Anion-Dependent Polarization and Piezoelectric Power Generation in Hybrid Halide MAPbX 3 (X = I, Br, and Cl) Thin Films with Out-of-Plane Structural Adjustments.

Anion-dependent differences in the electromechanical energy harvesting capability of perovskite halides have not been experimentally demonstrated thus far. Herein, anion-dependent piezoelectricity and bending-driven power generation in high-quality methylammonium lead halide MAPbX 3 (X = I, Br, and Cl) thin films are explored; additionally, anisotropic in situ strain is imposed to improve energy harvesting under tensile bending. After applying the maximum in situ strain of -0.73% for all the halide thin films, the MAPbI 3 thin-film harvester exhibited a peak voltage/current of ≈23.1 V/≈1703 nA as the best values, whereas MAPbBr 3 and MAPbCl 3 demonstrated ≈5.6 V/≈176 nA and ≈3.3 V/≈141 nA, respectively, under identical bending conditions. Apart from apparent ferroelectricity of tetragonal MAPbI 3 , origin of the piezoelectricity in both cubic MAPbBr 3 and MAPbCl 3 is explored as being related to organic-inorganic hydrogen bonding, lattice distortion, and ionic migration, with experimental supports of effective piezoelectric coefficient and grain boundary potential. Conclusively, piezoelectricity of the cubic halides is assumed to be due to their soft polarity modes and relatively low elastic modulus with vacancies contributing to space-charge polarization. In the case of ferroelectric MAPbI 3 , the distortion of PbI 6 octahedra and atomic displacement within each octahedron are quantitatively estimated.