Nanostructured Semiconductor Oxides for Dye-Sensitized Solar Cells

Abstract

One dimensional structures particularly TiO2 nanotubes (TiNTs) are extraordinary materials for dye-sensitized solar cell (DSSC) devices. The objective of present research work is to fabricate the highly ordered anatase TiO2 nanotubular arrays and their hybrid structures for developing the efficient device. In the first part, this work represents an attempt to optimize the parameters necessary for self-organization and ordering of tubes. In the second part of this thesis, combined structure of anodic TiO2 nanotubes, TiO2 nanoparticles (TiNTs-TiNPs) and TiO2 nanotubes combined with Ag nanoparticles (TiNTs-AgNPs) have been synthesized. These ordered and porous structures led to the improved performance of DSSCs. The nanotubes are prepared by a two-step anodization carried out in an aqueous electrolyte containing ethylene glycol and ammonium fluoride at a range of anodization voltages of 30-60 V. It was observed that TiO2 nanotubes self-branched during anodization of Ti metal carried out at a fixed applied potential. Considering different applied voltages, the TiO2 nanotubes prepared at 50V are comparatively more uniform and tend to grow straighter, whereas, numerous turns and pore branching are observed at other applied voltages. In this way, the best order in the tube arrays is found at anodization voltage of 50 V shown by the quantitative analysis as well. In addition, the nanotubes prepared by second-step anodization have shown higher crystallinity and with anatase as the major crystalline phase. TiNTs prepared at 50V are further investigated to study the influence of temperature on their morphology. An investigation by the SEM images reveals that if the temperature is kept constant during the anodizing experiment, variation in the average tube diameter is significantly reduced. Degree of pore arrangement, pore size and oxide thickness increased with the increase in temperature as observed at a range of electrolyte temperatures fixed between 5 to 40 ºC. However, if the temperature is not controlled during the anodization experiment, then due to the exothermic nature of the reactions to the formation of TiNTs, the temperature of electrolyte keeps on increasing. This work highlights the importance of fixed processing temperature during the anodization experiments in order to develop an ordered array of nanotubes with a uniform tube diameter. iv For the fabrication of TiNTs-TiNPs, a facile combination of hydrothermal and chemical vapor deposition methods was utilized. Ordered TiNTs fabricated at 50V were subjected to the hydrothermally produced gaseous environment in an autoclave with diluted TiCl4 solution at its bottom. Vapors of TiCl4 were allowed to react chemically with water vapors for predefined time durations at 180 ºC that resulted in the deposition of TiO2 nanoparticles on tubes’ surface and side walls. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed that for one hour reaction duration, nanoparticles were evenly coated on the walls of nanotubes, whereas, longer durations tend to deteriorate the tubular structure. Consequently, the ordered TiNTs-TiNPs array produced after one hour coating has shown better performance for dye-sensitized solar cell DSSC) in back illumination mode with 130 % increase in efficiency as compared to the device based on bare TiO2 nanotubes. The same photoanode has higher reflective properties with higher scattering ability. The solar cell based on photoanode prepared by 1 hour hydrothermal chemical vapor deposition treatment exhibits higher external quantum efficiency and effective charge transport properties. The combined structure of TiNTs and Ag nanoparticles were produced by depositing ApNPs onto the anodically produced TiNTs by pulse electrodeposition method. It was observed by scanning electron microscopy and energy dispersive X-ray analysis that size, distribution and concentration of AgNPs varies with applied current density. We have compared geometrical features and agglomeration of AgNPs by loading these particles on TiNTs array with nanograss, a disordered morphology, present at the tube top. SEM results reveal that AgNPs agglomerate in the form of nano-trees, nano-spheres and nano-cubes. It seems that disordered morphology at top surface of nanotubes favors the formation of clusters of AgNPs with novel shapes. The hybrid structures of TiNTs-AgNPs were subsequently employed as photoanode for DSSCs under one sun illumination in back illumination mode. The results of solar cells testing suggest that the ordered and open 1D TiNTs combined with uniform dispersion of AgNPs of size 20 nm show significantly increased photocurrent and conversion efficiency in DSSCs due to the combined effect of effective electron transport and enhanced plasmonic effect caused by AgNPs.