Doping Mechanism and Electronic Structure of Alkali Metal Doped Tris(8-hydroxyquinoline) Aluminum

Abstract

We investigated the electronic structures of alkali metal doped Alq(3) molecules prepared by codeposition of Na and tris(8-hydroxyquinolinato)aluminum (Alq(3)) (Na:Alq(3)) using in situ synchrotron radiation photoelectron spectroscopy. The doping ratio of Na to Alq(3) (R-Na) was 0, 0.3, 0.6, 0.8, 2.7, 3.4, or 3.9. The work function, calculated from photoemission spectra, remained at 3.6 eV +/- 0.03 eV for all samples, while the energy of the highest occupied molecular orbital increased from 2.2 to 2.85 eV as the doping ratio increased from R-Na = 0 to R-Na = 2.7. The work function and valence band spectra indicated that there is no band bending or surface work function change due to an alkali doping effect, in contrast to the findings of previous reports. The doped layer was composed of an n-type organometallic complex according to the analysis of O ls and N ls spectra. The N-type doping effects shown in the N ls spectra of coevaporation samples were reflected in the schematic band diagram, so the energy difference between the Fermi level (E-F) and the lowest unoccupied molecular orbital (LUMO) decreased by 0.64 eV. The schematic band diagram demonstrates that a monotonic shift of the LUMO toward E-F was observed with increasing doping, which is in contrast to general n-type doping effects in inorganic semiconductors. Also, we experimentally observed increased electron transport characteristics of alkali metal doped Alq(3). The operating voltage at 100 mA/cm(2) decreased from 11.9 V (R-Na = 0) to 9.6 V, and the luminance at 10.5 V increased from 3575 cd/m(2) (R-Na = 0) to 9675 cd/m(2) when the Na:Alq(3) film (R-Na = 2.7) was inserted between Alq(3) and LiF/Al.