Control of the Molecular Orientation in EML Using Substrate Temperature for Improving the Electro-Optical Characteristics and Lifetime

In this paper, we controlled the molecular alignment property ofthe EML layer in horizontal direction by changing the substrate temperature (T_sub) during deposition at -4, 20, and 40 °C and observed the effect of molecular orientation characteristics of the EML on the electro-optical characteristics and the lifetime of OLED devices. The color property of the devices fabricated indifferent T_sub show same properties, but the optical performance of the cells fabricated at -4 °C was better than those fabricated at high temperatures as we expected due to the horizontal component of the molecular orientation. In particular, as for the life time of the cell, we observed over 4 times longer lifetime of the cell fabricated at -4 °C than the cell fabricated at 40 °C. To analyze the cause of the increase in lifetime, we also observed the surface roughness and grain boundary of the EML fabricated in each T_sub conditions. In addition, more detail data including binding energies between the molecule and deformation data using major molecule weight are also presented either. As a result, we confirm that the T_sub during deposition strongly affects to the molecular anisotropy factor and we can improve the electro-optical and lifetime performance of the OLED cell through low T_sub.