Investigation of physical properties of mesoporous NANO-Ti02 for dye-solar
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Date
2008
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Abstract
The nanocrystalline dye sensitized solar cells, so called «Gratzel cells* are based on a wide band gap «3.0-3.2eV» semiconductor TiO2 in its nano-scaled form. These nano-sized TiO2 are sensitized by a dye monolayer so to increase its spectral response and are immersed in a redox electrolyte for charge transfer. At the present moment, the best overall sunlight to electrical energy conversion efficiency under direct and diffuse sunlight laboratory devices is about 7-11%. It is well acknowledged in the literature that the low efficiency of standard large dimension Gratzel solar cell which varies in the range of 2-5% is caused by the excitonic fast recombination. In order to improve this conversion efficiency and reach values comparable with standard PV technologies, it is necessary to minimize this charge recombination at TiO2-dye interface in addition to an optimized optical path management. The conversion efficiency is related to the Incident Photon-Current Efficiency «IPCE» via the charge carrier free mean path «L» and the absorption coefficient or of the dye-TiO2 system.
IPCE = 7—f
Where L is related to charge carrier lifetime «£» and its diffusion constant «D» and mobility «u»:
Hence nano-TiO2 with substantial shape anisotropy such as TiO2 nanorods/ nanotubes or 1-D porous TiO2 structures would exhibit less interfacial scattering as a result of a larger lifetime and efficient light trapping. This trapping effect would offer a better electron transport in the nano-TiO2 electrodes as a result of faster photoresponse and higher electron collection efficiency. This research project focuses
mainly on engineering meso-porous 1-D type TiO2 n a no-structures with an obvious shape anisotropy for possible application in DSCs application which is expected to minimize the fast electron/hole pair recombination process. In this work we report the high-density arrays of mesoporous anatase TiO2 nano-structures derived using the combination of Evaporation Induced Self-Assembly (EISA) and sol-gel methods using Pluronic P123 «EO23 -PO70-EO23» as a surfactant and Titanium isopropoxide «Ti-O'Pr4 » as a starting material . The mesoporous TiO2 films were prepared by dip coating method using ITO coated float glass as a substrate.
The phase and crystallinity information was determined from small angle grazing incident-XRD. The average crystallite size of the TiO2 nanopartides was estimated using Scherer's equation. X-ray reflectivity (XRR) and grazing incidence small angle x-ray scattering were used to follow the order and disorder in the formation of the mesoporous nano-structured TiO2 and also to track the evolution of the films structure before and after calcinations. Optical analyses were done using UV-VIS-NIR spectroscope. The Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) were used to observe the surface morphology and surface roughness of the mesoporous TiO2 nanostructures, respectively. Elemental analyses were done using X-Ray Photoelectron Spectroscopy (XPS). Photoluminescence (PL) measurements were done using an Argon laser source at an excitation wavelength of 476 nm in order to study the defects present in the samples.
Description
Thesis presented in fulfillment of the requirements for the degree of Master of
Sciences at the University of Zululand, 2008.
Keywords
Mesoporous nano-Ti02, Dye-solar