Notre Dame Chemist Prashant Kamat has long been a proponent of harnessing sunlight to produce pollution-free energy. Among his peers he has earned a reputation as an expert in developing light harvesting nanoassemblies.  Last winter Kamat found himself dealing with an altogether different form of light: the limelight.

Kamat drew attention in the popular presswith the announcement that he had significantly improved the efficiency of certain solar cells by wedding carbon nanotubes to a film made of titanium-dioxide nanoparticles.

This scaffolding, or structure, of carbon nanotubes and titanium dioxide attached to an electrode, provides a significant improvement over other methods to convert ultraviolet light into charge carriers, namely electrons and holes. These photogenerated charge carriers can be harnessed to create a current in solar cells.

With this structure using nanoparticles alone,  Kamat was able to double the efficiency of conventional solar cells This break through was considered to be an important milestone in the development of new generations of solar cells.

During the last decade, nanomaterials have emerged as promising candidates for constructing light-harvesting materials, like solar cells. Carbon nanostructures have unique electrical properties and stability. These properties of single-walled carbon nanotube (SWNT) are especially ideal for energy conversion devices.

In a typical photochemical solar cell, a semiconducting film serves as a photoactive electrode that, upon excitation with visible light, generates electron-hole pairs. As an electron is driven to the counter electrode, the other charge carrier (the hole) is scavenged by a redox couple present in the electrolyte, thus generating a photocurrent.

Photoconversion efficiencies of only 11 percent were reported from those photochemical cells constructed from nanocrystalline semiconductor materials.

Kamat and his colleagues at the Radiation Laboratory at the University of Notre Dame formed a mat of carbon nanotubes on an electrode. The nanotubes serve as a scaffold on which the titanium oxide particles are deposited.  “This is a very simple approach for bringing order into a disordered structure,” Kamat and his coworkers wrote in the journal Nano Letters. 

The new carbon nanotube and nanoparticle system is not yet a practical solar cell. That's because titanium dioxide only absorbs ultraviolet light; most of the visible spectrum of light is reflected rather than absorbed.

But Kamat and his coworkers are designing ways to modify the nanoparticles to absorb the visible spectrum by applying a one-molecule-thick layer of light-absorbing dye to the titanium-dioxide nanoparticles.

In another experiment Kamat and fellow Notre Dame chemist Ken Kuno, coated the nanoparticles with cadmium selenide quantum dots--tiny semiconductor crystals. Unlike conventional materials in which one photon generates just one electron, quantum dots have the potential to convert high-energy photons into multiple electrons.

Kamat developed a solar cell made from cadmium selenide (CdSe) quantum dots assembled onto nanostructured titanium dioxide films. CdSe quantum dots absorb visible light and inject electrons into the TiO2 particles to generate photocurrent.

Theoretically, the procedure of charge carrier multiplication in quantum dots could achieve photo-conversion efficiencies greater than 100 percent. Kamat is far from achieving this goal, but he says that this is the first step to capitalize on some of the unique properties of quantum dots and pave the way for developing the next generation of solar cells that are more efficient, cheap and practical.

Kamat’s solar photochemistry research is funded by the Department of Energy.