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UBC Theses and Dissertations

ZnO nanostructures for sensing and photovoltaic devices Mohseni Kiasari, Nima

Abstract

In this PhD thesis, vertical arrays of zinc oxide (ZnO) nanowires (NWs) are synthesized in a CVD system and then deposited on patterned electrodes using dielectrophoresis (DEP). The nanowire devices illustrate 4 orders of magnitude increase in conductivity when exposed to ultra violet (UV) irradiation of 1220 μW/cm². The UV response has a fast component, due to electron-hole generation, as well as a slower component, attributed to the release of oxygen. Moreover, due to the increased electron density in the presence of UV, the type of oxygen species on the surface of ZnO changes to more reactive negative ions. In addition, when the pressure is decreased to 0.05 mBar, the conductivity of the NWs increases ∼ 2 and 3.5 times for NWs with 300-nm and 100-nm diameter, respectively. For the first time, UV irradiation is used to improve the carbon monoxide (CO) sensing properties of ZnO. When exposed to 250 μW/cm² UV irradiation, not only the sensitivity increases more than 75%, but also a repeatable and recoverable response is obtained, which is due to formation of more reactive oxygen ions. For the same reason, when the temperature is elevated, higher sensitivity to CO is achieved. The devices demonstrate exponential sensitivities of more than 5 decades to 60% increase in relative humidity (RH) at room temperature, which is a record for ZnO NW based RH sensors. A novel, low-cost and simple technique is developed for fabrication of sensors based on solution processed ZnO nanoparticles (NPs) by simply sketching the electrode lines and painting the NP ink. Sensors show 2000 times increase in conductivity when exposed to 1220 μW/cm² UV irradiation and more than 200% increase in current when exposed to 5-mins of CO pulse at room temperature. Furthermore, this thesis presents efficient (3.8%) inverted organic photovoltaic devices based on a P3HT:PCBM bulk heterojunction blend with improved charge-selective layers. ZnO NP films with different thicknesses are deposited on the transparent electrodes as a nano-porous electron-selective contact layer. The optimized inverted devices show exceptional short circuit current, which is related to increased quantum efficiency.

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Attribution-NonCommercial-NoDerivs 2.5 Canada