Category Solar Energy 5

DSSC coupled to 1DPC

Although the coupling of inverse opals to dye sensitized electrodes demonstrated an increased IPCE with respect to that of a reference cell (Nishimura et al., 2003), the main drawback of these 3D structures is the difficult assembly process to achieve reasonable reflecting periodic materials, which leads usually to thick structures (between 5-10 micron thick). This might have a deleterious effect on charge transport and recombination through the cell. Very recently, new types of one-dimensional photonic crystals (1DPC) have been prepared by alternate deposition of either mesoporous (Choi et al., 2006); (Fuertes et al., 2007) or nanoparticles (Wu et al., 2007); (Colodrero et al., 2008) based films. These structures

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Fig. 6...

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Three dimensional ordered structures

In 2003, Mallouk and co-workers proposed the use of a novel type of optical elements to improve the efficiency of DSSC. Their approach was based on the coupling of a particular type of 3DPC to dye-sensitized nc-TiO2 films (Nishimura et al., 2003). This approach consisted of integrating a 3 pm layer of a TiO2 ordered porous structure, known as inverse opal (see figure 5). By doing so, IPCE was shown to increase with respect to that of a standard cell used as reference (Figure 6a)...

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Periodic structures

An alternative to the use of disordered structures to enhance diffuse light scattering is the introduction of porous materials in which a periodic variation of the refractive index has been built up. As it will be shown next, periodic structures allows to achieve high reflectance within the cell at targeted and well-defined wavelength ranges, which may prevent the drawback of the loss of transparency. Also, in some cases, highly reflecting structures can be only a few hundreds of nanometres thick, which reduces the potential problems of increase of resistance and reduction of the photovoltage...

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Diffuse scattering layer

TiO2 working electrodes used in DSSC are composed of 20 nm size crystallites. These electrodes are essentially transparent since visible light is not scattered for titania particles of sizes on the order of the few tens of nanometers. In fact, the incident photons that are not absorbed by the dye sensitized electrode are either lost through the counter electrode or partially absorbed by the electrolyte solution. From a photo-chemical point of view, this implies that part of the reagent (light) is wasted. The first attempt to collect these escaping photons were based on the use of polydisperse packings of sub-micron size spheres as highly diffusive reflecting layers (see figure 4)...

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Effect of increase optical absorptance on the efficiency of the cell

Optical approaches to raise JSC, and therefore efficiencies, are based on the increase of optical absorption caused by either an enlargement of the photon path length through the working electrode or light trapping effects occurring within the TiO2 electrodes. JSC can be attained by integrating the product of the ratio between the solar spectral irradiance and the photon energy density, F (X), and the photon-to-current conversion efficiency, IPCE, of the cell over the wavelength of the incident light:

Jsc =f q4(X)F (X)IPCE (X) dX (3)

Here q is the electron charge and ^ (X) is a factor that accounts for the losses at the air- substrate interface...

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Approaches to light management in DSSC

In the last years, many attempts using different modifications of the originally proposed cell have been made in order to improve its performance, most of them based on the use of different semiconductors (Tennakone et al., 1999), dyes (Wang et al., 2005) or ionic conductors (Wang et al., 2004). However, the extremely delicate sensibility of the charge transport and recombination dynamics to any alteration of the nature of the interfaces present in the cell should be considered (Haque et al., 2005). For instance, some of the most important routes of research have focused on the molecular engineering of suitable dyes having broader absorption spectra that show a better matching to the solar spectrum and higher molar extinction coefficients (Wang et al...

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Brief description of DSSC

Photovoltaic devices have become a promising alternative energy source in the last decades. They are expected to increasingly and significantly contribute to overall energy production over the coming years. The photovoltaic field, dominated mainly by inorganic solid-state junction cells, is now being challenged by the emergence of new devices based on nanocrystalline and conducting polymer films, which offer a very low-cost fabrication and attractive features such as transparency, flexibility, etc. that might facilitate the market entry. Among all of them, dye sensitized solar cells (DSSC) are devices that have shown to reach moderate efficiencies, thus being feasible competitors to conventional cells.

DSSC combine the optical absorption and charge-separation processes by the association o...

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Photon Management in Dye Sensitized Solar Cells

Silvia Colodrero, Mauricio E. Calvo and Hernan Miguez

Instituto de Ciencia de Materiales de Sevilla Consejo Superior de Investigaciones Cientificas-Universidad de Sevilla

Spain

1. Introduction

Solar energy is nowadays one of the most promising future energy resources due to the depletion of fossil fuels, which supply the major part of all energy consumed worldwide. Among the different types of solar cell technologies, dye sensitization of mesoporous oxide based films has attracted a great deal of interest in the last few years because of the possibility it offers to achieve moderate efficiency devices at very low cost, being therefore an interesting alternative to conventional p-n junction solar cells...

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