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

Abstract

The Schottky–Mott (S–M) rule has been successfully applied to evaluating work functions or Fermi energy levels of dedoped or lightly anion (p-)doped films of poly(3-arylthiophenes) (PATs) such as poly(3-phenylthiophene) (PPT) and poly[3-(4-methylphenyl)thiophene] (PMPT). It is found that the Schottky 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 probe technique are almost independent of substrates (Al, Pb, In, Bi, Pt, Au and ITO) and in fairly good agreement with the value of 5.0 eV estimated from the S–M rule. For PMPT and poly[3-(4-fluorophenyl)thiophene] (PFPT), however, their work functions are dependent on the substrate metals, indicating non-alignment of Fermi levels at the metal/PMPT or PFPT junctions under open-circuit conditions. Correction for the Fermi energy differences at the junctions reduces the metal dependencies. Corrected work functions for PMPT films are almost the same with the value of 4.9 eV estimated from the S–M rule, whereas those for PFPT are about 0.3 eV greater than the value of 5.1 eV. The discrepancy still remained for PFPT after the correction is ascribable to surface dipole layers due to the F substituents at the PFPT/air interface. Edges of the valence and conduction bands of the PAT films are estimated 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 and their interfaces with metals in air are depicted.