Interface Engineering in Organic Electronics

University dissertation from Linköping : Linköping University Electronic Press

Abstract: Organic electronics is a field covering all applications and devices where one or several of the active components are made of organic material, such as organic light emitting diodes, organic solar cells, organic thin film transistors, organic magnets for spintronics etc. In all of the applications mentioned above, transport of charges across both inorganic/organic and organic/organic interfaces play a key role for device performance. In order to achieve high efficiencies and longer life-times, proper matching of the electronic energy levels of the different materials is needed.The aim of the research presented in this thesis has been to explore different routes to optimize interface energetics and gain deeper knowledge of the mechanisms that govern charge transport over the interface. Photoelectron spectroscopy (PES) is a method well suited to study both interactions between different materials taking place at surfaces as well as interface energetics.One way to achieve proper matching of interfaces energy levels is by adding a dipole layer. In the three first papers presented in the thesis, the method of adding a monolayer of small organic molecules to change the work function of the surface is investigated. We start with a model system consisting of a nickel surface and PPDA molecules where we have strong interaction and mixing of orbitals between the molecule and the metal surface. The second system consists of a gold surface and TDAE molecules with weaker interaction with integer electron transfer and finally in the third paper an organic surface VPP-PEDOT-Tos is modified, with TDAE, to create a transparent low work function organic electrode. In the fourth paper, we focus on gaining deeper understanding of the Integer Charge Transfer (ICT) model and the mechanisms governing the alignment of energy levels at organic/(in)organic interfaces and in the fifth paper we continue to challenge this model by using it to predict the behavior of a bilayer device, in terms of energy level alignment.