AmericanChemicalSociety.com

The overall aim of our ACS-PRF proposal and the work of our group in general, is the elucidation of fundamental aspects of exciton generation and charge transfer in organic molecular systems. Our studies are first and foremost a basic research effort, yet the implications could lead to the discovery of transformational improvements in organic molecular photovoltaics.  In particular, our fundamental research could lead to advancement of organic-based photovoltaics that are inexpensive, easily processed, and stable replacements for silicon-based semiconductors.   The primary focus of our work is on singlet fission, in which a singlet excited state evolves into a pair of triplets on the same or neighboring molecules without change in net spin.  If the process occurs with high efficiency, and if the triplet excited states subsequently lead to charge separation, it has been established that the solar cell efficiency can increase nearly 50%.  Improvements of this magnitude are crucial for making organic-based solar cells competitive with the prevalent inorganic silicon-based cells.

We have focused our work on organic molecular fibers and aggregates in solution.  The fibers and aggregates are intimately connected to molecular films of working solar cells, because intermolecular interactions dominate the photophysics in all of these systems.  The aggregates provide some advantages over thin films. First, one can readily form different kinds of aggregates in solution.  Second, a liquid solution allows one to probe the effect of environment (e.g. solvent polarity) on photophysics.  Similar studies are difficult or impossible with thin films.  Third, it is possible to explore size effects with aggregates. Lastly, a liquid offers a practical advantage in pump-probe spectroscopy including the ability to flow fresh sample to the laser focus, which minimizes the possibility of thermal damage to the sample.

The most exciting outcome of our work during the period of the project has been the discovery of high-yield singlet fission in a self-assembled molecular aggregate of carotenoids. (Wang &Tauber J. Am. Chem Soc. 2010 132 13988–13991)  The technique that we employed in this discovery was picosecond resonance Raman spectroscopy, which has both the specificity and sufficient time resolution to reveal that the triplet yield at early time is very likely in the range of 90-200%.  We are now preparing an extensive manuscript that compares several kinds of carotenoid aggregates, and reports results from both femtosecond transient absorption spectroscopy as well as resonance Raman spectroscopy. 

An additional achievement has been the discovery of singlet fission in a perylene diimide (PDI) aggregate.  This advance is important because the PDI dyes are generally much more robust than the carotenoids, thus they are primary candidates in organic molecular photovoltaics. This work has been presented in two meetings (ACS spring 2010; Materials Research Society 2010).

During the term of the proposal, three new graduate students have benefitted from partial support of the ACS-PRF grant.  These students gained valuable skills, including design and construction of a picosecond pump-probe resonance Raman spectrometer that was built during this period.  The support as a whole has helped launch my independent research group into an exciting field.