Interplay between microscopic structure and intermolecular charge transfer processes in polymer-fullerene bulk-heterojunctions

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In this follow-up project we plan to perform systematic studies of charge transfer (CT) processes in polymer semiconductors and their blends with electron acceptor moieties. Our investigations are aimed at clarifying the correlation between CT and film morphology on the microscopic level. In the first application period, we developed a unique set of tools to study film structure on the nanometer scale as well as excited states in organic bulk heterojunction solar cells. We implemented and applied an optically detected magnetic resonance setup, and found surprising results related to formation of triplet excitons with different spatial extent in the conjugated polymer rr-P3HT (regio-regular poly-hexylthiophene). Triplet excitons are generally considered as loss factors in organic photovoltaics. We determined spatial extents of these triplet states ranging from few Ångstroms (molecular triplet excitons) to several nanometers. The latter we identify with CT excitons, or polaron pairs, the precursor state of free charges in organic solar cells—and thus crucial for their performance. The different triplet states occur depending on the film morphology, which we studied by scanning force microscopy (SFM) based nanotomography. SFM-nanotomography was for the first time adapted to study conjugated polymers and donor-acceptor blends by development of a suitable plasma-etching recipe. So far we demonstrated 3D volume imaging with 10 nm resolution of a P3HT:PCBM blend. Although the results of the first proposal period are novel and interesting, the microscopic nature of the triplet states remains unresolved.