Surface functionalization with amino based self-assembled monolayers : tailoring electrode/molecule interfaces for organic electronics

摘要

Control of the energetics at interfaces is one of the key steps that improve the efficiency of light-emitting and energy-conversion devices based on organic materials. In this context, the present work is embedded in the field of organic electronics with a special focus on the electrode/semiconductor interface. Efficient electron injection and extraction is still a crucial mechanism that deserves improvement. Electron transport through this interface is dominated by the energetic injection barrier Delta_e. In order to gain control of this barrier, polar molecules have been used in this thesis to tailor the work function of the electrode. Three topics are addressed: (i) Understanding the chemical and electrical interface of N,N-dialkyl dithiocarbamates (CnDTC) on the noble metals Au, Ag and Cu. (ii) Work function tuning using tailor-made dithiocarbamates. (iii) Introduction of first results to generate low work functions of the transparent conductive indium tin oxide (ITO) based on a modified chemical structure of DTCs. To study these systems, a combination of X-ray and ultra-violet photoemission spectroscopy (XPS/UPS) with additional results from density functional theory (DFT) has been employed. Charge transport in electronic devices has been investigated using organic thin film transistors based on the n-type material PTCDI-C13.(i) Prior to this thesis, the class of dithiocarbamates (DTC) have been reported by our research group to produce very low work functions of gold down to 3.2 eV. This result marks the starting point for the present work. The assembly of N,N-dialkyl dithiocarbamates on the three noble metals Au, Ag and Cu has been analyzed with XPS. Therein, densely packed monolayers are identified on Au and Ag, while a disordered layer is observed on Cu. Nonetheless, a common work function of 3.5 eV is revealed for CnDTC with n = 4 on all metals with UPS. Only those molecules with the shortest alkyl chains, i.e. n = {1,2}, were able to achieve 3.2 eV on Au and Ag. The final work function of the modified metals results from a complex interplay of molecular arrangement (surface coverage, orientation), interface dipole formation and the energy level alignment at the interface. The detailed comparison of C2DTC as a model system on the three noble metals generates a deeper insight into the chemical and electronic structure. Based on experimental and theoretical results, a common hybridization of the frontier S 3p orbital with those of the frontier metal d-states is identified to generate a metal/molecule interface state, that remains irrespective of the initial metal work function.(ii) A new set of dithiocarbamates are introduced and the effect of different polar groups attached to the molecular backbone is discussed. The molecular structure of dithiocarbamates has been modified using, among others, aromatic and fluorinated substitutions to induce different dipole moments. These directly affect the resulting work function of a modified electrode and enables the generation of a broad range of available work function values for noble metals using differently substituted dithiocarbamates.(iii) At last, the step towards modification of transparent conductive oxides based on the molecular structure of dithiocarbamates is explored for the commonly used indium tin oxide (ITO) as substrate. For metal oxides, a different linker group is required to bind to this kind of surface. Therefore, amine based carboxylic acids have been investigated as first candidates to lower the work function of ITO. A reduction down to 4.0 eV is demonstrated. This result may serve as guideline to create new surface modifiers that should generate even lower work functions similar to those of dithiocarbamates on noble metals.