I. INTRODUCTION

The piezoelectric nanogenerator devices that operate particularly on flexible substrates have attracted great attention as a useful method for easily harvesting the random mechanical energy that is usually generated in our surrounding environments^[1,2]. Many piezoelectric nanogenerator devices have been fabricated using zinc oxide (ZnO) which is one of the most widely used piezoelectric materials and also a key component^[2-6]. In general, their output voltages that are generated by mechanical movements or vibrations are very important for high-efficiency energy harvesting applications.

In order to improve the output voltage of a piezoelectric nanogenerator device, it is critical to prevent the induced piezoelectric potential in the piezoelectric nanogenerator device from being reduced due to an undesirable charge flow through an external circuit or within the device itself. As a solution, the reliable contact potential barriers between piezoelectric ZnO nanostructures and electrodes were formed, using various insulating materials such as polymethyl methacrylate (PMMA), aluminium oxide (Al₂O₃), and aluminium nitride (AlN)^[2-5]. Especially, the use of AlN thin film as an interlayer has been demonstrated to be a promising way of forming a desirably high potential barrier mainly due to its high dielectric constant (ɛ = 9) and large Young’s modulus (E = 330 GPa)^[5]. Besides, a modified scheme of vertically integrated nanogenerator (VING) devices has also been reported in an effort to increase the output voltages^[3,6].

In this work, we present a new tandem-type vertically integrated nanogenerator (TVING) device with five layers of alternately stacked thin films (AlN/ZnO/AlN/ ZnO/AlN) in comparison with the conventional vertically integrated nanogenerator (VING) device with three layers of stacked thin films (AlN/ZnO/AlN) as well as the conventional nanogenerator (NG) device with a single pure ZnO layer. Here, all the AlN and ZnO thin films were deposited in-situ in a radio frequency (RF) magnetron sputtering system. Compared to both the VING and NG devices, the TVING device has shown better performance in terms of the output voltages (measured using a bending/releasing cycle test system) and the potential distributions (obtained using a simulation tool). Specifically, the TVING devices exhibited about twice larger output voltages than the VING devices, while the NG devices showed the smallest output voltages among them. Moreover, the TVING devices could be formed by integrating two individual VING (or VING-like) devices through a simple in-situ process of depositing two different thin films, as shown in Fig. 1(c). Thus, this approach appears to be effective in fabricating even highly integrated forms of ZnO-based micro-energy harvesting devices.

II. EXPERIMENTS

We fabricated three different kinds of nanogenerator devices in this work. As shown in Fig. 1, each schematic device structure is defined as (a) an NG device with a single layer structure (ZnO), (b) a VING device with a three-layer structure (AlN/ZnO/AlN), and (c) a TVING device with a five-layer structure (AlN/ZnO/AlN/ZnO/ AlN). Here, all the AlN and ZnO thin films were deposited on flexible substrates made of indium tin oxide (ITO)-coated polyethylene naphthalene (PEN) using a radio frequency (RF) magnetron sputtering technique with AlN and ZnO targets (each 4-inch diameter) while the substrates were rotating at 8 rpm to enhance the uniformity of the deposited films. The base pressure of the deposition chamber was kept to be approximately 3.5×10^-6 Torr using a turbomolecular pump to remove any possibly existing particles and impurities in the chamber. Also, prior to each film deposition, the presputtering of AlN and ZnO targets was conducted to remove any possible contaminations on each target. Then, the AlN and ZnO layers were deposited at the powers of 180 W and 200 W and under the pressures of 18 mTorr and 15 mTorr, respectively. In addition, prior to the formation of each silver (Ag) top electrode, all the samples (NG, VING, and TVING) with the as-deposited AlN and ZnO films were thermally annealed simultaneously at 130°C for 1 hour, followed by the formation of each Ag top electrode (its effective area of 2.8 cm×1.3 cm) using Ag paste through the doctor blading technique (known as one of the non-vacuumed printing methods). Finally, they were all cured at 100°C for 30 minutes. More process details were summarized in Table 1.

The output voltage of each device was measured using a digital multimeter (Keysight 34401A) for various strain rates in a mechanical bending and releasing system, as shown in Fig. 2. Two lines of copper (Cu) wires contacted to the Keysight 34401A were connected respectively to the top and bottom electrodes of each device. The microstructures of the deposited AlN and ZnO thin films were characterized by the low-resolution transmission electron microscopy (LRTEM, JEOL JEM-ARM200F). The top of each analyzed sample was coated with a platinum (Pt) protection layer (seen as a thin black layer on the topmost of each TEM image) for the TEM measurements. The depth profile of the chemical compositions for each device was also obtained using an energy-dispersive X-ray (EDX).

III. RESULTS AND DISCUSSION

Fig. 3 shows the cross-sectional LRTEM images (left side) and EDX depth profiles (right side) respectively of three different devices (NG, VING, and TVING) fabricated in this work. The cross-sectional LRTEM images demonstrated that the three different NG devices were well fabricated as designed, further confirmed by the depth profiles showing the existence of the representative three elements, i.e., zinc (Zn), aluminium (Al), and indium (In) in the ZnO, AlN, and ITO layers, respectively. Based on this analysis, it was found that both the ZnO and AlN films in the three different devices (NG, VING, and TVING) were deposited in their thickness ranges of about 150~170 nm and 15~50 nm, respectively.

Fig. 4 shows the output voltages measured from the three different devices (NG, VING, and TVING). To extract the output voltages, each device was periodically bent by a strain of 0.08% at various strain rates of 0.01% s^-1 (black), 0.02% s^-1 (green circle), 0.03% s^-1 (blue square), and 0.04% s^-1 (red star) and subsequently released. Both sets of the positive and negative pulses were produced through a single cycle with the consecutive steps of bending and release, respectively.

It was also observed in this work that the output voltages of all three different nanogenerator devices (NG, VING, and TVING) tended to increase as the strain rate increased. When the two individual layers of AlN films as a potential barrier material were inserted respectively into the upper and lower sides of the VING device (Fig. 4(b)), there was a significantly large increase in the output voltage, compared to that of the NG device without any AlN layer (Fig. 4(a)). Those two AlN interlayers, acting as a reliable electron-blocking layer, seem to play an important role in blocking an undesirable flow of electrons, thus preventing a degradation of the piezoelectric potential induced in the ZnO layer^[5]. As shown in Fig. 4(c), the TVING device proposed in this work exhibited a significantly large output voltage than the VING device. Moreover, the peak-to-peak voltages for NG, VING, and TVING devices were respectively measured to be ~0.02 V, ~0.07 V, and ~0.12 V on average at a strain rate of 0.04% s^-1.

As shown in Fig. 5, we also compared the two potential distributions for the VING and TVING devices that were extracted using a simulation tool (COMSOL Multiphysics) assuming that both devices were under the same strain conditions. As a result, along the strain direction in the ZnO piezoelectric films of each bent device, a significant piezoelectric potential was observed to be induced due to the piezoelectric effect. In this simulation, the potential differences between the upper and lower AlN layers of the VING and TVING device structures were found to be 0.06 V and 0.11 V, respectively. Thus, the simulation result seems to be in good agreement with the measurement result, meaning that the TVING device can generate about two times higher output voltages than the VING device.

Hinchet et al. proposed that the energy conversion performance of the nanogenerator devices, which can be expressed as mechanical energy transfer ($\eta_{M}$) and electrical energy transfer ($\eta_{E}$) was affected by the insulating interlayer design^[7]. The energy transfer yields can be given by

where $d_{x}$, $E_{x}$, and $\varepsilon_{x}$ are the thickness, Young’s modulus, and dielectric constant of the layer $x$, respectively. Also, the subscripts $a$ and $b$ indicate the insulating interlayer and piezoelectric layer, respectively. The Eqs. ((1), (2)) imply that large $E_{a}$, $\varepsilon_{a}$, and small $d_{a}$ values are required for the insulating materials to maximize the energy stored in devices.

The AlN insulating interlayer proposed by our research group has comparatively high $E_{a}$ (330 GPa) and $\varepsilon_{a}$ (9)^[5]. We also designed a remarkably thin AlN interlayer (15~50 nm) when compared with ZnO layers (150~170 nm) for the TVING device. Furthermore, the TVING device can be fabricated simply through the vertical integration of two individual VING (or VING-like) devices (here, named VING1 and VING2), as shown in Fig. 1(c). From this point of view, the AlN interlayer inserted between the two adjacent layers of ZnO films for the TVING device seems to play an important role in enhancing the mechanical and electrical energy transfer performance, eventually leading to the improvements of output voltages.

IV. CONCLUSIONS

We have proposed a novel tandem-type vertically integrated nanogenerator (TVING) device with five layers of alternately stacked thin films (AlN/ZnO/ AlN/ZnO/AlN). The TVING device has exhibited about twice larger output voltages than the conventional VING device with three layers of stacked thin films (AlN/ZnO/AlN). In addition to its higher device performance, the TVING device could be formed simply by the vertical integration of two same or similar individual VING devices. From this standpoint, this approach seems promising for the fabrication of high-quality ZnO-based energy harvesting devices.