Discovery of the Crystal Phase Transition on Lead-free Cesium Manganese Bromine Perovskite Nanocrystal by Solvent Concentration

We have been demonstrated the crystal phase transition for lead-free cesium manganese bromine perovskite nanocrystal synthesized by the modi�ed hot-injection method due to change the concentration of solvent (trioctylphosphine; TOP). The compositions to be synthesized were determined by the amount of TOP solvent, and the structure phase of nanocrystal was changed from hexagonal CsMnBr 3 to tetragonal Cs 3 MnBr 5 as the amount of TOP solvent increased. The emission peaks of CsMnBr 3 and Cs 3 MnBr 5 nanocrystals were observed at 650 nm (red) and 520 nm (green), respectively. After a durability test at 85 °C and 85% humidity for 24 h, the lead-free perovskite CsMnBr 3 nanocrystal powder maintained its initial emission intensity, and the metal halide Cs 3 MnBr 5 nanocrystal powder exhibited an increase in red emission due to the post-synthesis of CsMnBr 3 nanocrystals.

Despite these advantages, however, the presence of Pb, a toxic element, in perovskite nanocrystals raises critical concerns with regard to commercialization. This is because the heavy metal Pb can affect human health and the environment [14,15] [17][18][19]. These encouraging results obtained via exploration of lead-free perovskite nanocrystals indicate a major imminent breakthrough in the fabrication of new optoelectronic devices.
In this study, to obtain lead-free inorganic halide perovskite nanocrystals, we focused on replacing Pb with the transition metal Mn. Mn has an ionic radius (0.083 nm for 6 coordination) similar to that of Pb (0.119 nm for 6 coordination), as well as a low toxicity, and has therefore attracted attention as a substitute for the B site in halide perovskites. Cesium manganese bromide perovskite nanocrystals, such as the red-emitting CsMnBr 3 and green-emitting Cs 3 MnBr 5 , have been reported to exhibit excellent optical properties [20][21][22][23]. In this paper, we synthesized red-emitting CsMnBr 3 and Cs 3 MnBr 5 perovskite nanocrystals using the modi ed hot-injection method and investigated the optical properties of these perovskite nanocrystals. Furthermore, we investigated the phase-tunable synthesis from Cs 3 MnBr 5 to CsMnBr 3 with controlling the amount of solvent.
Results And Discussion  shifts toward green emission, implying that our study can achieve tunable the color between green and red by simply controlling the amount of TOP solvent.
To evaluate the thermal and chemical stability of the CsMnBr 3 and Cs 3 MnBr 5 nanocrystals, a durability test was performed under high-temperature and high-humidity conditions of 85°C and 85%, respectively, for 24 h. The PL behaviors of the CsMnBr 3 and Cs 3 MnBr 5 nanocrystals after the durability test are shown in Fig. 5 (a). The lead-free perovskite CsMnBr 3 nanocrystal powder exhibits extremely high durability and maintains its initial PL intensity. In comparison, the PL intensity of the metal halide Cs 3 MnBr 5 nanocrystal powder does not decrease but rather increases in the red emission (CsMnBr 3 ) region. It is considered that the materials that remained unreacted during the preparation of the nanocrystals reacted to form more CsMnBr 3 nanocrystals due to the high temperature during the durability test. Both nanocrystals exhibited no change in the PL peak wavelength and FWHM values after the durability test.
The structures of CsMnBr 3 and Cs 3 MnBr 5 are considered to have remained intact even after the durability test because no difference was observed between the XRD patterns before and after the test. The XRD patterns of the CsMnBr 3 and Cs 3 MnBr 5 nanocrystals before and after the durability test are presented in Fig. 5 (b). Both samples exhibited identical XRD patterns before and after the test, indicating that the CsMnBr 3 and Cs 3 MnBr 5 structures were not degraded during the durability test.

Conclusions
We discovered the crystal phase transition of lead-free cesium manganese bromine perovskite nanocrystal from hexagonal CsMnBr 3 to tetragonal Cs 3 MnBr 5 due to change the concentration of TOP  Characterizations The crystal structure of the CsMnBr 3 and Cs 3 MnBr 5 nanocrystal was identi ed using Xray powder diffraction (XRD; Rigaku SmartLab) analysis, and the microstructure and morphology of the nanocrystals was characterized using transmission electron microscopy (TEM; JEOL). The photoluminescence (PL) spectrum was recorded at room temperature using a uorescence spectrophotometer (PSI, DARSA PRO-3400), and the emission spectrum was obtained at an excitation wavelength of 365 nm. The lead-free cesium manganese bromine perovskite nanocrystal powder was prepared using the nanocrystal solution; a durability test was performed at 85°C and 85% humidity for 24 h.
Page 11/12  (a) PL spectra of the high-temperature and high-humidity testing of lead-free perovskite CsMnBr3 (red) and metal halide Cs3MnBr5 (green) nanocrystals at 85 °C and 85% humidity for 24 h, and (b) XRD patterns of the CsMnBr3 and Cs3MnBr5 nanocrystals before and after the durability test.