Recent and near future smart electronic products get closer to the user, moving toward personal elec-tronics, portable electronics and flexible/wearable electronics. The key merit of these smart products is that they are present and available almost all the time at the nearest to the user, as well as in the convenient type to use. This means that they need the sustainable and wireless powering system. The flexible property of the device necessarily demands the flexible/wearable powering system. The flexible self-powered system using the ambient renewable energy can meet their requirements very well. Among them, mechanical energy has received attention as a feasible source of sustainable power for independent and wireless flexible/wearable devices. The recent advances in the nanotechnology and the broad distribution of wireless personal electronic devices open up the door to micro/nano scale energy harvesting, leading to the self-powered electronic devic-es that can operate wirelessly, independently, and sustainably. Especially, over the past decade, significant progress has been made in the energy harvesting devices based on the nanomaterials and the nanostructures, starting from the ZnO-based nanogenerators (NGs).
Flexible piezoelectric Zinc Oxide (ZnO)-based NGs are innovative approaches for harvesting random mechanical energy over a wide range of frequencies that is usually generated in our environment. In recent years, the vertically-integrated ZnO-based flexible NGs have extensively studied due to their attractive fea-tures including the simplicity and complementary metal oxide semiconductor (CMOS) compatibility of the manufacturing process, robustness, easiness of stacking. The key issues in the vertically integrated nanogenerator (VING) design are mainly about the formation of the more reliable potential barrier and the increase of conversion efficiency for the inherently n-type semiconductor ZnO. Besides, the low-cost, simplicity, and biocompatibility both in the fabrication process and device materials have also been extensively discussed in this research field.
This dissertation has presented the results of our research work in developing new approach for the fabrication of micro-energy harvesting devices, i.e., nanogenerators (NGs), based on ZnO for flexible/wearable device applications. The VING devices were implemented on the flexible substrates.
We have presented an in situ N-doping method for modulation of the free carrier density in inherently n-type-grown ZnO. Based on the adoption of this method, we also fabricates free carrier-modulated ZnO:N thin film-based flexible nanogenerators (NZTF-FNGs). As a result, the free carrier-modulated NZTF-FNGs show a significant performance improvement when compared with the conventional ZnO thin film-based flexible nanogenerators (CZTF-FNGs). This is believed to be a result of both the substantial suppression of the screening effect in the bulk of the ZTFs and the formation of more reliable Schottky barriers at their interfac-es, which is all mainly caused by the N-doping process. In addition, these NZTF-FNGs are verified via charg-ing tests to be well qualified for high-efficiency micro-energy harvesting applications. Consequently, we be-lieve that this N-compensatory doping approach will play an important role in the realization of micro/nano devices for high-efficiency micro-energy generation and harvesting applications.
In the following part, the four types of Ag-based top electrodes are fabricated on the prepared ZnO thin films by different process conditions or techniques. As a result, we have found that the oxidation of Ag-based electrodes in the ZnO thin film-based FNGs can contribute to the improvement of the output voltage and the local oxidation at the interface can induce a significant performance improvement. In addition, the ZnO thin film-based FNGs with Ag-paste printed top electrodes can provide the cost-effective and easy manufacturing processes despite showing the output performance rather short than the ZnO thin film-based FNGs with the Ag electrodes that are locally oxidized at the interfaces by an in situ sputter deposition. These findings can suggest an effective oxidation strategy of Ag-based electrodes in thin film-based FNGs, thereby increasing their practical applicability.
We have studied the performance improvement of ZnO thin film-based FNGs through design and se-lection of insulating interlayer materials. In particular, an aluminum nitride (AlN) insulating layer is newly adopted as an electron blocking layer in the device design. It is confirmed that the use of AlN thin interlayers at the interface of the ZnO nanorods (NRs) and the contact electrode causes a great improvement in terms of the output voltage. It is believed that the AlN interlayer can protect the piezoelectric potential generated over the ZnO NRs from reducing by short circuit or leakage current across the interface from the electrode. In addition, the effect of AlN thickness on the electric potential and the output voltages of FNGs are investigated; we study the operation mechanism by observing the output voltage performance of ZnO thin film-based FNGs with the position-controlled AlN interlayers. Consequently, we can confirm the best peak-to-peak output voltages in the ZnO thin film-based FNGs with the bottom AlN interlayer of 30nm thickness in our work, which are observed to be ~1.4V on average under periodic bending/release motions of a given strain condition. This result is the significantly improved values by more than 2 orders of magnitude compared to the ZnO thin film-based FNGs without AlN interlayers.
This research works are expected to be effective approaches in further improving the performance of ZnO thin film-based FNGs, leading to the feasible harvesting devices for flexible/wearable applications.