New Light Anode Materials In The Application Of Dye-sensitised Cells

The first high-performance dye-sensitized solar cell (DSSC), invented by Michael Gratzel at Ecole polytechnique federale de Lausanne (EPFL) in the1990s, shows great promise as a cheap alternative to expensive silicon solar cells. As a result, many researchers in both academia and industry are focusing their research on the development and the exploitation of the DSSC devices. In this thesis, the study about novel photoelectrodes is carried out, and functional semiconductor materials are successfully applied into DSSC. The main results included in this dissertation are summarized in the following:1. Carbon nanotube fiberWe first made a family of novel organic solar cells with excellent performance from the flexible, light-weight, ultrastrong fiber. Compared with traditional solar cells fabricated from rigid plates or recently explored flexible films/fibers, nanotube fiber solar cells demonstrate some unique and promising advantages.Firstly, as the building nanotubes are highly aligned, the fiber shows excellent electrical properties, which provide the novel solar cell with higher Jsc, better IPCE, and higher η. Secondly, nanotube fibers show excellent mechanical properties, much stronger than Kevlar and comparable to the strongest commercial fibers of zylon and dyneema in tensile strength. Thirdly, these fibers are flexible, light-weight, and weavable and have tunable diameters ranging from micrometers to millimeters.2. Zn/W doping and fill factor (FF)As an important factor, the analysis on FF is relatively rare. Although it is widely accepted that FF is sensitive to the series resistance (Rs) and the resistance correlating with the semiconductor photoelectrode, which includes both the property inside the semiconductor and on its interface with dye and electrolyte, is usually unclear and indirect.Two series of DSSC were used in this study:a) cells based on bare and Zn-doped TiO2, with Zn content ranging from0.5%to10%; b) cells based on bare and W-doped TiO2, with W content ranging from0.1%to2%. We find that FF increases with the doping amount of Zn2+but decreases with the doping amount of W6+in TiO2. A single-diode model is established to analyze the dependence of FF on Voc and ideal factor (m). This model well explains the changes of FF caused by the doping element and amount, that is, FF increases with the ratio of Voc/m. 3. Fe2+/Fe3+DopingDoping of metal atoms into semiconducting materials plays a crucial role in improving the efficiency of dye-sensitized solar cells (DSSC), but the reported papers are mainly focusing on the single valence state of metal ions but rarely on the multi-valence state of metal ions. This work demonstrates0.5wt%Fe2+-doped TiO2are detected Fe2+-Fe3+ions co-existed, as compared to the typical single valence state of metal ions (Fe3+)-doped DSSC. The multi-valence-doped DSSC improves short-circuit photocurrent (Jsc) by17.6%and power conversion efficiency by17.2%. As for all Fe doping, the conduction band (CB) of TiO2moves downward (i.e., positive shift). DSSC based on the Fe-doped anatase TiO2show an advantage in modifying surface polarity that causes surface positively charged, which is responsible for larger dye adsorption and uniform particle size distribution and facilitates charge transfer between Fe2+and Fe3+. The electron lifetime in the DSSC is significantly improved and the charge recombination is repressed by the Fe-doping, resulting in improved open-circuti voltage. As compared with the undoped DSSC, the improved fill factor (FF) by9.6%obtained at Fe2+/Fe3+-doping, is attributed to the increased ratio Voclm, which is in good agreement with the experimentally observed FF increase.4. Double-Layer Coating of SrCO3/TiO2Surface modification plays a crucial role in improving the efficiency of dye-sensitized solar cells (DSSC), but the reported typical surface treatment is TiCl4-treated TiO2. This work demonstrates a two-step treatment of the nanoporous titania surface with strontium acetate [Sr(OAc)2] and TiCl4in order, each step followed by sintering, corresponding to a double layer of SrCO3/TiO2coated on the TiO2surface. As compared to the typical TiCl4-treated DSSC, the Sr(OAc)2-TiCl4treated DSSC improves Jsc by17%, Voc by18mV, and η by20%. These results indicate that the Sr(OAc)2-TiCl4treatment is better than the often used TiCl4treatment for fabrication of efficient DSSC. Owing to the blocking effect of the SrCO3layer on electron recombination with I3-ions, the charge recombination rate constant of the Sr(OAc)2-TiCl4treated DSSC is half that of the TiCl4-treated DSSC, accounting well for the difference of their Voc.The improved Jsc is also attributed to the middle SrCO3layer, which increases dye adsorption and may improve charge separation efficiency due to the blocking effect of SrCO3on charge recombination.