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Solution-Based Synthesis of Nb Sol-Gel and Its Application in Doping Tio_2 For Photoanode of Dye Sensitized Solar Cells
dye-sensitized solar cell
Dye sensitized solar cells (DSSCs) have received attention as a potential alternative for converting solar energy into electricity. The performance of energy conversion efficiency in DSSCs is mainly determined by the mesoporous TiO2 photoanode, which provides not only an enormous surface area for dye chemisorption but also electrons transportation. However, the electrons transport in DSSCs is greatly influenced by the electron recombination from the electrolyte of redox couple, especially triiodide (I3 -). The most happened route for the I3 - to capture the electrons, so called back reaction or charge recombination, particularly for the liquid-electrolyte system of DSSC is happened at the liquid-solid interfaces of electrolyte and mesoporous TiO2 film, which provides the great area for dye adsorption but meanwhile also increases the interfaces for charge recombination. The electrons at LUMO state from dye and those injected into ECB of TiO2 could be trapped by I3 - or release florescence. According to the literature, the characteristics of the porous TiO2 nanoparticles (NPs) including the morphology and size, inter-particle connectivity, pore structure, and electric structure are greatly vital in determining the final photovoltaic performance of DSSCs. In order to address these problems, several strategies, such as dye sensitizing, heterostructure, ion doping, etc., have been developed. Among them, the ion doping is believed to be the most economical and facile method to optimize the performance of TiO2 NPs by simply modifying its structure, which has been widely used in the photocatalysis, biological engineering and gas sensors fields, but there are relatively few studies applied in DSSCs. Several methods have been tried to modify nanostructured TiO2 NPs by metal ions in order to further improve the efficiency of DSSCs, enhance electron transportation and suppress charge recombination. Of these doping elements, the Nb element stands out to show great potential in improving DSSCs performance due to the synergistic advantages of superior electrical conductivity, similar atom radii with Ti, high valence favorably enhancing free carriers and excellent ability in stabilizing the phase structure and tailoring the optical properties. However, one determined and important limitation for the practical application of the presently reported Nb-doped TiO2 NPs applied in DSSCs is that the starting materials either use expensive niobium ethoxide or the method to prepare such materials commonly adopts the hydrothermal technique which includes multi steps and long time procedure, thereby leading to high preparation cost and low production efficiency. To further improve the performance of TiO2 electrode, we firstly develop a scalable synthesis of TiO2 slurry from commercial and low-lost powder by ball-milling to make the particles down to 30nm. The Nb-doped TiO2 NPs was then made by simply mixing TiO2 paste with sol gel contained of niobium oxide used as the DSSC photoanode. This facile method is suitable for mass production. By optimizing the doping amount of Nb into TiO2 NPs, the adjustment of conduction band minimum (CBM) to a positive-shift and Fermi level closer CBM enhance the electron injection and the improved electron conductivity facilitates the electron transport, leading to 12% improvement of photo conversion efficiency of Nb-TiO2 NPs compared with the standard TiO2 NPs DSSCs.
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