**4.1 The working mechanism of a PENG**

The working mechanism of a general piezoelectric nanogenerator having the metal–insulator–metal structure along with the formation of dipoles under applied force is schematically portrayed in **Figure 4**. In a non-poled piezoelectric material, the dipoles will be randomly orientated, while the orientation of those dipoles will be changed according to the applied electric field direction under poling. Initially, there

**Figure 4.** *Schematic illustration of working mechanism of OIHP-based PENG.*

#### *Organic/Inorganic Halide Perovskites for Mechanical Energy Harvesting Applications DOI: http://dx.doi.org/10.5772/intechopen.105082*

will be no generation of output from the PENG without any applied strain owing to the absence of potentials (or in equilibrium state) at the electrodes. However, when mechanical stress is applied normal to the PENG, the piezoelectric material undergoes compressive deformation leading to the generation of dipoles within the active material. The subsequent dipolar polarization results in a piezoelectric potential difference between the two electrodes of the nanogenerator. This, in turn, leads to the flow of charges from one electrode to the other electrode through the external circuit by producing an electrical output signal. When the applied force is withdrawn, the piezoelectric potentials vanish and the electrons flow back to the original position and generate an electrical output signal with the opposite polarity. This output generation is a cyclic process under applied cyclic pressures. In addition, the output of a PENG significantly depends on the material and also some external parameters like applied pressure and frequency.

#### **4.2 The working mechanism of a TENG**

A TENG can effectively transform the irregular and randomly distributed mechanical energy into usable electricity via coupling contact electrification with electrostatic induction [18]. In general, when two different materials are in contact with each other, chemical bonds will be formed between the interface of those materials, leading to charge (i.e., electrons, or ions, or molecules) transfer from one material to another because of the difference in their electron affinities [40]. When the two surfaces are separated from each other, the potential drop in the triboelectric charges induces charges into the electrodes via electrostatic induction effect. The potential difference between the electrodes drives electrons to flow between the two electrodes, thus generating triboelectricity form the devices. Based on this principle, four kinds of TENGs with different modes of operations have been developed, as shown in **Figure 5**. In the contact–separation mode, the first invented operation mode of TENG, two dielectric films are placed face to face, and metal electrodes are deposited on the opposite surfaces of the dielectric layers (**Figure 5a**). The TENG operates when the force is applied normal to the device. In the lateral sliding mode, the device structure is similar to that of contact–separation mode (**Figure 5b**), and the TENG operates when the two films keep sliding against one another. This sliding operation offers more efficient charge transfer compared to that offered by the contact–separation mode [41]. By contrast, single-electrode mode is designed to work independently and can be moved freely (**Figure 5c**). This mode is composed of a moving dielectric film and an electrode film connected to the ground. When the top dielectric film approaches towards and/or departs from the bottom electrode, the distribution of the local electrical field may change by generating a potential difference between electrode and ground. This leads to a flow of electrons between the ground and electrode and generates electricity. The freestanding triboelectric-layer mode consists of two symmetrical electrodes underneath a moving dielectric layer that has electrodes of similar sizes (**Figure 5d**). In this mode of operation, no direct physical contact between the two triboelectric layers can be realized, which tends to extend the lifetime of the TENG [42]. Among these modes, the OIHP-based TENGs developed so far were constructed and operated based on vertical contact–separation mode [43, 44]. In the OIHP-based TENGs, the OIHP film fabricated on the electrode-coated substrate is a triboelectric material and is naturally separated from the counter triboelectric material using spacers. Furthermore, when a piezoelectric material like OIHP is used to construct a TENG, the dipoles formed by mechanical deformation of a piezoelectric material under an applied force promote the

#### **Figure 5.**

*Schematic depiction of a–d) the four fundamental operating modes of TENG, e) vertical contact–separation mode of OIHP/OIHP-polymer composite based TENG.*

generation of more charges onto the surface of the OIHP film during TENG operation, as schematically portrayed in **Figure 5e** [8].
