| Summary: | 博士 === 國立臺北科技大學 === 能源與光電材料專班 === 106 === Solar energy is the largest source of carbon-free energy that can be converted into heat and electricity. Since the early 1990s, dye-sensitized solar cells (DSSCs) have received a great deal of attention as a promising alternative photovoltaic technology because of their expected low costs and reduced energy input in manufacture. Considerable efforts have been made to improve the energy conversion efficiency, by developing or modifying DSSC components, such as sensitizers, photoanodes, electrolytes, and counter electrodes. However, a number of challenging issues remain, such as new optimized structures are required as DSSCs evolve. DSSCs harvest solar energy and convert it into electricity through a suitable dye sensitized semiconductor photoanode material. The aims of this thesis are to design, develop and investigate new semiconductor TiO2 architectures to use as photoanodes in different types of DSSCs.
The main objectives of this work were: (i) Enhancement of power conversion efficiency of TiO2-based dye-sensitized solar cells on various acid treatments, (ii) Investigation of ammonium salt effects on TiO2 photoanodes and their application for DSSCs (iii) A novel hybrid nanocomposite as highly efficient photo-anode for DSSCs.
Firstly, the surface modification of the TiO2 photoelectrode film is one of the promising ways to improve the photovoltaic performance of DSSCs. Fluorine containing compounds such as trifluoroacetic acid was used to carry out the acid treatment of TiO2 powder, to enhance the properties of photoanode. In order to investigate the effect of trifluoroacetyl group, the TiO2 nanopowders are also treated with different acids such as acetic acid, nitric acid, hydrochloric acid, and sulfuric acid and their properties compared. The TiO2 powders treated with both acetic acid and TFA have possessed smooth surface morphologies as well as enhanced particle dispersions with reduced particle sizes. Photoelectrodes prepared for these two kinds of TiO2 powders accommodated high amounts of dye loading and exhibited excellent light transmittance (wavelength region of 400–600 nm). Electrochemical impedance spectroscopy analysis showed the smallest radius of the semi circle which indicates the enhanced rate of electron transport for the cell based photoelectrode with trifluoroacetic acid treated TiO2 powder. The solar cell from the untreated TiO2 film showed the power conversion efficiency of 8.86% and the highest efficiency of 9.51% was achieved by the cell fabricated from trifluoroacetic acid treated TiO2 film respectively.
In addition, to increase the surface porosity of the TiO2 electrode, the effect of ammonium carbonate (AC) salt based TiO2 pastes were studied. Various concentrations of ammonium carbonate were employed and the solar cell with 7 wt % AC-TiO2 based photoanode was found to achieve the best overall photo-conversion efficiency. To further investigate the influence of different ammonium salts on TiO2 electrodes, a series of 7 wt% ammonium salts such as ammonium acetate (AA), ammonium bicarbonate (AB), ammonium hexaflurophosphate (AP), and tetraethyl ammonium bicarbonate (TAB) were compared and utilized to prepare the corresponding TiO2 pastes. The surface analysis of SEM image revealed that TiO2 using 7 % wt AC (7% AC-TiO2) increased porosity of the film. The efficiencies of individual cells were tested and it was found that the 7% AC-TiO2 based device showed the maximum energy conversion efficiency of 10.08%, compared to that (9.27 %) of the DSSC without AC.
Further, Carboxylic acid-functionalized multi-walled carbon nanotubes-polyindole/Ti2O3 hybrid nanocomposite (f-MWCNTs-PIN/Ti2O3) is utilized to prepare a photo-anode for dye-sensitized solar cells (DSSCs). Interestingly, TiO2 (f-MWCNTs-PIN/Ti2O3/TiO2) hybrid nanocomposite based photo-anode for DSSC reported power conversion efficiency (PCE) of 8.6%, which is higher compared with un-doped TiO2 photo-anode. The corresponding short-circuit photocurrent density (Jsc) of 18.30 mA cm-2, open circuit voltage (Voc) of 0.71 V, fill factor (FF) of 0.66, and dye absorption amount is 0.16 x 10-6 mol cm-2 respectively. The obtained results suggest that f-MWCNTs-PIN/Ti2O3 hybrid nanocomposite based TiO2 is a suitable photo-anodic material for DSSCs applications.
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