| Summary: | In this study, a glass/indium tin oxide (ITO)/formamidinium-methylammonium-cesium (FA-MA-Cs) tri-cation lead iodide perovskite/poly(methyl methacrylate (PMMA)/Al memory device with a controlled composition of (FA<sub>0.75</sub>MA<sub>0.25</sub>)<sub>1-x</sub>Cs<sub>x</sub>PbI<sub>3</sub> (x = 0–0.1) is demonstrated to exhibit bipolar resistive switching behavior. The tri-cation organic–inorganic metal halide perovskite film was prepared by a one-step solution process in which the amount of Cs was varied to modify the property of FA<sub>0.75</sub>MA<sub>0.25</sub>PbI<sub>3</sub>. It was found that the microstructure and defect properties of films are highly dependent on the contents of FA, MA, and Cs in the perovskite. The results found that 5% CsI doping is the optimized condition for improving the quality of FA<sub>0.75</sub>MA<sub>0.25</sub>PbI<sub>3</sub>, forming a high quality tri-cation perovskite film with a smooth, uniform, stable and robust crystalline grain structure. The resistive switching on/off ratio of the (FA<sub>0.75</sub>MA<sub>0.25</sub>)<sub>0.95</sub>Cs<sub>0.05</sub>PbI<sub>3</sub> device is greater than 10<sup>3</sup> owing to the improved thin-film quality. Moreover, for the 5% CsI doped FA<sub>0.75</sub>MA<sub>0.25</sub>PbI<sub>3</sub> films, the endurance and the stability of retention are better than the non-doped film. The improved microstructure and memory properties are attributed to the balance stress of FA/MA/Cs with different ionic size. It suggests the potential to achieve a desired resistive memory property of tri-cationic perovskite by carefully adjusting the cation ratios.
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