The roles of thermally evaporated cesium carbonate to enhance the electron injection in organic light emitting devices

Abstract

The properties of thermally evaporated cesium carbonate (Cs2CO3) and its role as electron injection layers in organic light emitting diodes were investigated. According to the ultraviolet photoemission spectra (UPS), the Fermi level of tris-(8-hydroxyquinoline)-aluminum (Alq3) after being doped with Cs2CO3 shifts toward or into the lowest unoccupied molecular orbital as a result of chemical reaction and charge transfer between Cs2CO3 and Alq3, which lowers the electron injection barrier and improves the current efficiency. As for whether Cs2CO3 being decomposed during the evaporation, we found that Cs2CO3 molecules were deposited on the substrates without decomposition, regardless of the evaporation rates, based on the signature features of carbonate groups and ionization energies measured in UPS spectra and the binding energy shifts of core level electrons. The reaction mechanisms between Cs2CO3 and Alq3 are also proposed. Since Cs2CO3 is not only used in the electron injection layer but also in converting high work function materials to cathodes, we further quantitatively investigated the work function modification of indium tin oxide (ITO) with deposition of Cs2CO3 at the surfaces. We found that while 0.5 Å thick Cs2CO3 is sufficient to reduce the electron injection barrier of Alq3, the thickness needed to convert ITO surface to low work function cathode is about 10 Å.