Studies of ZnO nanostructure-based dye sensitized solar cells (DSSCs)

Abstract

The energy crisis and soaring oil price have triggered the development of photovoltaic technology. Silicon (Si) based solar cell devices have dominated the market for the past decades. However, the high demanding and processing cost of crystalline Si have limited their applications and at the same time encouraged the development of simple and low cost alternatives. Dye-sensitized solar cells (DSSCs) utilizing nanostructured materials have been considered as one of the promising candidates for photovoltaic devices. In this dissertation, a low temperature hydrothermal growth method to synthesize one dimensional ZnO nanorods arrays (ZNAs) on indium doped tin oxide (ITO) substrates from a zinc salt for DSSCs application is demonstrated. By adjusting the reaction times, ZNAs with different morphologies resulted. The length of nanorods (NRs) increased directly proportional to the total reaction times along the [0001] direction of its hexagonal wurtizite (WZ) structure, whereas the diameters of NRs increased at a slower rate. However, the thickening effect of NRs reduced the surface area for dye loading in DSSCs application and worsened the performance of the cell. The morphologies of hydrothermal growth NRs were not only affected by the conditions of hydrothermal growth but also the ZnO seed layer which acts as homogeneous nucleation sites for the growth of NRs. Through the tuning of the Ar:O2 ratio in the seed layer deposition process using a pure Zn target by radio frequency (r.f.) magnetron sputtering, ZnO seed layers with different properties resulted and subsequently influenced the NRs based films. The growth rate of NRs was found to be increased with the oxygen flow rate during the seed layer deposition process. The orientation of the NRs along the [0001] direction perpendicular to the ITO substrate was improved at the same time. In a relatively short period of the hydrothermal growth reaction, hybrid NRs and nanosheets (NSs) complexes could result. To further optimize the morphologies for DSSCs application, a highly branched polymer additive, poly(ethylenimine) (PEI), was added to the hydrothermal growth solution bath. The growth of NRs along the c-axis perpendicular to the substrate was promoted and the existence of NSs structure in the film remained for a longer reaction time. The above ZnO nanostructures based films grown on the ITO substrates served as the photoelectrode for fabricating DSSCs. They were dipped into a commercial ruthenium dye, known as N719, for the dye adsorption process. After the application of an iodinebased electrolyte, with a platinum catalyst and fluorine doped tin oxide (FTO) as the counter-electrode, the sandwiched DSSCs were sealed and tested using simulated sunlight of AM 1.5. The current-voltage responses of the cells were recorded. The overall conversion efficiencies ranged from 0.3% to more than 1.2% and were found to be highly dependent on the morphologies and nanocrystalline structures of the ZnO based photoelectrodes. These findings helped in optimizating the performance of ZnO nanostructures based DSSCs fabricated by a low cost and simple route.