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Organotin halide perovskites are developed as an appropriate substitute to replace highly toxic lead-based hybrid perovskites, which are a major concern for the environment as well as for human health. However, instability of the lead-free Sn-based perovskites under ambient conditions has hindered their wider utility in device applications. In this study, we report a predominantly stable lead-free methylammonium tin bromide (MASnBr 3 ) perovskite that has air stability over 120 days without passivation under ambient conditions. Further, the feasibility of this predominant air-stable MASnBr 3 perovskite for use in the harvesting of mechanical energy is described with the fabrication of an ecofriendly, flexible, and cost-effective piezoelectric generator (PEG) using MASnBr 3 −polydimethylsiloxane composite films. The fabricated PEG exhibits high performance along with good mechanical durability and long-term stability. This flexible device reveals a high piezoelectric output voltage of ∼18.8 V, current density of ∼13.76 μA/cm 2 , and power density of ∼74.52 μW/cm 2 under a periodic applied pressure of 0.5 MPa. Further, the ability of PEG to scavenge energy from various easily accessible biomechanical movements is demonstrated. The energy generated from PEG by finger tapping is stored in a capacitor and is used to power both a stopwatch and a commercial lightemitting diode. These findings offer a new insight to achieve long-term air-stable Sn-based hybrid perovskites, demonstrating the feasibility of using organotin halide perovskites to realize highly efficient, ecofriendly, mechanical energy harvesters with a wide range of utility that includes wearable and portable electronics as well as biomedical devices.

Ferroelectric domain walls in single-crystal complex oxide thin films are found to be orders of magnitude slower when the interfacial bonds with the heteroepitaxial substrate are broken to create a freestanding film. This drastic change in domain wall kinetics does not originate from the alteration of epitaxial strain; rather, it is correlated with the structural ripples at mesoscopic length scale and associated flexoelectric effects induced in the freestanding films. In contrast, the effects of the bondbreaking on the local static ferroelectric properties of both top and bottom layers of the freestanding films, such as domain wall width and spontaneous polarization, are modest and governed by the change in epitaxy-induced compressive strain. Single-crystal complex oxide ferroelectric (FE) thin films exhibit high spontaneous polarization, coherent switching, and low coercive field-a unique set of properties ideal for the manufacture of energy-efficient, beyond-CMOS logic and memory components, [1] as well as future quantum computing devices. [2] One of the primary bottlenecks in this field stems from the great difficulty in producing high-quality interfaces between these single-crystal oxides and the singlecrystal silicon-the prevalent substrate in microelectronics, hindering their widespread use. Recent developments in complex oxide layer transfer techniques (LTTs), however, may make the monolithic integration of not only FE materials possible [3,4] but also other single-crystal functional oxides such as half-metals, high T c superconductors, and multiferroic materials-even down to monolayer Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))