After industrial revolution, energy consumption sharp increased and each kind of energy was developed one after another especially fossil fuel since 19th century.
However, these energy source storages are limited and going to be exhausted this century. Also, the highly usage of fossil fuel caused serious environmental pollution and ecological damage. United Nation Framework Convention on Climate Change (UNFCCC) and Kyoto Protocol clearly emphasize the importance of renewable energy development.
Renewable energy including waterpower, wind power, solar, biologic energy, terrestrial heat, ect. and these energy can transfer into electric power, heat, chemical power and fuel. In all kinds of energy, solar energy is almost inexhaustible and without environment pollution. The supply of energy from the Sun to the Earth is gigantic: 3×1024 joules a year, or about 10,000 times more than the global population currently consumes, which means converting 0.1% of the Earth’s surface with solar cells with an efficiency of 10% would theoretically satisfy our present needs. [1]
Nevertheless, the solar energy from the sun cannot be use efficiently. Part of the sun power would be absorbed by the Earth’s atmosphere or reflected to space. The Air Mass (AM) value is set to describe the spectrum (not necessarily the intensity) of sunlight at particular latitude. It is defined as the distance through the atmosphere that the light from the sun travels in order to reach the solar cell. This is expressed relative to conditions at the equator, where the sun is almost directly overhead, and where the light is therefore described as AM1.0. Thus in space, without atmosphere, the spectrum is referred to as AM0.
For most terrestrial applications, the generally accepted solar cell testing
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standard is that of AM1.5 conditions. In addition, we usually also specify the intensity of the light, integrated over the spectrum, as being 100mV/m2 (1 sun). When using solar concentrators, such as the intensity might be increased by a factor of 1000, but the shape of the spectrum would remain AM1.5. [2]
Fig. 1-1 The path length, in units of Air Mass, changes with zenith angle. [3]
Even though the sun energy is hard to be used efficiently, solar power still offers a realistic solution to energy problems and that is the reason solar cells have attracted extensive attention and fast developed. Solar can be transformed to electricity, fuels and heat through varied solar utilization, without noise and it produces no air pollution. Massive solar power conversion would ensure abundant energy and safe clear environment for future generations.
In all kinds of solar cells, silicon (Si) base solar cells are more mature developed and main products on the market. However, the coast of silicon base solar cells is higher than other energy generation method and only be used at specific situation. It is
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important to development more low coast and high conversion efficiency solar cells for mass energy generation for popular use.
Gratzel et alet al. developed a new kind of solar cell, dye-sensitized solar cell, which has relatively high conversion efficiency, simple fabrication process, low production coast and transparency in 1991. [4] Great conversion efficiencies (above 11%) have been reported based on mesoporous nanocrystalline TiO2 film, ruthenium sensitized dye, triiodide/iodide redox couple in organic solvent as the electrolyte and platinum coated counter electrode. [5, 6] Because its easy production process and transparency can be design to combine with various electronic devise and become portable final products.
The performance and efficiency of the DSSC depend on many factors such as the platinum layer of the counter electrode, the TiO2 layer surface morphology and the structure, dye molecules, the status and component of the electrolyte and so on.
Mesoporous nanocrystalline TiO2 films provide large surface area for dye adsorption, electrical connection with the redox electrolyte, electron diffusion and transportation.
As the pore size and porosity of the TiO2 film increases, the diffusion of electrolyte becomes easier, but the effective surface area decrease and the amount of dye adsorb less. Therefore, the morphology of the TiO2 film plays a very important role in the high efficiency DSSC. There is a precise balance between pore size, porosity, effective surface area and thickness of TiO2 film to achieve optimum DSSC performance.
Different shape and size of TiO2 particles were used to fabricate high efficiency DSSCs. The use of small TiO2 particles and large pore size was reported to yield good electron transport and high current efficiency of DSSCs. [7] Recently, TiO2 nanotubes [8, 9] are introduced to enhance conversion efficiency for DSSCs due to its increased surface area for dye absorption. However, the syntheses of TiO2 nanotubes or special
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shape TiO2 are time-consuming multistep process, which is difficult to scale up for production. In contrast, a simple and efficient method to control the TiO2 film morphology is to add dispersive polymer into TiO2 paste to form larger pores, higher porosity, and more surface area. [10, 11] Most commonly used dispersive polymers are polyethylene glycol (PEG) and polystyrene. Some reports show the relationship between dispersive polymer addition and their effect on TiO2 characteristic [12-14].
However, there are only few reports talking about the dispersive polymer addition effect on the porosity and pore size in TiO2 films, resistances inside DSSCs and the cell performance.
In this study, for modifying the pore size and porosity of TiO2, different molecular weight PEG and solvent were used in TiO2 paste and different PEG burn out rate were applied after TiO2 films coating. TiO2 films with different pore size and porosity were prepared by coating commercial TiO2 nanoparticles (P25) on FTO conducting glass using doctor-blade technique. Appling PEG and different solvents in TiO2 paste is an easy way to control the TiO2 film pore morphologies without change any process in produce DSSCs. The porosity and pore size of TiO2 films and the photochemical characteristics of DSSCs with these TiO2 films were investigated. The correlation between TiO2 film pore morphologies and DSSC performance were discussed. Finally, the pore morphology of TiO2 film was optimized for the best performance of DSSC.
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