Device physics of organic light-emitting diodes

This work investigated several aspects of OLED device physics. The mechanisms responsible for the efficiency enhancement typically seen when a dye molecule is doped into the emission layer were examined. By comparing the spectra and efficiencies of single-layer devices for varying dopant concentrations, it was found that both charge transfer and energy transfer from the host molecule to the dye dopant are important processes. The measured efficiencies for photoluminescence and electroluminescence were found to be consistent with a simple model developed to explain the functional dependence on the dopant concentration.; Work was also done on the enhancement of electron injection from an aluminum cathode using a thin layer of LiF. A double-layer device with the blue emitter DPVBi showed a factor of 50 enhancement in quantum efficiency upon insertion of a LiF layer. This technique has important practical application since it allows for the use of an environmentally-stable aluminum cathode while retaining high device efficiency.; The effect of the ionization potential of the hole transport layer on the efficiency of a double-layer device was also investigated. TPD side-group polymers were used as the hole transport layer. The device efficiency was shown to increase as the ionization potential of the hole transport layer was pushed further from the work-function of ITO. This trend was attributed to an improved balance between the injection rates of holes and electrons. A Monte Carlo simulation of a single-layer device was developed which demonstrated the importance of balanced injection to obtain high efficiency.; Drawing upon these results, an optimized OLED was fabricated which exhibited a luminous efficiency of 20 lm/W for green emission. This is one of the highest OLED efficiencies reported as of the date of this writing.