Keeping it simple: Fabrication of three-layered, purely quantum dot-based light-emitting electrochemical cells

Abstract

Light-emitting electrochemical cells (LECs) form a cheap and easily produced alternative to (organic) light-emitting diodes ((O)LEDs) due to their simple device structure and the ability to form an in-situ pi- n junction. Usage of quantum dots (QDs) as a luminophore in LECs is expected to result in devices with improved emission properties and stability compared to polymer-based LECs. LECs that use QDs as their only luminophore have, however, not been operated successfully without the presence of additional charge carrier injection layers and/or the inclusion of polyvinylcarbazole (PVK) as host (and light-emitting) polymer, yet. In this thesis, I show that three-layered, purely QD-based LECs can be fabricated and operated fruitfully. A crucial step to achieving this was carrying out a ligand exchange (LE) on the QDs, replacing the long aliphatic ligands that passivate the QD surface with short BF4- ions and thereby improving the conductivity of the QD film. LECs were then produced using the ligand exchanged QDs, both with and without additional charge carrier injection layers. The three resulting devices were confirmed to operate as LECs and show light emission at positive bias. The current density and electroluminescence (EL) intensity increase as the applied bias is increased for all three the LECs. The three types of LECs were compared on their electrical response and emission. Turn-on voltages and stability windows were also determined for the three types of devices. Improvement of LEC performance is suggested to be achieved by enhancing the photoluminescent quantum yield (PLQY) of the QDs, excluding side reactions that might take place in the LEC under operation or optimizing the current device structure.