into a system. Different nanogenerators have different electrodes and materials, which makes it difficult to realize device miniaturization and decrease the device cost. To solve this issue, in 2015, Prof. Ya Yang and coworkers first invented the coupled nanogenerators, which have the same materials and electrodes but different energy scavenging functionalities [9]. One‐structure‐based coupled nanogenerators can be fabricated by using the various physical properties to simultaneously/individually harvest various energies in the environment wherever/whenever energy types are available, where the core of the coupled nanogenerators is the multifunctional materials.
1.3.1 Pyroelectric and Photovoltaic Coupled Nanogenerators
Some ferroelectric materials have both pyroelectric and photovoltaic properties such as LiNbO3 [34–36], Pb(Zr,Ti)O3 (PZT) [37–39], BaTiO3 (BTO) [40–42], and BiFeO3 [43–45]. Owing to the wide bandgaps of about 2.7–4 eV, these ferroelectric materials are more suitable to be used as photodetectors instead of as solar cells. When the light is illuminated on the ferroelectric materials, the light‐induced heating can generate pyroelectric current, and photovoltaic current can be also observed. The pyroelectric and photovoltaic coupled nanogenerators are based on understanding the mechanisms about how to couple the two current/voltage signals in the ferroelectric materials.
Here, we used the ferroelectric BaTiO3 material as an example to describe the pyroelectric and photovoltaic coupled nanogenerators [46]. The Ag and ITO were utilized as the bottom and top electrodes of the device, respectively. As illustrated in Figure 1.7a, there will be no current signals when the dT/dt = 0, where T is the temperature and t is the time. The reason is that there is no change of the polarization in the BaTiO3 material, so that no output current signals can be seen in the external circuit. When the temperature variation dT/dt is larger than zero, the corresponding polarization in the BaTiO3 material will be decreased, indicating that the electrons in the external circuit can move from the bottom Ag electrode to the top ITO electrode to deliver a pulsed output current signal. The pyroelectric current signals are AC signals, which can only flow in the external circuits.
For the photovoltaic effect, the spontaneous polarization‐induced electric field can effectively drive the light‐induced electrons and holes to separate and flow to the two electrodes, as displayed in Figure 1.7b. Owing to the built‐in electric filed in these ferroelectric materials, only one material can realize the appearance of photovoltaic current instead of p–n junctions. As depicted in Figure 1.7c, the pyroelectric effect induced by light illumination can modulate the charge transfer processes of the light‐induced electrons and holes such as generation, separation, and recombination. As a result, the coupling enhancement “1 + 1 > 2” for the output current signals can be seen in this system due to the coupling effect between the pyroelectric current and the photovoltaic current signals. The new physical effect of ferro–pyro–phototronic effect can be found in this process.
1.3.2 Multi‐effects Coupled Nanogenerators
Multi‐effects coupled nanogenerators are focused on multifunctional materials such as ferroelectric materials. As illustrated in Figure 1.8, Prof. Ya Yang and coworkers invented the coupled nanogenerators in 2015 [9], representing a new research field in scavenging multiple energies in the environment. By utilizing the piezo–tribo–pyro–photoelectric effects, the multi‐effects coupled nanogenerator with one structure and one material can be used to scavenge various energies at the same time. The core of multi‐effects coupled nanogenerator is the multifunctional material, where the first material that was used for the coupled nanogenerator is the ferroelectric PbZrxTi1‐xO3 (PZT) material [9]. As depicted in Figure 1.9a, the coupled nanogenerator consists of a PZT material, a polyamide material, and two electrodes, where the PZT material has the piezoelectric, pyroelectric, and photovoltaic effects as the active material. The polyamide film as the triboelectric material was utilized to generate the triboelectric current/voltage signals. The top electrode consists of the ITO film together with Ag nanowires–polydimethylsiloxane film. Figure 1.9b presents the produced charge amounts in the different parts of the coupled nanogenerators. Owing to the pyroelectric effect, the produced charge amount is about 0.39 μC. The produced charge amount is about 1.37 μC due to the photovoltaic effect and that due to piezoelectric and triboelectric effects is about 5.91 μC. When the different energies are applied on the device, the produced charge amount can be up to 7.06 μC, which is much larger than that of individual energy scavenging methods.
Figure 1.7 Working principle of pyroelectric and photovoltaic coupled nanogenerators. (a) Schematic diagram of the mechanism of pyroelectric effect. (b) Schematic diagram of the mechanism of photovoltaic effect. (c) Schematic diagram of the mechanism of pyroelectric and photovoltaic coupled effect.
Source: Reproduced with permission from Ma et al. [46]. Copyright 2017, John Wiley and Sons.
Figure 1.8 Illustration of multi‐effects coupled nanogenerator toward multi‐energies scavenging.
Source: Reproduced with permission from Zhang et al. [9]. Copyright 2017, John Wiley and Sons.
Figure 1.9 One‐structure‐based multi‐effects coupled nanogenerator. (a) Schematic diagram of the designed one‐structure‐based multi‐effects coupled nanogenerator. (b) Transferred charges of the different devices.
Source: Reproduced with permission from Zhang et al. [9]. Copyright 2017, John Wiley and Sons.
The ferroelectric PZT material is toxic due to the presence of Pb. It is necessary to look for other high‐performance ferroelectric materials without Pb for the coupled nanogenerators. It has been reported that a polyvinylidene fluoride (PVDF)‐based one‐structure‐based coupled nanogenerator has been utilized to scavenge mechanical and thermal energies at the same time by using tribo–piezo–pyroelectric effects [47]. By charging a capacitor, the coupled nanogenerators have the best charging performance. However, the piezoelectric constant of the PVDF is too small with about 20 pC/N. Ji et al. reported a ferroelectric BTO material‐based multi‐effects coupled nanogenerator [48], where the piezoelectric constant of BTO can be larger than 300 pC/N. On comparing the charging curves of the different conditions, it is seen that the coupled nanogenerator exhibited faster charging performance than the other individual effects.
As compared with the other hybridized nanogenerators with simple physical integrations, the one‐structure‐based multi‐effects coupled nanogenerators have more advantages such as simpler structure, smaller volume, and lower cost, representing a new research trend in multifunctional materials‐based all‐in‐one multiple energy harvesting in our living environment. Moreover, these coupled nanogenerators have potential applications in multifunctional sensor systems. Owing to the multifunctionalities
