**5. Conclusion**

**4.2. Piezoelectric energy harvester**

106 Ferroelectrics and Their Applications

on PI layer showed 2*Pr*

Energy harvesting, which collects useful energy from wasted energy sources, is the most promising technology to provide solutions for the shortage of a fossil fuel, various environmental problems, and the improvement of energy efficiency in smart grids. Piezoelectric energy harvesters have been actively studied due to their easy integration with MEMS and integrated circuit technologies [70–74]. The cantilever type is the most typical structure of a piezoelectric energy harvester, in which a piezoelectric material is deposited onto a rigid Si cantilever, and a proof mass is located at the free end of the cantilever. The electricity can be generated by a conversion of kinetic energy from the mechanically stimulated proof mass via the piezoelectric material. However, the typical rigid-body-based cantilever type energy harvester has a large resonance frequency due to its high spring constant. Thus, it is not for harvesting energy from human activity or low-frequency ambient vibration. In this regard, flexible PZT appears as a promising candidate for flexible energy harvesting applications.

Cho et al. presented a microfabricated flexible and curled PZT cantilever using d33 piezoelectric mode for vibration-based energy harvesting applications [70]. The proposed energy har-

of approximately 47.9 μC/cm2

optimal conditions of resistive load (6.6 MΩ) and resonant frequency (97.8 Hz), the fabricated

electrodes. The PZT thin film

and 78.8 kV/cm, respectively. At

vester consists of PI layer, PZT thin film, and interdigitated IrO<sup>x</sup>

and 2*Ec*

**Figure 17.** Illustration of flexible PZT film-based NG [74].

This review on low-temperature processing of solution-derived PZT films summarized major approaches to decrease the crystallization temperature below 450°C. The success would mitigate the integration of PZT films in electronics devices and diminish a possible loss of stoichiometry and consequent worsened functional properties due to either evaporation of volatile species (lead-, alkali-oxides) or possible interface reactions. The approaches described here include chemical and physical treatments for the precursor solutions, as-deposited, and as-pyrolyzed films by solvothermal synthesis, UV-light treatment, laser, microwave, and flash-lamp-assisted annealing. It reveals that design of functional precursor solutions, which are photo-sensitive and possibly decomposable at a low temperature, is critical to the production of high-quality PZT films.

Combination of low-temperature solution-processed PZT films with facile micro-/nanopatterning techniques would open new opportunities for low-cost, large-area transparent, flexible ferroelectric/piezoelectric devices such as nonvolatile memory, piezoelectric sensor/ actuator/transducer, and energy harvesters.
