Light absorption in 1D and 2D nanostructured organic semiconductors

Organic photovoltaics will be one of the corner stones in solving the increasing world's energy demand issue. For this reason, a better understanding of the detailed mechanisms involved in energy conversion in the employed nanostructured materials is still desirable. In this context, one-dimensional and two-dimensional nanostructured organic semiconductors may reveal intriguing opto-electronic properties due to their morphology and the nanoscopic confinement. In this regard, supramolecular self-assembly has proven to be a powerful tool to control the obtained nanostructures, by using secondary interactions such as hydrogen bonding or the phase segregation between molecular segments of different nature, which requires functionalization of the pi-conjugated chromophore. The present thesis starts with the preparation of functionalized oligothiophenes. The synthesis of pi-conjugated systems is rendered difficult by their inherently low solubility. In the first part of the present thesis, we therefore developed a convenient synthetic pathway for chemically functionalized oligothiophenes up to the octathiophenes, by employing chlorendylimidyl active esters as both temporary protecting groups and solubilizing groups. From the obtained soluble precursors, a large library of oligothiophene derivatives carrying polymer-oligopeptide substituents was straightforwardly prepared in a divergent synthesis. This simple substitution served to induce aggregation into one-dimensional nanowires that comprised a single stack of oligothiophenes arranged in a helically twisted, parallel-displaced fashion. This molecular arrangement was in turn found to result in the facile photo-induced formation of charge-carriers. Thus, charge generation under illumination and decay in the dark were characterized for oligothiophene nanowires with varying molecular parameters. Notably, derivatives with an even number of thiophene units accumulated a higher concentration of charges, up to 4 mol%. Thus, aggregation into one-dimensional nanostructures with supramolecular helicity can be regarded as a useful tool to promote a cofacial stacking of chromophores and facilitate charge generation. In parallel, we investigated two-dimensional nanostructures obtained from an acceptor-donor-acceptor triad based on perylene-bisimide and quaterthiophene substituted with flexible polymer chains. The latter served to reliably induce aggregation into smectic lamellar phases. Within the lamellae, the triad segments were arranged with a tilt angle relative to the layer normal in order to accommodate packing constraints. Light absorption was followed by transient charge separation between the two types of chromophores. Charge recombination was slower in thin films than in solution and even slower for a derivative with longer polymer chains and higher degree of long-range order. These findings demonstrate that the phase segregation observed in rod-coil systems can be used to improve the nanoscale order and could be beneficial for applications in organic photovoltaics.

Frauenrath, Holger
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis6646-4

 Record created 2015-05-13, last modified 2018-01-28

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