Dye-sensitized solar cells

From Wikipedia, the free encyclopedia

!Dye-sensitized solar cells are photoelectrochemical cells that use photo-sensitization of wide-band-gap mesoporous oxide semiconductors. These cells were invented by Michael Graetzel et al.¹⁾ in 1991 and are also known as Graetzel cells.

These cells are extremely promising because they are made of low-cost materials and do not need elaborate apparatus to manufacture. The cells have a simple structure that consists of two electrodes and an iodide-containing electrolyte. One electrode is dye-absorbed highly porous nanocrystalline titanium dioxide (nc-TiO₂) deposited on a transparent electrically conducting substrate. The other is a transparent electrically conducting substrate only. The cells have been compared to photosynthesis because they use the redox reaction of the electrolyte. The energy conversion efficiency of the cells has not yet reached the level of silicon solar cells. The current energy conversion efficiency is about 10%, as was reported by Graetzel et al. It is thought that the energy conversion efficiency can rise to 33% in theory.

Commercial applications, which were held up due to stability problems, are now forecast in the EU PV (photovoltaic) Roadmap to be a significant contributor to renewable electricity generation by 2010.

[edit] Principle

Solar light passes through an electrically conductive glass electrode, and dyes adsorbed to the electrode are irradiated. When a dye absorbs light, one of the electrons in the dye transits from a ground state to an excited state. This phenomenon is called photoexcitation. The excited electron makes a jump from the dye to the conduction band in TiO₂. This jump occurs very rapidly; it takes only 10^-15 seconds. In TiO₂, the electron diffuses across the TiO₂ film, reaches the glass electrode, goes through conducting wire, and reaches the counter electrode. The dye molecule, after having lost an electron to TiO₂, is oxidized, i.e., has one less electron than before. The dye molecule recovers its initial state by receiving one electron from an iodide ion, in turn oxidizing the iodide to iodine. This iodine diffuses to the counter electrode, receives one electron, and becomes an iodide ion again. This is how a dye-sensitized solar cell converts solar energy into an electric current that passes through an external circuit.

New developments (August 2006) As an alternative to conventional inorganic photovoltaics, dye-sensitized solar cells use an encapsulated nanoparticulate layer in conjunction with a nonvolatile highly conductive ionic liquid. Unfortunately, ionic liquids showing high conversion efficiency when used in these new solar cells are not thermally or chemically robust and can suffer efficiency losses. But researchers at Ecole Polytechnique Fédérale de Lausanne (Lausanne, Switzerland) have used a robust new ionic liquid called 1-ethyl-3 methylimidazolium tetrocyanoborate (EMIB(CN)4) as the ionic liquid for a successful solar cell that achieves 7% energy-conversion efficiency in full sunlight, even after thermal and light aging.

To prove the chemical and thermal robustness of their solar cell, the researchers subjected the devices to heating at 80°C in the dark for 1000 hours, followed by light soaking at 60°C for 1000 hours. After dark heating and light soaking, 90% of the initial photovoltaic efficiency was maintained—the first time such excellent thermal stability has been observed for an ionic liquid electrolyte that exhibits such a high conversion efficiency. Contrary to silicon solar cells whose performance declines with increasing temperature, the dye-sensitized solar-cell devices were only negligibly influenced when increasing the operating temperature from ambient to 60°C.

[edit] References

M. Graetzel. "A high molar extinction coefficient charge transfer sensitizer and its application in dye-sensitized solar cell". DOI:10.1016/j.jphotochem.2006.06.028.
¹⁾Brian O'Regan & Michael Graetzel, Nature, 353 (24), 737 - 740 (24 October 1991). DOI:10.1038/353737a0
A. Kay, M. Grätzel, J. Phys. Chem. 97, 6272 (1993).
G.P. Smestad, M. Grätzel, J. Chem. Educ. 75, 752 (1998).