**Abstract**

Organic/inorganic halide perovskites (OIHPs) have recently emerged as promising candidates for the creation of high-efficiency electronic and optoelectronic devices, having superior performance because of their unique features such as excellent optical and electronic properties, cost-effective fabrication, solution-processing, and simple device architecture. The noteworthy dielectric and ferro/piezoelectric properties of OIHPs have enabled the design of mechanical energy harvesters (MEHs). Considerable research has been conducted on using OIHPs in the field of piezoelectric and triboelectric nanogenerators. In this chapter, we describe the potential of OIHP materials, such as organic and inorganic halide perovskites, for harvesting ambient mechanical energy and convert it into electrical energy. Furthermore, the crystal structure of OIHPs along with their dielectric, piezoelectric, and ferroelectric properties are discussed in detail. Recent innovations in OIHP-based MEHs are also summarized. The role of OIHP-polymer composites in enhancing the performance and operational stability of nanogenerators is discussed. Certain issues and challenges facing contemporary OIHP-based MEHs are stated, and finally, some directions for future developments are suggested.

**Keywords:** OIHP, piezoelectricity, triboelectricity, mechanical energy, nanogenerator

## **1. Introduction**

The rapid progress in artificial intelligence and internet-of-things technologies has increased the demand for portable and sustainable energy sources that can enable perpetual operation [1, 2]. Mechanical energy harvesters (MEHs) that convert abundant mechanical energy from the environment (wind, raindrops, water flow, and vibrations) as well as from human motions (walking, jogging, and running) into electricity, is considered a promising solution to alleviate the energy crisis for supplying power to low-power consumed portable electronics [3]. In particular, piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs)

have received immense attention as they efficiently convert mechanical energy into electricity for powering portable and wearable electronics [4]. Furthermore, their simple structure, easy fabrication process, high energy-conversion efficiencies, flexibility, and mechanical robustness makes them well-suited for energy generation [4, 5]. PENGs transform mechanical energy into electricity by generating electric dipoles through the deformation of piezoelectric materials, while TENGs convert mechanical energy into electricity effectively through a coupling of contact electrification and electrostatic induction. In the past decade, several flexible MEHs have been demonstrated using diverse piezoelectric nanostructured materials including ceramics (e.g., PbZrxTi1−xO3 (PZT) and BaTiO3), semiconductors (e.g., ZnO and CdS), and polymers (PVDF and its derivatives) [6–9]. Among these materials, perovskite-structured ceramics (i.e., ferroelectric materials) are commonly used materials for constructing efficient PENGs and sensors because of their strong dielectric and ferroelectric/piezoelectric properties [10, 11].

In recent years, organic/inorganic halide perovskites (OIHPs) have emerged as promising materials for solar cells with extremely high-power conversion efficiencies over 25% because of their unique optical and electrical properties while having a simple solution process [12]. Besides the discovery of the intriguing ferroelectric and piezoelectric properties of OIHPs have accelerated their recent application in PENGs [13–15]. In 2016, the first thin-based PENG was reported based on the solution-processed ferroelectric MAPbI3 thin films [14]. Subsequently, several flexible PENGs based on OIHP thin films and OIHP–polymer composite films have been developed [13]. OIHPs also exhibit impressive dielectric properties, which is one of the essential features for fabricating efficient TENGs. The first TENG based on MAPbI3 displayed lightdependent triboelectric output characteristics with a moderate performance [15]. Later, a series of commonly used Cs-based perovskites were applied in TENGs owing to their stability compared to organic perovskites [16, 17]. The light-active nature along with the ferro/piezoelectric properties of OIHP harnesses the light-dependent output characters of PENGs as well as TENGs, which allows the nanogenerators to be used as bimodal sensors for sensing pressure and light [18].

In this chapter, we introduce the viability of OIHP materials for mechanical energy harvesting in the form of nanogenerators. The crystal structure and dimensionality of OIHPs along with their dielectric, piezoelectric, and ferroelectric properties are discussed in detail. In addition, the operating mechanisms of OIHP based-MEHs (PENG and TENG) are discussed. Furthermore, the recent progress of various MEHs based on a broad range of OIHP/OIHP–polymer composite materials is summarized. Finally, a brief glimpse into current challenges and future developments for OIHPbased nanogenerators is provided.

## **2. Structure and dimensionality of OIHPs**

The term "perovskite" represents a class of materials originating from the mineral calcium titanate (CaTiO3) and having a crystal structure of ABO3, which were discovered in 1839 by Gustav Rose [19]. Oxide perovskites are widely used in various dielectric, ferroelectric, piezoelectric, and pyroelectric applications. However, OIHPs differ from inorganic ceramic perovskites by containing halide anions in place of oxide anions. Three-dimensional (3D) OIHPs also have the general crystal structure of ABX3 (**Figure 1a**) [20], in which A represents an organic or inorganic monovalent cation (e.g., methylammonium (MA+ = CH3NH3 + ), formamidinium (FA+ = CH (NH2)2), or

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

#### **Figure 1.**

*Schematic representation of a) typical ABX3 type 3D OIHP structure, b) structures of 2D OIHPs of A*′*mAn-1BnX3n+1 (with the value of n increasing from n = 1 to n =* ∞*) [24].*

cesium (Cs+ )), B denotes a divalent metal cation (e.g., Pb2+ or Sn2+), and X indicates a halide anion (Cl, Br, and I). In the crystal structure, A-site cations are connected with 12 neighboring X, while B-site is coordinated by 6 X anions to form cuboctahedral and BX6 octahedral geometries, respectively. The formation of the perovskite structure and its stability can be evaluated by the Goldschmidt tolerance factor (t) and the octahedral factor (μ) [21]. The tolerance factor is given by t = (rA + rX)/√2(rB + rX), where rA, rB, and rX are the ionic radii of A, B, and X, respectively. The octahedral factor is given by μ = rB/rX, which is directly correlated with a BX6 octahedron. The tolerance factor and octahedral factor values of OIHPs are expected to be in the range of 0.813 < t < 1.107 and 0.44 < μ < 0.90 [13], respectively. OIHPs tend to form ideal cubic, orthorhombic, and hexagonal structures when 0.8 < t < 1.0, t < 0.8, and t > 1, respectively [13, 22]. 2D perovskites or layered perovskites are formed by introducing large organic functional groups into the 3D structure (**Figure 1b**) and have received immense attention because of their excellent ambient stability [23]. These 2D perovskites can be prepared using a mixture of small cations that forms perovskite and a large organic cation that forms the layered metal halide. The general chemical formula for these layered perovskites is A′mAn–1BnX3n+1, where A′ is a monovalent (m = 2) or divalent (m = 1) long-chain organic cation (e.g., aromatic or aliphatic alkylammonium), which acts as a spacer. A, B, and X are cations and anions similar to the ones in 3D OIHPs, and n specifies the number of perovskite layers. Here, n = ∞ corresponds to a 3D structure, n = 1 represents a 2D structure, and other values of n denote a quasi-2D structure [24].

## **3. Structure and dimensionality of OIHPs**

#### **3.1 Dielectric properties**

A dielectric is referred as an insulating material that is polarized under an applied external electric field. The response of a dielectric material to an applied field is expressed in terms of permittivity. The dielectric constant or relative permittivity (εr) of a material is usually obtained from the ratio of its permittivity to the permittivity of a vacuum (ε0). Materials having a large dielectric constant have the ability to develop higher polarization for an applied electric field. In general, inorganic materials are well-known dielectric materials. In recent times, OIHPs are attracting extensive attention to be used as dielectric materials because of their simple low-temperature synthesis process. ABX3-structured materials exhibit higher dielectric constant values owing to the ease of polarizing the cell structure. Specifically, distortion of the edge-sharing BX6 octahedra in the ABX3 structure can produce an electric dipole between the A and B sites. OIHPs demonstrate impressive dielectric properties analogous to ceramic perovskites, but the values are relatively lower owing to the existence of polar organic cations in the center of the perovskite structure, which can introduce orientational disorder and polarization. The dielectric properties of various lead (Pb) and lead-free OHIPs have been previously investigated experimentally [14, 25, 26]. Kim et al. measured the temperature-dependent dielectric properties of MAPbI3 thin films and confirmed their tetragonal-cubic phase transition. MAPbI3 revealed a dielectric constant value of 52 at 100 kHz [14]. Furthermore, structural tuning of MAPbI3 also affects its dielectric properties. For example, with the partial incorporation of Cl into MAPbI3, the dielectric constant of the resultant films increased to 90.9 at 100 kHz (**Figure 2a**), while Br-incorporated MAPbI3 films exhibited a dielectric constant of 71.6 at 100 kHz [25]. In addition, a structural transition was also observed for partially incorporated Fe2+ into MAPbI3 from the dielectric study [26]. The partial replacement of Pb2+ with Fe2+ ions exhibited a tetragonal–cubic phase transition

#### **Figure 2.**

*Dielectric constants and dissipation factors of a) Cl-doped MAPbI3 thin films [25], b) Fe2+-incorporated MAPbI3 thin films, c) MAPbI3-PVDF composite films, e) Piezoelectric response in MAPbI3 thin films, and Piezoelectricity in MASnBr3 thin films; e) Piezo-amplitude, f) Piezo-phase hysteresis loop, and g) Piezoresponse [30].*

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

as discovered by the frequency dielectric study of Fe2+-incorporated MAPbI3 thin films (**Figure 2b**). The dielectric constant (εr) at 100 kHz of MAPb1-xFexI3 films continuously increased as the Fe2+ content increased to x = 0.07, attaining a maximum value of 107. It then decreased for larger Fe2+ content, indicating the ferroelectric-to-paraelectric phase transition for x = 0.07. The dielectric properties of OHIP-polymer composites were also investigated by a few researchers. In general, the heterogeneous materials interfaces in polymer composite films can induce an interfacial or a Maxwell–Wagner–Sillars polarization that results in an abrupt change of the total dielectric constant [27, 28]. As the MAPbI3 content in PVDF polymer increased, the dielectric constant of the MAPbI3–PVDF composite rapidly increased because of large dipole–dipole interactions (**Figure 2c**) [29]. In addition, the interaction between the organic action of MA<sup>+</sup> and -CF2- between the perovskite and PVDF leads to the self-orientation of polymer chains, enabling the nucleation of the electroactive phase that results in the formation of a spontaneous polar β-phase in the composite films. Similarly, the dielectric properties of leadfree MASnBr3–PDMS composite films were also examined for various percentage weights of MASnBr3 (0 to 25 wt.%) [30]. As the MASnBr3 increased from 5 to 15 wt.% content, the dielectric constants of composite films progressively increased and achieved a maximum of 36.23 for 15 wt.% at 1 kHz. By contrast, the dielectric constant values of composite films decreased with a higher loading amount of MASnBr3 owing to increased agglomeration of MASnBr3 particles, which leads to the leaky nature of composite films. The tuning of the dielectric and piezoelectric properties of OIHPs by a simple solution process makes them suitable to be used to construct efficient mechanical energy harvesters.

#### **3.2 Piezoelectric properties**

The piezoelectric effect refers to the capability of certain materials to generate electric charges under applied mechanical stress, which is also known as the direct piezoelectric effect. Conversely, an electric field applied to the material induces mechanical strain, which is called the converse piezoelectric effect. This unique property of piezoelectric materials allows their use as sensors and actuators. The structural requirement for a material to exhibit piezoelectricity is non-centrosymmetric. Piezoelectricity was first demonstrated in 1880 by brothers Pierre and Jacques Curie. The direct piezoelectric effect is observed in many natural crystalline materials such as Rochelle salt, quartz, topaz, and human bone. In addition, many engineered materials, in particular, inorganic perovskite materials with a structure of ABO3 including PZT, BaTiO3 and (K,Na)NbO3, exhibit a noticeable piezoelectric effect, and are widely studied for several applications [6]. The piezoelectric response or piezoelectric energy-harvesting capability of any piezoelectric material can be determined by its piezoelectric coefficient and is proportional to the dielectric constant and polarization (i.e., d33 α εrPr) [13].

OIHPs also exhibit relatively good piezoelectric properties similar to inorganic ceramic perovskites, but the values are comparatively lower. In recent years, some researchers have investigated the piezoelectric properties of OIHPs in order to determine their potential in various device applications. Kim et al. investigated the piezoelectric coefficient (d33) of solution-processed polycrystalline MAPbI3 films using piezoresponse force microscopy (PFM) and reported a d33 of 5.12 pm/V [14] (**Figure 2d**). A single-crystalline device may provide direct evidence regarding the piezoelectric properties of OIHPs, whereas studying the piezoelectric properties of polycrystalline films using PFM could face challenges owing to inaccurate estimation of tip contact area and other artifacts arising from surface topography and crystal orientation [27]. In this regard, Dong et al. verified the piezoelectric effect in single-crystalline MAPbI3 by depositing two parallel facet gold electrodes [28]. They obtained a d33 of 2.7 pm/V along the (001) direction for single-crystal MAPbI3 using the laser interferometry method. Compositional tuning of OIHPs also substantially altered their piezoelectric properties. The partial replacement of Pb with Fe in MAPbI3 improved the d33 of 17.0 ± 6.0 pm/V for MAPb1-xFexI3 (x = 0.07) [26]. Similarly, there was a considerable improvement in d33 of 20.8 pm/V observed for lead-free MASnI3 by substituting Sn in the Pb-site in MAPbI3 [31]. Likewise, lead-free MASnBr3 perovskite displayed a d33 of 2.7 pm/V [30] (**Figure 2e–g**). The slanted butterfly shape in amplitude loop of the MASnBr3 is caused from the electrochemical properties of defects, vacancies, and ions, which indicate that the MASnBr3 possess both electrochemical and piezoelectric properties. According to Ding et al., replacing the A and X site in MAPbI3 with FA and Br, respectively, significantly increased d33 (25 pm/V) for FAPbBr3 having a particle size 50–80 nm, which is a five-fold enhancement over MAPbI3 [32]. In addition, inorganic CsPbBr3 films also revealed a higher d33 of 40.3 pm/V after poling than organic MAPbI3 films [33]. As a subclass of 3D OIHPs, the piezoelectric properties of 2D perovskites (vacancy-ordered double perovskites) are also recently attracting significant research attention owing to their excellent ambient stability compared to 3D OIHPs. Although 2D OIHPs exhibit superior ambient stability, very few piezoelectric studies have been focused on the recently evolved 2D OIHPs. For instance, solution-processed (ATHP)2PbX4 displayed ferroelectric behavior with a large d33 of 76 pC/N and a giant piezoelectric voltage co-efficient (g33) of 660.3 × 10−3 V.m/N [34]. In addition, most 2D OIHPs have ferroelectric natures, thus displaying superior piezoelectric properties as a subclass of piezoelectric materials.
