Session:

Easy-generation of stable charge carriers in organic semiconductors is a key challenge towards modern high-performance devices for electronics, photovoltaics, spintronics, sensing, and catalysis applications. In recent years, covalently tethering an appropriate dopant to a semiconductor of interest to achieve self-doping by charge transfer upon simple photoexcitation has emerged as a promising attractive strategy. In the case of self-n-doping, perylene bisimide (PBI) derivatives have garnered the most attention due to their high electron affinity and strong absorption in the visible range, as well as their good chemical robustness and excellent photostability. Several studies described the photogeneration of anions in amine or ammonium-functionalized PBIs. However, in these few examples, only charge lifetimes of tens of minutes, at best, were reported. Additionally, the role of the chromophores arrangement in this photo-n-doping process remains unclear, and a more thorough understanding is required to design efficient self-n-doped materials. Our laboratory has previously revealed the formation of unusually long-lived positive photo-charges in self-assembled nanowires from electron-rich semiconductor molecules substituted with chiral oligopeptide-polymer side chains. The doping was attributed to a photo-induced electron transfer to the lateral amide bond acting as an acceptor and it was demonstrated that the formation of well-defined helical, one-dimensional aggregates is one decisive prerequisite for its occurrence. Recently, we discovered that the same molecular architecture and self-assembly strategy can also lead to self-n-doping of supramolecular nanowires based on well-chosen electron-poor chromophores. Dicyanoperylene bisimide (CN2-PBI) along with tetrachloroperylene bisimide (Cl4-PBI) were substituted with oligopeptide-polymer side chains and their supramolecular aggregates were investigated by a combination of spectroscopies as well as atomic force microscopy and X-ray scattering. While Cl4-PBI aggregates show no photo-doping, illumination of the CN2-PBI nanowires leads to the formation of air-stable radical anions with extraordinarily long lifetimes on the order of hours that can accumulate to concentrations of up to 7 mol%. In addition, devices prepared from these aggregates exhibit macroscopic transport properties typical for a strongly doped electron conductor with a highly persistent photocurrent. The comparison of these two systems reveals that the formation of mixed Frenkel/Charge-Transfer excitons in the aggregates plays an essential role in the efficiency of the photo-n-doping. Moreover, this conclusion was further confirmed to be a universal condition by the study of two additional self-p-doping systems. These results prove that the macroscopic electronic properties are not only inherent to the employed molecular structure, there are an “emerging property” that is strongly affected by the intermolecular chromophore arrangement which thus needs to be carefully designed as well in self-doped organic electronic materials.

Co-Author(s): Holger Frauenrath (École Polytechnique Fédérale de Lausanne (EPFL) ))

Lucile Chassat

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