Tris(8-hydroxyquinoline)aluminum (Alq₃) has been widely applied as a good electron-injecting layer (EIL) in organic light-emitting diodes. High-sensitivity 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.

We propose a method, called hν-dependent high-sensitivity ultraviolet photoemission spectroscopy, to observe the density of states (DOS) in a very wide range from HOMO to extremely weak gap states (10²² to 10¹⁵ cm⁻³ eV⁻¹ in density of states). The method was applied to a p-type semiconducting polymer. A series of spectra for hν = 4.4–7.7 eV were recorded, and the DOS was obtained by overlapping the spectral part with a similar line shape between adjacent photon energy spectra to eliminate the photon energy dependence of the photoionization cross section. This method can be applied to both organic and inorganic materials, providing useful information about the DOS of functional materials.