Work function and energy level alignment tuning at Ti 3 C 2 Tx MXene surfaces and interfaces using (metal-)organic donor/acceptor molecules Thorsten Schultz 1 , Peer Bärmann 1 , Elena Longhi 2 , Rahul Meena 3 , Yves Geerts 3,4 , Yury Gogotsi 5,6 , Stephen Barlow 2,7 , Seth R. Marder 2,7,8 , Tristan Petit 1 and Norbert Koch 1,9 1 Helmholtz-Zentrum Berlin, Germany 2 School of Chemistry and Biochemistry, Georgia, USA 3 Université Libre de Bruxelles, Belgium 4 International Solvay Institutes for Physics and Chemistry, Brussels, Belgium 5 A.J.Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania, USA 6 Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, USA 7 Renewable and Sustainable Energy Institute, University of Colorado Boulder, Colorado, USA 8 Department of Chemical and Biological Engineering and Department of Chemistry, Two-dimensional MXenes, withTi3C2Txbeing the most prominent member, show properties that make them promising for a manifold of applications, including electrodes in light-emitting diodes, solar cells, and field-effect transistors based on organic semiconductors. In these cases, the work function of MXenes plays an important role in the energy level alignment to the subsequently deposited organic layer, as it determines the electron and hole injection barriers. Therefore, methods for controlling theTi3C2Txwork function should be developed. We demonstrate that, by using thin layers of (metal-)organic donor/acceptor molecules, the work function ofTi3C2Txcan be tuned over a range of>3eV. This enables tuning the energy level alignment to a subsequently deposited organic semiconductor, all the way from intrinsic Fermi level pinning at the highest occupied molecular energy level (minimal hole injection barrier) to pinning at the lowest unoccupied level (minimal electron injection barrier). Furthermore, it is shown that a predominantly oxygen-terminated surface does not lead to an extraordinary high work function, in contrast to theoretical predictions. The proposed strategy may greatly expand the use of MXenes in conjunction with organic hole and electron transport layers in optoelectronic devices. University of Colorado Boulder, Colorado, USA 9 Humboldt-Universität zu Berlin, Berlin, Germany
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