Dye Sensitization of Colloidal Semiconductor Electrodes

Dye sensitization of electrodes is an old area of science with a rich history. The field has experienced renewed interest owing to the development of high surface area colloidal semiconductor electrodes. These materials yield impressive solar conversion efficiencies when employed in regenerative solar cells that have already found niche applications and have the real possibility of replacing traditional solid-state photovoltaics. Thus for the first time in history a solar cell designed to operate on a molecular level is useful from a practical point of view. It is also likely that other applications in the growing areas of molecular photonic materials will arise. 

The photcwurrent measurements establish that injection of charges occurs from the excited state of the dye to the conduction band of the semiconductor, but does not say anything about the identity of the dye excited state involved. Luminescence and time-resolved (transient) absorption studies on dye-semiconductor colloids and dye-coated films on conducting glass electrodes have been useful to establish the identity of the excited state and energetics of the sensitization process. 

ZnO is obviously one of the best candidates among semiconductors for DSSC applications as it can be synthesized easily and inexpensively into different shapes and sizes using various methods, and is environment friendly and stable indefinitely. Photovoltage and photocurrent measurements using modulated illumination on dye-sensitized ZnO solar cells have shown tens to hundreds of times faster electron transport in nanorod array electrodes compared to nanocrystalline particulate electrodes assembled from colloidal nanoparticles, with the electron lifetimes being only slightly smaller [237]. 

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