Dye-sensitized Solar Cell

Wide band gap semiconductors have potential applications in many areas due to its higher thermostabilization and photochemical stability. Although these semiconductors have lower light absorption abilities, they can be expanded to the visible region by virtue of the strong visible light absorption capability of suitable dyes, which anchored onto the semiconductor surface. This phenomenon is defined as Dye Sensitization of Semiconductors. The research on sensitization of wide band gap semiconductors using dyes has a long history. Most of these early studies were performed on dye sensitized bulk semiconductors. For the absorption of solar rays and charge injection into the conduction band (CB) of a semiconductor by a sensitizer attached as a monolayer to the surface of an oxide film there is a fundamental problem of the limited light-capture cross-section of the dye sensitized electrodes. As a result, the solar to electricity conversion efficiency of this type system is rested on lower level.

The breakthrough of dye-sensitized devices as solar cell was based on the exchange of the bulk semiconductor by mesoporous structures of high surface area, which allowed a drastic enhancement of the amount of adsorbed dye and developed by the research group of Prof. Micheal Graetzel at Ecole polytechnique fdrale de Lausanne (EPFL, Switzerland) since 1980s. Furthermore the development of stable ruthenium complex sensitizers bearing attachment groups which allow the chemical bonding of the sensitizer to the surface of the TiO₂ and using iodide containing redox systems lead to a drastic improvement in charge injection efficiency and dye regeneration. Based on these innovations, O'Regan and Graetzel reported this low-cost solar cell based on dye-sensitized colloidal TiO₂ films with an overall efficiencies of 7.1~7.9 % under simulated solar irradiation (AM 1.5) in 1991. Now the highest efficiency of this type solar cell with a working area of about 1 cm² is up to 11 %.

Typical DSC is based on a sandwich structure, which consists of photoanode, electrolyte and cathode. The photoanode is usually a wide band gap semiconductor film of nanocrystalline morphology that is anchored by a monolayer of light-absorbing material (the dye sensitizer). Photo-excitation of the sensitizer results in the injection of an electron into the conduction band of the semiconductor. The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an ionic liquid containing most frequently the iodide/triiodide couple as a redox system. The redox system is regenerated in turn by the reaction with the electrons at the counter electrode which have passed through the external circuit. The voltage generated under illumination corresponds to the difference between the quasi-Fermi level of the electron in the solid and the redox (Nernst) potential of the electrolyte. Overall, the device generates electric power from light without suffering any permanent chemical transformation.

The DSC does use the same concepts and similar structures as the photosynthesis center of green plant in order to harvest and convert solar energy. The chlorophyll and the sensitizer in DSC have the same function of harvesting the sunlight. The nanocrystalline semiconductor in DSC mimics the function of thylakoid in chloroplast. These structures serve as the support of sensitizers, which can enhance the light-capture cross-section and collect/transfer electrons. This photoelectrochemical device could be considered as a molecule-electron pump, which is drived by sunlight. Learning from the concepts used by green plants to make solar cells is a dream of human beings. After 20 years of development, the dream is coming true and will benefit our life in the near future.

Working Principle and Schematic Structure of the DSC