School of Chemistry - Theses
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ItemIncorporation of Quantum Dots into Optoelectronic Devices: Ligands as Charge Brigands( 2019)With their tunable and almost monochromatic emission over the whole visible spectrum, colloidal II-VI semiconductor quantum dots (QDs) are attracting significant interest as novel electroluminescent materials in light-emitting diodes (LEDs). Being processable in solution allows the deposition of large-scale films at low cost and brings QDs in a competitive position to their organic counterparts. Organic ligands capping QDs are used to maintain colloidal stability during synthesis and provide passivation in solution. While ligands remain invisible under optical characterisation, this thesis provides insights into the mechanisms by which ligands impact charge carrier dynamics within a light-emitting diode. In doing so, robust and bright QDs emitting at 410 nm are synthesized, passivated with ligands of various lengths and electrical conductivity and incorporated into an LED architecture. Based on Impedance Spectroscopy measurements ligands have been identified as a major obstacle for charges transiting to the QD to produce light: ligands act as brigands in trapping charges and prevent efficient charge recombination. A novel capacitance behaviour is described and attributed to the accumulation of charge carriers within the ligands during operation. Together with an inductive response in the impedance plane changes in capacitance can be used as a diagnostic tool to determine the recombination efficiency in a quantum dot light-emitting diode (QLED). By driving a QLED with a rectangular pulse accumulated charges lead to a delayed luminescence peak when the bias is turned off and can thereby be visualised. When a 5 nm thick aluminium layer is added into the hole transport layers, trapped charges can open a memory window and add a new device functionality to a QLED. This thesis concludes with ideas to overcome the ligand-dependent charge accumulation and suggests a novel type of QDs for a more efficient device performance.