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

The structure of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT · 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 · 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 · PSS. The thickness of PSS layer is estimated at 35 ± 5 Å. The elimination of the PSS shell reduces PEDOT · 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′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-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 · PSS an effective hole-injection material.