Light-harvesting in single conjugated polymer chains and semiconductor nanocrystals.

摘要

Light-harvesting characterizes the efficient absorption of light and subsequent transport of excitation energy to a reaction center and thus the two basic processes relevant to photovoltaics. In this work, light-harvesting is studied on the fundamental single particle level of two technologically relevant semiconducting material classes---nanocrystals and conjugated polymers. The approach allows for the identification of structure-property relations which can usually not be resolved in ensembles of the same materials.;In short, the main results comprise (1) a tool to track intramolecular energy transfer in single polymer chains; (2) aspects for a better understanding of intrachain energy transfer; (3) the confirmation of an oligomer-like chromophore picture in single polymer absorption which, however, demands some alterations; and (4) an all-optical classification to identify and structurally characterize subgroups of nanocrystals with favorable light-harvesting properties.;Specifically, photoluminescence excitation (PLE) spectroscopy under intraparticle energy transfer conditions is performed on semiconductor nanotetrapods and dye-endcapped polymer chains, both model light-harvesting systems. This way, the structural origin of hampered energy funneling to the tetrapod core as well as the distribution and interaction of polymer chain subunits (chromophores) is resolved, respectively. For the first time, the energetic distribution and vibronic coupling of absorbing chromophores is measured on single polymer chains. While the conventional chromophore picture can be confirmed by several excitation properties, unexpectedly broad single chain absorption, remote interchromophoric coupling, and a correlation between chromophore length and exciton funneling efficiency offer valuable experimental results leading to a better understanding of intrachain excitation energy transfer. For the study of this process as well as exciton self-trapping in single polymer chains, a new spectroscopic method is established: combined single molecule fluorescence and surface-enhanced Raman scattering (SERS) spectroscopy. A comparison of both simultaneously acquired spectral signatures in the time or frequency domain can track excited state relaxation within a single polymer chain. The work is complemented by a simple screening method to identify substrates which are well suited for SERS.