Towards (Supra) Molecular Organic Electronic Devices: the Key Role of Scanning Probe Microscopies


Well-defined conjugated materials play an important role in the growing field of organic electronics because their precise chemical structure and conjugation length give rise to well-defined properties and facilitate control over their supramolecular organization. The building of nanoscopic and mesoscopic architectures represent a starting point for the construction of (supra)molecular electronicsor even circuits, through surface patterning with nanometer-sized objects. It clearly appears that the solid-state properties of organic electronic materials are determined not only by those of individual molecules but also by those of ensembles of molecules. The ability to control the supramolecular architectures is thus essential for optimizing the properties of conjugated materials for their use in “supramolecular electronics”; this is primordial for technological applications in nanoelectronics.

In this talk, we report on the observation by Atomic Force Microscopy (AFM) of nanoscale architectures obtained in the solid-state from solutions of molecularly-dissolved conjugated materials (oligomers, polymers, and block copolymers) or from nanostructures already existing in solution, and demonstrate that they can organize onto a surface over lengthscales from nanometers to several microns, forming semiconducting nano-objects by p-stacking processes. During these processes, the interplay between the conjugated molecules, the solvent and the substrate surface is one key-parameter governing the formation of these supramolecular assemblies. Moreover, molecular modelling calculations are essential for a better understanding on how the molecules are organized within these nanostructures and therefore rationalize the experimental data

By using AFM-derived techniques (such as Conducting AFM, Peak Force TUNA, Kelvin Probe Force Microscopy), electrical properties can be also measured at the local scale together with the morphological characterization of the samples. For instance, for photovoltaic organic devices, we spatially resolve by KPFM under ultrahigh vacuum (UHV) the surface photo-voltage in high efficiency nanoscale phase segregated photovoltaic blends of photoactive polymers. The spatial resolution achieved, represents a tenfold improvement over previous KPFM reports on organic solar cells. By combining the damping contrast to the topographic data in non contact AFM under UHV or using Peak Force TUNA in a glovebox, surface morphologies of the interpenetrated networks are clearly revealed. We show how the lateral resolution in KPFM can be significantly enhanced, allowing a direct visualization of the carrier generation at the donor-acceptor interfaces and their transport through the percolation pathways in the nanometer range. Some other practical examples such as field effect transistors and (white)-light emitting diodes will be presented during the talk.