(Invited) Halide Perovskite Thin Films via Alternate Laser Deposition: Change in the Physical Properties with the Layered Structures

Abstract

We have formed methylammonium lead iodide (MAPbI3) perovskite by alternating irradiation of PbI2 and CH3NH3I (MAI) with an infrared laser in an ultra-high vacuum to elucidate the growth mechanism and realize structural control of MAPbI3 [1]. In this study, we found that MAPbI3 contains both tetragonal and orthorhombic phases, and the ratio of these phases systematically changes depending on the thickness of alternating layers of PbI2 film.In the sample preparation, the PbI2 and MAI powder targets were alternately irradiated with an infrared laser (λ = 808 nm) to convert them into molecular beams, and then alternately deposited on a synthetic fused silica substrate at room temperature. The deposition rate of PbI2 was controlled to be about 0.1 nm/s using a quartz crystal microbalance thickness monitor. The deposition amount of MAI was adjusted by changing the deposition time while the evaporation pressure was fixed at 4×10-4 Pa. Four different multilayer samples consisting of [PbI2/MAI] × n (n = 1, 2, 5, 10) were prepared under the condition that the total thickness of PbI2 was 300 nm.By analyzing the crystal structures by X-ray diffraction measurements, the prepared samples were found to have the following two unique features: (1) In the samples with n = 1, unreacted PbI2 was present in addition to MAPbI3, while in the samples with n = 2, 5, and 10, only MAPbI3 was observed, indicating a complete solid-phase reaction. (2) The full width at half maximum and peak position of the diffraction peak for MAPbI3 at 2θ = 14.0° ~ 14.1° were different among the samples, and thus we performed the analysis by peak separation. As a result, it was determined that the tetragonal (002) and orthorhombic (020) diffraction peaks were included.Defining r tetra and r ortho as the ratio of the tetragonal and orthorhombic diffraction peak areas to the total peak area, it was found that r tetra : r ortho changes significantly with the thickness of the PbI2 layer. Specifically, r tetra : r ortho =[0.64 : 0.36], [0.39 : 0.61], [0.20 : 0.80], and [0.25 : 0.75] for n =1, 2, 5, and 10, respectively. It is noteworthy that the orthorhombic MAPbI3, which usually appears below 161 K due to the phase transition [2], is contained in the MAPbI3 film formed at room temperature, and that the mixing ratio of tetragonal and orthorhombic phases varies systematically with the thickness of the alternating deposited PbI2 films.[1] K. Kawashima et al., Sci. and Tech. of Adv. Mater. 18, 307 (2017).[2] N. Onoda-Yamamuro et al., J. Phys. Chem. Solids 51, 1383 (1990).