Photoelectron spectroscopy of thin films of PEDOT-PSS conjugated polymer blend: a mini-review and some new results

We present an overview of the photoelectron spectroscopy studies of thin fi lms of the commercially important, electrically conducting polymer blend po ly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate), PEDO T-PSS. The issues discussed include the study of thermal effects, the influ ence of hydrochloric acid on the chemical and electronic structures of the films, phase segregation, as well as the energy level alignment at interfac es employing a PEDOT-PSS layer. All of these issues are important in applic ations of PEDOT-PSS as a hole-injecting electrode in polymer-based, light-e mitting devices. Among the most important results are the identification of the three chemically different species in pristine PEDOT-PSS, namely poly( 4-styrenesulfonic acid), poly(sodium 4-styrenesulfonate) and poly(3,4-ethyl enedioxythiophene), the conversion of the sodium salt into free poly(styren esulfonic acid) upon HCl treatment, and the decomposition of the free sulfo nic acid component (presumably through loss of SO3H) induced by annealing. It is also shown that phase segregation occurs in the PEDOT-PSS system, res ulting in a predominance of PSS in the surface region. This issue has been studied using different approaches, including X-ray photoelectron spectrosc opy studies of the sulfur S(2p) and oxygen O(1s) core levels, ultraviolet p hotoelectron spectroscopy of the valence band region combined with referenc e measurements and quantum chemical calculations, as well as variable photo n energy investigations of sulfur S(2p) core levels. It is demonstrated tha t, in the context of the energy level alignment at the polymer-metal interf aces, PEDOT-PSS shows metallic-like behavior. Due to the latter, the insert ion of a thin PEDOT-PSS layer between the hole-injecting electrode ITO and a polymer layer of poly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene) l eads to the lowering of the barrier for hole injection, independent of the work function of the underlying ITO. PEDOT-PSS is also used to show the ali gnment of the electrochemical potential across metal-polymer-metal structur es. (C) 2001 Elsevier Science B.V. All rights reserved.