The mechanism of work function tuning with covalently bound self-assembled monolayers : a study on electrode functionalization for the optimization of organic electronics

The performance of organic electronics depends critically on the interplay of the device interfaces. Organic devices are usually based on a layer stack of electrodes and organic semiconductors. The focus of the present work is the electrode-organic interface. By the energetic control of this interface the efficiency of organic devices can be significantly enhanced. Indeed, a decisive factor is the alignment of the electrode work function with the conduction orbital of the adjacent organic semiconductor. The energy-level alignment can be achieved by various approaches. This work focuses on the insertion of a monomolecular interlayer, a so-called self-assembled monolayer (SAM). Such a layer covalently binds to the electrode surface and has the ability to modify the electrode work function while retaining the bulk properties of the material. To get a deeper understanding of this interface control, the mechanism of work function tuning by SAMs is regarded in detail. To this end, experiments are put forward to promote insights ranging from the SAM formation up to the theoretical description of SAM induced work function shifts. First, the formation of a SAM is studied experimentally. By means of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Fourier transformed infrared (FTIR) spectroscopy the surface coverage, the covalent bond as well as the orientation of the molecules in a SAM are investigated. This study is done for the carboxylic acid 4-dimethylaminobenzoic acid (4DMABA) on indium-tin-oxide (ITO) electrodes. Following the successful SAM analysis, the work function change upon modification is regarded. As a starting point, the work function tuning of ITO electrodes is studied. This is done employing carboxylic acids as SAM molecules and is investigated by XPS. Systematic variations are applied to the molecules, which result in drastic work function reductions for the different molecules. For a theoretical understanding an established model is applied to the data obtained. This model relates the work function change to the molecular dipole moment. In the course of this work, another model is developed for an additional description of the mechanism of work function tuning. This model relies on the energetic positions of the frontier molecular orbitals of the SAM molecule. Employing this model the prediction of the resulting work function by insertion of a carboxylic acid SAM at an ITO-organic interface becomes possible. Subsequently, the efficiency improvements in a real device are analyzed. Hence, the modifications are applied to organic thin film transistors with ITO source and drain contacts and PTCDI-C13 as the active organic material. Since the contact resistance directly depends on the energetic barrier at this interface, it is a good measure for the energy-level alignment at the electrode-organic interface in the device. Therefore, this quantity is analyzed in detail and shows a huge improvement upon application of the energetically matching modification. To draw a full picture of the mechanism of work function tuning, the models applied are transferred to further SAM systems. This is done for phosphonic acid SAMs on ITO. The data are described on the same level as the carboxylic acid ones. Furthermore, a small set of molecules is applied to further transparent conducting oxides (TCOs) to generalize the mechanism to the whole class of TCOs. Finally, results for the modification of noble metals, silicon and TCOs are merged and show the overall possibility to specifically tune electrode work functions for enhanced device performance. At the end, a modular construction kit is introduced to design a SAM for specific applications and thus tailor the interfaces of a device towards highest efficiency