**4.4 OIHP-based TENGs**

TENGs have been intensively utilized as flexible power sources and self-powered sensors [8]. Two dissimilar triboelectric nature materials lying at the extreme opposite ends of the triboelectric series are usually employed to fabricate TENGs in order to achieve higher output power. In particular, materials with high dielectric properties are suitable for realizing efficient TENGs, which are designed as part of capacitors [8]. Therefore, OIHPs are recognized as one of the most promising candidates for developing efficient TENGs because of their remarkable dielectric and piezoelectric properties along with low-temperature synthesis [52]. The first OIHP-based TENG developed using MAPbI3 perovskite is operated as a self-powered photodetector based on the combined properties of photoelectric and triboelectric effects [15]. The TENG comprising of two triboelectric parts (Cu/PET as a negative triboelectric material and MAPbI3/TiO2/FTO as a positive triboelectric material) as illustrated in **Figure 8a** is operated in a fundamental vertical contact–separation mode. This TENG generated a triboelectric peak-to-peak output voltage of 8 V under mechanical pushing in darkness because of triboelectrification. The output is immediately decreased by nearly 37.5% (~5 V) under illumination with a light-intensity of 100 mW/cm2 , giving rise to a high responsivity of 7.5 V/W due to photogenerated charges in the light-active MAPbI3 film (**Figure 8b**). Further, the compositional tuning and electrical poling of perovskite materials can significantly improve the triboelectric performance of TENGs, because compositional modification and ion migration under poling process both tend to alter the conductivity of the OIHP films, which in turn changes the surface potential and electron affinity of those films [43]. Clearly, as shown in **Figure 8c**, the conductivity of the MAPbI3 perovskite film can noticeably change to either p-type (MAI rich) or n-type (PbI2 rich) by regulating the MAI/PbI2 ratio during the precursor synthesis [43]. This concept can further extend to TENG applications to realize high-performance TENGs. The composition-tuned MAPbI3 perovskite is paired with PTFE and nylon (PA6) polymer films to develop TENGs (**Figure 8d**). Here, the

#### **Figure 8.**

*a) Schematic representation of MAPbI3-based TENG, and b) corresponding light-dependent triboelectric output performance [15]. c) Schematic illustration of n and p type conversion in MAPbI3 films by controlling of MAI/ PbI2 ratio, and d) schematic diagram of MAPbI3-based TENG with the counter triboelectric parts of PTFE/ PA6. e) Schematic drawing of MAPbI3-PVDF composite-based TENG in single-electrode mode operation, f) corresponding light-dependent triboelectric output voltage signals under constant pressure of 300 kPa [52].*

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

500-nm-thick MAPbI3 film acts as a triboelectric positive friction layer while pairing with the triboelectric negative PTFE film in PT-PVK TENG and generates a peak output current density of 61.25 mA/m2 . By contrast, the MAPbI3 film acts as a triboelectric negative friction layer while pairing with the triboelectric positive PA6 in PA-PVK TENG and generates a peak output current density of 21.5 mA/m<sup>2</sup> with opposite polarity compared to that of PT-PVK TENG. The poling process further enhances the triboelectric output of PT-PVK TENG. The device generated a maximum output voltage, current density, and peak power density of 979 V, 106 mA/m2 , and 24 W/m<sup>2</sup> after an optimal compositional tuning (MAI/PbI2 ratio of 2) and poling process (EP = 4 V/ μm). Similarly, the TENGs fabricated using composition tuned-Cs based perovskites demonstrate notable variations in their triboelectric output depending upon A-site or B-site, or halogen modification [44]. The TENG (glass/FTO/CsPbBr3-yCly//PVDF/Ag) demonstrates increasing output performance with increasing Cl content and reaches an output similar to that of only CsPbCl3-based TENG owing to increased electrondonating ability with increasing doping amount. Here, perovskite acts as a triboelectric positive layer, while PVDF films act as a triboelectric negative layer.

As in the case of the OIHP–polymer PENGs, the OIHPs were further composited with polymer materials to improve the long-term operational stability along with air-stability of OIHP–polymer TENGs. To this end, several TENGs with different structures, materials, and modes of operations were developed and their feasibility to harvest the mechanical energy was demonstrated. The flexible single-structure multifunctional device with the structure of MAPbI3-PVDF/Au-IDE/SEBS can harvest mechanical energy and simultaneously sense multiple external stimuli like light and pressure (**Figure 8e**) [52]. The TENG in a single-electrode mode generates an output voltage of ∼44.7 V, a current density of ∼4.34 μA/cm<sup>2</sup> , and a power density of ∼59.52 μW/cm<sup>2</sup> under cyclic contact–separations in darkness. Furthermore, the triboelectric output gradually increases with increasing light-intensity and reaches a maximum voltage of 67.9 V (**Figure 8f**), current density of 7.44 μA/cm<sup>2</sup> , and power density of 158.34 μW/cm<sup>2</sup> at a high light intensity of 3.23 mW/cm<sup>2</sup> . This significant enhancement in triboelectric output is because of the combined photoelectric and triboelectric properties of the MAPbI3–PVDF active layer. Under mechanical pushing, when the pushing stack (Al2O3/Al-stack) touches the surface of the SEBS polymer, contact electrification results in the generation of charges with opposite polarities on the surfaces of the pushing stack as well as SEBS polymers. Concurrently, the active piezoelectric MAPbI3–PVDF layer undergoes deformation, thus generating dipoles. Owing to the combined triboelectric and piezoelectric effects, the charges will be induced on the Au electrode, leading to a higher potential difference between the electrode and the ground. The resultant potential difference allows the flow of electrons through the external circuit to the ground, thus generating improved outputs. However, when the TENG is illuminated under the applied pressure, the induced triboelectric charge allows the rapid injection of photogenerated charge carriers from MAPbI3 into the Au electrode. This results in a significantly higher triboelectric output under illumination compared to darkstate. Similarly, the fabricated MAPbI3–PDMS composite e-skin-based TENG is highly capable of harvesting mechanical energy and producing neural-stimulating electrical signals without relying on an external power supply [53]. The triboelectric output performance of e-skin significantly increases as the bending radius increases and shows high output voltage and currents of 0.659 V and 8.94 nA, respectively, for a bending angle of 60°. In addition, the device displays strain-dependent and

light-stimulated voltage variations, which enable the device to operate as a selfpowered pressure and physiological sensor application. More recently, a stretchable, breathable, and long-term stable hybrid MEH has been developed based on ecofriendly, 2D layered lead-free Cs3Bi2Br9, PVDF-HFP and SEBS composite (LPPS-NFC) nanofibers prepared via an electrospinning process [54]. Here, the strong electron-accepting nature of perovskite materials acts as a nucleating agent and improve the crystallinity and polar β-phase of PVDF polymers. The developed composite nanofibers can efficiently harvest the mechanical energy in piezoelectric as well as triboelectric modes. The LPPS-NFC stretchable device with the structure of Spandex/Ag-SEBS/LPPS-NFC//Al generates much larger peak-to-peak outputs with a voltage of 400 V, current density of 1.63 μA/cm<sup>2</sup> , and power density of 2.34 W/m<sup>2</sup> in the hybrid mode based on the combined piezoelectric and triboelectric effects of composite film. Furthermore, the LPPS-NFC based MEHs reveal excellent stability and are able to produce stable outputs even under harsh mechanical deformations like washing, folding, and crumpling, indicating the superior potential of these LPPS-NFC-based MEHs for use in smart textile-based wearable devices. All these results demonstrate the high potential of hybrid perovskites as triboelectric materials, given their superior dielectric property and stepping forward for highperformance TENG platforms.

## **5. Current challenges and future prospects**

Plenty of research effort has been expended in the study of OIHP nanogenerators to prove the potential of OIHPs as promising active materials for mechanical energy harvesting. The development of OIHP-based mechanical energy harvesters can substantially advance IoT and AI systems. OIHP nanogenerators can operate as sensors that have a wide range of utility in environment monitoring, health monitoring, motion detection, robotics, e-skin, and human-machine interactions. Furthermore, those nanogenerators can supply power to conventional batteries in small-scale and portable electronic devices. However, the key factors that need to be resolved currently in the field of OIHP-based MEHs are air-stability, encapsulation, toxicity, mechanical sturdiness, and moderate performances. Future developments in this field are likely to be focused on the following aspects. First, the output performance and energy conversion efficiency of OIHP-based MEHs should be improved to meet the requirements of small-scale/portable devices. Second, in order to be implemented in wearable devices, the OIHP nanogenerators should be highly flexible, stretchable, and lightweight, and must be able to withstand harsh environs. Third, the eco-friendly nature and low toxicity of OIHP-based devices is a key characteristic for use in health monitoring/biomedical devices; thus, the need of lead-free OIHPs for MEH applications is necessary. Systematic investigations of OIHPs having various dimensions can introduce a new platform for designing high-performance nanogenerators. Controlling the dielectric and ferro/piezoelectric properties of various OIHPs via compositional and structural engineering can also assist nanogenerators to improve energy conversion efficiencies. Furthermore, it has been recognized that layered 2D OIHPs have better piezo/ferroelectric properties along with decent moisture and air stability compared to the 3D OIHPs owing to the presence of long-chain organic cation molecules. It is expected that flexible and stretchable self-powered systems with dynamic sensing properties are the future direction of wearable electronic devices. Therefore, integrating OIHPs with flexible piezoelectric polymers

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

will aid in the construction of air-stable, mechanically robust and high-performance nanogenerators.
