Fabrication and characterization of zinc oxide (ZnO) nanowires array and its applications

Abstract

Since the discovery of carbon nanotubes by Iijima in 1991, extensive efforts have been put on the research of the synthesis and characterization of different kinds of materials in nano-scale. The new phenomena or properties found on nanostructured materials attract great interest not only on their potential applications, but also on theoretical studies. Recently, great attention has been paid on the research of nanostructures of different semiconducting materials, such as Si (group IV), GaN (group III-V), ZnO (group II-VI), etc. ZnO is recognized as a promising candidate for a broad range of applications in optoelectronics, gas sensing, piezoelectrics, etc. ZnO is a semiconductor material with wide bandgap (3.37 eV) and high exciton binding energy (60 meV). These properties make ZnO a suitable candidate for short wavelength photonic/optoelectronics applications. Recently, a huge variety of ZnO nanostructures has been fabricated. However, commercially-viable applications based on ZnO nanostructures are scarce due to the high cost, critical processing conditions, and complicated processing techniques. In this study, ZnO nanowires arrays were fabricated with a simple, highly reproducible and controllable hydrothermal method. The nanowires were found to be vertically-aligned and grown along the c-axis. More importantly, the nanowires covered uniformly across the whole substrate. This allows the fabrication of devices over a large area, and it is advantageous especially in applications like light-emitting diodes, or photovoltaics, etc. The resultant arrays were utilized in two practical applications: visible-blind UV photodetectors (PD) and dye-sensitized solar cells (DSSC). The UV PD consisted of a ZnO nanowires array p-n homojunction. The junction was composed of hydrothermally grown n-type ZnO nanowires array covered with p-type Al, N co-doped ZnO film by a sol-gel method. The homojunction exhibited good rectification characteristics, with reverse leakage current and rectification ratio of ~5 µA and ~150, respectively at a bias of 3 V. The PD had peak responsivity to light with wavelength of 384 nm with UV-visible responsivity ratio of ~70 at an operating bias of -3 V. The ZnO nanowires-based DSSC was built similar to the TiO2-based Grätzel cell. Under AM 1.5 simulated sunlight with intensity of ~100 mW/cm2 (1 Sun), the DSSC had an open-circuit voltage (VOC) of ~0.68 V and a short-circuit current density (JSC) of ~3.5 mA/cm2. The fill factor (FF) and light conversion efficiency (ƞ) were ~0.42 and ~1% respectively. In this work, a simple but reasonably effective encapsulation was employed in our cells. The efficiency of the cells remained stable at a range of 0.9-1% after storage time of over a month, suggesting that the cells were effectively encapsulated.

Date of Award	15 Jul 2011
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shuit Tong LEE (Supervisor)