Category SOLAR CELLS – DYE-SENSITIZED DEVICES

Liquid electrolytes

The electrolyte is generally composed with oxidation-reduction of I-/I3- where LiI, NaI, alkyl ammonium iodine or imidazolium iodine is used for materials of I – ion. For instance, 0.1M LiI, 0.05M I2, and 0.5M tert-butyl pyridine (TBP) are mixed in acetonitrile solution or 3- methoxypropionitrile, propylenecarbonate, y-butyrolaqctone, N-methylpyrrolidone as alternative solvents. I – ion is responsible for offering electrons for holes generated in dye molecule’s HOMO level, whereas the oxidized I3- ion accepts electrons that reach counter electrode to be reduced (Snaith & Schmidt-Mende, 2007).

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Electrolytes

The key composition elements for DSSC include fluorinated tin oxide (FTO) which is used for either electrode substrate, nanoparticulated oxide semiconductor layer like TiO2 and ZnO, sensitizer, metallic catalysts like platinum which plays the role of the opposite electrode and the electrolyte which includes redox couple and it is positioned between the two electrodes. The composition and the form of the electrolyte have great affect on the total energy conversion efficiency. The majority of the proposed DSSCs is based on liquid electrolytes with a variety of solvents where an overall maximum efficiency of ~12% was finally achieved. Nevertheless, there are still questions which own an answer about the stability and sealing in order to prevent the leakage of the solvent...

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Sensitizers

The dye plays the important role of sensitizing the semiconductor in the visible and infrared region of solar light. For this reason several requirements have to be succoured at the same time such as, broad absorption spectrum, good stability, no toxicity, good matching of the HOMO, LUMO levels of the dye with semiconductor’s bottom edge of conduction band and chemical potential of redox system of the electrolyte. Besides, the chemical bonding between the dye and semiconductor’s surface is absolutely necessary for effective electron transfer. The ideal sensitizer for nanocrystalline TiO2 particles has to absorb all the light below a threshold wavelength of about 900nm...

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Nanocrystalline semiconductor

In DSSC technology a variety of nanocrystalline mesoporous metal oxides have been used such as TiO2, ZnO, SnO2 and Nb2O5 (Sayama, et al., 1998, Jose, et al., 2009). Despite the fact that some of them exhibited promising results in cells’ performance only titanium dioxide has extensively used because of some advantages which are only present in this oxide. TiO2 performs excellent thermal stability; it is impervious to chemicals and non-toxic and finally a cheap material. The common crystalline form in application to solar cells is the anatase although a mixture of anatase/rutile form is often used mainly by the formation of very active commercial Degussa-P25 powder. Rutile has proved to be less active as it is less chemical stable than anatase form...

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DSSCs’ basic components

The basic structure of a DSSC, as it is referred in previous section, is consisted of two glass electrodes in a sandwich configuration. For the first electrode (negative) a nanocrystalline n – type semiconductor, typically titanium dioxide film is deposited on a transparent conductive glass (TCO) (Fig.2) and then a dye-sensitizer is adsorbed and chemically anchored in order to sensitize the semiconductor in the visible. For this purpose, the dye sensitizer bears carboxylate or phosphonate groups, which interact with surface -OH groups on the titanium dioxide. Several efforts have been made to apply dyes of various structures; however, Ru-bipyridine complexes have established themselves as choice sensitizers (Xia & Yanagida, 2009). This is the negative electrode of the solar cell...

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Principles of operation and cell structure

The working principle of a DSSC substantially differs from that of a conventional solar cell based on silicon. In silicon solar cell a p-n junction by joining semiconductors of different

(a) (b)

Fig. 1. (a) Principle of operation for a DSSC and (b) an energy diagram of DSSC’s operation.

charge carriers’ concentration in a very close contact is necessary. In this case the processes of light absorption and charge transport are caused in the same material. In DSSCs, these
fundamental processes are occurred in different materials which avoid the premature recombination of electrons and holes. As these processes do not happen at the same material ultrapure materials are not required for a high performance DSSC...

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Dye Sensitized Solar Cells as an Alternative Approach to the Conventional Photovoltaic Technology Based on Silicon – Recent Developments in the Field and Large Scale Applications

Elias Stathatos

Technological-Educational Institute of Patras, Electrical Engineering Department, Patras,

Greece

1. Introduction

Utilization of renewable energies is of major importance because of the increase in fossil energy costs in combination with carbon dioxide reduction preventing global warming. The importance of the solar energy can be considered as the sustainable energy which may successfully satisfy a part of the energy demand of future generations. The 3×1024 joule/year energy supply from sun to the earth is ten thousand times more than the global need. It means that the use of 10% efficiency photovoltaic cells could cover the present needs in electricity covering only the 0.1% of earth’s surface (Wu, et al. 2008)...

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DSSC performance using carbon nanofiber counter electrode

The TiO2 photoanode contained a blocking layer, a TiCl4-treated nanocrystalline TiO2 layer (Solaronix Ti-Nanoxide HT/SP) and a light scattering layer (Dyesol WER4-0). After sintering, the photoanode was soaked in a dye solution made of 0.5 mM Ruthenizer 535- bisTBA dye (Solaronix N-719) in acetonitrile/ valeronitrile (1:1). The photoanode was then assembled with carbon nanofiber counter electrode using a thermoplastic sealant. The F/fF electrolyte was finally injected into the cells. The reference DSSC devices with sputtered Pt layer (40 nm) as counter electrode were also fabricated for comparison in the same method.

The J-V curves of carbon nanofiber and Pt DSSCs are shown in Figure 7a, tested under AM

1.5 solar simulator illumination at 100 mWcm-2...

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