Highly conductive and low-work-function polymer electrodes for solution-processed n-type oxide thin-film transistors

We present an n-doped poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymer and its application in n-type oxide thin-film transistors (OxTFTs) as a source and drain electrode material. A reduced molecule of a cationic dye, methyl red (MR), was used as an effective solution-processed n-type dopant. The sequential de-doping and doping of the initially p-doped PEDOT:PSS polymer with the reduced MR (r-MR) effectively removed positive charges via cancellation by the added electrons. As a result, the electron conductivity of PEDOT:PSS increased from 3.4 S/cm to ∼51 S/cm, while its work function decreased from 4.8 eV to 3.5 eV. This is one of the lowest values of the work function reported for PEDOT:PSS. The n-doped PEDOT:PSS films were eventually applied as a suitable material to fabricate the contact electrodes of solution-processed bottom-gate top-contact amorphous indium-gallium-zinc oxide-based OxTFTs. The resultant devices exhibited electron mobility over ten times better compared to those with undoped PEDOT:PSS electrodes. Therefore, we suggest this method as a highly suitable and low-cost technique for improving electron transport in PEDOT:PSS and all solution-processed conductors. Further investigations with this method are expected to expand the application of PEDOT:PSS to other sectors of optoelectronics.