Nanogenerators Based on Organic–Inorganic Heterojunction Materials
Md Masud Rana, Asif Abdullah Khan, and Dayan Ban
Scavenging sustainable power by converting ubiquitously available unutilized energy to usable electrical energy holds promise to meet ever-expanding energy demands as the conventional fossil energy sources are being quickly exhausted. Energy harvesting based on nanotechnology [ 1 ] is attracting intensive interest and attention for two primary reasons: (i) the potential to realize self-powered electronics [1-4] as portable devices, sensors, and implantable biomedical devices which typically consume very low electrical power  and (ii) the potential to reduce global dependency on energy sources based on fossil fuels . Over the past decades, researchers have been investigating different ambient energy harvesting technologies based on electromagnetic , electrostatic [8-9], and piezoelectric methods [lO-l 1]. The contact electrification effect was first recorded as early as 2600 years ago, and the first demonstrated piezoelectric effect was reported by Pierre and Jacques Curie in 1880. Nevertheless, the unprecedented potentialities of these effects in energy harvesting applications had not been fully revealed for a very long time. Following the first demonstration of the PENG in 2006  and the triboelectric (contact-electrification) nanogenerator in 2012 , significant efforts have been devoted toward accomplishing a brand new era of self-powered electronics by using organic-inorganic heterojunction nanomaterials to build sophisticated micro/nano systems [14-23].
Triboelectric nanogenerators (TENGs) based on the coupling effect of contact electrification between two different materials and electrostatic induction have emerged as a viable technology to convert ambient mechanical energy into electrical energy. TENGs has numerous advantages, including large power density, high energy conversion efficiency, versatile options for material selection, lightweight, low cost, etc. They have been successfully used as self-powered sensors in wind speed sensing, micro liquid biological and chemical sensing, vibration monitoring, transportation and traffic management, motion tracking, powering biomedical microsystems, among others [24-26]. The contact electrification-induced surface charge density (SCD) is defined as the key figure of merit for TENGs, which originates from the different work functions between two materials. Therefore, increasing surface area by creating nanostructures, designing nanomaterials with high energy-storing capabilities, or having dense surface states are the efficient routes toward a highly-efficient TENG. A state-of-the-art TENG can produce a power density of up to 500 W/m2 . However, in contact-separation-triggered TENGs, the air breakdown effect can significantly limit the SCD on triboelectric surfaces .
On the other hand, PENGs, which have compact and flexible working modes, are a very promising alternative solution. When mechanical stress is applied on a piezoelectric material, the centers of positive and negative charges are separated, thus creating polarization-induced piezo-potential. PENG research is focused on manipulating material structures (porous, nanowires, etc.) by lithography, etching, or others to improve stress distribution profiles or growing materials with very high inherent spontaneous polarization. PENG device performance has been improved by a series of structure- driven techniques, such as adopting nanowires (NWs) , aspect ratio tuning, film porosity modulation through a multi-stage etching process , cascading multiple devices , and reducing charge-screening effects . However, PENGs suffer from comparatively lower output performance than other existing harvesters (e.g., TENGs, electromagnetic generators) and operate preferably in higher frequency regimes.
The energy conversion efficiency and power output can be further improved by combining harvesters of different types. Hybrid NGs integrate different harvesters in a single unit, where several energy sources can be leveraged either simultaneously or individually. This approach provides a continuous supply of power through harvesting renewable and green energy resources and helps maximize energy utilization.
Integration of a PENG and a TENG, or a PENG and a solar cell (SC), or a PENG, a TENG, and a SC yields some of the notable hybrid energy harvesters. In this chapter, different organic-inorganic heterojunction-based NGs are introduced. The device working principle, design and fabrication, and some promising applications are discussed in detail.
Fundamentals of Nanogenerator
NGs efficiently transform mechanical energy into electrical power/signal, which has broad applications in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics, and artificial intelligence . TENGs are generally based on contact electrification. When two dissimilar materials are brought into contact, electrostatic charges are created on the material surfaces due to the different electron affinities of the materials. When the two materials are subsequently separated, the developed voltage forces the electrons to flow between two electrodes, generating an alternating current in the TENG. When mechanical stress is applied, a piezoelectric material is polarized, creating a piezo-potential. The physics behind NGs can be explained using Maxwell’s equations.
Ampere’s circuital law with Maxwell’s addition is
where H is the magnetic field and D is the displacement field.
Here P is the polarization field and E is the electric field. Therefore, Maxwell's displacement current can be defined as:
The first part on the right of Eq. 9.3 gives the birth of electromagnetic waves. The second part relates to the output of the NG. If the SCD of a PENG is o;), and there is no external electric field, the displacement current is reduced to 
Equation 9.4 denotes the observed output current in PENGs. In TENGs, the electrostatic field built by the triboelectric charges (with a SCD of o(.) drives the electrons to flow through an external load, resulting in an accumulation of free electrons in the electrode o,(z, t). c,(z, t) is a function of the gap distance z(t) between the two dielectrics. The corresponding displacement current is 
This is the observed current for TENG. dz/dt depicts the speed at which two triboelectric layers contact each other. This basic theoretical understanding of NG
FIGURE 9.1 A tree idea to illustrate Maxwell’s displacement current: the first term edE/dt is responsible for the electromagnetic waves theory, and the second term dPJdt is related to energy and sensor applications, such as NGs. Reproduced with permission . Copyright 2020. Elsevier.
operation is vital to further modeling and analysis, which unveils enormous potentialities for meeting future energy demands as shown in Figure 9.1.
Piezoelectric Nanogenerators Based on Organic–Inorganic Hybrid Nanomaterial
Basic Concept of PENGs and Its Operating Principle
Originating from Maxwell’s displacement current, the PENG concept was coined by Prof. Wang in 2006 , where an array of zinc oxide (ZnO) nanowires (NWs) grown on a metallic substrate was bent by a conductive atomic force microscope (AFM) cantilever probe, as shown in Figure 9.2.
The tetrahedrally coordinated Zn2+ and O2- were accumulated layer-by-layer along the c-axis (Figure 9.2a). At its original state, the charge center of the anions and cations coincide with each other. Once an external force is applied, the ZnO NW structure is deformed and stretched on one side while compressed on the other side, w'hich accumulates negative and positive charges on the respective sides. Therefore, the
FIGURE 9.2 Mechanism of piezoelectricity, (a) Atomic model of the wurtzite-structured ZnO. (b) Different piezopotential in tension and compression modes of the PENG, (c) Numerical calculation of the piezoelectric potential distribution in a ZnO NW under axial strain. Reproduced with permission . Copyright 2017, Wiley-VCH. (d) Band diagram for the charge outputting and flowing processes in the PENG. Reproduced with permission . Copyright 2017, Wiley-VCH.
negative and positive charge centers are separated and form an electric dipole leading to a piezoelectric potential (Figure 9.2b). If an external load is connected to the deformed material, the free electrons are driven to partially screen the piezoelectric potential and flow through the external circuit to realize a new equilibrium state [35-36]. The resultant piezopotential is observed through the formed Schottky barrier between the AFM probe tip and semiconducting ZnO NW, which forces the electrons to flow between the electrodes, through an external circuit (Figure 9.2c-d).
Since this pioneering work, intensive research efforts were made to enhance output power generation from ZnO NW-based PENGs as it is environmentally friendly, easy to grow at low temperatures, and is self-poled. In contrast, most of the ferroelec- trics require high-temperature processing conditions .
Material Design Criteria and Techniques for Performance Enhancement
The key points for better device performance have been attributed to the higher aspect ratios of NWs: Schottky barrier formation between the top electrodes and ZnO due to the higher work function of a PdAu electrode than other commonly used metals and reduced charge-screening effects. As the tensile stress and compressive stress induce negative and positive piezopotential, respectively, an approach stems from creating pn-junction ZnO NWs rather than intrinsic ones to reduce local charge-screening effects inside the NWs . However, the lower piezoelectric coefficient (d„ ~ 12 pC/N) and the fragile nature of ZnO NWs, nanobelts, or nanorods are limiting their applications as a high-performance renewable power source. On the contrary, polymer materials like polyvinylidene fluoride (PVDF) and polyvinylidene fluoride trifluoro-ethylene (PVDF-TrFE) are promising piezoelectric materials that have higher flexibility, piezoelectric coefficients, and long-term reliability. Altering the microstructures of such piezoelectric films to enhance strain-dependent piezoelectric polarization has proven to be an effective energy-harnessing mechanism. Film porosity modulation through a multi-stage etching process, aspect ratio tuning, and cascading multiple devices are remarkable structure-driven techniques, further pushing the piezoelectricity limit. For example, by using random and highly porous (50%) PVDF structures (through etching process), Mao et al. enhanced the output voltage and current of PENG to ~ 11.1 V and 9.7 pA. respectively, which is higher than lithography-assisted porous PVDF NW array . Recently, Yuan et al. presented a cascade-type six-layer rugby-ball-shaped PENG and improved the output performance to 88.62 Vp p and 353 pA, setting a record value for multilayer PENGs . Although piezoelectricity can be enhanced by these strategies, optimally unifying appropriate mechanical and electrical properties in a single piezoelectric film remains a challenge. Growing organic-inorganic molecular perovskite solution or synthesizing an organic- inorganic perovskite single-crystal has recently achieved record d„ coefficients (-185 pC/N)  by surpassing their inorganic counterparts, for example lead zirconium titanate (PZT), lead magnesium niobate lead titanate (PMN-PT), and barium titanate (BTO). By dispersing highly piezoelectric nanoparticles (NPs) in a flexible polymer, composite films can be developed, which has been proven as an attractive, easier approach in terms of fabrication scalability, device flexibility, improved mechanical strength, and enhanced electrical output and stability.