Chemical and thermal treatment of PEDOT:PSS thin films for use in organic light emitting diodes

We have investigated the effects of thermal and chemical treatments on the optical and electrical properties of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS), a doped polymer widely used as a buffer layer to increase the stability and the charge injection in organic light emitting diodes (OLEDs). Thermal treatments carried out on thin films in the temperature range of 120-250 degreesC did not strongly modify the structure of the polymer as proved by Raman and infrared measurements while their resistivity increased with increasing temperature. Chemical treatment of different durations by 10% hydrochloric acid showed a strong increase in the conductivity but the structure of the polymer was generally preserved. X-Ray photoelectron spectroscopic analysis of the treated films indicated a partial change of their composition, which favoured the electronic conduction upon acid treatments. When using PEDOT:PSS as a buffer layer on indium tin oxide substrates, a diffusion of indium was observed in the as-deposited polymer films. A thermal treatment most probably increased the diffusion and an acid treatment removed the diffused metal. A combination of these treatments appears to be the best way to improve the quality of the films for their use in OLEDs. (C) 2003 Elsevier B.V. All rights reserved.