UCB chemistry professors James McCusker and Charles Shank, who is also the director of Berkeley Lab, along with graduate student Alvin Yeh, co-authored a paper in the August 11 issue of the journal Science in which they described a time-resolved spectroscopic study that followed the evolution of a photo-induced charge-transfer state in an inorganic chromophore. Their findings show, for the first time, two distinct factors that contribute to this evolution; one which is strongly influenced by the chromophore's immediate surroundings, and the other which is influenced only by the molecule's internal electronic structure.

A TYPICAL SOLAR CELL CONSISTS OF A COVER GLASS WITH AN ANTI-REFLECTIVE LAYER, A FRONT CONTACT TO ALLOW ELECTRONS INTO A CIRCUIT AND A BACK CONTACT TO ALLOW THEM TO COMPLETE THE CIRCUIT, AND THE SEMICONDUCTOR LAYERS WHERE PHOTO-INDUCED CHARGE TRANSFER PROCESSES TAKE PLACE. Image courtesy Dept. of Energy

"Although this work is very fundamental in nature, it suggests that medium-induced charge localization could be an important component of photo-induced charge transfer in a variety of settings that employ inorganic compounds as chromophores," says McCusker. "Since this is the necessary first step in almost any scheme one can come up with to convert light into usable energy, we believe that our results will help shape the way people think about this aspect of the problem."

In the process of photo-induced charge transfer, incident light upon a molecule redistributes electron density to create the chemical potential necessary for energy conversion. This process is central to a wide range of physical and chemical phenomena including photosynthesis in plants, and also forms the basis of the photovoltaic effect in semiconductors.

"Prior to our work, very little was known concerning the dynamics of photo-induced charge-transfer in inorganic chromophores," McCusker says. "But its only been within the last 10 years or so that the study of processes on such short time-scales has been experimentally accessible."

Understanding the dynamics behind photo-induced charge-transfer in inorganic chromophores is especially important for certain classes of solar cells where light absorption by dye-sensitized wide-bandgap semiconductors such as titanium oxide forms the basis for photoelectric conversion. Recently there has been considerable interest "both theoretically and experimentally" McCusker says, in the "dynamics of solvation." That is, scientists want to know what happens when the effects of incident light upon the chromophore and the dynamics of its nearby molecular neighbors take place on a comparable time-scale.

In their Science paper, McCusker, Shank, and Yeh report their observations of the factors that contributed to the formation of a charge-transfer state following the absorption of light by an inorganic chromophore in solution on a time-scale of less than one trillionth of a second. A chromophore known as [Ru(bpy)₃]²⁺, which is the prototype for the most widely used inorganic chromophores in sensitized solar cells, was photoexcited with flashes of light that were a mere 25 femtoseconds in duration. (A femtosecond is to one second what one second is to roughly 30 million years.) The chromophore's absorption of this light was then monitored as a function of time.

"Analysis of the data revealed that the excited electron is initially delocalized over all three bpy ligands, but eventually becomes trapped on a single ligand due to the rapid motion of the molecules in the surrounding solvent which occurs in response to the charge-transfer event," says McCusker. "In contrast, electronic relaxation from the initial excited state of the compound to lower energy states appears to occur independent of this charge localization process."

Whether or not the intramolecular effects of photoexcitation are totally independent of the environmental effects when it comes to localizing the charge-transfer is still not clear, the scientists report, but the identification of solvation dynamics as having a role to play answers some long-standing questions about the dynamics of charge-transfer states in inorganic chromophores. The researchers say it is possible that the surrounding medium might also play a similar role in localizing or directing charge-transfer states in the organic chromophores of biological systems.

"In this circumstance, the nearby residues within the proteins would act in much the same way as the solvent does in our experiment," says McCusker. "We have no evidence that this is the case, but it's interesting to think about."

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

For more information, Professor James McCusker can be reached by e-mail at mccusker@socrates.berkeley.edu