X-ray photoelectron spectroscopy study of the metal/polymer contacts involving aluminum and poly(3,4-ethylenedioxythioph
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Salaneck Department of Physics, Linko¨ping University, SE-581 83 Linko¨ping, Sweden
M. Fahlman Department of Science and Technology, Linko¨ping University, SE-601 74 Norrko¨ping, Sweden (Received 18 December 2002; accepted 19 February 2003)
The contact formed between aluminum and poly(3,4-ethylenedioxythiophene)– poly(styrenesulfonic acid) (PEDOT–PSS) derivatives was studied using x-ray photoelectron spectroscopy. The aluminum/PEDOT–PSS contact contains an interfacial layer formed by chemical reactions between aluminum and mainly poly(styrenesulfonic acid) (PSSH). These chemical interactions were studied with the help of model systems (PSSH, benzenesulfonic acid, and sodium benzenesulfonate). The preferred reaction site of aluminum is the SO3− and SO3−H+ groups of the PSS chains, giving rise to C–S–Al(–O) and C–O–Al species. The resulting contact formed consists of an insulating aluminum/PSS layer and a thin region of partially dedoped PEDOT–PSS. There is significant aluminum diffusion into films of the highly conducting form of PEDOT–PSS that have substantially less PSS at the surface. Hence, no (thick) aluminum/PSS layer is formed in this case, though the PEDOT chains close to the aluminum contact will still be partially dedoped as for the aluminum/ PEDOT–PSS case. I. INTRODUCTION
One vision for future polymeric and organic electronic devices is to replace the metal electrodes with polymers or organic materials with low enough sheet resistance so that devices become purely organic. This is because polymeric (organic) devices offer potential advantages over their inorganic counterparts in terms of ease of fabrication (possibly reel-to-reel processing), mechanical flexibility, and recycling. In the present state-of-theart, organic devices typically contain organic materials interfaced with metals. 1–3 One of the more commonly used materials in organic electronics is poly(3,4– ethylenedioxythiophene)–poly(styrenesulfonic acid) (PEDOT–PSS). PEDOT is a good hole conductor that is transparent in its p-doped state and blue-black (band gap approximately 1.6 eV) in its neutral state.4 PSS is typically added to p-doped PEDOT to act as a counter ion and to improve solubility in water.4 The electroactive polymer blend PEDOT–PSS is used in several applications including polymer light-emitting devices (as hole injecting layer at the anode side),5 field-effect transistors (electrodes),6,7 photographic film (AGFA-Gevaert, Mortsel, Belgium, antistatic coating), solid electrolyte capacitors (electrodes),8 photovoltaic devices (electrode),9 and J. Mater. Res., Vol. 18, No. 5, May 2003
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electrochemical transistors and displays (electrodes and “active” material).10 PEDOT–PSS can be used as both anode and cathode material, depending on the application. For many applications where high currents or low losses are required, PEDOT–PSS is not electrically conducting enough to be used on its own. Instead real metals have to be used to carry the current in buzz-lines, etc. Even so, the
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