The organic-metal interface : design of an inverse photoemission spectrometer for the investigation of the energy level alignment and the electronic structure

  • Die Organik-Metall Grenzfläche - Design eines inversen Photoemissionsspektrometers zur Untersuchung des Energieniveauausgleichs und der elektronischen Struktur

Jacobi, Carolin Claudia; Wuttig, Matthias (Thesis advisor); Mayer, Joachim (Thesis advisor)

Aachen (2017, 2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

The development of organic-based devices has received considerable interest over the past few years and has led to commercial products that are, nowadays, indispensable in daily live. In the course of device design, organic-metal as well as organic-organic interfaces play crucial roles for charge injection and transport in these devices. In order to further improve the device performance, their electronic structure, chemical properties, and electrical behaviour must be fully characterised and understood. The present work is embedded in this context with a special focus on the organic-metal interface. Four distinct topics are addressed:(I) First an inverse photoemission spectrometer was constructed and successfully put into operation in the course of the thesis presented. Inverse photoemission spectroscopy (IPES) enables investigation of the unoccupied part of the density of states. Thus, combining it with ultra-violet photoelectron spectroscopy (UPS) the complete density of states can be probed. During preparation of the thesis presented, both methods have been applied in order to study the frontier electronic states of noble metal and organic surfaces.(II) A discussion on the electronic structure of the metallic substrates builds the groundwork in order to understand the effects occurring at the organic-metal interface. Since the electronic states of noble metal surfaces are well-known, the presented data serve, furthermore, as prove that the IPE spectrometer is properly working and obtains reliable results.(III) Before the focus is put on the organic-metal interface, also the electronic structures of bulk-like thin films are discussed. In this context, a combined UPS/IPES study was applied to the molecule of interest namely N,N’-dimethyl-3,4,9,10-perylene dicarboximide (DiMe-PTCDI or PTCDI-C1). Its electronic structure is compared to the measured electronic states of its core molecule perylene and to two perylene derivatives - 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) and N,N’-dioctyl-3,4,9,10-perylene dicarboximide(PTCDI-C8). The obtained results reveal that due to the high-lying LUMO of perylene with respect to Fermi level EF , there is a large injection barrier for electrons from metal electrodes. As a consequence, perylene shows mainly p-type electrical transport. Furthermore, it is presented that substitution of perylene with electron-withdrawing anhydride or imide groups significantly lowers the molecular orbitals with respect to the vacuum level. Here, the impact on the electron affinity is much stronger than on the ionisation energy. Thus, PTCDA as well as DiMe-PTCDI and PTCDI-C8 possess a low-lying LUMO with respect to EF resulting in an n-type behaviour of these molecules. The replacement of the oxygen atom in the anhydride group of PTCDA by the less polar methylimide group in the case of DiMe-PTCDI leads to a small increase in energy of the molecular orbitals with respect to the vacuum level. This results in a slightly smaller electron affinity as well as in a decreased ionisation energy in the case of DiMe-PTCDI. The lengthening of the alkyl chain, when going from DiMe-PTCDI (PTCDI-C1) to PTCDI-C8, has only a minor impact on the electronic structure.(IV) Last but not least, the effects occurring at the interface between DiMe-PTCDI and the noble metals silver and copper is studied regarding its chemical and electronic interaction. Therefore, additionally to UPS and IPES measurements also X-ray photoelectron spectroscopy (XPS) is applied. Furthermore, the growth behaviour of the DiMe-PTCDI molecules on the respective substrate is investigated with atomic force microscopy (AFM). On silver, DiMe-PTCDI shows Volmer-Weber growth. Furthermore, a strong chemisorption between the DiMe-PTCDI molecules and the underlying silver substrate is observed. This strong chemical interaction leads to the formation of a covalent bond between the molecule and its substrate. The formation of the covalent bond, in turn, results in a filling of the LUMO of the isolated molecule and thus becomes the HOMO of the thin film. For DiMe-PTCDI grown on copper, Volmer-Weber growth and LUMO filling is also observed. However, XPS measurements reveal a weaker chemical interaction in comparison to DiMe-PTCDI grown on silver. While in the latter case the whole functional group - the oxygen and nitrogen atoms - contributes to the interaction with the substrate, for DiMe-PTCDI grown on copper a chemical interaction with the underlying substrate is only observed for the nitrogen atoms.

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