# Hybrid nanogenerators Based on Organic–Inorganic Hybrid Nanomaterial

ORGANIC-INORGANIC HYBRID NANOMATERIAL

## Basic Concept of HNGs and Its Operating Principle

Hybrid NGs integrate different types of harvesters in a single unit, where several energy sources can be leveraged either simultaneously or individually, making it possible to use whatever energy is avaliable at the time. This approach provides a continuous supply of power through renewable and green energy resources and helps maximize energy utilization to achieve a stable electrical output. Within a hybrid cell, constituent units can be connected in series or in parallel to enhance the voltage or current, respectively.

Extending from the basic understanding of NG operation from Maxwell’s equations, a specific theoretical model for hybrid NGs was developed in 2017. Song etal. [79] presented the first theoretical model for piezo-tribo-based hybrid NGs, and later extended the analytical approach based on the following three conditions: (a) PENG and TENG are separate units, (b) tribo-charges are uniformly distributed on the electrode, and (c) electric potentials are regarded as a closed loop in a circuit. Considering the above three conditions, it is possible to write from Figure 9.12b that [80]

where Va, VPT and Vpv are the voltages across the air gap, the PTFE layer, and the PVDF layer, respectively, and V, and V2 are the voltages across the external resistors. The test cases for this hybrid NG can be assumed from Figure 9.12 that after applying the external impact from time t = 0, the top electrode EDI makes contact with PTFE at t = /1 (after ДГ, time from t = 0). then the whole device starts deforming and reaches its highest deformed stage at Г = t2 (after At-, time from t = t).

So, after time t = t2 the device starts releasing and at t = ?3 (after At3 time from t = t2) the device reaches its same position again like t = t, and finally at t = t4 (after

FIGURE 9.12 Theoretical analysis approach for piezo-tribo hybrid NG. (a) Motion characteristics of external load and (b) PENG-TENG hybrid device. The dynamics is used for EDI and ED2 are x, and x2 respectively. Reproduced with permission [80]. Copyright 2019, IEEE.

Дt4 time from t = 13) EDI starts breaking contact with PTFE and reaches the initial position, thus completing one cycle of operation. In this approach, x, and x2 will be used to express the dynamics of EDI and ED2 (top electrode of PVDF-based PENG). If the transferred charges in ED,, ED, and ED, are 0,, 0, and Q3 respectively, and assuming the initial condition (0, = 0, = 0 and 0, = oS) at (t = tO = 0), then the V-Q-x relationship for the top TENG unit can be expressed as [В 1 ]

where s is the surface area, dr] is the effective thickness constant for PTFE (с1,л = d/er), G,. is relative permittivity, and a is the SCD. Assuming the initial condition (0i = 0з = 0 and 0, = as) at (1 = t0 = 0), Eq. 9.9 can be solved as

This is the charge transfer equation for the TENG unit and is valid within the time range of ДI, and Д14.

Similarly, by using the same approach and boundary condition, the charge transfer equation for the PENG unit is found as [81]

It is clear from Figure 9.12a that Eq. 9.11 is valid for 1 = 0 and t = Д/, + Д/2 because the PENG unit operates in that period. From the charge transfer equation for the PENG and TENG units, it is possible to obtain the current and voltage expressions for this hybrid NG. This theoretical approach for hybrid NGs can be used to qualitatively [81] describe the PENG and TENG-based hybrid NG: however, it should be used with caution as it could yield substantial errors when predicting PENG characteristics. Another important limitation is that the output of the two units is simply superimposed in this model, and the potential synergetic effect in hybrid NGs is ignored.

## Various Approaches Taken to Design High-Performance HNGs

Solar [82] and vibration [83] energies are most commonly available in the ambient environment. However, vibrations generate power only while motion persists and solar energy is significant only when optical illumination is sufficient. The nanotechnology-based compact hybrid energy cell (СНЕС) can individually and concurrently harvest vibrations and/or solar energies [84-85]. In typical hybrid energy harvesters, the components that scavenge different types of energy are designed and fabricated independently, following distinct physical principles. Due to their different output characteristics, each energy harvesting modality requires its own power conversion and management circuitry. For example, PENGs have large output impedance and can produce high voltage but low current, while SCs have small output impedance, with high current but low voltage [86]. Designing compact cells that can effectively and simultaneously harvest energy from multiple types of sources will increase their applicability and levels of output power.

A cascade-type transparent vibration/solar energy cell synthesized on a polyethylene naphthalate (PEN) flexible substrate is presented [87]. The cascade-type CHEC’s monolithically integrated two-terminal structure substantially suppresses the large interfacial electrical losses typically encountered in mechanically stacked devices. Furthermore, integrating the SC on top of the PENG significantly enhances the output power density, through effective, simultaneous, and complementary harvesting of ambient strain and solar energies. The cell consists of a vertically aligned n-p ZnO homojunction NW-based NG and a hydrogenated nanocrystalline/amorphous silicon (nc/a-Si:H) n+-i-p+ junction SC. The device’s full inorganic heterostructure improves chemical stability and mechanical durability. It can function as a sensor, a SC, or a NG.

The ZnO homojunction NWs are grown hydrothermally [88]. A SiN buffer layer and aluminum-doped ZnO (AZO, 2 wt.% Al203 + 98 wt.% ZnO) layer are deposited onto a pre-cleaned polyethylene naphthalate (PEN) substrate using radio-frequency (RF) magnetron sputtering at 150 °C. To obtain p-type ZnO NWs, a doping reagent, lithium (Li) nitrate (75 mM), is added to the solution (heavily /Муре). Additionally, the n-n homojunction NWs are prepared with an intrinsic (effectively /Муре) NW growth procedure for use as control samples in the experiments [88]. The solar component of the CHECs consists of a stack of n+-i-p+ nc/a- Si:H thin-film layers, deposited on top of the synthesized n-p and n-n homojunction ZnO NWs by PECVD at a substrate growth temperature of 150 °C. To minimize electromagnetic interference, the two copper wires connected to the device under test are twisted together. All measurements are conducted at ambient room temperature.

Figure 9.13a shows a schematic diagram of a fabricated СНЕС and its architecture. An equivalent circuit of the СНЕС, showing the NG and SC connected in series, appears in Figure 9.13b. The nc/a-Si:H n+-i-p* layers are integrated directly on top of the underlying lithium-doped ZnO NW layer. Two types of ZnO NWs are employed in the device fabrication: ZnO n-p homojunction NWs and ZnO n-n homojunction NWs. Figure 9.13c shows a photograph of the patterned array of CHECs with varying side lengths (from 1 mm to 1 cm) and insulation separation. This array configuration provides the basis for effectively comparing the output for a range of CHECs. Figure 9.13d shows a cross-sectional helium ion microscope (HIM) image of a typical СНЕС and confirms the monolithic and seamless integration between the nc/a- Si:H n*-i-p* layers and the underlying ZnO NW layer. The ZnO NWs are functioning as the piezoelectric material for mechanical energy conversion and as the electron transport layer for photocurrent collection of the SC component. Figure 9.13e shows top-view HIM images of the as-grown n-p and n-n homojunction ZnO NWs, revealing uniform growth of high-density and vertically aligned NWs. The average length

FIGURE 9.13 (a) A schematic diagram of а СНЕС made of n-p homojunction ZnO NWs

grown on a flexible substrate (cross-sectional view), (b) A schematic showing an equivalent circuit of the hybrid energy cell, (c) A photograph of patterned СНЕС arrays, (d) A cross- sectional helium ion microscopy (HIM) image of a fabricated СНЕС, (e) HIM images of the n-p (top) and n-n (bottom) homojunction ZnO NW arrays. Reproduced with permission [87]. Copyright 2016, Elsevier.

and diameter of these NWs are ~750 nm and 80 nm, respectively. This monolithic СНЕС can exploit piezo-potential under compressive strain and photovoltaic potential under ambient optical illumination, to generate electrical power. The CHECs, when placed solely under optical illumination, function as traditional SCs and produce continuous photocurrent output. The photocurrent flows from the n+-nc-Si layer to the />+-nc-Si layer, or from the left (the bottom) to the right (the top), as illustrated in Figure 9.13a. The hybrid energy cell’s potential to charge capacitors, power LEDs, and drive wireless sensor nodes is illustrated using the n-p CHECs under ~ 10 mW/cm2 illumination and an acceleration amplitude of 3 m/s2 at 3 Hz frequency. Their pulsed voltage output is rectified using a full-wave bridge.

Figure 9.14a shows the charging curves of a 10 pF capacitor charged by a 1 cm- sized СНЕС. Under optical illumination only, the capacitor can be charged from 0 V to 0.61 V in less than 0.3 seconds. Voltage remains constant afterward (left inset. Figure 9.14a). Under mechanical excitation only, the voltage across the capacitor increases slowly and almost linearly, reaching ~1.27 V in 580 seconds (right inset. Figure 9.14a).

Under combined optical and mechanical input, the СНЕС charges the same capacitor to a voltage of 2.0 V in 920 seconds. The comparison indicates that the hybrid cell can effectively compensate for the lower voltage output of the SC component. To enhance the CHEC’s output, six cells are integrated with series to charge a 1000 pF capacitor. The capacitor is then deployed to power eight blue and three white LEDs connected in parallel. The emitted light lasted for 0.5-1.0 seconds and is captured against the background, in Figure 9.14b.

The CHECs’ capacity to sustainably drive a wireless sensor node is tested on a commercial EH-LINK wireless sensor (strain gauge) node (LORD Corporation). On this node, the output of six CHECs connected in series is first rectified by the full- wave bridge. The charge is stored in the 1000 pF capacitor. A custom-made full Wheatstone bridge is implemented using four 350 £2 commercial strain gauge sensors (Vishay precision group) (Figure 9.14c) to measure the strain at the instrumented root of a cantilever beam.

The wireless strain sensor node is used to transmit the measured strain signal to a USB base station connected to a computer that acquire and record data. Figure 9.14d shows the recorded strain signals obtained from this experimental setup. The strain in the beam is measured by the wireless sensor node powered by an electronic circuit consisting of the CHECs, the capacitor, and the full-wave bridge. Depending on whether the beam is under mechanical excitation or not, measurable strain signals are recorded (lower graph) or not (upper graph, Figure 9.14d). When the excitation frequency of the beam is set to 3 Hz and the acceleration amplitude to 3 m/s2, the intermittently measured strain is about 1600 pc. These results demonstrate that the CHECs are capable of powering commercial electronics.

This work presents a compact hybrid energy cell (СНЕС) made of an inorganic SC monolithically integrated with a ZnO PENG. Employing n-p junction-based ZnO NWs in the NG component improves the piezoelectric voltage output of the CHECs by more than two orders of magnitude (138 times). This cascade-type ZnO n-p homojunction NW СНЕС represents a significant step toward effectively combined energy harvesting from the ambient environment, offering a flexible power supply for self-powered electronics.

### Organic–Inorganic Hybrid NG

A PENG and a TENG can be integrated to synergistically harvest mechanical energy and convert it into electricity. The NG device performance is therefore further improved.

FIGURE 9.14 (a) The charging curves of a 10 pF capacitor being charged by an n-p individual СНЕС. The insets are the curves for the NG and SC components, separately, (b) A photograph of eight blue and three white LEDs before and after being powered by a charged 1000 pF capacitor, (c) A photograph of two commercial strain gauges (the front-side of a Wheatstone bridge) incorporated into the wireless sensor node, (d) The measured strain signals (top) without vibration and (bottom) with vibration from the wireless strain gauge sensor. Reproduced with permission [87]. Copyright 2016. Elsevier.

This synergistic phenomenon was recently demonstrated in a hybrid NG on a shim substrate, which integrates both piezoelectric and triboelectric components based on inorganic p-n junction ZnO nanostructures and nanostructured organic PTFE film, respectively [88]. In this design, individual components can be operated independently or concurrently. Moreover, when operated concurrently, component performance is mutually enhanced, enabling more efficient conversion of mechanical energy into electrical energy in a single press-and-release cycle. When triggered with 25 Hz frequency and 1 G acceleration of external force, the PENG component generates a peak-to-peak output voltage of 34.8 V, which is ~3 times higher than its output when it acts alone. Similarly, the TENG component generates a peak-to-peak output voltage of 356 V under the same conditions, which is higher than its initial output of 280 V when acting alone. The NG unit produces an average peak output voltage of 186 V, a current density of 10.02 pA/cm2, and an average peak power density of 1.864 mW/cm2 when operated in the hybrid configuration. The device can even produce an average peak-to-peak voltage of ~ 160 V from normal hand movement when placed under a wristband fitness tracker, and ~670 V from human walking when placed within a walker’s shoe. Figure 9.15 shows the step-by-step fabrication of the hybrid NG.

The fabrication of the entire device can be divided into two components: the PENG component and the TENG component. Figure 9.16a presents a 3D schematic of the curve-shaped TENG integrated on top of a flat-shaped piezoelectric device. The piezoelectric part consists of the Al/р-н junction-type ZnO nanostructures covered with PMMA/AZO/Cr/shim substrate from the bottom of the structure, and the

FIGURE 9.15 The fabrication process of the hybrid NG. (aMg) Step-by-step progress toward the piezoelectric component of the device, (h)-(k) Fabrication steps for the triboelectric component. (1) Integration of piezoelectric and triboelectric components to form the hybrid structure. Reproduced with permission [88]. Copyright 2019, Elsevier.

FIGURE 9.16 (a) 3D schematic of the curved-shaped hybrid device, (b) The average peak- to-peak output voltage from the PENG and the TENG components of the device at different frequencies, respectively, (c) The output voltage from the PENG and the TENG components in a single press-and-release cycle, respectively. Reproduced with permission [88]. Copyright 2019, Elsevier.

triboelectric part consists of Polyethylene terephthalate (PET)/Cu/PTFE (with nanostructures on the surface)/nanoporous shim from the top of the structure. Since the p-n junction-type ZnO nanostructures have demonstrated better piezoelectric performance compared to pristine ZnO NWs [37,89], a low temperature, cost-efficient, and straight forward hydrothermal method is used to grow the p-n junction-type ZnO nanostructures as reported earlier [61,87]. Hence, both the piezoelectric and triboelectric output signals can be leveraged simultaneously within one press-and-release cycle as shown in Figure 9.16c, enhancing the energy conversion efficiency of the device. Before starting the device characterization, that is, output voltage, short-circuit current, output power, charging capability, etc., the optimized operating frequency is determined by sweeping the frequency of the applied mechanical vibration from 1 Hz to 250 Hz as shown in Figure 9.16b. Both of the piezoelectric and triboelectric components yield their highest output voltage at 25Hz, thus subsequent measurements are taken at this frequency unless otherwise mentioned. When the output performance is measured separately, the piezo-tribo hybrid energy harvester unit (2 x 2.5 cm2 in dimension) produces piezoelectric and triboelectric peak-to-peak output voltages of ~34.8 V and ~356 V, respectively, as shown in Figure 9.17a-b. However, when the two units are combined in parallel using hybrid operation mode, the unit produces a peak-to-peak output voltage of -106V as shown in Figure 9.17c, which is higher than that of the

PENG unit alone but lower than that the TENG unit alone. This can be attributed to the mismatch between the internal resistances of the piezoelectric and TENG units. It is well established that when two power sources are connected in parallel, the power source with a lower internal impedance dominates the output voltage [87]. In addition, at the testing frequency, the phase difference between the voltages from the two components results in voltage cancelation and thus degrades the output voltage. To mitigate the foregoing issues, two bridge rectifier units are utilized to collect the electrical signals from the two components separately (Figure 9.17d) as well as their combined output during hybrid operation mode (Figure 9.17e). The full-wave bridge rectifiers do not allow voltage degradation due to the different internal resistances and eliminate the voltage cancellation effect from the phase mismatch [30-31,44].

FIGURE 9.17 Output voltage of (a) piezoelectric, (b) triboelectric, and (c) hybrid NG without using rectifiers, (d) Concurrent measurement method for piezo and triboelectric outputs, (e) The piezoelectric and triboelectric outputs are combined in parallel for the hybrid output measurement. Measured piezoelectric, triboelectric. and hybrid output voltages (f) and short- circuit currents (g) using the rectifier circuits. Reproduced with permission [88]. Copyright 2019, Elsevier.

Figure 9.17f—g shows the rectified piezoelectric, triboelectric, and hybrid output voltages and short-circuit currents under a periodic mechanical vibration at 25 Hz frequency, 1 G acceleration, and 5 mm peak-to-peak hammer displacement. Finally, to demonstrate its long-term mechanical stability and reliability, the device is tested over 200,000 cycles over 4 consecutive weeks. The consistent output voltage waveforms from both the TENG and PENG components of the hybrid device, with no perceivable performance degradation over time, clearly demonstrates the long-term stability and robustness of the device. The high output as well as the fast charging characteristics confirm that the hybrid energy harvester can be used as a pow'er source for storing electrical energy from mechanical vibrations in the surroundings, demonstrating great potential in the field of self-powered systems or sensor networks.