Low Temperature Synthesis of Nanostructured Oxides for Dye Sensitized Solar Cells

Zhao, Chao (2016) Low Temperature Synthesis of Nanostructured Oxides for Dye Sensitized Solar Cells. Doctoral thesis, Northumbria University.

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Abstract

Dye-sensitised solar cells (DSSCs) based on metal oxide semiconductor photoanodes (i.e. TiO2, ZnO) offer a promising route for low-cost and transparent solar cells,
especially suitable for indoor/outdoor applications, building and automotive integrated electricity generation. Apart from developing new dyes (or absorbers) for increasing absorption and stable fast-regenerated electrolytes, improving metal oxide photoanodes has also gained great research attention. In a conventional DSSC, the mesoporous TiO2 photoanodes assembled by TiO2 nanoparticles can support large surface areas for
sufficient dye (absorber) loading, thus result in reasonably good solar cell performance. However, the poor pore-filling of large sized molecules (i.e. solid electrolyte) and inefficient electron transport lead to significant photo-generated charge recombination thus loss of photo-generated energy. Despite the reasonably good electron transport ability of the currently used particulate-based photoanodes, the requirements of high temperature processes for these photoanodes significantly limit the substrate and material choices. In this thesis, a low-temperature strategy was designed to synthesise crystallised metal oxide (ZnO and TiO2) nanostructures with controllable morphologies to be used as photoanodes with improved electron transport abilities for DSSCs.

ZnO nanorods (NRs) with tailored nanostructure (i.e. growth direction, aspect ratio and surface distribution density) were synthesised on pre-seeded substrates using zinc salt based aqueous solutions at temperatures generally lower than 95 oC. The influences of reaction temperature, pH, concentration, reaction duration and additives were systematically investigated. The detailed studies of nanostructures, morphology, crystallinity and properties of the ZnO NRs led to an improved understanding of the synthesis process.

Anatase TiO2 modification layer was achieved using a plasma ion assisted deposition (PIAD) without external heating or subsequent annealing. Effects of the deposition parameters (duration, gas flow rates and plasma energy) on TiO2 film properties (optical, structural and chemical activities) have been studied. By combining two lowtemperature processes (aqueous solution growth of ZnO NRs and PIAD of crystalline TiO2 nanostructures), nano-sculptured ZnO-TiO2 nanostructures were achieved. The ZnO NRs were covered with a layer of anatase TiO2 to form core-shell and foxtail-like nanostructures. These nanostructured photoanodes showed an improved electron transport as well as suppression of recombination capability in the DSSCs assembled by these photoanodes.

A novel in-situ microfluidic control unit (MCU) was designed and applied in the aqueous solutions synthesis process, which provided an easy way to localize liquidphase reaction and realise selective synthesis and direct growth of nanostructures, all in a low-temperature and ambient pressure environment. The morphology of the nanostructures was controlled by varying the amount of additivities supplied by the MCU. This achieved a facile fabrication of Al-doped ZnO (AZO) nanoflakes vertically grown on flexible polymer substrates with enhanced dye loading and electron transporting capabilities. Flexible DSSCs with a significant enhancement (410% compared to ZnO NRs based devices) in the power conversion efficiency were obtained using the AZO nanoflakes photoanodes of 6 µm thickness, due to the enhancement in electron transport capability of the photoanodes and reduction in the recombination process.

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