Research: Nanocrystalline TiO2

TEM of TiO2

Nanocrystalline titanium dioxide photoelectrodes are of interest because they offer a potentially very inexpensive method for converting sunlight into electrical energy. The TiO2 itself has a band gap that is too large to absorb much sunlight, so it is sensitized by adsorption of an inorganic, ruthenium-based, dye. The dye absorbs the light, and the excited state injects a charge carrier into the TiO2. Before the electron in the TiO2 can back react with this injected charge carrier, the Ru(III) species reacts with a mobile redox-active ion in the solution, regenerating the Ru(II) complex. The redox-active species is typically iodide triiodide in a nonaqueous solvent. This oxidized redox species diffuses to the counter electrode, where it is reduced, closing the cycle. Work is extracted by passing the injected electron through an external load before allowing it to reduce the oxidized form of the redox couple at the counterelectrode solution interface.

Pioneering work by Gratzel and co-workers at the EPFL in Switzerland has shown that such devices can display up to 10% energy conversion efficiencies, in a potentially extremely inexpensive device structure. Many people believe that if these cells are sufficiently stable and efficient, that they could offer a truly low cost approach to photovoltaic-type solar energy conversion and storage. Our goal is to understand several fundamental, but to date not well-elucidated, facets of such systems and to ultimately design improved energy conversion devices through this detailed understanding of what controls their microscopic behavior.

Dye-sensitized Solar Cell

Questions of interest in our laboratory currently include:

  1. What controls the rate of charge injection from the dye into the semiconductor?
  2. What controls the rate of back reaction of injected electrons with oxidized species in the solution?
  3. Why is the iodide iodine redox couple the only redox shuttle that works well in these cells?
  4. Can the iodide iodine system be replaced by other redox species that would allow more efficient use of the available energy in the photosensitized electrode systems?
  5. What controls the rate of electron percolation through the TiO2 film and how do charge carriers "know" to move towards the back electrode as opposed to recombining with the species in the electrolyte?

To address some of these questions, we are studying a series of Os- and Ru-based metal complexes. This allows us to vary systematically the ground state redox potential, the excited state redox potential, and the electronic coupling to the electrode surface. We are investigating the effects that these variations have on the performance of the photoelectrochemical cell. We have performed an investigation of how these changes affect the steady-state current vs voltage properties of the cells, and also have performed nanosecond and femtosecond studies of the dynamics of charge injection and recombination at these interfaces. In addition, we are attempting to understand and block the back reaction of electrons in the TiO2 with oxidized species in the electrolyte and to block this reaction through controlled surface functionalization processes. Finally, we are synthesizing a novel series of linked donor-acceptor complexes in an attempt to suppress recombination and to open up the use of other redox couples than iodide iodine and thereby obtain significant improvements in energy conversion efficiency from these types of systems.