Work function and energy level alignment tuning at Ti 3 C 2 T x MXene surfaces and interfaces using (metal-)organic donor/acceptor molecules

Two-dimensional MXenes, with ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{\mathrm{T}}_{x}$ being 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 the ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{\mathrm{T}}_{x}$ work function should be developed. We demonstrate that, by using thin layers of (metal-)organic donor/acceptor molecules, the work function of ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{\mathrm{T}}_{x}$ can be tuned over a range of $>3$ eV. 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.