Grätzel's cells

   In contrast to p-n junction photovoltaic devices where the semiconductor assume both the task of light absorption and charge carrier transport the two functions are separated here. Light is absorbed by a sensitizer, which is anchored to the surface of a wide band semiconductor. Charge separation takes place at the interface via photoinduced electron injection from the dye into the conduction band of the solid [2].

   At the heart of the system is a mesoporous oxide layer composed of nanometer-sized particles which have been sintered together to allow for electronic conduction to take place. The material of choice has been TiO2 (anatase) although alternative wide band gap oxides such as ZnO, and Nb2O5 have also been investigated [2].

    Attached to the surface of the nanocrystalline film is a monolayer of the charge transfer dye. Photo excitation of the latter results in the injection of an electron into the conduction band of the oxide. The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an organic solvent containing redox system, such as the iodide/triiodide couple. The regeneration of the sensitizer by iodide intercepts the recapture of the conduction band electron by the oxidized dye. The iodide is regenerated in turn by the reduction of triiodide at the counterelectrode the circuit being completed via electron migration through the external load [2].

    Overall the device generates electric power from light without suffering any permanent chemical transformation [2].

    A recent alternative embodiment of the DSSC concept is the sensitized heterojunction usually with an inorganic wide band gap nanocrystalline semiconductor of n-type polarity as electron acceptor, the charge neutrality on the dye being restored by a hole delivered by the complementary semiconductor, inorganic or organic and of p-type (solid state DSSC) polarity [2].