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crystals, leaving randomly island-like large grains and an overall low coverage of a perovskite film on the substrate, as manifested by the top view SEM image in Figure 2a. However, the perovskite films prepared by employing the SiO2 scaffolds display significant improvement in uniformity, continuity and coverage. Figure 2b presents the morphology of the periodic SiO2 scaffold, which is fabricated by replicating the ordered PS monolayer and inherits the hexagonal order from the initial PS nanosphere template. The inset shows a high-magnification SEM image of the scaffold, wherein each unit looks like a nanobowl. Further investigation focuses on the impact of the spin-coating process on the morphologies of the perovskite films. Figures 2c and 2d present the morphologies of the perovskite films fabricated via the SD and the DD methods on the as-prepared SiO2 nanobowl micro-arrays with a unit size of 450 nm, respectively. The UV-Vis absorption spectra (Figure S1) could confirm the formation of the perovskite films on the nanobowls, and also exhibit the optical performance for different filming methods. The SD method is by far the commonly reported method for spincoating, which can be interpreted as dripping droplets on a static substrate and then rotating the substrate. The SD method offers a conventional and reproducible process to prepare thin film, though its disadvantage in controlled growth of ultra-thin film is also obvious. The film fabricated by the SD method (Figure 2c) displays a clear additional capping layer on top of the scaffold. The cross-sectional SEM image of the perovskite film also gives a clear evidence of both fully filled nanobowl and the existence of capping layers (inset in Figure 2e), and the outline of the nanobowl is marked out with the red dashed line. In comparison, the DD approach has recently been proved more effective in controlling film thickness due to a faster solvent evaporating rate compared with the SD method.27 The film fabricated by the DD method (Figure 2d) exhibits a full coverage and a uniform distribution of the crystal domains. The sharp and


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defined edges of the nanobowls observed in the inset indicate the absence of capping layer on the scaffold. Nevertheless, the cross-sectional SEM image exhibits that MAPbBr3 materials only fill into the upper part of the nanobowl, leaving a cavity on the bottom, as shown in the inset of Figure 2f. The entire structure looks like a nanobowl supporting a continuous perovskite film, which suggests that the precursor solution may not fully fill in the nanobowl.

Figure 2 SEM images for (a) a perovskite film directly deposited on a flat substrate, without the SiO2 scaffold. (b) SiO2 scaffold and its magnified images (inset). (c) and (d) The perovskite films fabricated by the SD method and the DD method, respectively. The insets are the corresponding magnified images. (e) and (f) Schematic illustrations of the formation mechanism of the capping layer and the suspended MAPbBr3 film via SD (e) and DD (f) method, respectively. The insets


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are the corresponding cross-sectional SEM images of the MAPbBr3 films in (c) and (d) respectively. From the above results, we infer that the formation of the suspended MAPbBr3 film via DD method follows the mechanism as illustrated in Figure 2f. After droplets of the precursor solution landing on the rotating nanobowls, large amounts of excess solution is spun out, leaving only a small amount of solution on the nozzles of the nanobowls due to capillary-induced adhesion,28 which forms a thin liquid film on top of the nanobowl nozzles and imprisons air inside the nanobowls. During the spinning process, the airflow on the surface of the thin liquid film and the convection current in the thin liquid film enhance the evaporation rate,29 and promote fast nucleation and growth of the intermediate particles in the liquid film. Thereby, a suspended transition-state film can be obtained, with its edges supported by the nozzles of the nanobowls and cavities on the bottom of the nanobowls. On the other hand, the formation of the capping layer via the SD method is schematically illustrated in Figure 2e. Because the DMF solvent is low viscous, when the droplets of the precursor solution land on the static structures, they would rapidly infiltrate the pores before the scaffold starts spinning. The precursor solution is trapped inside the nanobowls due to the restriction of the nanobowl structures, and tends to form a thick liquid film to fill in and cover the nanobowls. As a result, after the evaporation of the solvent, intermediate particles would assemble into the transition-state film, which fills up the pores with the generation of an additional capping layer on top of the scaffold. In the above assumption, the capillary-induced adhesion and the evaporation rate are two critical factors for the formation of a suspended MAPbBr3 film. The former determines whether it is possible to form a suspended liquid film on top of the nanobowl nozzles and imprison air inside the nanobowls, while the


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latter determines whether the assembling process of the intermediate particles could largely complete before the precursor solution enters into the nanobowls. Impact of capillary-induced adhesion on the formation of suspended films The capillary-induced adhesion is of great importance for the formation of the suspended perovskite film. If the force is not large enough to support the initial liquid film of the precursor solution, the final state of the perovskite film obtained after annealing cannot be suspended. In order to further confirm the above concept, we investigate the effect of reducing the adhesive strength on the morphologies of the final perovskite film. A bare Si wafer, nanobowl microarrays using nanospheres with diameters of 450 nm (NB-450) and 1200 nm (NB-1200) are utilized to study the influence of adhesive strength on the morphology of the MAPbBr3 film, respectively. The adhesive strength is indirectly inferred through the surface wettability of the precursor solution on the scaffolds,30 which can be characterized by measuring the contact angle (CA) between the precursor solutions and the surfaces. In general, a larger CA value reflects higher adhesive strength. The CA measurements of the precursor solutions on the bare Si, NB450 and NB-1200 are shown in Figure 3, and the CA results are 20.56°, 55.96°, and 39.02°, respectively. Therefore, the adhesive strength of the precursor solutions on the three substrates follows a similar trend that NB-450 generates stronger surface adhesion than the other two substrates, which is supported by the principle that the diameter and the height of nanobowls change equally and the adhesive strength decreases with height extending and diameter increasing.31


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Figure 3 Photos of precursor solution droplets on (a) the bare Si wafer, (b) the NB-450, and (c) the NB-1200. Then, we investigate the impact of the adhesive strength on the morphologies of the annealed perovskite films. The morphologies of the MAPbBr3 films grown on the NB-450 and NB-1200 scaffolds are shown in Figure 4. The corresponding samples are referred to “NB-450P” and “NB-1200-P”, wherein “P” represents the formation of MAPbBr3 perovskite films. Noticeably, the crystal domain size and the continuity of the NB-450-P are significantly different from those of the NB-1200-P. The top-view SEM image of the NB-450-P (Figure 4a) exhibits smooth surface and unobvious boundaries of the crystal domain within nanobowls. The crosssectional SEM view of the NB-450-P in the inset of Figure 4a displays a suspended perovskite film and a cavity beneath the film. However, when increasing the diameter of the scaffold to 1200 nm, MAPbBr3 materials could not completely fill the nanobowls. Thus, materials inside the nanobowls exhibit many large crystal grains with voids among them, similar to those in Planar-P (Figure 2a). Besides, the edge of the nanobowls can also serve as the crystal nucleation site to drive the crystal growth, as indicated by the red arrow in Figure 4b. The cross-sectional SEM view of the NB-1200-P in the inset of Figure 4b illustrates that large grains of MAPbBr3 are distributed discretely in the nanobowls.


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Figure 4 Top-view SEM images of (a) NB-450-P and (b) NB-1200-P. The insets are the corresponding cross-sectional SEM images. On the basis of the above observations, we propose different crystal growth mechanisms for the NB-450-P and the NB-1200-P, as schematically illustrated in Figure 5. When taking into account the structural parameters of the nanobowls,31 the adhesive force (F) of the liquid film on NB-450 (LF-450) is stronger than that on NB-1200 (LF-1200). In addition, the gravity force (G) of the LF-450 is less than that of the LF-1200 due to the smaller area of the liquid film at the same thickness on each unit. Therefore, the LF-450 could keep its initial suspended form more easily than the LF-1200. Considering that the total amount of the solvent in the LF-450 is relative fewer and the time the solvent takes to evaporate is less, there suggests the possibility that the solvent in the LF-450 would have evaporated and the transition-state film has formed from the intermediate particles on the nozzles of the nanobowl before the liquid film has the chance to enter the nanobowl. The resulting NB-450-P structure looks like a nanobowl supports a continuous perovskite film.


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Figure 5 Schematic illustration of the process of formation of MAPbBr3 perovskite films based on NB-450 and NB-1200. The upper panel is the illustration about NB-450, and the bottom panel is the illustration about NB-1200. On the other hand, subjecting to the larger gravity force, the LF-1200 trends to fall and infiltrate into the nanobowl. Moreover, it takes more time for the solvent within the NB-1200 to evaporate. Due to the capillary effect, the materials preferentially crystallize at the gas-liquidsolid interfacial area in the nanobowl at first. Subsequently the rest of the solution crystallizes at the internal liquid-solid interfacial area of the nanobowls. Due to the relatively spacious space and the volume shrinkage after the solvent evaporation, the nucleation and crystal growth are relatively slow and would occur in parallel. The resultant crystals within the nanobowl is larger and discrete, similar to those growing on a flat substrate. Therefore, it is reasonable to assume that capillary-induced adhesion has a decisive influence on keeping the liquid film to remain suspended. Impact of solvent evaporation rate on the formation of suspended films


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The solvent evaporation rate is another key parameter that determines the formation of suspended film by affecting the nucleation rate. However, the solvent evaporation rate is tricky to monitor, especially during the spinning. Fortunately, the solvent evaporation rate can be indirectly evaluated by the crystal size both in the transition-state film and in the crystalline perovskite film, because the rapid evaporation rate of the solvent will induce a burst of nucleation to form a vast number of small crystals with relative narrow size distribution in the transition-state film, and the intermediate particles simultaneously formed would serve as nucleation sites to be converted into the perovskite crystals during the annealing process. Hence, we intend to evaluate the evaporation rate through the crystal size and size distribution in the transition-state films as well as the crystallinity and mean size of perovskite crystals in the polycrystalline perovskite films by employing in situ PL and XRD characterization. In situ optical characterization has been widely employed to study the growth kinetics of the crystals.32 Here, we adopt a dedicated in situ PL characterization to monitor the growth dynamics of the crystals during annealing. Figure 6 shows the tendencies of PL peak and FWHM of the transition-state films during the annealing process at 80℃on different substrates, corresponding to Si, NB-450, and NB-1200, respectively. According to the quantum confinement effect of semiconductor, when the crystal size is comparable with or smaller than the Bohr diameter of the bulk material, their bandgap becomes size dependent.33 Therefore, the PL emission wavelength and the FWHM of the PL peak refer to the average size and the size distribution of the crystals inside the transition-state films. All three samples have emission wavelength shorter than or close to 534 nm during annealing, which indicates that the crystal size is smaller than the Bohr diameter according to the literature.34 Thereinto, the transition-state film on NB-450 shows a relative shorter initial wavelength and narrower FWHM, which leads us to suspect that the


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evaporation rate of the solvent in LF-450 is the fastest among the three samples. A reasonable interpretation is that the LF-450 owns a minimum amount of solvent as well as a quite small surface area, which could enhance the mass transport within the liquid film sitting on the nozzles and accelerate the solvent evaporation rate, making the NB-450-P to remain its initial suspended form after annealing process.

Figure 6 Evolution of the PL emission peak wavelength and the FWHM of the transition-state films on (a) the bare Si wafer, (b) the NB-450, and (c) the NB-1200, respectively, during the annealing process at 80℃. For all three samples, the PL emission peak exhibits continuous red-shift till 534 nm (emission wavelength of bulk MAPbBr3 35) during annealing, indicating that the crystal size is continuously increasing. This phenomenon could be explained by the gradual conversion of the intermediate particles into perovskite and the continuous growth of the initial nuclei with the newly transformed perovskite. The PL peak will stop shifting and exhibit the maximum value when the crystal size reaches the Bohr radius. The calculated bandgap from the shift of wavelengths is within 2.35-2.32 eV, referring to a size change from about 3.9 to 4.4 nm. It has demonstrated strong quantum size effect. The time to reach the maximum PL peak as well as the change tendency of the FWHM reflect the different growth kinetics for the perovskite crystals on various substrates.


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As shown in Figure 6a, the transition-state film of the Planar-P takes only 12 min to attain the maximum PL peak, and its FWHM decreases linearly. This phenomenon is contributed to the utilization of a Si substrate, which has a high thermal conductivity (1.35Wm-1K-1)

36

and

transfers heat quickly from the hot plate to the transition-state film. Hence, the crystals in the transition-state film of the Planar-P grow quickly. On the other hand, since the SiO2 (0.17Wm1

K-1)

37

that forms the nanobowl-like scaffolds has much smaller thermal conductivity, the

transition-state films on the scaffolds take longer time to reach thermal equilibrium. As shown in Figure 6b, the transition-state film of the NB-450-P needs 20 min to reach the maximum PL peak. Notably, its FWHM increases in the first 6 min, and then decreases till 18 min. The possible reason for the different trend is that the N2 beneath the suspended film has poor thermal conductivity (0.0228Wm-1 K-1)

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, and the heat from the hot plate mainly transfers to the

suspended film through the contact area between the scaffold and the film. Therefore, there exists an uneven distribution of heat field from the edges to the middle of the suspended film, which induces the different growth rate of the crystals in the film, and results in the observed increase in FWHM at the first stage. When the suspended film achieves thermal equilibrium, the size distribution of the crystals trends to improve, and the FWHM starts to decrease until the crystal size is close to the Bohr radius. However, the crystals in the transition-state film for NB1200-P grow at a much slower rate and the change in FWHM indicates a complicated growth kinetics (Figure 6c). It could be due to the very irregular initial distribution of particles inside the nanobowls. It should be noted that the in situ PL spectra can only evaluate the size and size distribution of the crystals smaller than their Bohr radius. Hence, we employ XRD to characterize the crystallinity of the annealed perovskite films and calculate the crystal size by the Scherrer


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equation. XRD patterns of the annealed perovskite films on the scaffolds and on the planar Si substrate are measured to characterize the crystallinity of perovskite films, as depicted in Figure 7a. In XRD spectra of the three samples, two distinct diffraction peaks at close to 15° and 30° are observed (labelled inverted triangles), which are assigned respectively to (100) and (200) planes of MAPbBr3 perovskite,39 confirming the formation of MAPbBr3 perovskite on the substrates. A XRD signal with peak position located at 33° is the same as that of the bare SiO2 scaffolds (labelled circles). It is noteworthy that Planar-P generates the widest FWHM of the (100) plane, indicating its poor crystal quality, and the scaffolds could help to improve the crystal quality of the perovskite film.

Figure 7 (a) XRD spectra of Planar-P, NB-450-P, NB-1200-P, and the SiO2 scaffold only, and (b) their XRD profiles near (100) (upper) and (200) (bottom). The average size of the perovskite crystals could be calculated by the Scherrer equation as follows,40

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where K, D, γ, θ and B are the Scherrer constant of 0.943, the average crystal size, the Xray wavelength of 0.154 nm, the Bragg diffraction angle, and the full width at half maximum (FWHM), respectively. The D value for Planar-P is 19.8±1.6 nm, but it increases to 37.8±0.7 nm and 22.1±1.2 nm for the NB-450-P and the NB-1200-P, respectively. This result is closely related to the growth space of the perovskite crystals. The D value is the smallest when with no structures to limit the crystal growth. However, take the case of the NB-450-P, the nozzles of the nanobowl structures provide a narrow space, which promotes neighboring small particles to form big grains during the annealing process. According to the lattice constant

41

and the calculation based on the Bragg equation, the

peaks corresponding to the (100) and (200) planes are 14.98° and 30.15° respectively, which coincide with the values of the NB-1200-P (14.98° and 30.15°). In contrast, it can be seen from Figure 7b that the center of the peaks show a shift to the lower diffraction angle for the Planar- P (14.85° and 30.09°) and a slight shift to the higher diffraction angle for the NB-450-P (15.02° and 30.16°). Three black dash lines are drawn to show such disaccord between the peaks. The shift of XRD peaks is a signature for the presence of lattice strain in the perovskite films, which shifts to the higher angle indicating the shorter neighboring lattice planes induced by the compressive strain, and the shift moving to the lower angle indicates the longer neighboring lattice planes induced by the tensile strain.42 The lattice strain in the perovskite films originates from the large thermal expansion mismatch between the perovskite (1.57x10-4K-1)43 and the substrates. The perovskite film is formed by a phase transformation at temperature of 80℃. During the process of cooling the perovskite film formed at 80℃ to room temperature, both the perovskite film and the substrate trend to contract, but the different shrinkage degree will raise


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the concern of lattice strain in the perovskite films. For the Planar-P, it deposits on a Si substrate, which has a small thermal expansion coefficient (3.57 x10-6 K-1)

44

, and will hinder the

perovskite from contracting, resulting in a tensile strain in the Planar-P. The thermal expansion coefficient of the SiO2 materials made in the nanobowl-like scaffolds is 3.7 x10-6 K-1 45, which is also smaller than that of the Si substrate. Nevertheless, the NB-450-P is suspended on the nanobowl nozzle, and we assume that the N2 imprisoned inside the cavity beneath the film serves as the substrate for the NB-450-P. The thermal expansion coefficient of N2 is 93.15 K-1,38 bigger than that of the perovskite, which means the shrinkage rate of the imprisoned N2 is faster than that of the perovskite, leading to a compressive strain in the NB-450-P. It is worth mentioning that the peak shift of NB-450-P is much smaller than that of the Planar-P, which is attributed to the NB-450-P free of the restriction of the rigid substrate on the film, and strain relaxation achieved by the flexible N2 inside the nanobowl. On the other hand, for the NB-1200-P, the perovskite film crystallizes along the internal walls of the nanobowl. The internal walls have a concave-shaped surface area, which is helpful to reduce the residual strain in the NB-1200-P.46 In addition, discrete crystal domains in the NB-1200 could release the lattice strain more easily because of less bonding of perovskite to the internal walls. Therefore, the peak positions of the NB-1200-P match the calculated results in the literature. The characteristic peak shift indicates a good agreement with the film morphologies observed by SEM and the previous kinetics study by in situ PL. Improvement on optical performance of the suspended perovskite film Compared with the conventional perovskite film, the suspended perovskite film proves to have better optical performance from both aspects of the simulation and the experiment. 3D FDTD simulations are performed to investigate the light extraction efficiency (LEE) and the light


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intensity distribution of the perovskite films under illumination. Three samples are selected to create the models for comparison, including NB-450-P (Sample 1) and NB-1200-P (Sample 2) produced by the DD method, and the perovskite film on the NB-450 (Sample 3) by the SD method. The structure parameters of the models are set to match the corresponding experimental structures. The patterned perovskite films on the nanobowl micro-arrays are assumed to be periodic in the XY-plane with a triangular lattice symmetry, and each unit cell of the period model is composed of a whole and six half circles as shown in Figure 8a. Periodic boundary conditions (PB) are applied at the side walls, and both the top and the bottom boundaries are closed by perfectly matched layers (PML) in the Z-directions. Dipole sources with the emission wavelength of 534 nm are located in the center of the perovskite films. Planar monitors are placed on top of the spheres along the plane parallel to the substrates (Figure 8b), and are utilized to record the light intensity distribution and the LEE. Figure 8c-e illustrate the simulated intensity distribution of electromagnetic field when the three samples undergo the PL tests, and their LEEs registered by the monitors are 51.81%, 22.87%, and 33.28%, respectively. It is evident that the electromagnetic field of Sample 1 is distributed more uniformly, while that of Sample 2 and 3 has relatively intensive distribution. Similarly, the corresponding LEE values also indicate significantly enhanced light extraction ability of the Sample 1 (51.81%) than that of Sample 2 (226%) and Sample 3 (156%). Furthermore, the PL characterization confirms that the fluorescence intensity of Sample 1 outperforms over the other two samples, as shown in Figure 8f. The enhanced optical properties of Sample 1 could be attributed to its unique suspended structure. Since the refractive index of MAPbBr3 is 2.45 at 534 nm

47

, much larger than that of air (n=1), it is commonly agreed that,

when a light propagates from a medium with high refractive index into one with low refractive


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index, the phenomenon of total internal reflection will occur at the interface. For Sample 2 and 3, MAPbBr3 materials almost fill up the nanobowls. It means that a portion of the light generated in the MAPbBr3 materials is tend to be trapped in the high refractive index MAPbBr3 rather than emit. Nevertheless, benefiting from the suspended structure of Sample 1, the air cavities beneath the film can effectively increase the probability of the light escaping from the MAPbBr3 film. Taking both the simulated and experimental results into account, we conclude that the unique structure of the suspended film contributes to the uniform intensity distribution of electromagnetic field and the enhanced capacity of redirecting the trapped light inside the film. Our finding suggests that the morphology of the suspended perovskite film has a high potential in the optoelectronic devices.

Figure 8 (a) Schematic of top view of the model used for FDTD simulation, and the red dotted box highlights the simulated domain. (b) The cross-sectional view of the simulated suspended structure. An X-oriented dipole source is located in the center of the perovskite, and a monitor highlighted by the red dotted line is placed along the plane parallel to the substrate. Simulated intensity distributions of electromagnetic fields of (c) Sample 1, (d) Sample 2, and (e) Sample 3, respectively. The insets are the corresponding models. (f) PL spectra of the three samples.


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CONCLUSION In summary, we demonstrate the fabrication of a suspended MAPbBr3 film supported on the nanobowl micro-arrays with a unit size of 450 nm by using a dynamic-dripping approach. Study on the growth mechanism reveals that both the capillary-induced adhesion and the solvent evaporation rate play the dominant role in the morphology control and the formation of a suspended MAPbBr3 perovskite film. The capillary-induced adhesion supports a suspended liquid film on top of the nanobowl nozzles with a small unit size of 450nm. Such liquid film on top of a large unit size nanobowls, e.g. 1200 nm, becomes unstable due to weaker adhesion force and a larger gravity force. The solvent evaporation rate is critical to the crystal growth kinetics in liquid film, which determines the extent of completion on the transition-state film formation before the solution enters into the nanobowls. In situ PL characterization is employed to investigate the growth kinetics of the crystals in the transition-state films during the annealing process. XRD peak shift of the crystalline perovskite films grown on different substrates is further utilized to establish the relationship between the film formation process and the lattice strain. These studies draw the conclusion that both DD approach and microstructural parameters are key factors in controlling the film morphologies and achieving high quality MAPbBr3 films. This study provides insights into the influences of the interfacial forces and the evaporation dynamics on the kinetics of OHPs films formation, paving the way of a new film growth and morphology control strategy to improve the optical performance of optoelectronic devices. ASSOCIATED CONTENT Supporting Information.


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The Supporting Information is available free of charge on the ACS Publications website. Additional UV-vis absorption spectra (PDF) AUTHOR INFORMATION Corresponding Author *Shu Xu. E-mail: [email protected] *Wengang Bi. E-mail: [email protected] ACKNOWLEDGMENT The authors would like to thank the financial support from the National Natural Science Foundation of China (No.51672068, 51703017), the Hebei Natural Science Foundation (No.B2016202229), the Scientific Innovation Grant for Excellent Young Scientists of Hebei University of Technology (No.2015003), the Financial Grant from the China Postdoctoral Science Foundation (No.2016M601302) and Scientific Research Foundation of the State Human Resource Ministry for Returned Talent Chinese Scholars (No. CG2015030001). REFERENCES (1) Xiao, Z.; Kerner, R. A.; Zhao, L.; Tran, N. L.; Lee, K. M.; Koh, T. W.; Scholes, G. D.; Rand, B. P. Efficient Perovskite Light-Emitting Diodes Featuring Nanometre-Sized Crystallites. Nat. Photonics 2017, 11, 108-115. (2) Hoang, M. T.; Pham, N. D.; Han, J. H.; Gardner, J. M.; Oh, I. Integrated Photoelectrolysis of Water Implemented on Organic Metal Halide Perovskite Photoelectrode. ACS Appl. Mater. Inter. 2016, 8, 11904-11909. (3) Meng, L.; Yao, E. P.; Hong, Z.; Chen, H.; Sun, P.; Yang, Z.; Li, G.; Yang, Y. Pure Formamidinium-Based Perovskite Light-Emitting Diodes with High Efficiency and Low Driving


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For Table of Contents Use Only

Fabrication and Growth Mechanism of Uniform Suspended Perovskite Thin Films Chong Geng, Fangfang Li, Yuemei Fan, Lijing Zhang, Shuangshuang Shi, Zi-Hui Zhang, Yonghui Zhang, Shu Xu*, and Wengang Bi*

TOC graphic：

Synopsis：

We demonstrate the fabrication of large area and uniform CH3NH3PbBr3 suspended films on a periodic microstructure via the dynamic-dripping method. The suspended perovskite film proves to have a better optical performance from both aspects of the simulation and the characterization.


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Fabrication and Growth Mechanism of Uniform Suspended Perovskite

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