Interface dipole formation in organic surface functionalizations : tailoring metal contacts for organic optoelectronic devices

摘要

This study shows a clear pathway for a successful engineering of metal-organic contacts in organic electronics. Therefore thiolate-based self-assembled monolayers (SAM) were investigated with a dedicated set of experimental methods (Photoemission Spectroscopy and Fourier Transform Infrared Spectroscopy). The analysis of the sample structures requires the decisive infrastructure of an ultra-high vacuum system. Besides the work at the facilities at the National Institute of Standards and Technology (NIST) in Gaitherburg, MD, USA, a new setup was installed at the I. Institute of Physics (IA) at the RWTH Aachen University in Aachen, Germany, during the course of this work. The interpretation of the findings was derived by density functional theory calculations. I particular the detailed modeling of the interfacial region leads to a comparative experimental and theoretical study. Following the previously formulated task of molecular engineering the coupling between gold surfaces and thiolate as well as dithioate linker groups is described in the first part of the results. More precisely, the SAM formation of benzylmercaptan (BM) and dithiobenzoic acid (DTBA) is compared. Both molecules share a common phenyl backbone but differ in the linker end group. The single sulfur atom in BM is substituted by a double sulfur (dithio) moiety in DTBA which is expected to form a resonant bond structure with the substrate beneath. At first, the electronic structure of the SAMs is characterized. Especially the electronic alignment process of the frontier molecular orbitals and the work function are discussed. In the second step the two different molecules are employed as electrode functionalizations of pentacene based field effect transistors. The performance of the transistors as well as their suitability for ambipolar transport is compared to values found in unmodified transistors. Thereby the considerable differences found for the electronic alignment within the two SAM systems are summarized in the concluding discussion. Eventually the question is answered if the carbodithioate linker group can advance to be a new benchmark coupling group to enhance ambipolar transport in organic electronic devices. In the second part of the results the findings on the dithioate linker group are picked up. Dithiocarbamates (DTC) depict a novel class of molecules in the context of metal surface functionalization. In an approach to combine the resonant linker qualities of the carbodithioate, a high polarity of a C-N bond as well as the robustness of chelating agents, the SAM formation of these compounds on noble metal surfaces is pursued in the first step. Second, the characteristic energy level alignment in the DTC SAM on gold is determined. Thereby a record value of 3.2 eV for the resulting work function is discovered and explained by a surface potential analysis at the level of DFT theory. Third, with the prospect of significantly increasing the electron injection into n-type organic semiconductors the impact on PTCDI-C13 thin film transistors is studied. The performance of these OTFTs is directly compared for DTC covered and uncoated electrodes. In conclusion, DTC monolayers are identified as efficient coupling layers at metal and n-type organic semiconductor interfaces.