Photoemission study of direct hole injection into Alq3 by PEDOT:PSS polymer anode

Combined ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) measurements have been performed on the interface between the electron transport material tris-(8-hydroxy quinoline) aluminum (Alq3) and the conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), indium tin oxide (ITO) and polycrystalline gold. The estimated hole injection barrier φh is smallest at the PEDOT:PSS/Alq3 contact and largest at the Au/Alq3 contact. In the absence of chemistry at each interface, as indicated by XPS, φh are attributed to Fermi level pinning by interface states. The relative differences in φh are helpful in elucidating recent device measurements that show direct hole injection into Alq3 from PEDOT:PSS [P.A. Lane, G.P. Kushto, Z.H. Kafafi, Appl. Phys. Lett. 90 (2007) 023511].