A comprehensive review of flexible piezoelectric generators based on organic-inorganic metal halide perovskites

Organic-inorganic metal halide perovskites (OMHPs) have been extensively investigated over the past few years, not only because of their key features such as ease of solution-based processing, strong optical absorption, tunable bandgap, high carrier mobility, a long diffusion length, etc., and also because of their multi-faceted electronic applications with unprecedented power conversion efficiencies (PCE) in the field of photovoltaics. Apart from the impressive photovoltaic and optoelectronic applications, the applications for OMHPs have recently diverged towards mechanical energy harvesting applications owing to high levels of dielectric and piezoelectric properties that are relatively analogous to those of inorganic piezoelectric materials. Despite these assets, a fundamental understanding of physical behaviors such as ferroelectric and piezoelectric properties remains a key challenge that hampers the development of OMHP-based piezoelectric energy harvesters. This review updates the progress in nanogenerators for mechanical energy harvesting based on OMHPs and includes materials classifications and structural properties, as well as an overview of dielectric, ferroelectric, and piezoelectric properties based on the available literature. In addition, the numerous methods used to synthesize high-quality perovskite films and crystals are also addressed. In the overview of piezoelectric generators (PEGs), various approaches to PEGs based on OMHPs and flexible polymer composites are introduced. For flexible PEGs, fabrication, structural design, factors determining output, and applications are reviewed. Finally, some essential experimental features, perspectives, and issues in this exciting field are discussed and suggested.