46755-AC10
Breaking the glass ceiling of organic photovoltaics by controlling temporal fluctuations in light-harvesting in single macromolecular complexes
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.
Research in first year:
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, which are currently being evaluated.
The research of studying light harvesting in single polymer chains is topical and relevant to a range of devices. The results from the first year have been instrumental in defining future research directions of the group.
The project served to fund a dedicated and gifted graduate student, Mr. Manfred Walter, who has made excellent progress over the past year and is nearing the completion of his graduate PhD work. So far, three publications are submitted/in press arising from the support of the PRF.
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 is in press in Nano Letters. 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.
