Theoretical and experimental study of light-emitting electrochemical cells based on ionic transition-metal complexes. From the molecule to the device

Resumo

In this thesis, the progress in the field of Light-Emitting Electrochemical Cells (LECs) during the last four years is described. LECs are devices that can be applied in lighting systems. The configuration of a LEC is simple. It consists of a thin layer of an ionic electroluminescent material sandwiched between two air-stable electrodes. Upon applying a low bias these devices emit light with a high power efficiency. LECs are cheaper to produce compared to Organic Light-Emitting Diodes (OLEDs). However, their performance level is inferior. Hence, the aim of this thesis is to enhance the state of the art performance of LECs and to increase the fundamental understanding about them. This thesis is divided in eight chapters. In the first chapter, the concept of electroluminescence and a brief description of the history of LECs are introduced. Subsequently, a detailed description of the LECs mechanism and the main characteristics of the ionic transition-metal complexes (iTMCs), iridium(III) complexes, are presented. The second chapter summarises the main concepts which are used for the theoretical and photophysical characterization of the iridium(III) complexes studied in this thesis. Also the parameters for the device characterization are introduced. In the third chapter, the theoretical and photophysical characterizations of two archetype iridium(III) complexes are described. Also, the performances of LECs using these two complexes are discussed. These two complexes are considered as archetype due to their simple chemical structure. This chapter is used as a reference for evaluating the results presented in the following chapters. The fourth chapter concerns the color emission of LECs and presents different ways to obtain blue and deep-red LECs, which were scarce prior to this thesis. The fifth chapter is dedicated to the efficiency of LECs. The strategy of introducing bulky groups is adopted. Additionally, a new plausible way for improving the efficiency is also presented. The sixth chapter refers to the stability of LECs, which prior to this thesis was around a few hours. It has been improved by the use of iridium(III) complexes with a supramolecular cage conformation together with steric impediments that minimize the degradation of this type of complexes. Exploiting these approaches LECs with stabilities of thousands of hours are presented in this thesis. The seventh chapter treats the turn-on time of LECs, which is in general too long. We focus especially on the fact that up to now each attempt to reduce it negatively affects the device stability. In the end, the eighth chapter reflects all the obtained advances during this thesis. Additionally, the advances made by other groups are presented.