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 °C 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.