Solvent-mediated aggregate formation of PNDIT2: decreasing the available conformational subspace by introducing locally highly ordered domains.

The high-mobility n-type donor/acceptor polymer PNDIT2 is well-known to form aggregates in solution depending on the solvent used. To gain additional insight into this process, we probed the local environment of triplet excitons in two different solvents and with two different polymer chain lengths using time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Results clearly show aggregation to introduce a high degree of local order in the polymer and to dramatically enhance the delocalisation of the exciton. Furthermore, triplet exciton delocalisation is only affected by the solvent used and hence by aggregate formation, not by chain length. Finally, aggregation changes the mode of delocalisation from intrachain to interchain when forming aggregates, the latter mode dominating as well in thin films. Taken together, TREPR proves to be a valuable tool for investigating aggregation and order in polymers on a molecular length-scale, ideally complementing preceding optical data.

Ordering of PCDTBT revealed by time-resolved electron paramagnetic resonance spectroscopy of its triplet excitons.

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy is shown to be a powerful tool to characterize triplet excitons of conjugated polymers. The resulting spectra are highly sensitive to the orientation of the molecule. In thin films cast on PET film, the molecules' orientation with respect to the surface plane can be determined, providing access to sample morphology on a microscopic scale. Surprisingly, the conjugated polymer investigated here, a promising material for organic photovoltaics, exhibits ordering even in bulk samples. Orientation effects may significantly influence the efficiency of solar cells, thus rendering proper control of sample morphology highly important.

Probing Exciton Delocalization in Organic Semiconductors: Insight from Time-Resolved Electron Paramagnetic Resonance and Magnetophotoselection Experiments.

Delocalization of excited states of organic semiconductors is directly related to their efficiency in devices. Time-resolved electron paramagnetic resonance spectroscopy provides unique capabilities in this respect because of its high spectral resolution and capability to probe the geometry and extent of excitons. Using magnetophotoselection experiments, the mode of exciton delocalization, along the backbone or parallel to the π-π stacking direction of the conjugated polymers, can be revealed. We demonstrate the robustness of this approach by applying it to building blocks of a prototypical conjugated polymer showing a symmetry of their excited states being far from ideal for this experiment. This renders magnetophotoselection superior to other approaches because it is applicable to a wealth of other organic semiconductors. The insight gained into exciton delocalization is crucial to the structure-function relationship of organic semiconductors and directly relevant for developing highly efficient materials.

Direct S0→T Excitation of a Conjugated Polymer Repeat Unit: Unusual Spin-Forbidden Transitions Probed by Time-Resolved Electron Paramagnetic Resonance Spectroscopy.

A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. (Time-resolved) electron paramagnetic resonance (EPR) is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior resolution compared to optical methods. We present here evidence for a direct S0→T optical excitation of distinct triplet states in the repeat unit of a conjugated polymer used in organic photovoltaics. These states differ in their electronic structure from those populated via intersystem crossing from excited singlet states. This is an additional and so far unconsidered route to triplet states with potentially high impact on efficiency of organic electronic devices.