Ionic Liquid Based Electrolytes for Dye-Sensitized Solar Cells

1.1 New type of solar cell: dye-sensitized solar cells (DSSCs) The rising price of fossil fuels, together with their rapid depletion and the pollution caused by their combustion, is forcing us to find sources of clean renewable energy. Fortunately, the supply of energy from the sun to the earth is gigantic, i.e., 3 × 1024 joule a year or about ten thousand times more than what mankind consumes currently. This means that only 0.1% of the earth’s surface with solar cells with an efficiency of 10% would suffice to satisfy our current needs (Hamakawa, 2004; Grätzel, 2001). Therefore, solar power is considered to be one of the best sustainable energies for future generations. There are already a number of terrestrial applications where photovoltaic devices provide a viable means of power generation. Photovoltatic devices are based on the concept of charge separation at an interface of two materials of different conduction mechanism. To date photovoltaics has been dominated by solid-state junction devices, usually in silicon, crystalline or amorphous, and profiting from the experience and material availability resulting from the semiconductor industry. However, the expensive and energy-intensive high-temperature and high-vacuum processes is needed for the silicon based solar cells. Therefore, the dominance of the photovoltatic field by such kind of inorganic solid-state junction devices is now being challenged by the emergence of a third generation of solar cell based on interpenetrating network structures, such as dye-sensitized solar cells (DSSCs) (Grätzel, 2001; O’Regan et al., 1991). DSSCs have been extensively investigated since O’Regan and Grätzel reported a 7.1% solar energy conversion efficiency in 1991 (O’Regan et al., 1991). DSSCs offer particular promise as an efficient, low cost alternative to Si semiconductor photovoltaic devices and represent a specific type of photoelectrochemical cell. The advantages of DSSCs are that they do not rely on expensive or energy-intensive processing methods and can be printed on flexible substrates using roll-to-roll methods. Instead of using a single crystal semiconductor, DSSCs rely on a thin mesoporous film (10-15 μm thick) of nanocrystals of a metal oxide, most often TiO2, which is sensitized to visible light with a molecular light absorber. The sensitized nanoparticles are combined with a redox active electrolyte solution and counter electrode to produce a regenerative photoelectrochemical cell. By using the traditional liquid electrolyte, the DSSC has achieved an 11.5% efficiency record (Chen et al., 2009), encouraging the surge to explore new organic materials for the conversion of solar to electric power. However, presence of liquid electrolytes in traditional DSSCs has some problems such as a less long-