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.
exception. Furthermore, this polymer has a remarkable electron mobility of up to µ e = 0.85 cm 2 Vs −1 , [8] which is correlated with a high degree of molecular order on the micro-and nanoscale. [9,10] The degree of (de)localization of excitons in organic semiconductors is of outstanding importance, as it directly relates to efficiency. Depending on the desired application, either localized or delocalized excitons are sought-after. In a simple picture, delocalization is directly connected to planarity and conjugation, as the orbital overlap is maximized for parallel orientation of adjacent p z orbitals. [11] However, deviations in both directions are known, such as strong conjugation despite large torsional angles [12] as well as exciton delocalization confined to a rather small part of the available π system. [13] Eventually, (de)localization highly depends on structure and ordering primarily on the mole cular scale, as organic semiconductors show normally rather restricted large-scale ordering as compared to their inorganic and often highly crystalline counterparts. However, planarity within a molecular backbone resulting in high molecular order does not necessarily lead to higher carrier mobility. [14] Therefore a detailed understanding of the electronic structure of polymers and their building blocks is essential to develop efficient materials for organic electronics.To gain insight into the electronic structure of polymers, starting with their building blocks has proven to be valuable. [15] To the best of our knowledge, no systematic spectroscopic study of PNDIT2 starting from its building blocks has been performed yet. Here, we investigate four different building blocks of PNDIT2 and the polymer itself with two different chain lengths (see Figure 1 for chemical structures), using timeresolved electron paramagnetic resonance (TREPR) spectroscopy of their triplet excitons in combination with steady-state absorption spectroscopy and quantum-chemical calculations density-functional theory (DFT).Electron paramagnetic resonance (EPR) spectroscopy is intrinsically sensitive to the local environment of the electron spin, rendering it particularly well-suited to probe both electronic structure and local ordering on a molecular level. The key is its unrivalled sensitivity and selectivity for paramagnetic states not only allowing for unambiguous assignment of the spin multiplicity, but outperforming optical spectroscopy by far in terms of resolution. While conventional EPR spectroscopy can characterize structural features in the range Exciton delocalization in organic semiconductors, due to its direct relation to device efficiency, is of outstanding importance. Time-resolved electron paramagnetic resonance spectroscopy of light-induced triplet excitons gives access to the delocalization length in a unique way, connecting it to both, electronic structure and overall conformational flexibility. Systematically investigating building blocks of increasing length and comparing the results with the polymer deepens the unders...
Insight into the electronic structure of conjugated polymers used for organic electronics applications is of outstanding importance. Time-resolved electron paramagnetic resonance spectroscopy of light-induced triplet excitons provides access to the electronic structure with molecular resolution. Systematically investigating building blocks of increasing length and comparing the results with the polymer deepens our understanding of the structure–function relationship in organic semiconductors. Applying this approach to the copolymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) known for its efficiency and device stability reveals the electronic structure of the polymer as well as each of the smaller building blocks to be dominated entirely by the TBT moiety. Hence, the usual description of PCDTBT as a carbazole derivative is somewhat misleading. Furthermore, delocalization extends along the backbone, over at least two repeat units, and is consistent for singlet and triplet excitons, quite in contrast to other push–pull systems previously investigated. DFT calculations of the spin density distribution agree well with the experimental results and show the BP86 functional to be superior to B3LYP in the given context. The polymer and all its building blocks show a remarkable homogeneity that by ruling out aggregation phenomena is ascribed to a rather rigid and planar backbone geometry.
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 S→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.
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.
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