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Title: Arrays of ZnO nanowires for photovoltaic devices
Authors: Wang, Yunqi (王韞琦)
Department: Department of Physics and Materials Science
Issue Date: 2009
Course: AP4116 Dissertation
Programme: BEng (Hons) in Materials Engineering
Instructor: Prof. Igor Bello
Subjects: Nanowires
Zinc oxide
Photovoltaic cells
Abstract: The photovoltaic cells, also called solar cells, are electronic devices operated on principle of conversion of solar energy to electricity. These novel sources of energy becomes more increasingly important because firstly traditional fossil recourses will be exhausted for several decades, and secondly the conversion of solar energy to electricity do not produce environmentally unfriendly emission and represent unlimited source of energy. Therefore this work focuses on design, fabrication and study solar cells. The solar cells are based on zinc oxide nanowires with different devices architectures. Zinc oxide (ZnO) nanowire arrays were synthesized directly on electrically conducting and optically transparent indium tin oxide (ITO) films using solution and thermal evaporation methods. The ITO films were deposited on glass substrates. ZnO nanowires have been grown on the ITO in hydrothermal environment at 90 °C. Alternatively ZnO nanowires have also been grown on aluminum doped zinc oxide (AZO) buffer layers prepared on silicon (Si) substrates by thermal evaporation at 700 °C using a double-tube furnace system. Different characterization techniques were employed to investigate the properties of fabricated ZnO nanowires, which demonstrated that the ZnO nanowires synthesized by thermal evaporation method show higher degree of the vertical alignment on the substrates, and they comprise lower crystal defect densities. However, the low temperature environment of solution method which can induce less damage to the electrical property of the bottom electrode is more favorable to synthesize ZnO nanowire arrays used for constructing dye sensitized solar cells (DSSCs). Conventional DSSCs were designed and fabricated with using N719 dyed ZnO nanowire arrays and iodine based redox electrolyte. After coating platinum thin films on the counter electrodes, silver contact pads were deposited. The pads were used to bond wire leading for external electronic circuit to determine the device parameters and overall performance. The fill factor (FF) and energy conversion efficiency (η) of the DSSCs were improved from 0.28 to 0.36 and from 0.48 to 0.69 %, respectively, due to reducing photoelectron recombination at the contact interfaces. The performance investigation of the fabricated DSSCs under different illumination intensities indicated raising the power conversion efficiency η from 0.34 to 0.85 % as the intensity increased from 80 to 120 mW/cm². Gold (Au) nanoparticles coated ZnO nanowire arrays have been employed to fabricate dye free Schottky barrier solar cells. The Au nanoparticles enhanced the optical absorption of ZnO nanowires in the visible light region due to their surface plasmon resonance. Upon illumination, the photoelectrons generated within the plasmon excited Au nanoparticles were injected into the conduction band of ZnO, and the electron donors (such as I -) from the electrolyte would compensate the loss of electrons in Au nanoparticle surfaces, simultaneously. The fill factor FF of the fabricated dye free Schottky barrier solar cell reached about 0.5. Hybrid DSSCs have also been designed and fabricated using ZnO nanowires coated Au nanoparticles and applied ruthenium dye (N719) and redox electrolyte. Compared with DSSCs with bare ZnO nanowires, the open circuit voltage (VOC) was improved from 0.51 to 0.64 V due to blocking effect of the Schottky barrier formed at ZnO/Au interface. As a result, electron density increased at ZnO conduction band. The reduction of the short circuit current density (JSC) can probably be ascribed to the increased surface defects of ZnO nanowires after coating Au nanoparticles, and poor electron injection efficiency implying unsuitable bindings of ZnO and dye molecules. Controlling the characteristics of Au nanoparticles coatings is proposed to be vital for fabrication of hybrid DSSCs with enhanced performance.
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