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Transport is a central phenomenon in various scientific areas. Charge and exciton transport is at the heart of functionality of photovoltaic devices, field effect transistors, and light emitting diodes. In addition, controlling phonon transport is essential to enhance the performances of conversion of heat energy into electric energy, e.g. in thermo-photovoltaics and thermoelectric devices. Fuel cells, electrochemical sensors, electrochemical reactors, and electrochromic devices rely on the efficient proton transport.
Improving the performances of these devices and reducing the costs of materials and processing goes beyond mere technological challenges, as it implies a fundamental development beyond state-of-the-art (mostly inorganic) materials.
For many of these challenges organic (polymeric) materials provide innovative solutions, due to their chemical versatility, low costs, advantageous mechanical properties - such as flexibility - and good processability. For example, conjugated polymers have been receiving growing attention as optoelectronic devices and organic photovoltaics.
Polymeric materials and composites play a massive role in thermal management, and have been recently proposed as potentially interesting thermoelectrics. Exploiting the variety and versatility of organic materials for charge and thermal transport requires a deep fundamental understanding of their properties over several size scales, which range from the detailed electronic structure of small molecules or polymer segments, to the mesoscopic structure of polymer or composite films.
This workshop gathers scientists of different backgrounds and expertise, from quantum chemists to materials scientists and statistical physicists, who can provide an original view over several different aspects of transport phenomena in soft matter systems.
The workshop will be linked to the annual meeting, Mainz Materials Simulations Days (MMSD 2013), which is by now an established series of meetings organized by the Max Planck Institute for Polymer Research and Mainz University.
Motivated by the recent developments in the area of materials for energy production and storage, the field of organic semiconductors has seen a major expansion in the industrial sector as well as in the academic environment.
Conjugated polymers have been studied by chemists and physicists for almost a decade in order to develop various optoelectronic devices, such as light-emitting diodes [1–5], field effect transistors [6–13], optically pumped lasers [14–19], and organic solar cells [20–25]. The success of the field was acknowledged in 2000 when the Nobel Prize in chemistry was given to A. J. Heeger, A. G. MacDiarmid and H. Shirakawa for the discovery and development of conductive polymers . The rapid expansion of this
field has also been backed up by industrial needs: photovoltaic solar energy technology is, after wind power, the second fastest growing in the world.
The main issues hindering the development of organic optoelectronic devices are (i) low efficiency, which is often due to the low charge carrier mobility in the active layer of the device; (ii) stability of organic compounds under ambient conditions. While the second issue can be addressed by synthesis of air-stable compounds (HOMO energy level below the air oxidation threshold, ca -5.27eV) or proper encapsulation, the low charge carrier mobility is intrinsic for all organic materials. Indeed, van der Waals and pi-pi interactions, which are responsible for self-assembly of these materials, operate on the energy scale of the order of kT, and the entropy and thermal fluctuations are essential ingredients for their morphology development. A substantial amount of disorder leads to the localization of electronic wave functions and prohibits band-like charge transport. Charge carriers move via thermally activated hopping, with two to three orders of magnitude lower mobilities than in metallic compounds.
Molecular junctions and ordered self-assembled monolayers provide a complementary approach to organic electronics. In such junctions, the connection between the molecule and the electrodes greatly affects the current-voltage characteristics. Despite several experimental and theoretical advances, including the understanding of simple systems, there is still limited correspondence between experimental and theoretical studies of these systems. Of special interest is the recent discovery thermoelectricity in single
molecule junctions , and in general the conncection between structure, chemistry and local currents [28, 29].
Polymers and composites are massively exploited as insulators, due to the very low thermal conductivity. In contrast, recent works have demonstrated that it is possible to tune the thermal properties of single polymers, as well as of bulk systems over several orders of magnitudes, for example by mechanical strain or by arranging them in nanofibers [30, 31]. Experiments have also demonstrated that it is possible to achieve large contrasts in electrical and thermal conductivities using first-order phase transitions in percolated composite materials . In general, for many of the above-mentioned applications, one would want to tune independentely electrical and thermal conductivity and composite materials would provide an ideal platform in this sense. Yet, applications are still limited by poorly understood interface phenomena . A deeper understanding of electron and phonon transport in these materials would pave the way to a broader technological exploitment of their features.