Nano-Photovoltaics

Environmental concerns as well as economic and political considerations will shift the world`s energy consumption away from fossil fuels towards renewable energy sources. Solar energy has attracted tremendous attention in the recent years as it is one of the most promising approaches to solving our current energy problems. To overcome economic constraints that currently limit a broad acceptance of solar energy utilization, technological advancement - which relies upon basic investigations and the acquisition of profound mechanistic understanding about the fundamental processes occurring in photovoltaic systems - is indispensable. We plan investigations of nanometer-scale photovoltaic units, which can be analyzed and understood at the atomic level, and thus offer a tantalizing opportunity to probe and control fundamental photovoltaic processes of optical absorption, charge separation, and charge transport. For this purpose, we will use a powerful and new nanofabrication and nanocharacterization tool, consisting of a combined scanning tunneling microscopy (STM) / atomic force microscopy (AFM) stage with built-in optics at the junction region, and a high resolution scanning electron microscope which will additionally be used for Electron Beam Induced Deposition (EBID) to electrically contact deposited nanostructures. The present project constitutes a step towards a deeper understanding of photo-induced processes on the nanometer-scale and aims at the application of fundamental research to technologically relevant problems.

Molecular materials have attracted fundamental and technological interest for their application in photovoltaics, as well as in electronics, spintronics, and photonics. One of the distinctive features of molecular materials is the facile tunability of their electronic, magnetic, and optical properties over a wide range. Technological capitalization of the complex and dynamic interplay between chemical structure and physical and chemical function of molecular materials requires understanding and control at the atomic level. In our studies we have investigated at the atomic scale how molecular photovoltaic systems can be synthesized atom by atom and bond by bond via surface-supported reactions, how the chemical structure of such systems affects their electronic properties, and how gateable graphene substrates can be used to control molecular behavior.Using atomic force microscopy we were able to follow the bond rearrangements associated with cyclization and coupling reactions of organic molecules on metal surfaces. We imaged the chemical wireframe structure of reactants, intermediates, and products of different competitive reaction pathways. Based on these measurements we could directly derive the reaction mechanisms and - supported by theoretical calculations - we revealed how the energy propagation at the atomic scale drives the global reaction kinetics.Such reactions were used to synthesize surface-supported polymeric molecular chains, the chemical structure of which determines their electronic properties. Using scanning tunneling microscopy, we mapped a one-dimensional conducting channel along the backbone of these chains. Such states are important for charge transport in photovoltaic devices and will critically affect device performance.Moreover, we have constructed a prototypical hybrid molecule/graphene device. We have shown how the molecular levels can be shifted by the change of the gate voltage of this device. This allows to dynamically change the molecular behavior and thus control a variety of physical and chemical properties, such as the optical absorption, magnetism, molecular movement, or catalytic activity.