Multifunctional Plasma Treatment on Metal Oxide Thin Films and Nanostructures for Photovoltaic Device

• Main Authors: Chan, tarat, Naratip, 陳大方
• Other Authors: Chen, San-Yuan
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/55585528118587191422

博士 === 國立交通大學 === 材料科學與工程學系 === 100 === To satisfy the universal goal of improving solar-energy conversion efficiency, the need to develop and deploy large-scale, cost-effective, renewable energy is becoming increasingly important. In recent years, a solar cell consisting of organic or inorganic materials along with good transparent conducting oxide (TCO) films has achieved good power conversion efficiencies (PCE). Transparent conducting oxide (TCO) films or nanostructures serve as a window for light to pass through to the active material beneath and as an ohmic contact (electrode) for carrier transport out of photovoltaic. All detailed studies will become the focus in this thesis. In the first part, a spray deposition process was used to investigate the effect of oxygen content in the carrier gas on FTO film morphology and properties. The carrier gas containing various O2/N2 concentrations (0, 20, 50, 80 and 100%) led to significant change in thickness, size and shape of grain growth. The deposited films reach a low resistivity of ~10-4Ω-cm and a transmittance of 76%~96% at 550 nm. Finally, photovoltaic implement of dye-sensitized solar cells (DSSCs) and polymer-based solar cells reveals the particular behaviors such as charge transport, recombination, and collection properties respected to the surface and interfacial effects. However, in case of amorphous silicon solar cells (a-Si:H), it was found that a hydrogenated effect results in the deterioration of FTO film as TCO electrode. Therefore, a double-layered transparent conducting AZO/FTO thin film was investigated and the results suggest that the AZO film acts as a protecting layer for the beneath FTO film surface, providing an excellent epitaxial interface, from the direct bombardment of H ions and radicals. Moreover, following by a post-annealing treatment at 400oC, the degraded properties of FTO can be recovered via the reoxidization of Sn-O bounds, in which H-ions diffusion and Sn-O redox process was interpreted by XPS and SIMS analysis. In the second part, the investigation is mainly focused on a selective plasma-treatment technique for introducing surface functionalization, passivation, and etching to form nanorods (NRs), nanowires (NWs) and nanotubes (NTs). Herein, we fabricate the p-type SnO2-based transparent conducting electrode which was constructed by a novel approach of In and N co- doping by nitrogen plasma (5-40 min), to In-doped SnO2 films with several In contents (0, 3, 7, 15 and 30%). Depending on In doping concentration, (N, In)-codoped SnO2 can be modified to either p-type or n-type where N atoms primarily substitute in O sites in SnO2 with enhanced conductivity, Hall mobility and solubility of the In dopant. Significantly, such behavior is also exhibited in term of pn core-shell heterojunction, showing the symmetrical I-V curve with rectified diode characteristics. Furthermore, similar method was applied to investigate the key aspects in comparison between ZnO NWs and NTs under nitrogen plasma treatment at room temperature condition. Upon an extended treatment of 900 s, the ZnO NTs exhibit higher reliability of photoresponse, 20 times of NWs without deteriorated structure. This indicates that higher surface-to-volume ratio of NTs is critically important factor for inducing the surface modification, occupying the oxygen defects and impurities in the ZnO matrix by the presence of N ions. On the other hand, we proposed a selective oxygen-plasma-etching technique for the formation of ZnO-FTO heterostructure nanotubes using presynthesized ZnO nanorods (NRs) as sacrificial templates, and FTO nanoparticles are deposited onto the ZnO nanorods by a simple spray pyrolysis method. XPS analysis demonstrated that the oxygen-plasma treatment decreased the O2-/OH- concentration ratio, resulting in dissociation of the Zn-O bonds and the outward diffusion of Zn cations to form an interior hollow, which is related to the formation of the hydroxyl functional group, Sn-OH, at the FTO surface. Time-dependent photocurrent (I-T) measurements under ON-OFF cycles of UV illumination confirms a rectified photoresponse characteristic and a dark current increased by about 3 orders of magnitude over that of the unetched sample.