Metal Oxides and Their Composites for the Photoelectrode of Dye Sensitized Solar Cells

Abstract

Nowadays, solar energy is one of the most promising future energy resources in concerns to the sustenance of life on Earth and the depletion of fossil fuels. It is projected that the fossil fuel related CO2 emissions rise from 32 to 42 billion metric tons in 2007 and 2035 (EIA., 2010). This enormous amount of CO2 emission will leads to a severe climatic change of the world and therefore a greatest anxieties for the scientific era of 21st century. This rigorous apprehension leads the scientist for the development of solar cell that utilizes solar energy, a renewable and carbon free energy source. The solar energy strike to the earth in one hour is about 4.3x1020 J, which is higher than all the energy consumed in the planet (4.1x1020 J). Therefore, covering 0.1% of the earth’s surfaces with solar cell of 10% efficiency would satisfy the current energy demand (Gratzel., 2001). In general, solar cells can be classified as p-n junction semiconductor solar cells and organicbased exitonic solar cells (OESCs), in which polymer solar cell (PSC), dye sensitized solar cell (DSSC) and hybrid solar cell are included. In 1991, O’Regan & Gratzel first reported the dye sensitized nanocrystalline TiO2 solar cell (DSSC) based on the mechanism of a first regenerative photoelectrochemical processes with an efficiency of 7.1-7.9 %(under simulated solar light) (O’Regan & Gratzel., 1991). Since then, extensive researches have continued to increase the power conversion efficiency (PCE) of DSSC by incorporating n-type metal oxide semiconductors such as TiO2, ZnO, SnO2, Nb2O5, SrTiO3 etc and their composites as photoelectrode materials to achieve a reasonable efficiency of DSSCs of low cost, being therefore a promising alternative to conventional p-n junction solar cell. The wide band gap (Eg > 3eV) metal oxide semiconductors having suitable band position relative to sensitizer has been employed for the fabrication of DSSCs. The high surface area of nanoporous metal oxides facilitates the improvement of light absorption with improved dye loading for improved performance of DSSC. It is evident that the metal oxides employed for the fabrication of DSSCs has solar absorption below a threshold wavelength, ┣g (where, ┣g = 1240/Eg), i.e., they have absorption at ultraviolet region. On the other hand, dye is only responsible for the absorption of light at visible and near-infrared region. The strong absorption of light is attributed to the intramolecular charge transfer transition (ICT) from electron donating group to the anchoring acceptor group of dye. Therefore, the anchored