“Secondary doping” methods to significantly enhance the conductivity of PEDOT:PSS for its application as transparent electrode of optoelectronic devices

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the most successful conducting polymer in terms of the practical application. It can be dispersed in water and some polar organic solvents, and high-quality PEDOT:PSS films can be readily prepared through solution processing. In addition, PEDOT:PSS is highly transparent in the visible range and has excellent thermal stability. Nevertheless, PEDOT:PSS has a problem of low conductivity. The as-prepared PEDOT:PSS films from its aqueous solution have a conductivity of lower than 1 S cm⁻¹, which severely impedes the application of PEDOT:PSS in various aspects. It has been discovered that the conductivity of as-prepared PEDOT:PSS from its aqueous solution can be significantly enhanced by adding organic compounds like high-boiling point polar organic solvents, ionic liquids and surfactants or through a post-treatment of PEDOT:PSS films with organic compounds, including high-boiling point polar solvents, salts, zwitterions, cosolvents, organic and inorganic acids. Conductivity of more than 3000 S cm⁻¹ was recently observed on PEDOT:PSS films after treated with sulfuric acid. This conductivity is comparable to that of indium tin oxide (ITO), the conventional transparent electrode material of optoelectronic devices. In addition, PEDOT:PSS has high mechanical flexibility while ITO is a brittle material. Thus, PEDOT:PSS is very promising to be the next-generation transparent electrode material. This article reviews the methods to enhance the conductivity of PEDOT:PSS, the mechanisms for the conductivity enhancements and the application of the highly conductive PEDOT:PSS films in polymer light-emitting diodes and polymer solar cells.