Characterization of the PEDOT-PSS system by means of X-ray and ultravioletphotoelectron spectroscopy

Thin films of an electrically conducting polymer blend, poly(3,4-ethylenedi oxythiophene) oxidized with poly(4-styrenesulfonate), PEDOT-PSS, have been studied using X-ray and ultraviolet photoelectron spectroscopy. Special att ention has been paid to thermal effects and the influence of hydrochloric a cid on the chemical and electronic structures of the films, since these iss ues are important in applications of PEDOT-PSS as a hole-injecting electrod e in polymer-based light emitting devices. Three chemically different speci es were identified in pristine PEDOT-PSS, namely poly(4-styrenesulfonic aci d), poly(sodium 4-styrenesulfonate and poly(3,4-ethylenedioxythiophene). Th e HCl-treatment is shown to lead to the conversion of the sodium salt into the free poly(styrenesulfonic acid). Heating of PEDOT-PSS films resulted in a decomposition of the free sulfonic acid component (presumably through lo ss of SO3), whereas the sodium salt is thermally stable. It is shown that p hase segregation occurs in the PEDOT-PSS system resulting in a surface enti rely covered by PSS part of the PEDOT-PSS, as indicated by both XPS and UPS measurements. (C) 1999 Elsevier Science S.A. All rights reserved.