Spectroscopic study on sputtered PEDOT Æ PSS: Role of surface PSS layer

The structure of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT Æ PSS) consists of an insulating PSS layer surrounding doped PEDOT grains. In this study, X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) are used to investigate the composition and electronic structure of as-loaded and lightly sputtered PEDOT Æ PSS films. The sputtered film shows a significant increase in the PEDOT/PSS ratio (from 0.12 to 0.7) as well as a build-up of the density of filled states close to the Fermi level, consistent with the removal of the insulating PSS layer and the uncovering of the highly doped conducting PEDOT Æ PSS. The thickness of PSS layer is estimated at 35 ± 5 Å. The elimination of the PSS shell reduces PEDOT Æ PSS work function to a value equal to that of highly doped PEDOT. The effect of the PSS removal on the charge injection barrier at the interface with the hole-transport material N,N 0-diphenyl-N,N 0-bis(1-naphthyl)-1,1 0-biphenyl-4,4 0-diamine (a-NPD) is also investigated. The increase in interface dipole and hole-injection barrier suggests a stronger interaction between the two materials, and points out the key role of the PSS layer in making PEDOT Æ PSS an effective hole-injection material. 2006 Elsevier B.V. All rights reserved.