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The aim of the project was to study intramolecular light-harvesting processes in single pi-conjugated macromolecules such as conjugated polymers. To this end, a special class of polymers end capped with an emissive dye unit was used as a model system. Strong fluctuations with time in the energy transfer efficiency are observed, and it is proposed that these microscopic fluctuations ultimately limit the efficiency of molecular solar cells in which primary photoexcitations have to be transported efficiently to charge dissociation sites.

The PI relocated to Utah in July 2006 and was rapidly able to set up a unique single molecule spectroscopy facility designed to study the elementary photophysics of conjugated polymers. Further information on recent progress is available on the PI’s group web page. The project was awarded to coincide with the lab beginning to take data and a significant number of developments have been made since then. Most importantly, we have demonstrated the feasibility of studying the emission of a single molecule while varying the excitation wavelength over a wide range (of order 100 nm). Most recently we have succeeded in taking single polymer chain photoluminescence excitation spectra; the key results are to appear in Physical Review Letters in October 2009.

The research of light harvesting in single polymer chains is topical and relevant to a range of devices. The results gained have been instrumental in defining future research directions of the group.

The most important result so far is that we were able to demonstrate that the single molecule absorption of a polymer chain appears to be much broader spectrally than the emission. A qualitative description of the observations appeared in Nano Letters in 2008. The consequence of the observations is that different single molecules can be excited equally efficiently below a certain threshold wavelength. This means that the strong broadening of absorption spectra of conjugated polymers is an effect intrinsic to the absorption of the single chain, not a consequence of interchain disorder as previously thought. Efficient photovoltaics requires a broad absorption spectrum with minimal intermolecular disorder to efficiently harvest charge carriers. Our results underline how important conjugated polymers are going to be in reaching efficient photovoltaic devices.

On a more conceptual note, we were able to demonstrate a subtle influence of excitation energy of the donor on the emission properties of the acceptor. Conjugated polymers can be tens, if not hundreds, of nanometers in length, and it is very surprising to note that a change of the excitation energy on the backbone influences the spectroscopy of the remote end cap. These results underline the power of wavelength selective (i.e. site selective) single molecule excitation spectroscopy.

Further work carried out in the scope of the project has related to the occurrence of energy transfer and light harvesting in the presence of surface enhancement (i.e. of single molecules deposited on nanostructured silver surfaces, published in JACS 2008); and of light harvesting in inorganic CdSe/CdS semiconductor nanostructures. These quantum structures serve as illustrative model systems for comparison with the less well-defined polymers, and also exhibit a more universal spectral behavior in terms of light harvesting. The initial results in studying light harvesting in single semiconductor nanostructures has been written up and is presently being revised following positive initial reviews. 

The project has been instrumental in advancing the PI’s career, not least because it was awarded within less than of a year of arrival in Utah. Preliminary work from the project led to the award of a substantial Fellowship from the David & Lucile Packard Foundation, as well as a generous explorative grant from the Volkswagen Foundation (based in Germany). This Volkswagen project arose from a collaboration with a chemistry group in Germany (Prof. Hoger); the student Manfred Walter studied materials synthesized by this group using our single molecule approach (the collaborative results arising from PRF funding are published in JACS 2008). In addition, the PRF funding could be employed to rapidly promote Manfred Walter’s experience in making organic light-emitting diodes (OLEDs). Although this work was not directly linked to the PRF project, a minimal amount of seeding led to a substantial collaborative DoE project awarded this summer.

Based not least on the work enabled by the PRF funding, the PI was awarded early tenure in 2009 and is scheduled for promotion to full professor in 2010. 

Finally, the project funding greatly aided the progress of the dedicated student Manfred Walter. Manfred defended his thesis in 9/2009, having presented his PRF work at the 2009 ACS March meeting in Salt Lake City. Manfred was offered several different positions and is now training to become a patent attorney in Munich.