Synthesis of Organic–Inorganic Lead Halide Perovskite Nanoplatelets: Towards High‐Performance Perovskite Solar Cells and Optoelectronic Devices

dedicated equipment. Another method to prepare lead halide perovskite fi lms is to dip a pre-deposited lead halide fi lm into a solution of alkyl amino halide to convert the lead halides to their corresponding perovskites. [ 16 ] This method works well with a layered perovskite compound (RNH 3 ) 2 PbI 4 in which Ris an alkyl group with two or more carbon atoms in the chain, which makes it easier for alkyl amino halide molecules to diffuse into the lead halide octahedron network through the van der Waals gap within the organic bilayer and, as a result, the reaction could be done in a few minutes time. However, in the case of CH 3 NH 3 PbX 3 perovskite, due to the lack of a van der Waals gap in the threedimensional structure of lead halide, the required dipping time would be much longer ( i.e. , 1–3 hours). Such a long immersion time in an organic solvent could dissolve and/or disperse parts of the lead halide and the formed perovskite, reducing the quality of the perovskite fi lm. [ 2,17 ] In conventional semiconductors, there have been extensive investigations on the synthesis in forms of nano-platelets (2D), [ 18 ] nano-wires (1D), [ 19,20 ] nanobelts (quasi-1D), [ 21,22 ] and their extraordinary optical and electrical properties compared to their bulk counterparts, leading to a number of exciting applications in nano-electronics and nanophotonics. To the best of our knowledge, nobody has reported a lead halide perovskite species in two-dimensional platelets. Herein, we report for the fi rst time the synthesis of lead halide perovskite family nano-platelets with lateral dimensions from 5 – 30 μm and thicknesses from several atomic layers to several hundred nanometers. The CH 3 NH 3 PbI 3 platelets prepared by our method have an electron diffusion length of more than 200 nm, which is two times higher than the recently reported value for a fi lm prepared by conventional solution spin-coating. We believe that this new synthesis method will push forward the fundamental study of the lead halide perovskites family as well as to explore their new applications in opto-electronics. The presented method involves two steps: First, the growth of lead halide nano-platelets on muscovite mica utilizing van der Waals epitaxy in a vapor transport chemical deposition system. [ 23–25 ] Next, the as-grown platelets are converted to perovskites by a gas–solid hetero-phase reaction with methyl ammonium halide molecules. Figure 1 a shows the optical and scanning electron microscopy (SEM) images of lead halides grown on muscovite mica substrates. The difference in color corresponds to different thicknesses as shown in Figure 1 b for particular lead iodide platelets. The in-plane orientation of the platelets in the case of PbCl 2 and PbBr 2 (Figure 1 a: A, B) is evident of the van der Waals epitaxial growth on the muscovite mica substrate because of the three-fold symmetry of the mica Synthesis of Organic–Inorganic Lead Halide Perovskite Nanoplatelets: Towards High-Performance Perovskite Solar Cells and Optoelectronic Devices