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In situ investigation of growth methylammonium lead halide (MAPbI3-xBrx) perovskite from micro-droplets Yahui Li, Yifan Zhang, Zhenhao Zhao, Lili Zhi, Xiaobing Cao, Yi Jia, Feng Lin, Lei Zhang, Xian Cui, and Jinquan Wei Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00181 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018
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Crystal Growth & Design
In situ investigation of growth of methylammonium lead halide (MAPbI3-xBrx) perovskite from micro-droplets Yahui Li1‡, Yifan Zhang2‡, Zhenhao Zhao3‡, Lili Zhi4, Xiaobing Cao1, Yi Jia5, Feng Lin3, Lei Zhang3, Xian Cui1, Jinquan Wei1* 1. State Key Lab of New Ceramic and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China 2. Shenzhen Vanke Meisha Academy, No. 33 Huanmei Road, Yantian District, Shenzhen, 518000, Guangdong, P.R. China 3. Key lab of Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P.R. China 4. Department of Physics, Changji College, Changji 831100, Xinjiang, P.R. China 5. Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100094, P.R. China ‡ These authors contributed equally to this work. *Corresponding author:
[email protected] KEYWORDS: perovskite; crystallization; thin film; micro-droplet, in-situ
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Abstract The properties of organic–inorganic hybrid perovskites depend on their compositions greatly. Here, we in situ investigate the crystallization of methylammonium lead halide (MAPbI3-xBrx) from micro-droplets by adjusting the molar ratio of iodine to bromine in precursor solution, which help to control the morphology, crystallinity and physical properties of the perovskite materials. The composition of the precursor solutions has profound effects on the crystallization and perovskite morphology. It tends to form needle intermediate crystal of MAPbIxBr3-x·DMF from the precursor solution at low bromine concentration; while it tends to form flake or cubic perovskite of MAPbIxBr3-x at high bromine concentration. There are evident composition segregation of bromine when the perovskite and their intermediates grow freely from the solution at low temperature. At high growing temperature of 100 ℃, continuous perovskite films consisting of large crystal domains with sizes up to 100 microns are fabricated from the precursor solution.
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Crystal Growth & Design
1. Introduction Organometallic perovskites, especially methylammonium halide lead perovskite (MAPbX3, X=Cl, Br and I) are promising photovoltaic materials with unique properties, such as tunable band gap,
[1, 2]
high light absorption coefficient,
[3, 4]
defect densities and long carrier diffusion length.
high ambipolar charge mobility,
[6-8]
[5]
low
The MAPbX3 perovskites have been
widely used for high efficiency solar cells, [9-12] photo detectors, [13-15] light emitting diodes, [16-18] laser,
[19, 20]
and field effect transistors.
[21]
In the field of photovoltaics, the efficiency of
perovskite solar cells (PSCs) has improved to above 20% by composition engineering,
[22, 23]
especially partially replacing iodine with bromine. The band gap of perovskite depends on its elemental ratio in the MAPbX3 (X= Cl, Br and I), which can be tuned from about 1.5 to 2.3 eV by adjusting the elemental ratio of iodine and bromine
[24, 25]
. Although, the efficiency of PSCs
has exceeded 20%, high quality perovskite films are still greatly desired for higher performance. It is necessary to investigate the fabrication of MAPbI3-xBrx, which will help to enhance the performance of the perovskite devices significantly. Generally, the MAPbI3-xBrx perovskites were fabricated through solution processing,
[26-28]
where the relevant solvates were dissolved in organic solvents. By spin-coating, blade-coating, or spraying the precursor solution, [29-34] perovskite crystals and their intermediates nucleate and grow on the substrate easily. It is difficult to investigate the growth process of the perovskite during these processing due to its fast process basing on above techniques. Recently, we developed a facile method to in-situ observe growth of crystals from micro-droplets. [35] Here, we investigate the crystallization of MAPbI3-xBrx perovskite by varying composition in the microdroplets, which help for fully understanding the growth of perovskite and fabricating high quality perovskite films.
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2. Experimental section 2.1 Materials
N,N-dimethylformamide (DMF, 99.9%), PbI2 (99%), PbBr2 (99%) were purchased from Sigma-Aldrich. CH3NH3Br (MABr) and CH3NH3I (MAI) were purchased from Xi’an Polymer Light Tech Co.; Ltd (China). FTO-coated glass was purchased from Yingkou OPV Tech New Energy Co.; Ltd (China). All reagents were used as received.
2.2 Preparation of substrates and CH3NH3PbI3-xBrx perovskite precursor
In order to eliminate the effects of substrate on the growth of crystals, we jetted the perovskite precursor solution to the FTO glass coated with a compact TiO2 layer, which was usually used for fabricating PSCs. The FTO glass was cleaned with detergent solution, deionized water, acetone, and ethanol in sequence, followed by drying with nitrogen gas. A compact and flat TiO2 layer was deposited by spin-coating a mildly acidic solution of titanium isopropoxide (97%, Sigma-Aldrich) in ethanol at 2000 rpm for 30 s. After drying at 100 °C for 30 min, the TiO2/FTO substrate was annealed at 500 °C for 30 min. Perovskite precursor solutions were prepared by dissolving PbBr2, PbI2, MABr and MAI with different molar ratios of iodine to bromine in DMF. The concentration of Pb2+ in the DMF solution was kept at 1 mol/L. The molar ratio of bromine and iodine in the solutions was controlled to form a compound of MAPbI3-xBrx. All solutions were stirred at room temperature overnight before using.
2.3 Formation of micro-droplets and materials characterization
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Crystal Growth & Design
The solutions were jetted to the TiO2/FTO substrate to form micro-droplets with volume of pico- to nanolitres according to our previous report. [35] The growth of perovskite crystals was insitu observed and recorded by an optical microscope. The perovskite crystals were characterized by scanning electron microscopy (SEM, Zeiss Merlin Compact), transmission electron microscope (TEM, JEM 2100F), and X-Ray diffraction (XRD, Rigaku Smartlab), respectively.
3. Results and discussion Figure 1 shows a series of microscopic images of perovskite intermediates growing from a micro-droplet of the MAPbI2.33Br0.67 precursor solution with a volume of tens of picolitre on the TiO2/FTO substrate at room temperature. The corresponding growth process of the MAPbI2.33Br0.67 is also shown in Video S1. The micro-droplet spreads on the substrate quickly and from a tiny round pool due to good wettability between the solution and TiO2 substrate (Figure 1a). As DMF evaporates, some white crystals nucleate and grow from the edge of the droplet (Figure 1b), which form needle-like crystals. As the needle-like crystals grow toward the center of the droplet, some crystals nucleate inside the droplets, as shown by the circles in Figures 1b and 1c. As the crystals grow up, more and more crystals nucleate from the microdroplet and the needle-like crystals as time elongate, until the solvent completely evaporate (Figure 1d and 1e). Finally, the white crystals turn red in several seconds (Figure 1f). The needle-like crystals are perovskite intermediate of Lewis adducts according to our previous report
[35]
. The red crystals might derive from MAPbI3-xBrx perovskites according to their band
gap and light absorption. We increase the Br/I ratio in the precursor solution to 2:1. Figure 2 shows microscopic images of the crystals crystallization process from MAPbIBr2 precursor solution. The crystallization process from the MAPbIBr2 solution is quite different from that of the MAPbI2.33Br0.67 crystals in
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Figure 1. Some red granular crystals first precipitated from the edge of the micro-droplet (Figure 2b). Besides these red crystals, some white needle crystals are also observed in the solution. The white crystals turn red after the solution completely evaporate. It is noted that there are also tiny crystals among the granular and needle crystals due to limited transportation of solvates at the last second of solvent evaporation. In order to investigate the evolution of the crystallization process of MAPbI3 depending on the Br concentration, we adjust Br concentration in MAPbI3-xBrx precursor solution from x=1.33~3. The corresponding microscopic images of crystallization processes are shown in Figures S1 to S6. When x
