Application of self assembled monolayer technique to imporove hole transport in organic leds

Abstract

This thesis concentrates on the fabrication and characterization of highly efficient Organic Light-Emitting Diode (OLED) with surface modification of indium tin oxide (ITO) anodes by using self-assembled monolayer (SAM) technique. ITO has beencommonly used as an anode for OLEDs, because of the combination of high optical transparency and good electrical conductivity. That is why, ITO substrate is used in this study. On the other hand, the ITO film has often a rough surface, relatively low work function compared with the adjacent organic layer and its surface energy is not well matched to nonpolar organic film. For this purpose, in this work, eleven different SAM molecules were examined to modify ITO surface. Clearly, we aim to reduce the barrier of hole injection, lower the turn-on voltage and improve charge transfer characteristic at the anode-organic film interface using SAM molecules.The ITO work function with the SAM molecules was measured using Kelvin Probe Force Microscopy (KPFM). KPFM measurements were carried out to investigate if there is any increase in ITO work function. Atomic Force Microscopy (AFM) via semi-contact mode was examined the surface morphology of modified ITO. The transmittances of the ITOs with SAM were measured using an UV spectrometer. In addition to these surface analyses, the device performances were characterized by Keithley 236 and High-Resolution Spectrometer. Moreover, Space-Charge-Limited Current (SCLC) and Schottky models were used to analyze the I-V characteristics to calculate hole mobility and barrier heights, respectively.