Halide Tunablility Leads to Enhanced Biomechanical Energy Harvesting in Lead-Free Cs₂SnX₆-PVDF Composites

The main challenges impeding the widespread use of organic-inorganic lead halide perovskites in modern-day technological devices are their long-term instability and lead contamination. Among other environmentally convivial and sustainable alternatives, Cs₂SnX₆ (X = Cl, Br, and I) compounds have shown promise as ambient-stable, lead-free materials for energy harvesting, and optoelectronic applications. Additionally, they have demonstrated tremendous potential for the fabrication of self-powered nanogenerators in conjunction with piezoelectric polymers like polyvinylidene-fluoride (PVDF). We report on the fabrication of composites constituting solvothermally synthesized Cs₂SnX₆ nanostructures and PVDF. The electroactive phases in PVDF were boosted by the incorporation of Cs₂SnX₆, leading to enhanced piezoelectricity in the composites. First-principles density functional theory (DFT) studies were carried out to understand the interfacial interaction between the Cs₂SnX₆ and PVDF, which unravels the mechanism of physisorption between the perovskite and PVDF, leading to enhanced piezoresponse. The halide ions in the inorganic Cs₂SnX₆ perovskites were varied systematically, and the piezoelectric behaviors of the respective piezoelectric nanogenerators (PENGs) were investigated. Further, the dielectric properties of these halide perovskite-based hybrids are quantified, and their piezoresponse amplitude, piezoelectric output signals, and charging capacity are also evaluated. Out of the several films fabricated, the optimized Cs₂SnI₆_PVDF film shows a piezoelectric coefficient (d₃₃) value of ∼200 pm V^-1 and a remanent polarization of ∼0.74 μC cm^-2 estimated from piezoresponse force microscopy and polarization hysteresis loop measurement, respectively. The optimized Cs₂SnI₆_PVDF-based device produced an instantaneous output voltage of ∼167 V, a current of ∼5.0 μA, and a power of ∼835 μW across a 5 MΩ resistor when subjected to periodic vertical compression. The output voltage of this device is used to charge a capacitor with a 10 μF capacitance up to 2.2 V, which is then used to power some commercial LEDs. In addition to being used as a pressure sensor, the device is employed to monitor human physiological activities. The device demonstrates excellent operational durability over a span of several months in an ambient environment vouching for its exceptional potential in application to mechanical energy harvesting and pressure sensing applications.