Photoemission study of the sodium stearate/tris-(8-hydroxyquinoline) aluminum interface

Behaviors of amphiphilic sodium stearate (NaSt) molecules as a buffer material in tris-(8-hydroxyquinoline) aluminum (Alq)-based organic light-emitting diodes (OLEDs) have been studied by photoemission techniques. A 1.0 eV lowering of the vacuum level is measured when the Alq surface is covered with a 1.5 nm-thick NaSt layer, indicating that the NaSt molecule sticks on the surface with the Na end of its hydrophilic head pointing to the surface. Moderate anneal of the NaSt-covered Alq sample results in a more surface concentrated distribution of Na than N in the substrate, showing that the thermal energy the NaSt molecules gain may strengthen their amphiphilic arrangement, which is beneficial to efficient and stable operation of the OLEDs against degradation