ZnO Nanorods-Based Piezoelectric Energy Harvesters.
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Piezoelectric nanostructures of ZnO were employed for development of vibration energy harvesters. Columnar nanorod structures of ZnO, incorporated into various heterojunction-based device prototypes, were strained to generate voltage signals. The fabricated devices’ prototypes were based on different top electrodes such as: p-n junction-type Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS)/ZnO devices, metal-insulator-semiconductor type Poly(methyl methacrylate) (PMMA)/ZnO devices. Similarly, various bottom electrode materials based prototypes were also assembled: ZnO/indium tin oxide (ITO), ZnO/silver (Ag) and ZnO/zinc (Zn). The overall device design was based on flexible electrodes and substrates, due to which low temperature (below 100 °C) fabrication processes were implemented. Device performance measurement and characterisation techniques were explored and implemented to improve the reliability of results. These techniques included open-circuit voltage and short-circuit current output measurement, resistive load matching and impedance analysis. The analysed performance of energy harvester was assessed in relation to its constituent material properties. The parameters which affect the energy harvester performance were investigated and for this p-n junction-based (PEDOT:PSS/ZnO) devices were used. It was analysed that devices with optimum shunt (Rsh) and series resistance (Rs), which were in the ranges of 0.08 – 0.17 kΩ and 0.5 – 1.65 kΩ respectively, generated the highest peak open-circuit voltage (Voc) and peak power density (PL) of 90 – 225 mV and 36 – 54 μW cm-2. However, the p-n junction-based devices with low shunt resistance (Rsh), ranged between 0.2 – 0.3 kΩ, were considered to be affected with leakage losses, such as short-circuits. Therefore, these devices generated lower Voc and PL in the range of 20 - 60 mV and 2 - 16 μW cm-2. Similarly, the p-n junction-based devices with higher Rs, ranged between 0.3 – 0.6 kΩ, were adversely affected by I2R losses and therefore their generated power density was also dropped to 0.22 - 0.25 μW cm-2. In addition to parasitic resistance losses, the most significant phenomenon investigated in ZnO energy harvesters was, screening of polarisation ii charges in ZnO. The polarisation screening effects were observed to be related to the electrical properties of device components like electrode material type and conductivity of ZnO. Hence, the effect of electrode electrical properties on electric field screening was investigated. In this regard, device electrodes were varied and their effect on energy harvesting efficiency was studied. A comparison based on the performance of bottom electrodes like indium tin oxide (ITO), silver (Ag) and zinc foil on device performance was made. It was observed that due to lower screening effects of ITO, the ITO-based devices generated voltage output which was two orders of magnitude higher than the zinc foil-based devices. Similarly, the screening effects of top electrode materials, like PEDOT:PSS and PMMA, on device output generation were investigated. The PMMA-based devices generated average 135 mV which was higher than average 100 mV generation of PEDOT:PSS-based devices; which indicated that the PMMA-based devices had slower screening rate. On the contrary, the PMMA-based devices’ 7 times higher series resistance than PEDOT:PSS-based devices caused the PL of PMMA-based devices to be 0.4 μW cm-2, which was two orders of magnitude lower than 54 μW cm-2 generated by PEDOT:PSS-based devices. Further to electrode materials study, polarisation screening caused by electrical properties of ZnO was also anaylsed. In this regard, the surface-induced conductivity of ZnO was decreased by using surface coating of copper thiocyanate (CuSCN). The reduction in ZnO conductivity was considered to reduce the screening of polarisation charges. Consequently, the power density of ZnO devices was enhanced from 54 μW cm-2 to 434 μW cm-2.
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