Enhanced piezo-response of mixed-cation copper perovskites with Cl/Br halide engineering

Halide and cation engineering of organic-inorganic hybrid perovskites has shown a great potential for structural modulation of perovskites and enhancing their optoelectronic properties. Here, we studied the impact of Cl/Br halide engineering on the structural and piezoelectric properties of MA/Cs mixed-cation Cu-perovskite crystals. X-ray diffraction, Raman spectroscopy, and ¹³³Cs solid-state NMR were utilized to find out the nature of the perovskite crystal structure formation. Three distinct crystal structures were obtained depending on the Cl/Br content. High Cl content resulted in the formation of Br-doped (Cs/MA)CuCl₃ perovskite with the presence of paramagnetic Cu²⁺ ions. High Br content led to the formation of Cl-doped (MA/Cs)₂CuBr₄ perovskite with the presence of diamagnetic Cu⁺ ions. Equimolar Cl/Br perovskite content gave a novel crystal structure with the formation of well-dispersed diamagnetic domains. Compared to the high Cl/Br containing perovskites, the equimolar Cl/Br perovskite revealed the highest potential for piezoelectric applications with a maximum recordable piezoelectric output voltage of 5.0 V. The results provide an insight into the importance of mixed-halide and mixed-cation engineering for tailoring the perovskite structural properties towards a wide range of efficient optoelectronics.