Significant relaxation of residual negative carrier in polar Alq₃ film directly detected by high-sensitivity photoemission

Tris(8-hydroxyquinoline)aluminum (Alq₃) has been widely applied as a good electron-injecting layer (EIL) in organic light-emitting diodes. Highsensitivity photoemission measurement revealed a clear photoemission by visible light, although its ionization energy is 5.7 eV. This unusual photoemission is ascribed to Alq₃ anions captured by positive polarization charges. The observed electron detachment energy of the anion was about 1 eV larger than the electron affinity reported by inverse photoemission. This difference suggests that the injected electron in the Alq₃ layer is energetically relaxed, leading to the reduction in injection barrier. This nature is one of the reasons why Alq₃ worked well as the EIL.