Electronic structures of poly(3-arylthiophene) films and their interfaces with metals in air

The Schottky-Mott (S-M) rule has been successfully applied to evaluating wo rk functions or Fermi energy levels of dedoped or lightly anion (p-)doped f ilms of poly(3-arylthiophenes) (PATs) such as poly(3-phenylthiophene) (PPT) and poly[3-(4-methyl-phenyl)thiophene] (PMPT). It is found that the Schott ky barrier formation at the PAT/metal interface is not altered very much by surface states. Work functions of the PPT films measured by the Kelvin pro be technique are almost independent of substrates (Al, Pb, In, Bi, Pt, Au a nd ITO) and in fairly good agreement with the value of 5.0 eV estimated fro m the S-M rule. For PMPT and poly[3-(4fluorophenyl)thiophene] (PFPT), howev er, their work functions are dependent on the substrate metals, indicating non-alignment of Fermi levels at the metal/PMPT or PFPT junctions under ope n-circuit conditions. Correction for the Fermi energy differences at the ju nctions reduces the metal dependencies. Corrected work functions for PMPT f ilms are almost the same with the value of 4.9 eV estimated from the S-M ru le, whereas those for PFPT are about 0.3 eV greater than the value of 5.1 e V. The discrepancy still remained for PFPT after the correction is ascribab le to surface dipole layers due to the F substituents at the PFPT/air inter face. Edges of the valence and conduction bands of the PAT films are estima ted from the onset potentials for anodic p-doping and cathodic n-doping of the polymer films. On these bases electronic structures of the PAT films an d their interfaces with metals in air are depicted. (C) 2000 Elsevier Scien ce S.A. All rights reserved.