Mechanisms of injection enhancement in organic light-emitting diodes through insulating buffer

Three types of organic light-emitting diodes are fabricated. Tris-8-hydroxyquinoline aluminum $\left({\mathrm{Alq}}_3\right)$ is used as an electron-transporting layer $\left(\mathrm{ETL}\right)$ and sodium stearate $\left(\mathrm{NaSt}\right)$ as an electron-injecting buffer. The optimal thickness of $\mathrm{NaSt}$ for electron injection is different for cathodes of different metals, such as $\mathrm{Mg}$ , $\mathrm{Al}$ , and $\mathrm{Ag}$ . This is attributed to the different work functions of cathodes, which result in different initial barrier heights for electron injection from cathodes into $\mathrm{ETL}$ , and explained based on tunneling theory.