Systematic control of the electrical conductivity of poly (3,4-ethylenedioxythiophene) via oxidative chemical vapor deposition (oCVD)

Systematic variation in the electrical conductivity of poly (3,4 ethylenedioxythiophene) (PEDOT) was achieved by oxidative chemical vapor deposition (oCVD). For oCVD, both the oxidant, Fe(III)Cl-3, and 3,4 ethylenedioxythiophene (EDOT) monomer are introduced in the vapor phase. A heated crucible allows for sublimation of the oxidant directly into the reactor chamber. Spontaneous reaction of the oxidant with the monomer results in the rapid (> 200 nm thick film in 30 min) formation of pi-conjugated PEDOT thin films directly onto a temperature controlled substrate. As the substrate temperature is increased from 15 degrees C to 100 degrees C, increasing conjugation length and doping level of the PEDOT chains are observed by Raman spectroscopy. The X-ray photoelectron spectroscopy (XPS) study showed that the surface consists of PEDOT polymer chains doped with Cl- ions in the absence of metallic impurities such as Fe. This contrasts to the surface composition of commonly used PEDOT:PSS films, where the PSS segregates to the surface. Scanning transmission electron microscopy (STEM) with energy dispersive X-ray analysis (EDX) study clearly demonstrates that the bulk composition is uniformly maintained throughout thickness of the film and is identical to the surface concentration. Both the substrate and air side interfaces were abrupt. The doping level could also be controlled by changing substrate temperature and accordingly, the work function was tuned from 5.1 to 5.4 eV, a range desirable for optimizing hole transport by lowering the charge injection barrier. The controllability of doping level and work function of oCVD PEDOT offers great potential advantages for optimization of the performance of organic devices. (C) 2007 Elsevier B.V. All rights reserved.