**4.3 OIHP-based PENGs**

OIHP materials have only been recently applied to PENGs because of their favorable characteristics, which include high piezoelectricity, flexibility, large-area fabrication and low-temperature synthesis along with the biocompatibility of leadfree OIHPs [30, 31]. In addition, various OIHP-polymer composite materials were developed to achieve flexible PENGs with improved mechanical and air stability [29]. Although the output power of OIHP PENGs is very moderate and is lower than OIHP solar cells, with increasing research efforts, the output power has enormously increased with polymer composites. OIHP-based PENGs having the typical metal– insulator–metal structures similar to other piezoelectric materials based devices and were constructed on flexible plastic substrates using simple solution methods [45, 46]. Yoon et al. reported the first OIHP PENG using solution-processed MAPbI3 thin films as shown in **Figure 6a**. The PENG poled at an applied field of 80 kV/cm demonstrated an output voltage and current density of ~2.7 V and ~140 nA/cm2 under a mechanical pressure of 0.5 MPa (**Figure 6b**) [14]. Later, many researchers focused on improving the output performance of OIHP PENGs [25, 26, 47]. For example, the lateral-structured PENG with inter digitated electrode (IDE) patterns using a MAPbI3 active layer and ZnO & Cu2O-charge transport layers achieved improved output current values [47]. The device was poled under a low (12 kV/cm) electric field for 10 min and was

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

#### **Figure 6.**

*a) Schematic picture of MAPbI3-based PENG, and b) corresponding pressure-dependent piezoelectric output performance [14]. c) Schematic depiction of Cl/Br-doped MAPbI3 PENG, and d) piezoelectric output performance of 4Cl-doped MAPbI3 PENG [25]. Piezoelectric output performance of MASnBr3 PENG: e) output voltage and f) current density signals [30].*

able to generate a voltage of ~1.47 V and a current of ~0.56 μA under 0.2 MPa pressure. In addition, the output of OIHP PENGs can be further enhanced by controlling the dielectric and piezoelectric properties of perovskite material via the concept of functional-modification of perovskite. A high amount of Cl or Br doping into MAPbI3 perovskite leads to enhanced dielectric and piezoelectric properties, which results in better piezoelectric output performance from halide doped-MAPbI3 PENGs compared to the pure MAPbI3 PENG (**Figure 6c**) [25]. As shown in **Figure 6d**, the poled 4Cl-MAPbI3 PENG generated a particularly high output voltage and current density of ∼5.9 V and ∼0.61 μA/cm2 , respectively, because of the improved dielectric constant (εr = 90.9) and remanent polarization (Pr = 0.56 μC/cm2 ) of perovskite film. Similarly, the partial incorporation of Fe2+ into the Pb2+ sites of MAPbI3 perovskite using the simple solution method rapidly enhanced the piezoelectric output performances of PENGs [26]. As discussed earlier, with increasing Fe2+ content, the morphological and crystalline properties of the MAPb1-xFexI3 thin films were improved, leading to improvement of dielectric and piezoelectric properties up to a doping amount of 7 at.% (x = 0.07). After 10 at.% (x = 0.10) doping, the MAPb1-xFexI3 samples exhibited a structural transition from tetragonal to cubic; this was a paraelectric material and is unsuitable for PENG applications. However, as the Fe2+ concentration increased, the piezoelectric output performance of MAPb1-xFexI3 thin-film PENGs linearly increased

and achieved a maximum of 4.52 V for 7 at.%-doped PENG. In addition, the same 7 at.%-doped PENG demonstrated a much higher piezoelectric output of 7.29 V and 0.88 μA/cm2 after poling at an applied electric field of 30 kV/cm.

However, the high toxicity of Pb makes it inappropriate for direct application in the human body or real environs. Researchers have been searching for other lead-free materials in an effort to develop alternatives to lead-based nanogenerators. One of the emerging lead-free OIHP materials is Sn-based perovskite, which is eco-friendly, biocompatible, and has a large piezoelectric coefficient comparable to that of ceramic PbTiO3, which makes it a promising candidate for high-performance nanogenerators in the medical field [31, 48]. The poled lead-free MASnI3 PENG produced an output voltage of 3.8 V and a current density of 0.35 μA/cm2 under an applied pressure of 0.5 MPa [31]. Similarly, the lead-free MASnBr3 PENG displayed an output voltage and current density of 1.56 V and 0.58 μA/cm2 , respectively, under the same applied pressure of 0.5 MPa (**Figure 6e** and **f**) [30]. The generated low output from the MASnBr3 film is because of a lower piezoelectric coefficient of 2.7 pm/V compared to the MASnI3 d33 value of 20.8 pm/V [30, 31]. In addition to organic–inorganic halide perovskites (OHPs), some researchers have also explored inorganic halide perovskite (IHP) materials for PENG applications because of their decent environment stability compared to OIHPs. In particular, CsPbX3 has attracted considerable interest in device applications given its higher chemical stability than other perovskites. The CsPbBr3 nanogenerator was developed on a plastic substrate with the structure of PET/ITO/PDMS/CsPbBr3/ITO/PET and poled at an applied electric field of 25 kV/ cm [33]. The PENG demonstrated better output performance with an output voltage and current of 16.4 V and 604 nA, respectively, after optimized poling conditions. The same device was further able to sense selective motions, such as eye-blinking, throat movements, and finger motions of a human body, highlighting the potential of CsPbBr3 materials for physiological sensing applications.

Although many studies prove the potential of materials in harvesting mechanical energy for generating the electricity, the practical application of OIHP-based PENGs has not been realized so far because of their lower outputs. Furthermore, they are completely incompatible with irregular mechanical deformations. Hence, a key solution proposed was to create composite OIHP structures with polymer materials for the construction of high-performance and long-term air-stable nanogenerators that can withstand highly harsh environs. The first OIHP–PDMS composite-based PENG (PET/ITO/FAPbBr3-PDMS/Al) was developed by incorporating ferroelectric FAPbBr3 nanoparticles into a PDMS polymer, spin-coating the composite onto an indium tin oxide (ITO)-coated PET substrate, and integrating the film with Al foil acting as a top electrode [32]. This PENG demonstrated a maximum piezoelectric output voltage and current density of 8.5 V and 3.8 μA/cm2 , respectively, under pushing. Another group developed an eco-friendly PENG using lead-free MASnBr3-PDMS composite material (**Figure 7a**), which displayed a high piezoelectric output voltage of 18.8 V, current density of 13.76 μA/cm2 , and power density of 74.52 μW/cm2 under an applied pressure of 0.5 MPa (**Figure 7b**) [30]. In addition, the PENG exhibited enormous air-stability over 120 days and mechanical durability over more than 10,000 cycles. However, the non-uniform dispersion of perovskite materials in highly viscous polymers like PDMS may result in modest interactions between PDMS and perovskite crystals that could reduce the piezoelectric output performance of nanogenerators [13]. Soon after, researchers have made use of ferroelectric PVDF polymers to realize high-performance nanogenerators because of its ferroelectric nature [29, 49–52].

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

#### **Figure 7.**

*a) Schematic representation of MASnBr3-PDMS composite based PENG, and b) corresponding piezoelectric output performance [30]. c) Schematic diagram of MAPbI3-PVDF composite based PENG (inset is fabricated device), d) SEM image of 25 vol% MAPbI3-PVDF composite film, and e) corresponding load-resistance dependent piezoelectric output performance [52]. f) Frequency-dependent piezoelectric output voltage signals of FASnI3-PVDF composite PENG and g) Force-dependent piezoelectric output voltage and power densities of FASnI3-PVDF composite PENG and [50].*

PVDF, a semi-crystalline ferroelectric polymer, is mainly available in four phases (α, β, γ, and δ). Among these, the β-phase is one of the polar phases and is a highly electroactive phase with superior piezoelectric properties [49]. Hence, the MAPbI3 perovskite solution was mixed with PVDF solution and spin-coated onto the desired plastic substrates to construct PENGs (**Figure 7c**) [52]. The 25 vol% MAPbI3-PVDF composite films showed porous-like morphology with good dispersion of MAPbI3 nanoparticles into the PVDF matrix as shown in SEM image of **Figure 7d** [29, 52]. By increasing the volume fraction of MAPbI3, the dielectric and ferroelectric properties of composite films improved remarkably owing to the enhanced β-phase content of the PVDF matrix caused by the strong polar interactions or hydrogen bonding between MAPbI3 and PVDF. In addition, PVDF encapsulation significantly increased the air-stability of MAPbI3 perovskite over 6 months [29]. The 25 vol% MAPbI3- PVDF PENG, with the IDE-structure given in **Figure 7c**, generated a high piezoelectric output voltage, current density and power density of 33.6 V and 3.54 μA/cm2 , 41.18 μW/cm2 , respectively, at an applied pressure of 300 kPa, while demonstrating long-term operational stability and mechanical stability due to SEBS polymer passivation (**Figure 7e**) [52]. Furthermore, another eco-friendly PENG based on lead-free FASnI3-PVDF nanocomposite materials that has a high piezoelectric coefficient of 73 pm/V demonstrated a piezoelectric output voltage of 23 V [50]. The piezoelectric output of the same PENG is highly influenced by the applied frequency and force as shown in **Figure 7f** and **g**, respectively. Furthermore, the developed lateral-structured PENGs based on highly uniform CsPbBr3-PVDF composite fibers reveal a recordable piezoelectric output performance with an output voltage of 103 V and circuit current of 170 μA/cm2 , which is noticeably higher than many OIHP/OIHP-polymer materials [51]. The same composite PENG exhibits enhanced thermal/water/acid–base stabilities along with exceptional mechanical stability. These results open up a route for more simple and cost-effective production of high-performance PENGs using OIHPs and their polymer composite materials for mechanical energy harvesting and sensor applications.
