How Dye Solar Cells Work:
Conventional solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. The semiconductor performs two processes simultaneously: absorption of light, and the separation of the electric charges ("electrons" and "holes") which are formed as a consequence of that absorption. However, to avoid the premature recombination of electrons and holes, the semiconductors employed must be highly pure and defect-free.In contrast, dye solar cells (DSC, also referred to as dye sensitized solar cells) work on a different principle whereby the processes of light absorption and charge separation are differentiated. Due to their simple construction and non-vacuum manufacturing process, dye cells offer significant reduction in the cost of solar electricity. Solar-to-electric energy conversion efficiencies in excess of 10% have been obtained in research cells of this type.
The dye solar cell consists of two electrodes in a sandwich configuration (see diagram). On one of these electrodes, the photoanode, is a several micron-thick layer of nanocrystalline titania TiO2 that is deposited by screen printing and oven treatment in air. This compact layer is porous with a very high surface area, allowing monolayer absorption of dye molecules. The dye-coated electrode is then put together with another conducting electrode, the counter electrode, and the intervening space is filled with electrolyte (a redox electrolyte usually based on an organic medium containing iodine). The counter electrode is catalyzed with trace platinum or carbon, efficient electrocatalysts for the cathodic reduction of triiodide to iodide in the redox electrolyte. After making adequate provisions for current takeoff from the two electrodes, the assembly is sealed.
Respectable Efficiency, thanks to Nanostructure
A respectable photovoltaic efficiency is obtained by the use of the porous nanostructured titania layer of very high surface roughness. When light penetrates the photosensitized, semiconductor "sponge", it crosses hundreds of adsorbed dye monolayers. The end result is greater absorption of light and its efficient conversion into electricity. The dye is usually based on an organic complex of ruthenium which sensitizes the titania to visible light. An incoming photon causes a dye molecule to enter an activated state, and an electron is injected into the conduction band of the titania. The electron passes out of the cell via the conducting coating on the photoanode, does work in the external circuit and re-enters the cell via the counter electrode. There it reduces tri-iodide to iodide ion which diffuses to the activated dye molecule on the photoanode and gives up its electron. Tri-iodide is thus regenerated and the dye returns to its preactivated state ready for the next photon.