Charge-carrier Mobility in Self-assembled Monolayers
Subject Area Physical Chemistry of Solids and Surfaces, Material Characterization
The mechanism of charge transport in organic semiconductors (OSCs) is a topic of utmost importance for organic electronics. Although it is well-established that in the best-performing organic semiconductors the mechanism must be clearly different from the band transport in inorganic semiconductors like Si, Ge or GaAs, a better understanding of the mechanism is just about to emerge. Elucidating the underlying chemical and physical mechanisms and deriving strategies for the synthesis of novel molecules with improved charge carriers mobilities are, accordingly, a topic of enormous importance. Unfortunately the reliable measurement of intrinsic charge carrier mobilities in organic molecules is still a rather challenging topic as OSC-specimen are required, which are truly single crystalline. Grain boundaries within the specimen will immediately limit the mobilities and thus make a determination of true intrinsic values virtually impossible.
The goal of the project is to fabricate self-assembled monolayers using appropriately functionalized organic molecules based on tetraazapentacene derivatives , which are equipped with anchor groups for the immobilization on gold coated substrates. The synthesis of these molecules is carried out by the group of Prof. Uwe Bunz (Institute of Organic Chemistry, University of Heidelberg). After receiving the thiol- or thioacetate-functionalized tetraazapentacenes from the synthetic group, OSC self-assembled monolayers (SAMs) are prepared in the group of Prof. Christof Wöll (Institute of Functional Interfaces, Karlsruhe Institute of Technology). The self-assembled monolayers are characterized using different surface science methods, including infrared and photoelectron spectroscopy as well as near-edge x-ray absorption spectroscopy, a synchrotron-based method. In the next step an atomic force microscope (AFM, Dimension Icon from BRUKER, funded by the SFB 1249) which is mounted in the Wöll group at the IFG, is used to lithographically write insulating SAM patches in a self-assembled monolayer fabricated from the OSC-based molecules (Figure 1a). In this way two-dimensional conductive SAM patterns can be prepared, where the charge carrier mobilities are determined by recording the vertical conductivity through the layers as a function of size and shape of the conductive SAM patterns (Figure 1b) . Electron and hole mobilities can be determined separately by changing the polarity of the bias voltage. Using this approach it will be possible to systematically investigate a larger class of organic compounds and to determine factors which govern the mobilities of organic semi-conductors such as the chemical side groups of the OSC molecules or the pattern dimensions of the conductive SAM islands.
Figure 1: (a) Schematic principle of a tertaazapentacene derivative-based OSC island prepared by AFM supported lithography. (b) Resistor-model, which is used as base for the determination of charge carrier mobilities. (c) AFM for conductivity experiments financed by SFB 1249.
 Miao, S.; Appleton, A. L.; Berger, N.; Barlow, S; Marder, S. R.; Hardcastle, K. I.; Bunz, U. H. F.; Chem. Eur. J. 2009, 15, 4990-4993.
 Bashir, A.; Heck, A.; Narita, Feng, X.L.; Nefedov, A.; Rohwerder, M.; Mullen, K; Elstner, M.; Wöll, C.; Phys. Chem. Chem. Phys. 2015, 17, 21988-21996.