Thursday, June 12, 2014
3:00pm (110 INSCC)
Title: The Influence of Morphology on Excitons in Single Conjugated Molecules
The morphology of the conjugated polymer poly-(3-hexylthiophene), P3HT, is known to have a strong influence on its charge-carrier transport and optical properties. Both are crucial elements for the development of efficient photovoltaic devices. As a result P3HT quickly became one of the most studied materials in the field of organic electronics. The absorption spectrum of the bulk can be extended to higher wavelengths resulting in an increased collection of the solar radiation and thus improving solar cell device performance. This feature is often attributed to the formation of an electronic aggregate in the literature.
Whereas conventional optical spectroscopy leaves the role of individual polymer chains hidden in the ensemble, single molecule spectroscopy can be a powerful tool in revealing the properties of single entities comprising the bulk material. The advantage of this technique is employed to arrive at insight into individual building blocks of P3HT and unravel the disorder in the bulk.
Single chromophoric units of P3HT exhibit blinking as well as surprisingly stable temporal behavior with spectral linewidths narrowed down to only a few meV. Whereas organic dyes or polymer materials such as polyfluorene show inhomogeneous broadening of about 20 nm, in P3HT a constant linewidth and vibronic coupling energy of single chains over an unprecedented spectral range of almost 200 nm (0.8 eV) is observed. The results lead to a better understanding of the morphology of single chains and may clarify the necessity of the aggregate picture.