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 films of the commercially important, electrically conducting polymer blend poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate), PEDOT–PSS. The issues discussed include the study of thermal effects, the influence of hydrochloric acid on the chemical and electronic structures of the films, phase segregation, as well as the energy level alignment at interfaces employing a PEDOT–PSS layer. All of these issues are important in applications of PEDOT–PSS as a hole-injecting electrode in polymer-based, light-emitting 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-ethylenedioxythiophene), the conversion of the sodium salt into free poly(styrenesulfonic acid) upon HCl treatment, and the decomposition of the free sulfonic acid component (presumably through loss of SO 3H) induced by annealing. It is also shown that phase segregation occurs in the PEDOT–PSS system, resulting in a predominance of PSS in the surface region. This issue has been studied using different approaches, including X-ray photoelectron spectroscopy studies of the sulfur S(2p) and oxygen O(1s) core levels, ultraviolet photoelectron spectroscopy of the valence band region combined with reference measurements and quantum chemical calculations, as well as variable photon energy investigations of sulfur S(2p) core levels. It is demonstrated that, in the context of the energy level alignment at the polymer–metal interfaces, PEDOT–PSS shows metallic-like behavior. Due to the latter, the insertion of a thin PEDOT–PSS layer between the hole-injecting electrode ITO and a polymer layer of poly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene) leads 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 alignment of the electrochemical potential across metal–polymer–metal structures.