Fabrication, Characterization and Modelling of Organic Electroluminescent Diodes with Blue Emission. Application to the Design and Manufacturing of Organic Displays

Abstract

In the present work, electroluminescent organic diodes based on a new blue polyfluorene derivative have been fabricated, characterized and modelled. Passive matrix diodes have been fabricated adapting standard techniques such as photolithography, spin-coating and metal evaporation. Devices were fabricated for different active layer thicknesses and using different commercial organic materials for comparison. Electrical characterization was carried out in terms of current density ¿ voltage and impedance spectroscopy. A simple but efficient encapsulation method serves to significantly improve the better electrical response and achieve longer lifetimes. Optical characterization was carried out in terms of photoluminescence, electroluminescence spectra, luminance and CIE coordinates. Moreover, these measurements were taken during prolonged operation times in order to identify the origin of different degradation mechanisms. Furthermore, a new and original theoretical model has been developed for conduction in single carrier organic diodes with non-ohmic contacts, taking into account both, charge injection and bulk transport. The main contribution relies on the inclusion of a new term considering the electric field at the interface that accounts for the non-negligible injection barrier. This model has been validated by fitting it to experimental curves with different active layer thicknesses. Information of the active layer material and the device structure such as injection barrier height, mobility and other microscopic parameters have been extracted from the fits. Besides, a small signal model describing the frequency dependence of the admittance in organic diodes with non-ohmic contacts has been reformulated. Finally, a new technique to pattern the cathode of an organic display based on mechanical ablation has been developed. The experimental set up is based on a computer-assisted commercial equipment that includes a micropositioner and a spring probe. The system is customized to define the display cathode columns by removing the unwanted metal following a previously designed pattern. This technique was successfully implemented to fabricate a 7x5 pixel display prototype.